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HIGH ENERGY LASER (HEL) LETHALITY DATA COLLECTION

STANDARDS

HEL JTO

Lethality Technical Area Working Group

HIGH ENERGY LASER (HEL) LETHALITY DATA COLLECTION STANDARDS

February 2007

Contributing Authors

Dr. Craig T. Walters (CWA), Dr. Robert F. Cozzens (NRL), Dr. Brian J. Hankla (NSWCDD). Mr. Jay Howland (SMDC). Dr. William T. Laughlin (PSI). Mr. David N.

Loomis (DNL Consulting) Mr. Robert Roybal (AFRL). Mr. Barry Price (SPARTA). Dr. Ralph R. Rudder (CAVALLA),

Jorge E. Beraun (AFRL / DELE), Charles R. LaMar (USA SMDC), J. Thomas Schriempf (NAVSEA PMS 405)

THE DIRECTED ENERGY PROFESSIONAL SOCIETY P.O. Box 9874

Albuquerque. NM 87119-9874 505-998-4910 www.DEPS.org

http://www.DEPS.org

High Energy Laser (HEL) Lethality Data Collection Standards

The Directed Energy Professional Society PO Box 9874 Albuquerque, NM 87119-9874 505-998-4910 www.DEPS.org

Copyright © 2007 by the Directed Energy Professional Society. All rights reserved. Printed in the United States of America. This book, or parts thereof, may not be reproduced in any form without the permission of the publishers.

ISBN 0-9793687-0-7 ISBN 978-0-9793687-0-7

Joseph S. Accetta, Ph.D. Managing Editor

Information contained in this work was compiled from sources deemed to be reliable. However neither DEPS nor the authors guarantee the accuracy or completeness of any information published herein and neither DEPS nor its authors nor employers shall assume any liability for any errors, omissions or damages arising out of use of this information.

m

http://www.DEPS.org

PREFACE

This publication represents the first in a series of DEPS textbooks, handbooks and monographs on topics in Directed Energy. The project from which this work was derived was originally funded by the High Energy Laser Joint Technology Office (HEL-JTO). JTO was established in 2000 for the purpose of developing and executing a comprehensive investment strategy for HEL science and technology that would underpin weapons development. The JTO is currently sponsoring 80 programs across industry, academia, and government agencies with a budget of approximately $60 million. The competitively awarded programs are chosen to advance the current state of the art in HEL technology and fill technology gaps, thus providing a broad capability that can be harvested in acquisition programs hy the military services.

The information that was acquired in this program and many others before it forms a significant intellectual legacy of research in Directed Energy that spans nearly 40 years and represents the work of many dedicated scientists and engineers. These data collection standards were compiled by the HEL Joint Technology Office (JTO), Lethality Technical Area working Group (TAWG), Integrated Product Team (IPT) with significant reliance on the subject matter expertise of Drs. Robert Cozzens, Bill Laughlin, Ralph Rudder, and Craig Walters, all with significant experience in the field of HEL lethality. In addition to the current research published in the Journal of Directed Energy, a DEPS educational objective is to make this work available to the DE community through a series of publications on specific topics in Directed Energy. This first publication "High Energy Laser (HEL) Lethality Data Collection Standards" was published in loose leaf notebook form to take account of its likely use as a laboratory reference document that will be updated from time to time.

We acknowledge the effort and dedication of the authors and those DE researchers who have directly or indirectly contributed to this work over the years. We commend the foresight of the DEPS Board of Directors to expend the effort necessary to bring this information to the community. Lastly, we acknowledge the expert copy editing of Kathie Coogler-Prado.

Joseph S. Accetta David N. Loomis

Albuquerque, New Mexico February, 2007

IV

CONTENTS SECTION PAGE 1 INTRODUCTION AND BACKGROUND 1

: PURPOSE/SCOPE 1

3 DATA COLLECTION STANDARDS 1

3.1 MEASURED CHARACTERISTIC: LASER BEAM POWER AND PULSE FORMAT 1

3.1.1 Purpose of the Measurement 1 3.1.2 Technical Approaches to the Measurement 1 3.1.3 Calibration of Measurement Hardware 5 3.1.4 Data Acquisition and Logging 6

3.1.4.1 Measurement Bandwidth and Sampling Rate 6 3.1.4.2 Data Acquisition System Calibration 7 3.1.4.3 Quick-Look Data Extraction 8

3.1.5 Data Reduction 8 3.1.6 Uncertainty of Measurement Results 9 3.1.7 Output to Database 10

3.2 MEASURED CHARACTERISTIC: LASER BEAM SPATIAL PROFILE 11

3.2.1 Purpose of the Measurement 11 3.2.2 Technical Approaches to the Measurement 11

3.2.2.1 Beam Sampling for Replica Target Plane Irradiance Profile Measurement 12 3.2.2.2 Dedicated Target Plane Irradiance Profile Measurement 15

3.2.3 Calibration of Measurement Hardware 17 3.2.4 Data Acquisition and Logging ; 17

3.2.4.1 Image Acquisition 17 3.2.4.2 Image Acquisition System Calibration 18 3.2.4.3 Quick-Look Data Extraction 18

3.2.5 Data Reduction 19 3.2.6 Uncertainty of Measurement Results 19 3.2.7 Output to Database 20

3.3 MEASURED CHARACTERISTIC: WAVELENGTH 22

3.3.1 Purpose of the Measurement 22 3.3.2 Technical Approaches to the Measurement 22 3.3.3 Calibration of Measurement Hardware 23 3.3.4 Data Acquisition and Logging 24 3.3.5 Data Reduction 24 3.3.6 Uncertainty-of Measurement Results 24 3.3.7 Output to Database 25

3.4 MEASURED CHARACTERISTIC: TARGET ABSORPTANCE 26

3.4.1 Purpose of the Measurement 26 3.4.2 Technical Approaches to the Measurement: 26 3.4.3 Calibration of Measurement Hardware 27 3.4.4 Data Acquisition and Logging 28 3.4.5 Data Reduction 28 3.4.6 Uncertainty of Measurement Results 28 3.4.7 Output to Database 29

3.5 MEASURED CHARACTERISTIC: THERMAL COUPLING COEFFICIENT 30

3.5.1 Purpose of the Measurement 30 3.5.2 Technical Approaches to the Measurement 30 3.5.3 Calibration of Measurement Hardware 31 3.5.4 Data Acquisition and Logging 33 3.5.5 Data Reduction 33

CONTENTS (Continued)

PAGE rtainty of Measurement Results 35

Output to Database 35 SURED CHARACTERISTIC: PENETRATION TIME 36

6 J Purpose of the Measurement 36 Technical Approaches to the Measurement 36

Optical Methods 36 3.6.2.2 Video Cameras 39

Witness Thermal Response 40 3.6.2.4 Break Wires 40

3.6 J Calibration of Measurement Hardware 41 3.6.4 Uncertainty of Measurement Results 41 3.6.5 Output to Database 42

3.7 MEASURED CHARACTERISTIC: IMPULSE 43 3.7.1 Purpose of the Measurement 43 3.7.2 Technical Approaches to the Measurement 43 3.7.3 Calibration of Measurement Hardware 43 3.7.4 Data Acquisition and Logging 44 3.7.5 Data Reduction 44 3.7.6 Uncertainty of Measurement Results 44 3.7.7 Output to Database 45

3.8 MEASURED CHARACTERISTIC: PLASMA/PLUME ABSORPTION / SCATTERING OF THE LASER BEAM 46 3.8.1 Purpose of the Measurement 46 3.8.2 Technical Approach to the Measurement 46 3.8.3 Calibration of Measurement Hardware 49 3.8.4 Data Acquisition and Logging 49 3.8.5 Data Reduction 49 3.8.6 Uncertainty of Measurement Results 49 3.8.7 Output to Database 50

3.9 MEASURED CHARACTERISTIC: PLASMA FORMATION 51 3.9.1 Purpose of the Measurement 51 3.9.2 Technical Approach to the Measurement 51 3.9.3 Calibrations Required 52 3.9.4 Data Acquisition and Logging 53 3.9.5 Data Reduction 53 3.9.6 Uncertainty of Measurement Results 53 3.9.7 Output to Database 54

3.10 MEASURED CHARACTERISTIC: AMBIENT PRESSURE AND OPTICAL CONTAMINATION 55 3.10.1 Purpose of the Measurement 55 3.10.2 Technical Approaches to the Measurement 55 3.10.3 Calibration of Measurement Hardware 56 3.10.4 Data Acquisition and Logging 57 3.10.5 Data Reduction 57 3.10.6 Uncertainty of Measurement Results 57 3.10.7 Output to Database 58

3.11 MEASURED CHARACTERISTIC: AIR FLOW SIMULATION 59 3.11.1 Purpose of the Measurement 59 3.11.2 Technical Approaches to the Measurement: 59 3.11.3 Calibration of Measurement Hardware 61 3.11.4 Uncertainty of Measurement Results 61 3.11.5 Output to Database 62

VI

CONTENTS (Continued) SECTION PAGE

3.12 MEASURED/REPORTED CHARACTERISTICS: TARGET AND MATERIAL DESCRIPTION 62

3.12.1 Purpose of Specimen Description 62 3.12.2 Examples of Material Description and Characterization 64 3.12.3 Measurements and Accuracy 65 3.12.4 Organization by Materials Family 65 3.12.5 Pre-Test Material/Target Description 65 3.12.6 Post Test Material Damage Assessment 68

4 REPORTING UNCERTAINTY OF MEASUREMENTS 70

4.1 DEFINITIONS 70

4.2 EVALUATION OF UNCERTAINTY CONTRIBUTIONS 72

4.3 CALORIMETER CALIBRATION EXAMPLE 74

5 REFERENCES 76

6 ABBREVIATIONS AND ACRONYMS 77

Vll

FIGURES PAGE

ÏCRE 1. SCHEMATIC ARRANGEMENT FOR BEAM SAMPLING WITH WEDGE WINDOW 3

JURE 2. SCHEMATIC ARRANGEMENT FOR BEAM SPATIAL PROFILE MEASUREMENT WITH WEDGE WINDOW 14 JURE 3. SCHEMATIC OF TRAM REFLECTOMETER 27

FIGURE 4. ILLUSTRATION OF DATA REDUCTION FOR TCC MEASUREMENT 34 FIGURE 5. TARGET-HOLE TRANSMISSION DIAGNOSTICS 48 FIGURE 6. PEAK BRIGHTNESS TEMPERATURE FOR GRAPHITE / EPOXY 52 FIGURE 7. PROBABILITY DISTRIBUTION FUNCTIONS AND ASSOCIATED UNCERTAINTY ESTIMATES 73 FIGURE 8. UNCERTAINTY ESTIMATE FOR BLACK 8 BALL CALORIMETER AT CALIBRATION 74

TABLES

TABLE 1. MATERIAL FAMILY TREE 67 TABLE 2. COVERAGE FACTOR PROBABILITY 71 TABLE 3. UNCERTAINTY ESTIMATE AT CALIBRATION 74

vin

Introduction and Background

High Energy Laser (HEL), lethality, and vulnerability data have been taken by

DUS organizations for more than 30 years. Well-substantiated data are pertinent and

valuable to support service mission lethality analyses versus threat targets of interest.

However, a significant amount of this data is very poorly characterized and not useable.

The lethality community determined that a standardized methodology should be applied

to future lethality experiments to maximize the utility of the resultant data and minimize

waste.

Under the auspices of the High Energy Laser Joint Technology Office (HEL

JTO), an Integrated Project Team (IPT) consisting of personnel from each of the

services and several subject matter experts (SME's) was assembled and tasked with

developing detailed data collection standards for laser lethality testing. The team (each

with twenty or more years' of practical expertise) first identified those critical measurable

data that require standards. Individual team members were assigned responsibilities for

authoring draft standards. The drafts were subjected to critical reviews and refined into

final form. The resulting standards are presented in this document. It is strongly

anticipated and encouraged that experimentalists use these standards to plan, conduct,

analyze, and report future laser lethality tests.

It may be noted that the work reported here was part of a larger effort, sponsored

by the HEL JTO, to develop lethality documentation architecture. Other tasks included

(1) establishing a methodology for evaluating data from past tests and analyses, and (2)

developing a systematic mechanism to summarize both existing and future data. These

data collection standards are expected to aid in the first of the tasks and serve as a

benchmark for data evaluation.

Purpose / Scope

Of the potentially many measurable data associated with a particular laser

ithality experiment, the ones of importance are dependent on the laser being used, the

target, and the phenomenology being studied. The IPT identified a long list of

measurable data identified four primary categories 1 ) laser beam properties, 2) target

response, 3) test stand/facility properties, and 4) target/test specimen characteristics

and identified valuable secondary data.

1 ) Laser Beam

Laser Beam Power and Pulse Format

Laser Beam Spatial Irradiance Profile

Wavelength

2) Target Response

Target Material Absorptance

Thermal Coupling Coefficient

Penetration Time

Impulse

Plasma/Plume Absorption/Scattering of the Laser Beam

Plasma Formation

3) Test Stand/Facility

Ambient Chamber Pressure and Optical Contamination

Air Flow Simulation

4) Target/Test Specimen

Laser Target/ Test Specimen and Material Description

Standards under the first three categories are written in a consistent format that

covers the essentials associated with the measurement: the purpose, the technical

approaches to the measurement, calibration of measurement hardware, data acquisition

1

: egging, data reduction, and uncertainty of measurement results. Then, the format

for data entry to a proposed future database is specified.

The estimation of the uncertainty of the measurement results is particularly

critical to use in both data collection and in data analysis for making reliable predictions

of laser lethality in mission scenarios. In each of the standards, elements of the

measurement contributing to uncertainty of the result are listed. The list is not

exhaustive, but is meant as a guide. It is critical that the experimenter carefully estimate

these component uncertainties for a particular test and make appropriate combinations

according to the National Institute of Standards and Technology (NIST) guidelines to

report an uncertainty for the final result. A brief summary of the NIST guidelines for

reporting uncertainty of measurement results is presented in Section 4.0 for

convenience.

The IPT SME's recognize that with the continuing evolution of technology, the

state-of-the-art collection standards presented in this document will gradually become

obsolete. Consequently, the team recommends that periodic revisions and updates be

accomplished to reflect improvements and innovations as available.

Data Collection Standards

3.1 Measured Characteristic: Laser Beam Power and Pulse Format

3.1.1 Purpose of the Measurement

The measurement of laser beam power incident on the test article is one of the

most important diagnostics in lethality testing because the nature of the interaction of

the laser beam with the materials in the test article is critically dependent on the beam

irradiance at the interaction site. The beam irradiance is, in turn, a function of the beam

power and the spatial distribution of the beam at the interaction surface. For pulse

lasers, the temporal history of power also provides information on the actual pulse

widths and pulse repetition rates delivered. It should be noted that there is a very useful

International Standards Organization (ISO) standard for measurement of beam power,

energy, and temporal characteristics (ISO 11554)[11 which should be consulted by

experimenters. However, the focus of the ISO standard is primarily on characterizing

laser devices for consistent tabulation of manufacturer's specifications and does not

address all the needs specific to high energy laser lethality tests.

3.1.2 Technical Approaches to the Measurement

The general approach to measurement of laser beam power during a lethality

test is to sample the laser beam with a linear sampling device that splits off a known

small fraction of the main laser beam to form a diagnostic beam directed to a power

measuring instrument. As long as the sampling fraction is known accurately and stays

constant, the beam power measured in the diagnostic beam provides a direct measure

of the power delivered to the target during the test. If the sampling fraction varies,

uncertainty in the measured power increases proportional to the variation.

The sampling device should be the last element in the optical train before the

main laser beam is incident on the test article. If this is not feasible, the beam sampler

should be as close to being the final optical element as possible to minimize errors. The

nature of the beam sampler depends upon the laser wavelength and beam power level.

For low or moderate beam power levels, a beam sampling wedge window or

beam splitter with low reflectivity coatings on each surface is often suitable. The coated

wedge window has the advantage that two diagnostic beams are generated (e.g., one

for beam power and one for spatial profile). If the window is uncoated, multiple

reflections lead to an array of replica beams some of which have very large attenuation

factors for measuring high power beams.

Fused silica wedge windows are often used for visible/near infrared wavelengths

and ZnSe, Cleartran, NaCI, etc., windows have been used for longer wavelength

infrared. The primary requirements for the transmissive material are very low

absorptivity at the laser wavelength and that the surfaces have stable reflectivity and

good optical quality. The allowable upper bound on absorptivity may be estimated by

calculating the bulk heating that would result from the irradiance at the wedge from the

beam power and run time expected.

Heating of more than a few degrees C might change the calibration factor. This

can be verified by independent measurements to determine the uncertainty introduced.

Use of this type of sampler requires careful attention to maintaining the coating or bare

surface reflectivity stability in the environment of the test facility. A 10 percent change in

the reflectivity of the surface changes the calibration and power reading by 10 percent.

The humidity in the environment can change the dielectric coating thickness and

introduce uncertainty in the beam sampling fraction calibration if it is not controlled.

One strategy is to flow clean dry purge air over the optic. The angle of incidence on the

window must be controlled and sensitivity to variations in beam polarization must be

understood and accounted for.

The reflectance of coated windows and beam splitters may be very sensitive to

both angle of incidence and polarization state. The reflectance of the beam splitter

should be measured for the wavelength to be used, as well as the specific angle and

polarization state. Also, measurements should be made for additional angles so that an

error assessment can be made with regard to the precision of the incidence angle and

the divergence/convergence of the laser beam at the beam splitter.

For moderate laser powers, an uncoated beam splitter may be preferred because

at near normal incidence angles (up to about 1 D degrees), the reflectance is insensitive

to angle and polarization state. If the refractive index is known at the laser wavelength,

the reflectance may be calculated with very good precision. Care must be taken to

trace multiple reflections and capture all of the power in the diagnostic instrument.

A typical arrangement for beam power measurement with a wedge window beam

sampling element is shown in Figure 1. If a parallel face beam splitter is used, all of the

reflected power would be directed to the detector.

îeam Power Detector

Beam Sampling Wedge

Test Article

Figure 1. Schematic Arrangement for Beam Sampling with Wedge Window

The experimenter should be very cautious about placing a beam splitter too close

to the test article, especially for tests conducted at reduced atmospheric pressure or

vacuum. Contaminants may deposit on the exit surface of the splitter and alter its

reflectance (as well as its transmittance). In general, it is recommended that a beam

sampling element not serve the dual purpose of a vacuum chamber entrance window.

For atmospheric pressure measurements, a high velocity air knife is normally used to

protect the beam splitter from test article éjecta.

Several alternative approaches have been employed for sampling high-power

beams for beam power measurement during a test. Diagnostics employing low-

efficiency grating-like elements have been used to sample high-power beams. For

example, grooves placed on a beam steering or focusing mirror will produce a set of

secondary beams by diffraction with most of the power in the zeroth order directed to

target. A wire-type grating produces similar results, but the wires must actually be

small-diameter tubes carrying a coolant. Another grating type element is a beam

steering mirror with small holes arranged in a rectangular array. The holes transmit a

fraction of the beam through the mirror to the power measuring instrument. In some

cases, a beam steering mirror that is liquid cooled may be used as a power measuring

instrument. Careful measurement of the coolant inlet and outlet temperatures and the

flow rate provides a measure of the power lost from the beam by absorption in the

mirror coating. If the coating absorption fraction can be demonstrated to be stable, this

technique can be useful for high-power beams. The temporal resolution will be limited

by thermal response time of the mirror structure.

For laser transport optics employing dielectric mirrors, the small fraction of beam

power transmitted through the dielectric layer may be used as a diagnostic replica

beam. The substrate must be transparent to the laser wavelength and the back surface

of the optic must have the same optical quality as the front surface. Measurements

must be taken to assure that heating of the coating does not alter the coating

transmittance at high irradiance.

Having sampled the main laser beam in a stable manner, the diagnostic beam

power may be measured in real time by any of several commercially available power

meters. For example, if a 100 kW beam has a two percent sampled diagnostic beam, a

power meter of 2 kW capacity may be used for the power measurement. In some

cases, it may be necessary to sample the diagnostic beam to get the secondary

sampled beam down to a power level within the range of available power meters. In all

cases, it is important to follow the manufacturer's guidelines on peak power limits for the

detector sensing surface. Exceeding these limits will often damage the sensing surface

and change the calibration of the instrument. Note that most commercial power

measuring devices have a very slow response time and therefore are only suitable for

lasers with very steady output power. For lasers with rapidly varying power, it is

preferable to use a total energy calorimeter in conjunction with a suitable fast response

photo-detector. This combination will yield detailed data on the laser's power-time

history.

The approaches to beam power measurement discussed above were in the

Dntext of a continuous wave (CW) laser lethality test. The beam sampling techniques

for rep-pulse and single pulse tests are identical to those for CW tests; however, more

attention must be given to peak power effects on the materials employed. The power

measurement on the sampled beam may be quite different depending on the type of

test. For single-pulse tests or short bursts of pulses, a calorimeter is used in place of a

power meter. The total energy measured may be divided by the laser run time to get

the average power over the run. This is important information, but is not sufficient to

understand the laser effects on the material. If the pulse repetition rate is not too high

(< 2 kHz), a fast response pyro-electric detector may be used on a second diagnostic

beam to measure the total energy in each individual pulse. A measurement of relative

power versus time may be made on a third diagnostic beam with a photo-detector. This

relative power history can, in principle, be converted to absolute power versus time

through adjustment of a multiplicative constant that forces the integral of the power

history to match the measured total energy. Care must be taken to have sensors and

instrumentation with sufficient bandwidth to capture the data on all relevant time scales

of interest. The details of the approach depend heavily upon the nature of the pulse

format employed. For a rep-pulse test of long duration with high repetition rates, the

standard power meter captures most information of interest, with peak powers

determined by appropriate intermittent high-data-rate sampling of a photo-detector

viewing a second or third diagnostic beam.

3.1.3 Calibration of Measurement Hardware

Two types of calibration are required for accurate beam power measurement

during a lethality test: (1) calibration of the beam sampling fraction, and (2) calibration of

the main beam power meter or calorimeter. The beam sampling fraction is measured

by placing the main beam power meter (calorimeter) or laboratory standard power

meter (calorimeter) at a position in the beam train after all optical elements that are to

be used in the test.

Depending on the beam delivery optical train, this position could be well in front

of the target plane or beyond the target plane so that the beam may expand and fill the

power meter (calorimeter) aperture. The latter position may be used only in cases

where air breakdown is not expected at an intermediated beam waist. Simultaneous

measurement of the power in the main beam and diagnostic beam with the laser

running under conditions planned for the lethality test provides the best calibration of the

sampling fraction. This requires that the main beam power meter (calorimeter) be able

to handle the full power or energy of the laser.

If such a main beam power meter (calorimeter) is not available, the most capable

instrument available must be used and the calibration must be conducted at lower laser

power or energy. Care must be taken in estimating possible error introduced by low-

power calibration of the beam sampling fraction. In critical cases, a low-power

secondary monitor beam (slightly off-axis) could be used to monitor the sampling

fraction during the high-power laser run. To assure that the high-power laser lethality

test has not altered the beam sampling element, the beam sampling fraction should be

re-calibrated after the test.

The second calibration is calibration of the main beam power meter or

calorimeter. In many cases, this may be performed by the manufacturer who provides a

certificate of calibration that indicates traceability to NIST. For very-high-power

instruments that are often custom-made, direct calibration by NIST or a NIST-qualified

calibration service provider is appropriate.

3.1.4 Data Acquisition and Logging

The data acquisition approach depends heavily upon the laser type, run time,

and facility equipment. In general, if digital data acquisition hardware is not available at

the test site, it should be brought in by the experimenter. A wide variety of low-cost data

acquisition cards are available for small computers for this purpose. Several aspects of

data acquisition for the power measurement are discussed below.

3.1.4.1 Measurement Bandwidth and Sampling Rate

Most laser beam power measuring devices have a response time set by the

characteristics of the sensor (e.g., thermal response time) or the readout electronics (if

integral with the meter). The input bandwidth of the data acquisition instrument and the

digital sampling rate should be consistent with the response time of the power meter so

6

that the data acquisition system captures the power fluctuation information provided by

the power meter.

For CW laser measurements, this approach is straightforward. For repetitively

pulsed lasers, two or more channels of data acquisition are often used. One channel is

as described above, which captures the average laser power with an averaging time

equivalent to the power meter time constant. A second channel might record the output

of a pulse detector, such as a pyro-electric sensor. A pyro-electric sensor will typically

integrate the power delivered over the duration of the pulse and provide a measure of

the pulse energy in each pulse.

Commercial readout devices record the pulse energy at pulse repetition rates up

to 2 kHz. If the peak power in each pulse needs to be recorded, a third channel might

digitize and store the direct output from a fast photo-detector. The bandwidth and

sampling rate of the digitizer must be fast enough to capture all of the fluctuations within

the pulse. The approach for this third channel depends strongly upon the laser

pulsewidth, the pulse repetition rate, and the run time. For wide pulses (ms), complete

recording of the power history is usually feasible. For short pulses (ns) at low duty

cycle, burst digitization may be the best approach. For very short pulses (fs to ps),

pulse sampling techniques must be used.

3.1.4.2 Data Acquisition System Calibration In cases where the calibrated power meter (calorimeter) provides a calibrated

value directly in digital form, calibration of the data acquisition system is not necessary

except to check that the correct digital values are being passed to the system. If the

power measuring instrument outputs a calibrated analog voltage that must be digitized

by the data acquisition system, the data acquisition system itself should be calibrated by

a standard voltage reference source.

A baseline check should also be conducted, when feasible, wherein a laser test

is run as it would be in an actual test, but with the diagnostic beam diverted or blocked

from the power meter (calorimeter). This test determines a typical noise floor and

baseline offset for the power measurement and may help find potential sources of error

or drift (e.g., electrical noise, wind tunnel air, or beam back reflections affecting

the sensor).

3.1.4.3 Quick-Look Data Extraction

The data acquisition system must be set up so that key metrics of the power

history are available in real time to support changes in setup for subsequent tests.

These metrics might include total energy in the run, dwell time, and average power. A

quick plot of the power history may also be valuable. The metrics extracted from the

data are entered into the experimenter's data log spreadsheet to support real-time

tracking of test results.

3.1.5 Data Reduction

For CW lasers the data reduction step is straightforward. To obtain the beam

power incident on the test article, the raw power meter readings are simply multiplied by

the beam sampling calibration factor, F, which is given by F= T/f, where Tis the

transmittance from the sampling surface to the target and fis the fraction of the

radiation incident on the sampling surface that is delivered to the power measuring

instrument. When a calorimeter is used, the total energy incident on the test article is

equal to F times the energy measured by the calorimeter in the sampled beam. This

energy is then used to calibrate the relative-power temporal record acquired with a

photodiode. FEca¡ = kpjVPD (t)dt

where, Eca¡ is the calorimeter energy (in J), kp is the conversion constant

determined by the calibration (in W/V), and Vpoif) is the temporal record of the

photodiode (in V). The integral must be taken over the interval of the laser pulse. Care

must be taken to eliminate baseline offset or drift that erroneously adds to the integral.

The constant, kp, may then be applied to the voltage record to determine power versus

time.

A similar approach may be used for determining the power for repetitively pulsed

laser beams, but even more care must be taken to avoid baseline contributions to the

integral. In cases where a pulse energy detector was employed in addition to the photo-

•actor, the integral results may be tested against the individual pulse energy

-easurements.

In addition to power measurements, the power records provide measures of

actual dwell time, pulsewidth, and pulse repetition rates. Simple algorithms may be run

for the temporal records to extract these metrics as well as pulse rise and fall time if

needed.

3.1.6 Uncertainty of Measurement Results

The uncertainty in the measurement should be estimated by the experimenter

and included in the report along with the basis for the estimate. This uncertainty should

be based on all of the independent contributions to error associated with the

measurement method including the instrumentation, the data acquisition hardware,

calibration factors, and input information from other sources. The combined uncertainty

calculation should be performed and reported according to NIST guidelines. Factors to

be considered in the beam-power-measurement uncertainty estimate include:

• Power meter or calorimeter manufacturer's specifications on accuracy and

precision,

• NIST calibration reported accuracy values,

• Standard deviation of beam sampler calibration result,

• Baseline drift during the tests,

• Accuracy of beam sampling fraction estimate,

• Power stability (if using a power meter),

• Run time measurement precision (if using a calorimeter), and

• Measurement scale settings used in the tests.

The last item is very important because, if the full range of a digitizer is not used

or the full scale of an amplifier is not used, the result could be subject to significant

digital or amplifier noise error. Generally, the uncertainty in the result should be

reported in the units of the measurement rather than percentages and should be

calculated by the NIST guidelines.

3.1.7 Output to Database

The result of the power measurement is entered into the database in several

forms depending on the nature of the laser test.

Tabular Data

• ÇW Laser Test: Average Power, Maximum Power, Minimum Power, Dwell Time;

uncertainties

• Repetitive Pulse Laser Test: Average Power, Average Pulse Peak Power,

Average Pulse Energy, Dwell Time, Average Pulsewidth FWHM (full width at half

maximum power), Pulse Repetition Rate; uncertainties

• Burst Laser Test: Average Power, Macro-pulse Average Power, Average Micro-

pulse Peak Power, Dwell Time, Macro-pulse Pulse Width, Macro-pulse

Repetition Rate, Micro-pulse Width FWHM (full width at half maximum power),

Micro-pulse Repetition Rate; uncertainties

• Single Pulse Laser Test: Peak Power, Pulse Energy, Pulse Width FWHM (full

width at half maximum power), Rise Time (10% to 90% of peak power), Fall Time

(90% to 10% of peak power); uncertainties

File Data

• Raw temporal records of power versus time

10

3.2 Measured Characteristic: Laser Beam Spatial Profile


The measurement of laser beam spatial profile incident on the test article is very

important in lethality testing because the nature of the interaction of the laser beam with

the materials in the test article is critically dependent on the beam irradiance at the

interaction site. The beam irradiance is, in turn, a function of the beam power and the

spatial distribution of the beam at the interaction surface. In recent years, considerable

effort in the commercial laser community has been devoted to development of

standards for spatial profile measurement and standard metrics for characterizing

beams. The experimenter is advised to consult the ISO 13694[2J standard for additional

information on this subject.

3.2.2 Technical Approaches to the Measurement The general approach to measurement of laser beam spatial profile during a

lethality test is to sample the laser beam with a linear sampling device that splits off a

known small fraction of the main laser beam to form a diagnostic beam directed to a

beam profile measuring instrument. The diagnostic beam must form a replica target

plane that has a spatial profile identical to that of the target plane profile incident on the

test article. If forming the replica plane is impractical or the beam power is so high that

beam sampling devices are not reliably linear, dedicated tests with profile recording

material placed at the target plane may be conducted before and after the lethality test.

The latter method may be less accurate because the beam profile may change

between the lethality test and the dedicated profile measurement. An estimate of the

uncertainty introduced by sampling before and after a lethality test may be obtained by

doing a series of dedicated profile measurements at different times during a typical laser

run sequence and analyzing the irradiance profile variation.

The most robust approach would employ simultaneous beam sampling

diagnostics and dedicated target plane measurements to calibrate the beam sampling

technique. Both approaches are discussed below.

11

3.2.2.1 Beam Sampling for Replica Target Plane Irradiance Profile Measurement For the diagnostic beam approach, the sampling device should be the last

element in the optical train before the main laser beam is incident on the test article. If

this is not feasible, the beam sampler should be as close to being the final optical

element as possible to minimize errors. In either case, care must be taken to assure

that the diagnostic beam is a true replica of the main beam between the sampling

device and the target plane.

Furthermore, the replica target plane position must be established accurately, so

that there is a direct one-to-one correspondence between spatial dimensions in the

replica and the actual target planes. This may be done by careful measurement of the

distance of the replica and target planes from the beam sampling plane.

For slowly converging beams, the distance measuring process does not require

high accuracy (± 1 mm typical). For higher precision, the beam profiler may be placed

alternately to view the profile at the target plane and replica plane using very low power

beams. The replica plane position is adjusted until the beam image size agrees for both

planes. A hard aperture in the beam helps with this setup procedure. The nature of the

beam sampler will depend upon the laser wavelength and beam power level.

For low or moderate beam power levels, a beam sampling wedge window with

low reflectivity coatings on each surface is often suitable. The coated wedge window

has the advantage that two diagnostic beams are generated (e.g., one for beam power

and one for spatial profile). If the window is uncoated, multiple reflections lead to an

array of replica beams some of which have very large attenuation factors for measuring

high power beams.

Fused silica wedge windows are often used for visible/NIR wavelengths and

ZnSe, Cleartran, NaCI, etc. windows have been used for longer wavelength IR. The

primary requirement for the transmissive material is very low absorptivity at the laser

wavelength and that the surfaces have stable reflectivity and good optical quality.

The suitability of a given wedge window for a given power level may be

estimated by calculating the heating non-uniformity that would deflect a sampled ray by

one resolution element (pixel) in the plane of the beam profile sensor. This heating

12

level may be compared to the heating level anticipated due to the small amount of

absorption in the wedge for the planned beam irradiance on the wedge. Use of this

type of sampler requires careful attention to maintaining the coating or bare surface

reflectivity stability in the environment of the test facility.

Angle of incidence on the window must be controlled and sensitivity to variation

in the beam polarization must be understood and accounted for. The reflectance of

coated windows and beam splitters may be very sensitive to both angle of incidence

and polarization state. The reflectance of the beam splitter should be measured for the

wavelength to be used, as well as the specific angle and polarization state. In addition,

measurements should be made for additional angles so that an error assessment can

be made with regard to the precision of the incidence angle and the

divergence/convergence of the laser beam at the beam splitter.

For moderate laser powers, an uncoated beam splitter or wedge window may be

preferred because at near normal incidence angles (up to -10 deg), the reflectance is

insensitive to angle and polarization state. If the refractive index is known at the laser

wavelength, the reflectance may be calculated with very good precision. If a parallel

face beam splitter is used, one face must be antireflection coated to avoid image

distortion. For the uncoated wedge, care must be taken to trace multiple reflections and

capture only one replica beam for imaging.

A typical arrangement for beam spatial profile measurement with a wedge

window element is shown in Figure 2.

13

Spatial Profile Imager

Replica Target Plane

Beam Sampling Wedge Actual Target

Plane

Laser Beam

Focusing Lens

Test Article

Figure 2. Schemat ic Ar rangement for Beam Spatial Profile Measurement wi th W e d g e W i n d o w

The experimenter should be very cautious about placing a beam splitter too close

to the test article, especially for tests conducted at reduced atmospheric pressure or

vacuum. Contaminants may deposit on the exit surface of the splitter and alter its

reflectance (as well as its transmittance). In general, it is recommended that a beam

sampling element not serve the dual purpose of a vacuum chamber entrance window.

For atmospheric pressure measurements a high velocity air knife is normally used to

protect the beam splitter from test article éjecta.

Several alternative approaches have been employed for sampling high-power

beams for irradiance profile measurement during a test. Diagnostics employing low-

efficiency grating-like elements have been used to sample high-power beams. For

example, grooves placed on a beam steering or focusing mirror produce a set of

secondary beams by diffraction with most of the power in the zeroth order directed to

the target. Each higher order beam is a replica of the zeroth order beam and may be

14

used for spatial profile measurement. A wire-type grating produces similar results, but

the wires must actually be small-diameter tubes carrying a coolant.

For near-normal-incidence-angle laser transport optics that employ dielectric

mirrors, the small fraction of beam power transmitted through the dielectric layer may be

used as a diagnostic replica beam. The substrate must be transparent to the laser

wavelength and back surface of the optic must have the same optical quality as the

front surface. Measurements must be taken to assure that heating of the coating does

not alter the coating transmittance at high irradiance.

For measurements of beam profile on a replica beam, electronic imaging devices

are most suitable. The most cost-effective approach for visible/NIR laser beam profiling

is a simple industrial silicon CCD video camera. While infrared imaging arrays tend to

be more expensive, they are now available for most IR laser wavelengths of interest.

Imaging optics are arranged to re-image the replica target plane to the camera sensor

(as illustrated in Figure 2) with an appropriate de-magnification to capture the entire

irradiance profile footprint. Because of the high sensitivity of the camera sensor, high

levels of attenuation are usually used in the optical train.

If significant power levels remain in the diagnostic beam, these should be

rejected by secondary beam sampling or use of a low transmission reflector for the first

attenuator. An alternative method of capturing the beam profile for some beam

geometries (e.g., large spots) is to place a uniform Lambertian scatterer (e.g., a Macor

machineable ceramic plate) at the replica target plane. The scattered light (backward or

forward) is then imaged with the electronic imager at an appropriate magnification.

3.2.2.2 Dedicated Target Plane Irradiance Profile Measurement

In cases where it is impractical to form a suitable replica beam for spatial profile

measurements, a beam profiling material may be placed in the actual target plane in

dedicated tests before and after the lethality test. A typical profiling material is a linear

ablator, such as polymethylmethacrylate (PMMA) that has been used in profiling high-

power C02 laser beams. In using this approach, gas flow over the material is employed

to avoid beam absorption by ablated material. A short run of the laser produces a crater

in the profiling material. The run time should be such that the maximum depth of

15

^Nation does not exceed about 30 percent of the beam diameter. If the ablation is

Inear over the range of irradiance values encountered, the local recession depth in the

material is directly proportional to the local irradiance averaged over the time of the

laser run. This method is most useful for lasers that do not vary much during a run or

do not vary much from run to run, but is not as reliable as direct measurement of beam

profile on a replica beam during a lethality test.

Another dedicated target plane measurement technique employs a thin sacrificial

metal plate of low thermal conductivity that is coated on both sides with a uniform high-

temperature coating with an absorptivity near unity. t3] The front surface of the plate

absorbs the incident beam without ablation, the plate heats in proportion to the local

irradiance, and the back surface temperature profile is read with an infrared array

imaging camera.

The main consideration in designing the plate is to assure that the heat

conduction in the plate is one-dimensional over the laser exposure and temperature

measurement time interval, i.e., the plate thickness and the characteristic conduction

distance corresponding to the exposure time, d= {K!)V2, are much less than the profile

variation scale of interest (lateral resolution). Here K is the thermal diffusivity of the

metal plate and t is the exposure time.

Generally, exposures will be relatively short for high power beams, so care must

be taken in rapid shuttering of the beam, retargeting of the beam, or translation of the

plate to avoid profile distortion. If the required infrared imager is available, this method

is superior to ablation material methods, because the irradiance profile is rapidly

available in digital form without the need for crater depth measurements.

An alternative or supplementary technique for dedicated beam spatial profile

measurements is to place an aperture in the target replica plane and to position a power

or energy measuring device directly behind the aperture. This 'Power-in-the-Bucket'

measurement yields the average irradiance (or fluence) over the area being sampled.

Sequential measurements with different size apertures can provide more detailed

information on the beam's spatial structure (assuming it is consistent from one run to

the next). A rotating disk with apertures of various size arranged around a circle can

16

automate this process. Alternatively, measurement results can be used to validate

simultaneous measurements made with a spatial profile imager.

3.2.3 Calibration of Measurement Hardware Several calibrations should be undertaken to produce an accurate beam profile.

First, a precision reticle with horizontal and vertical scales should be placed at the

replica target plane. This may be used to obtain rough focus of the imaging system with

visible or ambient illumination. The imaging optics must then be adjusted to give a

sharp focus at the camera sensor for the laser wavelength. This can be done with a

short burst of the main laser or an alignment laser of the same wavelength. An image

of the reticle illuminated with laser light should then be recorded by the camera to

provide a calibration of the spatial dimensions in the image recording plane.

If an appropriate reticle is not available, a hard aperture or set of apertures of

known diameter may be used for the calibration.

For dedicated target plane measurements employing infrared cameras viewing

thermal sensing plates, distance scales may be calibrated with reticles or hard

apertures and temperature scales may be calibrated with simultaneous thermocouple

measurements on heated plates. The remaining calibrations are discussed below

under data acquisition system calibration.


The data acquisition approach for electronic imagers is based on frame-grabber

technology, while the data for beam profiling material are inherent in the craters

obtained with the material and require no data acquisition hardware. These approaches

are image acquisition, image acquisition system calibration, and quick-look data

extraction. .

3.2.4.1 Image Acquisition For CW laser tests, a stream of two-dimensional images is generated at the

frame rate of the electronic imager employed. These may be recorded on videotape,

stored in mass storage media in digital form, or grabbed directly at regular intervals for

archiving. In any case, the image is urti natety digitized and transferred to computer

17

storage with a frame grabber. For pulsed lasers operating at repetition rates less than

the framing rate, synchronization issues must be addressed. The camera clock must be

synchronized to the frame grabber clock and both must be asynchronously reset to

capture the pulse when it occurs.

3.2.4.2 Image Acquisition System Calibration

The most reliable beam profiling may be obtained by carefully assuring that the

entire camera-frame-grabber system is linear. This may be done by making sure that

the automatic gain control (AGC) on the camera is off and the gamma setting is set to

unity (linear response). The gain and offset of the frame-grabber should be set so that

a dark field gives a digital count of 1 or 2 and saturation of a camera pixel yields an

output value near 255 (for 8-bit digitizer).

Using any convenient illumination source, the replica target plane should be

illuminated at a level that just saturates the pixels and an image should be recorded.

Without changing the illumination, a calibrated attenuator should be placed in front of

the sensor and a second image should be recorded. The calibrated reduction in sensor

illumination should be reflected in a corresponding drop in digital count for the affected

pixels. Use of several attenuator values (or a step wedge) permits construction of a

calibration curve for relative pixel irradiance versus digital count.

A baseline check should also be conducted, when feasible, wherein a laser test

is run as it would be in an actual test, but with the diagnostic beam diverted or blocked

from the beam spatial profile camera. This test will determine a typical background

noise floor for the profile measurement and may help find potential sources of error.

3.2.4.3 Quick-Look Data Extraction

The data acquisition system must be set up so key metrics of the fluence profile

are available in real time to support changes in setup for subsequent tests. These

metrics might include effective beam diameter, peak-to-average irradiance ratio, and

margin between the maximum irradiance and camera saturation level. A quick plot of

the fluence profile may also be valuable. The metrics extracted from the data will be

entered into the experimenter's data log spreadsheet to support real-time tracking of

test results.

18


The primary output of the in-band imager-based spatial profile measurement is a

two-dimensional array of values corresponding to fluence received by the imager pixels.

The profile recording material crater may be processed by a profiling instrument to

produce a similar array of numbers. The array of numbers must be processed by an

algorithm to extract meaningful metrics of the beam profile.

Many commercial software packages have been developed for this purpose, but

they have been developed primarily for Gaussian beams. However, most commercial

software packages calculate the total counts (7) sensed by the camera.

Great care must be taken to be sure that this count does not include background

counts that can introduce large errors (i.e., check that background subtraction is done

accurately). From this value of T, the spatial calibration factors (Ax, Ay cm per pixel),

and measured power on target {PT), the irradiance (/) associated with the count (C) of

any pixel can be computed:

l = PTC/(T Ax Ay)

Application of the 'appropriate magnification factor to the data yields the

irradiance profile in the target plane. Care must be used in the interpretation of other

metrics calculated by commercial packages when analyzing flattop beams and other

beam shapes. Software has been developed in the laser effects community for flat-top

like beams based on analysis of the histogram of the array values. After application of

the appropriate algorithm, values such as effective beam area, effective beam diameter,

and peak-to-average ratio may be extracted for use in the database.

3.2.6 Uncertainty of Measurement Results The uncertainty in the measurement should be estimated by the experimenter





19


be considered in the beam-spatial-profile-measurement uncertainty estimate include

• Pixelization error in spatial calibration,

• Spatial resolution of sensor,

• Accuracy of replica target plane location,

• Uniformity of scatterer (if used),

• Uncertainty of temperature measurement (if thermal sensor plate is used),

• Linearity of camera-frame-grabber combination,

• Accuracy of background subtraction, and

• Measurement scale settings used in the tests.

The final item is very important because, if the full range of the frame-grabber is

not used, the result could be subject to significant digital error. Generally, the

uncertainty in the result should be reported in the units of the measurement rather than

percentages and should be calculated by the NIST guidelines.

3.2.7 Output to Database The result of the fluence profile measurement is entered into the database as

follows.

Tabular Data

• Effective Beam Area (area to be used in average irradiance or fluence

calculation); uncertainty in beam area

• Beam Area Fluctuation* (maximum beam area minus minimum beam area

divided by average beam area); uncertainty in beam area fluctuation

• Effective Beam Diameter (diameter to be used in average irradiance or fluence

calculation with a circular beam assumption), uncertainty in beam diameter

• Peak-to-Average Ratio (ratio for determining maximum irradiance or fluence from

average value), uncertainty in peak-to-average ratio

• Profile Type (Gaussian, flat-top. irregular)

• Profile Symmetry (circular, elliptical, square, rectangular, irregular)

* Obtained from multiple profiles acquired during a single laser run.

20

Data entry may depend upon profile type. For example,

Gaussian (Peak Irradiance, 1-a radius)

Top Hat (Average Irradiance, Peak-to-Average Ratio, Effective Area)

Irregular (Average Irradiance (inside the effective area), Peak-to-Average Ratio,

Effective Area).

File Data

• Raw image data collected as two-dimensional arrays

•

21

3.3 Measured Character ist ic: Wave length


Knowledge of the wavelength of the laser irradiating a target is important

primarily because the target's absorptance is wavelength-dependent. Model

calculations of the target's thermal response can be performed if the laser wavelength

and the spectrally-resolved absorptance of the target, its thermal-physical properties,

and the properties of its environment are known.

In addition, there are occasions where wavelength-scaling of laser effects data is

of interest.

Also, when radiometric methods are used to measure temperature, it is vital that

the spectral region used by the instrument not include the laser wavelength(s). Thus, it

Is clear that knowledge of the wavelength at which the data were obtained is required.


In almost all cases, the builder of the laser is able to provide data on the

wavelength characteristics of the laser. The experimenter should carefully examine this

information, and, if he is satisfied as to its validity, he may report it without making

independent measurements.

Some lasers provide monochromatic output while others lase on multiple lines.

For the latter, the spectral characteristics may vary with time during a run and/or may be

different depending on operational parameters of the laser. In these cases, it may be

desirable for the experimenter to make wavelength measurements in real time, during

each run.

Selection of the wavelength-measuring instrument depends on the part of the

spectrum to be measured. For wavelengths from -200 nm to 5 um, light-weight

commercial spectrometers are commercially available. These typically employ a grating

and a Czerny-Turner design. Light enters the instrument through a slit, whose width

may be varied. The light is then dispersed by the grating and delivered to an array of

detectors, for measurement of the spectrum.

22

For measurements in the ultraviolet (UV), visible and near IR (-200 to -1050

nm), charged-coupled device (CCD) arrays may be employed. At longer wavelengths,

the detectors may be InGaAs (0.9 um to as long as 2.5 um), PbS (1.0 to 3.0 um), or

PbSe (1.5 to 5.0 um). Spectrometer resolution depends on the slit width, the grating

design, and the detector size/number configuration. The sampling rate depends on the

detector temporal response characteristics.

For wavelengths greater than -2 um, Fourier Transform IR spectrometers may

be used. These typically employ a Michelson interferometer configuration that splits the

incident beam into two paths; one of fixed length and the other dithered at a high

frequency. The two beams are recombined and directed onto a detector. The signal

measured by the detector is the Fourier transform of the spectrum of the incident

irradiation. Instrument software performs the Fourier transform inversion to present the

experimenter with the actual laser spectrum.

Damage to the spectrometer is a valid concern for laser measurements, and a

very small fraction of the laser beam must be used. This may either be imaged directly

onto the spectrometer entrance slit or may be delivered via a fiber optic cable. The

fraction of the laser beam to be delivered depends on the laser power, laser

wavelength, and the spectrometer detector characteristics, and must be calculated

before measurements are made.


Absolute wavelength calibration may be performed with one or more 'standard'

sources of line radiation. These may be hollow cathode discharges or lasers whose

wavelength has been established independently. The exact choice(s) depends on the

wavelength of the laser to be measured. Ideally, the standard should provide a

wavelength close to that being measured.

In addition, it may be desirable to perform an absolute spectral irradiance

calibration. This relates the detector signal to the incident spectral irradiance

(W/cm2/nm). Independent laboratories (e.g., Optronic Laboratories, Inc.; Orlando,

Florida) can provide calibration services or it may be desirable to purchase a NIST-

23

traceable broadband light source standard (e.g., tungsten-halogen lamp) for use in

laboratory calibration.


Experimental data are acquired on a computer with a digital data acquisition

card. The data are voltage or counts for each wavelength element being sampled and

for each time step. The raw data may be converted to spectral irradiance through use

of the spectrometer's spectral irradiance calibration curve or look-up table.


For a monochromatic laser, the data of interest are the wavelength of laser

emission and (if resolvable by the instrument) the laser line width. For lasers with

multiple lines, it is important to determine time- and spectrally-resolved laser line data.

The experimenter first performs a temporal average to obtain the mean laser spectrum

during the test. The experimenter then makes a judgment as to the best way to report

the 'laser wavelength." This may be the wavelength of the dominant line, or the middle

of the spectrum, or a weighted mean for the spectrum, and the choice must be spelled

out in detail. Next, the experimenter calculates the laser's spectral width. This may be

the total wavelength spread, the values of the maximum and minimum wavelengths, or

the 1-G width of the spectrum.

Lasers with significant temporal variation of the spectral output necessitate

quantification of this property. This may involve multiple plots of the spectrum at

intervals during lasing, a single plot of the relative intensity of the primary line versus

time, or some other depiction that best illustrates the temporal variability of the laser's

spectral performance.






calibration factors, and input info- from other sources. The combined uncertainty

24


be considered in the wavelength measurement uncertainty estimate include

• Spectrometer wavelength calibration precision,

• Spectrometer resolution, and

• Accuracy of spectral irradiance calibration.

Generally, the uncertainty in the result should be reported in the units of the

measurement rather than percentages and should be calculated by the NIST guidelines.


The result of the wavelength measurement is entered into the database as

follows.

Tabular Data

Laser wavelength (in um). If the laser is monochromatic, this is the line-center

wavelength; if it has multiple lines, it is either the dominant line or the mean wavelength

Laser wavelength spread (in urn). If the laser is monochromatic, this is the line width. If

it has multiple lines this is the spread from shortest to longest wavelength

File Data

Plot of the temporal mean of the laser spectrum during the experiment. If it is

relevant, the experimenter may also provide data on the temporal variation of the laser's

spectral irradiance. This may be, for example, a plot of the irradiance of the dominant

line versus time.

25

Measured Characteristic: Target Absorptance


The absorptance of the target at the laser wavelength is needed for model

calculations of the target's thermal response. Ideally, the absorptance should be known

as a function of temperature. However, if this is not available, the initial absorptance is

still a vital input to the thermal model.

3.4.2 Technical Approaches to the Measurement:

For opaque (optically dense) targets (as is assumed to be the case for this

standard), the absorptance (a) at a given incidence angle (0) can be inferred from a

measurement of reflectance (p) at that incidence angle:

cc(6) = 1 - p(0)

There are a number of Government and commercial laboratories that perform

spectrally-resolved reflectance measurements. These give the directional

hemispherical reflectance (DHR) of a sample as a function of wavelength over a range

of interest. The sample is positioned in an integrating sphere and is illuminated at a

fixed incidence angle by a spectrally-resolved light source/1 Commercial and custom

reflectometers have the capability of heating the sample in-situ to temperatures as high

as -500 °C *2. Measurement uncertainty is typically in the range of 0.01 to 0.02.

An alternative approach, illustrated in Figure 3, has been pioneered by the Air

Force Research Laboratory. The instrument, known as the Thermal Reflectance of

Aerospace Materials (TRAM) facility, is a hemi-ellipsoidal, gold-coated dome. The

sample is heated by a laser whose wavelength is different from the wavelength of

interest (provided by the probe beam). The sample is located at one of the foci of the

hemi-ellipsoid and the entrance port of an integrating sphere is located at the other focal

point.

+' The absorptance of some materials is polariza» dependent, and therefore the light source should either be unpolarized or measurements should be made with orthogonal polarizations.

*2 Most laboratories restrict heating to tempenBae the threshold for decomposition.

26

Heater Beam

TRAM Dome

Delector

Video Camera

Heater Beam Reflector

Integrating Sphere

Figure 3. Schematic of TRAM Reflectometer

In order for the integrating sphere detector to distinguish the probe beam signal

from the heater beam, the probe beam is chopped at a relatively high frequency (-300

Hz) and a phase sensitive ('lock-in') amplifier is employed. Additionally, a filter having

high reflectance at the heater laser wavelength is positioned to in front of the detector.

A pyrometer or thermo-graphic imaging camera is employed to measure the

sample's temperature as it is heated.

Note that for partially transparent materials, one must also measure its

transmittance. Recent modifications to the TRAM instrument have permitted

simultaneous measurements to be made of reflectance and transmittance as the

sample is heated. However, this capability is still under development and not yet fully

proven.


Ideally, the reflectance of the material at the probe laser wavelength is known

from independent DHR measurements. Then a material sample is used exclusively for

reflectance calibration measurements with no heater beam.

27

If the material's reflectance is not known, diffuse and specular standards are

available to calibrate the TRAM instrument. Which is used depends on whether the

sample to be measured is primarily diffuse or specular in its reflectance characteristics.

The diffuse standards are NIST-traceable and the specular standards are indirectly

traceable to NIST standards.

3.4.4 Data Acquisition and Logging A detector (not shown in Figure 3) is used to monitor fluctuations in the probe

beam power. This signal and that from the integrating sphere detector are fed to phase-

sensitive amplifiers set at the frequency of the probe beam-chopping wheel.

A computer with a digital data acquisition card is used to acquire data from the

two amplifiers and from the pyrometer.


Custom software is routinely used to reduce raw data to reflectance,

absorptance, and temperature as a function of time. In addition, cross plots of, e.g.,

absorptance versus temperature, are easily generated.








be considered in the absorptance measurement uncertainty estimate include

• Accuracy of instrument calibration using reflectance standards;

• Beam alignment accuracy;

• Detector non-linearity and/or saturation;

• Detector response to black body radiation from the heated sample;

• Errors introduced by sample deformation as it is heated;

28

Changes in dome reflectance due to deposition of contaminants on the dome

interior;

• Reductions in the transmittance of the probe beam entrance port window due to

the deposition of contaminants; and

• Accuracy of temperature measurements, including the effects of contaminant

deposition on the pyrometer port window.



3.4.7 Output to Database The results of the absorptance measurement(s) are entered into the database as

follows.

Tabular Data

• Room temperature absorptance (dimensionless); uncertainty

File Data

Absorptance as a function of temperature.

•

29

Measured Characterist ic: Therma l Coupl ing Coefficient


The fraction of incident laser beam energy retained in a test article (average

thermal coupling coefficient) is one of several useful results that provide a simple check

point on modeling of the laser beam interaction with the test article. This measurement

is most useful for simple material coupon tests where an understanding of the

mechanisms of beam absorption and material removal is sought.

One example of its use might be the study of metal heating and ablation. At low

irradiance when there is no material removal, the thermal coupling coefficient (TCC)

should be identical to the surface absorptance averaged over the laser exposure time.

By plotting TCC versus average irradiance (or total fluence) obtained in a series

of tests with increasing average irradiance (or total fluence), one can observe the onset

and magnitude of absorptance change with surface chemical change (oxidation) and

topography change (melting). At the highest irradiance levels where ablation occurs,

TCC is dominated by heat conducted to the substrate behind the ablation front.


The general approach to measurement of average TCC in a lethality test is to

configure the test article (usuaiiy a coupon) as a calorimeter. The most meaningful

results are obtained if the coupon is very well thermally isolated and the laser exposure

time is short compared to the post-test coupon cooling time. The post-test temperature

history of the coupon may be used to extract the TCC value, if the thermal

characteristics of the coupon are known.

The measurement is best done with a flattop beam and a coupon size that is not

much greater than the beam diameter. If non-flat-top beams are used, thermal

transport models may be needed to help with interpretation of the results. The coupon

is configured in a manner such that it may be held in place with a minimum of thermal

conduction paths to the coupon holder. Typical approaches to holding the coupon

include setscrews with pointed tips and close fitting low-thermal conductivity frames

30

(e.g., shuttle tile). The coupon mass must be measured prior to and after the test and

some means of monitoring the post-test equilibrium temperature of the coupon must be

implemented. If one thermocouple is employed for the temperature measurement, it

should be placed near the edge of the coupon.

Multiple thermocouples may be used for redundancy and establishment of the

degree of temperature uniformity at thermal equilibrium. A calibrated infrared camera or

radiometer viewing the back surface of the coupon may also be employed for the

temperature measurement. The TCC measurements are usually conducted without

wind tunnel gas flow that would introduce large convection losses from the surface.

In cases where a vacuum chamber is available and the role of oxidation is not an

important objective of the investigation, these measurements should be performed in a

vacuum chamber. Then, convective losses can be essentially eliminated. This

approach to the measurement is to irradiate the sample with a temporally-stable laser

and allow the sample to reach a steady temperature (Tss)- For measurements in

vacuum with the sample held in a low conductivity mount, the dominant loss mechanism

is radiation. The thermal coupling coefficient (fTcc) is given by

, _q[(s/+s,fe-T:] JTCC ~ ,

where Cf and £r are the IR emissivities of the front and rear surfaces, Tw is the

temperature (K) of the vacuum chamber wall, o is the Stefan-Boltzmann constant (5.67

10'12 W/K4 cm2), and / is the incident irradiance (W/cm2). Note that this method does

not require knowledge of the target's specific heat or density.


In most cases, the only calibration required is a temperature calibration of the

thermocouple or radiative measurement instrumentation. Thermocouples may be

calibrated by comparison to a standard in an oven or simply placed in a well-stirred

liquid at known temperature. Radative measurement instruments should be calibrated

31

with a blackbody source. For radiative temperature measurements, it often is

necessary to know the IR emissivity of the surface being viewed.

For thin samples, it may be beneficial to coat the rear surface with a very

absorptive paint of known emissivity and view that surface with the instrument. The

anticipated temperature rise of the coupon should be taken into account when selecting

the temperature range for calibration of thermal sensors. Generally, the test coupon is

designed to keep maximum temperatures below a level where thermal radiative losses

to the environment would introduce excessive error.

Measurement of temperature rise of the coupon may be used to infer the total

energy retained in the coupon by the calculation,

Erc,=mCpAT

where Eret is the retained energy, m is the coupon mass after the test, Cp is

specific heat of the coupon material, and A Tis the measured temperature rise. The

thermal coupling coefficient is then calculated,

JTCC ~ p

where free is the thermal coupling coefficient and E-mc is the total incident laser

energy during the laser exposure. If the specific heat of the coupon is not well known,

calibration of the test coupon as a calorimeter may be needed to achieve a desired level

of accuracy.

This calibration may be performed by attaching a small heater wire to the coupon

and performing an electrical heat substitution calibration as is done for many laser

power meters and calorimeters. In this method, power is applied to the heater for a

fixed time, while current and voltage are measured with calibrated instruments. This

direct calibration method eliminates the need for thermal sensor calibration because a

direct correlation is provided between coupon retained energy and sensor output. In

32

application of this calibration method, the results must be adjusted for any mass loss

during the test.

The remaining calibrations are discussed below under data acquisition system

calibration.


Data acquisition for this measurement depends on the nature of the temperature

monitoring technique. For thermocouples, a conventional instrumentation amplifier and

analog-to-digital (A/D) voltage converter instrument may be used or a packaged

thermocouple data acquisition system may be employed.

Care must be taken to record cold junction temperatures and to select voltage

gain settings to capture (with good digital resolution) the post-test thermal transients

anticipated. The temporal resolution should be selected to resolve detail in the post-test

thermal equilibration period. The latter is usually not stressing on data acquisition

systems for typical coupons (sampling rates in the 1 to 1000 Hz range).

Calibration of the thermocouple data acquisition system may be done with

voltage standards or with temperature standards and thermocouples of the same type

as used in the test. If the temperature is monitored with an infrared camera, data

acquisition may be done by recording frames sequentially at rates similar to those

mentioned above. Calibration of the camera may be performed with a blackbody

calibration oven. With both types of instrumentation, pretest thermal data should be

recorded for the coupon to provide a measure of baseline fluctuation or systematic drift.


The data recorded from the measurement must first be converted to a

temperature versus time record with "zero" time set as the end of the laser exposure.

An exponential fit to the data set for time points after the coupon internal thermal

equilibration time period is then made to determine the thermal loss rate by conduction,

convection, and radiation to the coupon surroundings. The coupon equilibration time

33

may be estimated from the thermal properties of the coupon or through examination of

the post-test transient.

This method works best if the characteristic time constant for cooling to the

surroundings is much greater than the time constant for internal equilibration of the

coupon by conduction. If the fitted exponential relationship is back extrapolated to

"zero" time, the resulting temperature value is the equilibrated temperature that would

have resulted had there been no cooling to the surroundings. This temperature, minus

the initial temperature just before laser exposure, provides a measure of retained

energy (ATabove). The data reduction approach is illustrated in Figure 4.

If two thermocouples are attached to the target, one in the center and one at the

edge, these devices may be used to determine the time at which the sample has

equilibrated during cool-down.

25

20

to

S

| 10

-

-

1 • • • . . 1

\ y

~ ^ N . -

Retained Thermal Energy = 14.5 g * 0.45 J/g/K * 4.34 K = 28.3 J

Incident Energy = 66 J f-rcc = 0-43

Negligible Mass Removal

= 4.3448e"00099" No Air Flow

R2 = 0.9339

1 i l V. 1 i »

-5 10 15 20 25 30 35 40

Time (s)

45

Figure 4. Illustration of Data Reduction for TCC Measurement

34






calibration factors, and input information from other source (e.g., property data). The

combined uncertainty calculation should be performed and reported according to NIST

guidelines. Factors to be considered in the TCC measurement uncertainty estimate

include

• Thermocouple wire error limits (alloy variation),

• Cold junction variation,

• Temperature calibration uncertainty,

• Digitizing uncertainty,

• Exponential fitting uncertainty,

• Mass measurement uncertainty,

• Uncertainty in Cp value, and

• Uncertainty in E¡„c value.

Generally, the uncertainty in the result should be reported In the units of the


3.5.7 Output to Database The result of the TCC measurement is entered into the database as follows.

Tabular Data

• Thermal Coupling Coefficient, free , uncertainty of free

File Data

• Temperature History

35

Measured Characteristic: Penetration Time


When the large number of laser-induced target-damage events involve heating

the outer skin of the target until a hole is created, the laser continues on to damage

internal components, ignite fuel, release pressure, etc. Knowing the penetration rates of

materials as a function of thickness and laser irradiance is very important. Most

penetration time data come from basic laser effects experiments where small flat plates

of material are held in the desired environment, air or vacuum, and the laser melts or

ablates holes through the samples at various irradiance levels. Knowing the irradiance

levels and accurate penetration times yields the fluence or energy density required for

penetrating the material as a function of heating rate.


There are numerous ways to measure penetration time. The best choice

frequently depends on the laser waveform and the type of material being penetrated by

the laser.

3.6.2.1 Optical Methods

The fastest response and most consistent methods for measuring penetration

times involve the use of optical sensors. These can be configured in many different

ways.

(1 ) Radiometer viewing a small area within the heated zone.

(2) Photo-detector sensing high power laser light passing through the

sample and reflecting off of a diffuse scatter plate behind the sample.

(3) Photo-detector sensing a probe laser of another wavelength passing

through the hole in the sample.

(4) Photo-detector viewing the rear surface.

(5) Photo-detector looking across the back surface to sense expelled hot

debris.

36

It is essential to couple any measurement of penetration time to a signal of

shutter opening or first pulse trigger for a RP laser which determines the start of the

irradiation time.

3.6.2.1.1 Radiometers

The multicolor radiometer viewing a small area in the center of the laser spot

provides the temperature history of the heated surface, given a reasonable estimate of

the surface emissivity. At the penetration time all radiometric sensors, regardless of

their wavelength sensed, show a sharp drop in signal because upon sample penetration

the heated area being sensed disappears.

To obtain an accurate penetration time, the radiometer should be viewing an

area where the high power beam is most intense because the penetration should occur

first there. This viewpoint is less important for good conductivity materials such as

metals that tend to smooth the heating as a function of position for an irregular or

sharply peaked beam profile.

Multicolor radiometers with widely spaced narrow band sensors can be calibrated

so that a wide range of front surface temperature can be accommodated with multiple

channels able to sense the time of penetration.

Assuming the radiometer view direction is reasonably close to the high power

beam axis, the radiometer can also record the penetration of underlying surfaces as

well. This is particularly true if the underlying surfaces have radically different surface

temperatures while being heated by the same heating rate. One can identify when the

laser is exposing and penetrating each layer.

3.6.2.1.2 Photo-Detector

A photo-detector notch filtered to sense the high power laser light scattering off of

a diffuse scatter plate behind the sample will sense laser light passing through a hole in

the sample. The signal level varies with the beam irradiance at the scatter plate and the

size of the hole in the sample. Even for flat spatial irradiance profiles holes tend to start

small in the center and grow with time. The advantage of this method is that the onset

of laser radiation coming through the sample is sensed regardless of the hole location.

37

A possible disadvantage of this method occurs for partially transparent materials

(e.g., plastics). Transmission of the incident laser light may produce a false early

indication of penetration. A fiber resin composite, for example, may have its resin

completely degraded and laser radiation pass between fibers still not removed giving a

premature penetration signal. Some materials may transmit so much broadband

radiation from the heated front surface that this detection scheme cannot work properly.

3.6.2.1.3 Photo-Detector with Probe Laser For this method, a low power laser such as a HeNe at 0.63 um wavelength is

used to illuminate the area to be penetrated by the high power laser. A photo-detector

behind the target senses the probe laser. Narrow band-pass filters are employed to

reject as much of the broadband radiation from the hot spot as possible.

For accurate penetration times the probe laser must be aimed at the location on

the target front surface where the hole first appears. Even though the photo-detector is

placed as far behind the sample as possible, for materials with high melting or ablation

temperatures it may be difficult to get adequate rejection of the broadband radiation.

The probe laser can be modulated to assist in separating its signal from the

background radiation. For partially transparent materials the same issue of an early

indication of penetration may occur, if the probe laser is transmitted through an almost

penetrated sample prior to the actual penetration.

3.6.2.1.4 Photo-Detector Behind Sample

There are multiple ways of sensing the time of sample penetration with a photo-

detector behind the sample. The detector can be mounted to look across the rear of the

sample so that it senses heated debris coming off the back of the penetrated sample.

This works well because molten metal droplets and chunks of fibers tend to get blown

backward in an airflow environment and even in a vacuum environment.

Another method is to align the photo-detector to view the back of the target. In

general this is not too successful for an accurate penetration time because of the

broadband radiation coming from the back of the heated zone prior to penetration. The

type of material and conditions where this might be most successful would be an

38

opaque low conductivity ablating material being penetrated quickly by a high average

irradiance beam. The thermal depth behind the ablating surface would be minimal and

there would be no visible hot spot on the back until just before penetration. The

broadband radiation would still be sensed by the photo-detector but only for an

extremely short period of time.

This method is not recommended for metals or for difficult to ablate, high ablation

temperature material such as graphite epoxy. It could be made to work adequately for

low ablation temperature opaque plastics. To observe a precise penetration time, the

loss of hot material from the heated zone must be faster than motion of heat out and

around the melted/ablated hole so that there is a noticeable change in signal upon

penetration.

An alternative is to optically restrict the field of view of the sensor to a spot size

smaller than the heated area so that when the hole is created a large drop in signal is

observed. This requires imaging optics and approaches the radiometric measurement.

3.6.2.2 Video Cameras

Video cameras are employed to record the timing of specific damage events and

for an overall record of a laser effects or laser lethality experiment. Video cameras can

be employed for a primary measurement of penetration time but also are frequently

used as a backup penetration time measurement for one or more of the optical

methods. Multiple cameras are often used to record sample damage events. Video

cameras are mounted to view the front, back, and side of sample and components

being tested. Within the field of view, there must be a digital display showing the shutter

open (test start) time so that penetration time can be referenced to the irradiation start

time.

Video cameras are very sensitive to large changes in light level, and ND filters

are used to adjust the light level so that sample material removal events can be seen

while the irradiation is underway. Some materials at typical irradiance levels are

extremely bright, i.e., graphite epoxy irradiated by 1 kW/cm2 shows a front surface

temperature of about 3200 K. An ND filter combination has a lot of light to block and It

is typical that the sample is not visible until the high power beam is on and the sample

39

surface has come up to temperature. A penetration event should then be visible in the

video.

Sometimes, a video camera placed behind or at the side of the sample makes

viewing the penetration time easier. Properly filtered for each material, video cameras

can record the timing of numerous material damage events such as paint ablation, first

surface melting, and sample penetration. With improper filtering, the video cameras

record a totally white field of view and all timing data is lost. Most video cameras are

limited to 30 frames per second, although there are high speed videos available with

hundreds of frames recorded per second.

The video monitoring of a multiple layer component target can yield valuable

damage event timing data from changes in vapor plume shape, size, color, and

direction as various layers are penetrated. Pressure release upon penetration of any

pressurized component should be observable. Adjusting to the correct ND filtering level

is more difficult where a variety of materials are penetrated.

In addition to video cameras, thermo-graphic imaging cameras can provide

valuable data on the sequence of damage events, including the penetration time.

3.6.2.3 Witness Thermal Response

A high temperature thermocouple placed behind the sample in the high power

beam path or welded to a thin highly conducting metal plate placed in the beam shows

a rapid thermal response upon sample penetration. Once again, alignment of the

witness plate in the beam path after the beam passes through the sample is critical for

rapid time response. This method is not useful for short penetration events. Also, if the

beam is left on too long after the sample is penetrated, the witness plate or

thermocouple can be destroyed. If the witness plate is made thicker and more difficult

to damage, its time response will likely be too slow.

3.6.2.4 Break Wires

A variation on the witness plate idea is to mount fine wires in the beam path to

act as break wires when exposed by the beam. A simple electrical circuit detects

interruption of current flow when the wire breaks. Reasonable response time can be

40

achieved with 0.005-inch diameter wires with a highly absorbing surface stretched

across the beam path. Alignment must be done carefully, multiple wires may be

needed, and new ones must be installed for each test, making this a more labor

intensive method.


Optical sensor calibrations are not required for most penetration time methods.

However, as mentioned previously, all penetration time measurements must be

referenced to an irradiation start time. This should be a signal or sensor that records

actual shutter opening time, not a signal for the shutter to open. It is ideal for shutter

opening times to be a small fraction of the penetration times being measured.

If a shutter transit time is not known, it must be measured. Times of interest are

(1) first light through the shutter and (2) shutter completely open with all laser light

reaching the target. These times may be measured optically by breaking HeNe or LED

beam paths with the shutter blades. An alternative method is to extract the start time

information from the rising edge of the power versus time record. Timing data are

extracted from digital data acquisition hardware that is usually periodically calibrated by

the facility using calibration sen/ice providers. Simultaneous digital time stamping of all

data channels and video records provide a reliable means for extracting the timing data.








be considered in the penetration time measurement uncertainty estimate include

• Resolution for laser start time,

• Uncertainty in penetration event detection,

• Background light effects, and

41

• Detector temporal bandwidth.



3.6.5 Output to Database The result of the penetration time measurement is entered into the database as

follows.

Tabular Data

• Penetration time (s); uncertainty

File Data

• Detector temporal records

3.7 Measured Character ist ic: Impulse


Pulsed laser heating of a target produces a variety of phenomena including melt,

pyroiysis, decomposition, boiling, delamination, spallation, plasma formation, etc. As

target material is ejected from the surface, a recoil momentum (impulse) is imparted to

the target. This may be of importance, for example, in terms of stress wave damage to

the target or change of trajectory for target discrimination applications.


There are numerous techniques for measuring impulse and each has

advantages and disadvantages. Examples include ballistic pendulums, cantilevers, and

linear motion transducers that produce signals proportional to location (fiber optic,

capacitive, and inductive) or to velocity (linear velocity transducer gauges). The linear

velocity transducer (LVT) gauge is described in detail here to provide an example.

Information on the use of alternatives is widely available in the literature.

The LVT consists of the target attached to a magnetic shaft. The shaft is held by

low friction bearings and passes through a coil. Linear motion of the target/shaft

induces a voltage in the coil that is proportional to the instantaneous velocity. One of

the advantages of the LVT is that it is relatively insensitive to alignment accuracy or the

requirement that the impulse vector be parallel to the shaft axis.

The delivered'impulse is simply the product of the mass (target plus shaft) and

the initial velocity.


Calibration of the LVT requires determination of the proportionality constant

between the velocity and the induced voltage. Commercial LVT gauges usually are

provided with a calibration constant; however, direct calibration may be employed for

more accurate results. Mechanical means can be used to impart impulse to the LVT

assembly. The motion of the target/shaft can be observed with a camera operating at

sufficiently high speed to accurately determine its displacement as a function of time.

43

The distance-time data can be differentiated to obtain velocity as a function of time for

correlation with the voltage-time history.

Alternatively (or as a check of the determination obtained by the method just

described), a stop can be employed to restrict the total travel of the LVT. The voltage-

time history of the instrument is integrated from the start of motion to the time of

rebound from the stop. The distance traveled divided by the value of the Integral is the

proportionality constant.

3.7.4 Data Acquisition and Logging A computer with a digital data acquisition card is used to acquire the LVT-

induced voltage as a function of time. Independently, the target and the shaft are

weighed to obtain their total mass.

3.7.5 Data Reduction The LVT gauge voltage is multiplied by the proportionality constant (determined

by the instrument calibration measurements) to obtain velocity.

The LVT gauge may exhibit oscillatory behavior, particularly at the beginning of

motion. If this occurs, the recommended approach is to perform a linear fit to the data

and evaluate the fit at time zero to obtain the initial velocity. Statistics associated with

the fit may be used to estimate the uncertainty in the velocity determination.

The impulse is the product of the combined mass of the target and magnetic

shaft times the initial velocity.








be considered in the impulse measurement uncertainty estimate include

• Instrument calibration constant precision,

44

• Mass measurement accuracy, and

• Initial velocity determination.



3.7.7 Output to Database The result of the impulse measurement will be entered into the database as

follows.

Tabular Data

• Impulse (in dyne-s); uncertainty

45

3.8 Measured Characterist ic: Plasma/Plume Absorption / Scattering of the Laser Beam


There is some experimental evidence that under intense pulsed or very highly

focused continuous irradiation conditions, plume absorption or plume scattering of the

laser light is altering the incident beam profile at the sample surface. Smoothing of

crater profiles for selected materials has been observed. This has the effect of

decreasing the irradiance on the target surface by spreading out the beam and/or

absorbing the beam. The formation of plasma clearly blocks a significant fraction of the

beam.

Prior research suggests that blockage is more severe for larger (weapon sized)

spot sizes and short very intense pulses. A repetitively pulsed laser with short pulse

lengths is a condition where blockage may become significant even at fairly modest

average irradiance levels. Determining when scattering/blockage becomes significant is

an important experimental measurement aiding our understanding of laser material

interactions.

3.8.2 Technical Approach to the Measurement

The best methods for quantifying plume absorption/scattering are to irradiate a

sample with a small hole at the center of the laser irradiated spot, and (1) sample a

portion of the high power beam coming through the pinhole with a photo-detector, or (2)

measure attenuation of a probe laser passing through the plume and the pinhole at a

different but nearby wavelength than the high power beam. Sampling the high power

beam and the probe beam can be done simultaneously. This geometry assures that the

laser radiation being sensed has passed completely through the plume. This is

preferred to passing a probe beam sideways through the plume as a measurement

would be required at several locations at various distances from the target surface.

Collecting the radiation passing through the pinhole can be done by just putting a

sensor in the beam path or by using a lens to collect more radiation and focus it into the

detectors. The small collection angle and large collection angle methods are shown in

Figjre 5.

46

The HeNe laser beam sensor has a stack of narrow band filters to reject the

broadband radiation and the 1.06 um laser radiation. Narrow bandpass filters at 1.06

um may be needed on the 1.06 um sensors for broadband radiation rejection. Using

the large angle collection method with the focusing lens may require the use of an

attenuator to reduce the light intensity at the detectors.

Measurement of the plume attenuation as a function of time for a pulsed laser

requires precise knowledge of the pulse shape for comparison with the transmitted

pulse. This is not an issue for a continuous wave laser unless the output power

fluctuates or decays with time. It would be advisable to monitor the power or energy per

pulse throughout the measurement interval.

47

1.06 um

0.63 um

Target with 750 um Hole

0.633 urn Beamlet

1.06 um Detector

18 mrad Full-angle Acceptance

Fiber

1.06 um Beamlet

44 mrad Full-angle Acceptance

0.633 um Narrow Bandpass Filter Stack

Photomultiplier

G-7798

(a) Small angle

1.06

Target with 750 um Hole

F2

0.633 um

Reimaging Lens

Attenuator

1.06 um Detector

16° Full Angle Acceptance

1.06 um Mirror

0.633 urn' Narrow Bandpass Filter Stack

(b) Large angle

Photomultiplier

G-7799

Figure 5. Target-Hole Transmission Diagnostics

48


Plume attenuation is really a relative measurement. No absolute calibration of

sensors is required. A baseline value for no attenuation is established before the test or

at the start of the irradiation where no blockage of the pinhole can be assured by

independent means. Changes in laser power/energy per pulse/pulse shape must be

monitored during any kind of plume blockage experiment.


A computer with a digital data acquisition card or oscilloscope is used to acquire

the detector voltage as a function of time. The best results are obtained if the incident

power versus time is recorded using the same instrument on a separate synchronized

channel.


Data reduction is accomplished by taking the ratio of the transmitted power to the

incident power and multiplying the result by a calibration constant which forces the

result to be unity under the condition of no blockage.








be considered in the plasma/plume absorption and scattering measurement uncertainty

estimate include

• Broadband radiation contribution to signal,

• Detector dynamic range,

• Detector noise floor, and

• Pinhole diameter change during exposure.

49




The result of the impulse measurement will be entered into the database as

follows.

Tabular Data

• Maximum Plasma/Plume Attenuation; uncertainty

File Data


50

Measured Characteristic: Plasma Formation


When irradiating a material with a repetitively pulsed laser, if the pulse lengths

are short and both the irradiance and fluence per pulse are high enough, the hot vapor

plume above the sample surface can begin ionizing and absorbing laser radiation

directly. The temperature of the plume vapor increases further, causing more ionization

and further absorption. Eventually, the temperature and the amount of ionization in the

plume become high enough that the entire laser beam is absorbed in the plume with

none of the beam reaching the sample surface directly.

This plasma above the sample surface is at high pressure, and a dynamic

expansion of the plume occurs while the laser keeps depositing energy into the plasma.

Reradiated energy from the plasma over a broad range of wavelengths heats the

sample surface. The net surface heating is now modified because the plasma is

reradiating in all directions.1 It is important to understand the onset of plasma formation

for various laser wavelengths and waveforms.

3.9.2 Technical Approach to the Measurement

Studies of thermal coupling and impulse generation on sample surfaces from a

laser pulse historically included radiometric instrumentation to measure the surface

temperature and the onset of plasma formation. Multiple fast response photo-detectors

adjusted for a wide range of light levels may be required to resolve the target surface

thermal history during short intense plasma producing pulses. As peak surface

temperatures begin to exceed 5000 K, ionization of the plume begins. The observed

temperatures become less consistent than material vaporization values and they

continue to rise as fluence increases. Figure 6 shows temperatures in excess of 20,000

K above a graphite epoxy target irradiated by a short 1.06 um pulse. It also shows the

increased randomness in the observed peak temperature when plasma begins to form.

The observed temperatures are not the same for every pulse. With data for several

' Whether the coupling increases or decreases depends on the initial absorptance at the laser wavelength (ot¡). Most materials are very absorptive in the UV, where the plasma primarily radiates. Thus, even though the plasma radiates in all directions, if cti is low (less than -0.25), there may be a net increase in thermal deposition ('enhanced coupling').

51

pulses it becomes clear when the plasma is present. In Figure 6 plasma begins to

appear above about 8 J/cm2/pulse.

100,000

10,000 -

S 4,000

1,000

Fluence (J/cm /pulse)

100

G-7800

Figure 6. Peak Brightness Temperature for Graphite / Epoxy

3.9.3 Calibrations Required Although the onset of plasma could probably be determined with an un-calibrated

radiometer, it is always good practice to calibrate any radiometric instrument over as

wide a temperature range as possible. It is, however, very difficult to calibrate optical

sensors for blackbody temperatures above 3000 K.

The usual procedure is to calibrate radiometers over temperature ranges where

high power lasers heat and melt or ablate materials: room temperature to 2800 K. This

calibration really does not apply to a plasma situation where the temperature of a

volume is being measured.

Furthermore, with ionized species present, that volume is emitting copious

amounts of line radiation in addition to blackbody emission. Fortunately, the onset of

plasma formation can be found without worrying about a calibrated temperature

measurement at several thousand degrees K. If the emissivity is unknown, the term

52

brightness temperature is used to describe the measured "temperature." This is a

reminder that the precise temperature is unknown.

A useful approach is to pick a radiometer with highly non-linear response to

temperature in the temperature range of interest. For typical hot vapors and plasmas

this is easily done by using short wavelength filters (near UV to blue) and a silicon-

photodiode-based radiometer.


A computer with a digital data acquisition card or oscilloscope is used to acquire

the detector voltage as a function of time. The best results are obtained if the incident

power versus time is recorded using the same instrument on a separate synchronized

channel.


Data reduction is accomplished by correlating the peak detector voltage with

pulse energy for each pulse in the train. If a calibration of the radiometer in the 300 to

2000 K range is available, the brightness temperature should be correlated with pulse

energy. It should be recognized that the plasma temperature obtained by the latter

approach is an extrapolation and may not properly characterize the plasma state.

Based on the correlation, a threshold fluence range for plasma formation should be

identified.








be considered in the plasma formation measurement uncertainty estimate include

• Uncertainty in fluence measurement,

• Detector dynamic range, and

53

• Detector noise floor.




The result of the impulse measurement will be entered into the database as

follows.

Tabular Data

• Plasma Formation Threshold Fluence; uncertainty

File Data


• Pulse energy temporal records

54

3.10 Measured Characteristic: Ambient Pressure and Optical Contamination


Certain laser-target engagements occur in space or at high altitudes. To

simulate these conditions, experiments must be performed in a vacuum chamber,

evacuated to a pressure that matches, as closely as possible, the ambient conditions of

the engagement. Target heating may produce out gassing that increases the pressure

in the chamber. Thus, it is desirable to measure the initial pressure, as well as pressure

excursions that occur during the experiment. In some cases, it may be desirable to

suppress oxidation or combustion of the target. This may be accomplished by

conducting the experiment in an inert atmosphere, such as nitrogen or argon.


A variety of vacuum gauges are available from a number of commercial vendors.

These include manometers, thermocouple, Pirani, convection, hot cathode ionization,

and cold cathode ionization gauges. The selection of the gauge(s) depends on the

pressure range to be measured, the temporal response needed, the gas type being

monitored, susceptibility of the gauge to being contaminated (and ease of cleaning), etc.

If large changes in ambient pressure are expected during the experiment, multiple

gauges may be required to span the range of pressures produced.

The experimenter must be concerned, when conducting an experiment in an

enclosed chamber, with the accumulation of contaminants on chamber ports. These

are needed both for entrance of the laser beam itself and for viewing of the target by

instrumentation. Contaminants often reduce the transmittance of the optical flats and

may increase the probability of damage to the entrance window.

One way of mitigating this problem is to introduce a steady stream of purge gas

at the window location. In addition, the window may be mounted on a cylindrical tube

that will aid in channeling the gas flow. The mass flow rate of the purge gas should be

equal to or greater than the mass flow coming from the irradiated target. The vacuum

pumping port should be near the target, and the pumping capacity should be in excess

of the purge gas flow rate.

55

Another approach is to position a mirror at the far end of the chamber and orient

the target so that it faces away from the entrance port. This tends to protect the

entrance window from contaminants, but then exposes the turning mirror to target

effluents. In general, the target should be positioned as far as possible from any optical

element.

If protection of windows is not feasible, then the experimenter should determine

changes in window transmittance (and the reflectance/transmittance of any other optical

surface that may be contaminated during the experiment). Ideally, a probe laser at the

appropriate wavelength may be employed with a detector to make real-time

measurements.

Difficulties with this technique include forward scatter of laser light by the

contaminant film as it develops. This may be of sufficiently high magnitude that the

probe laser signal is overwhelmed. Use of a mechanical chopper and a phase sensitive

amplifier may mitigate this problem or a probe laser may be available at a nearby

wavelength and an interference filter may be used to reject the scattered primary laser

light. Also, the experimenter must account for the effects of contaminants that

accumulate on the detector during the test.

Lacking real time data, the experimenter may make in-situ before and after

measurements. Or he may be obliged to remove the window for a dedicated

measurement of transmittance change. Based on these data, corrections may be

estimated for the irradiance/power actually delivered to the target. Similar corrections

may be necessary for, e.g., the temperature response of a pyrometer or thermal

imaging camera viewing the target through a window.

If contaminants have altered the transmittance or reflectance of an optical

element, it should be carefully cleaned and then re-measured to verify that the cleaning

has been effective.


In general, highly precise vacuum measurements are not required and the

experimenter does not need to perform vacuum gauge calibration measurements.

Reputable manufacturers provide gauges and controllers whose calibration is NIST-

56

traceable. If the calibration of a gauge that has been in use for some time is in doubt, it

should be returned to the manufacturer for re-calibration.


Vacuum gauges'are capable of providing data over many orders of magnitude.

Therefore, controllers generally have user-selectable provisions for data output. These

include linear and logarithmic options. The choice selected depends on the data range

anticipated. A computer with a digital data acquisition card is used to acquire the

voltages produced by the gauge controller.


Post-test data reduction is required to convert voltages recorded from the

controller to pressures as a function of time. If only the initial pressure is of interest, this

may be read out visually from the controller or the voltage may be recorded and

reduced as just described.








be considered in the vacuum pressure measurement uncertainty estimate include

• Instrument calibration precision,

• Estimated changes in gauge calibration as the result of contaminant deposition,

• Temporal response characteristics of the vacuum gauge in comparison to the

rate of pressure change experienced, and

• Distance from the target to the gauge location; this becomes more important

when rapid pressure changes occur.

57



3.10.7 Output to Database The result of the pressure measurement will be entered into the database as

follows.

Tabular Data

• Initial pressure (in torr); uncertainty

File Data

• Pressure-time history during target irradiation, if deemed relevant

58

3.11 Measured Characteristic: Air Flow Simulation


HEL target engagements are frequently against targets flying through the air at

high velocities. To simulate the high-speed aerodynamic environment for a material or

component in flight being exposed to HEL radiation, airflow simulators have been

constructed and calibrated. These normally simulate subsonic flow from Mach 0.1 to

Mach 0.9 over an area of the target greater than the interaction zone. This simulates

the shear stress acting on the target from its flight through the atmosphere as well as

the convective heat loss from the heated interaction zone. The shear stresses may

assist any active mass removal mechanisms such as delamination or flowing melt on

the heated surface.

3.11.2 Technical Approaches to the Measurement:

The typical approach to airflow simulation is to supply air or nitrogen to a control

regulator and plenum or stilling chamber. Screens in the plenum chamber help to

homogenize the slow moving flow. The top, bottom and sides of the plenum converge

quickly accelerating the flow before it enters a straight three-sided flow channel. The

fourth side of the straight sided channel is open so that the high power laser beam can

enter and expose the sample. The sample is mounted on the opposite side (back) of

the channel exposed to the flowing air and acting as an extension of the back wall. The

sample is mounted with its front surface flush with the upstream inner surface of the

back wall such that the boundary layer above the sample is equivalent to flow past a flat

plate whose leading edge is located upstream at the plenum exit. In spite of the straight

channel having only three sides, the flow continues to be well characterized past the

sample location.

Both convective heat losses and shear stresses on the sample surface are well

represented by equations readily available in the literature for turbulent flow across a flat

plate where the characteristic length parameter is the distance from the leading edge.

The convective heat loss coefficient varies as (velocity)08 and the shear stress varies as

(velocity)2.

59

Flow channel simulators for high-power laser experiments have been designed

and built for samples from 3-in. squares to 12-in. squares. Subsonic are the most

common although small supersonic versions have been built. Another geometry used

for flow simulation is axial where the sample is a cylindrical object such as a missile

mounted on axis and inserted partially into a cylindrical nozzle. These have been built

up to 36 inches in diameter for large targets. Design or selection of an airflow simulator

for a set of HEL experiments depends on whether one is trying to simulate convective

heat losses to the air environment or shear stresses at the target surfaces. It is often

not possible to simulate both at the same time. For example, shear stress on a high

velocity high altitude missile can be simulated at one atmosphere pressure and a lower

velocity. But then the convective heat loss may be different than desired. If the

temperature of the target surface is measured, then the convective heat loss can be

estimated and the laser irradiance on the target surface adjusted to get the desired net

target heating rate.

The selection of airflow velocity can best be made with some prior knowledge of

the importance of shear stress and convective losses to the air stream and to the

target/material damage process. For experiments at low irradiance levels convective

heat losses can be very important. But at very high irradiance levels, convective losses

to the airflow become a very small percentage of the absorbed heat flux and are

therefore relatively unimportant to simulate accurately. Shear stresses may or may not

be important to certain target damage mechanisms. For melting metals, the melt

removal rate may depend on airflow velocity. For the brittle fracture of a ceramic

radome, surface shear stresses from the airflow are irrelevant.

Most airflow simulation facilities are operated using compressed and dried air

held in a large storage tank. These can also be operated with "makeup" air by mixing

oxygen and nitrogen in the standard 20% O2 - 80% N2 mixture. Sometimes it may be

desirable to match some partial pressure of oxygen at higher altitudes in order to

simulate the correct contribution of surface oxidation to a material damage process.

Virtually all airflow simulators used for laser lethality studies operate at atmospheric

pressure, so mixing less oxygen and more nitrogen together will reduce the oxygen

partial pressure to that found at the target altitude.

60


Whenever an air flow simulator channel is built or components changed it must

be calibrated. The flow velocity is varied by a regulator in the air supply line that adjusts

the plenum chamber pressure. The velocity and boundary layer thickness at the

sample location must be related to the regulator setting so the desired flow conditions

can be accurately set. Also in the three-sided flow channel the flow needs to be

mapped three-dimensionally. The velocity must be measured as a function of distance

from the sample surface out into the flow, the distance downstream from the plenum

and the distance across the channel (usually vertically). This provides a map of the

velocity profile and boundary layer development over the sample surface which shows

how far downstream in the three-sided flow channel a sample can be mounted and still

be in characterized airflow.

The device used to measure the air velocity is the pitot tube with pressure

transducers. The pitot tube should be small in diameter, i.e., 1/16 of an inch, and

mounted on a translation stage so the pitot tube can conveniently measure velocity as a

function of position through the flow channel. This is not a measurement that needs to

be performed routinely before each use of an airflow simulator for experiments, as the

flow velocity profiles will not change. Once the calibration of the plenum pressure and

regulator setting to the flow velocity is completed, then all that is necessary is to monitor

the plenum pressure to be sure the airflow velocity remains consistent.

Because the air is expanded rapidly in the flow simulator it is cooler than room

temperature. This temperature must be measured to obtain the correct air density for

the shear stress and convective heat loss correlations.


The uncertainty in the measurement should be estimated by the facility or the

experimenter and included in the report along with the basis for the estimate. This

uncertainty should be based on all of the independent contributions to error associated

with the measurement method including the instrumentation, the data acquisition

hardware, calibration factors, and input information from other sources. The combined

uncertainty calculation should be performed and reported according to NIST guidelines.

61

Factors to be considered in the air flow simulation measurement uncertainty estimate

include

• Precision of plenum pressure reading,

• Precision of regufator reading,

• Gas mixing valve precision,

• Pitot tube calibration uncertainty,

• Pitot tube location uncertainty, and

• Gas temperature measurement uncertainty.


measurement rather than percentages, and should be calculated by the NIST

guidelines.

3.11.5 Output to Database The result of the air flow simulation measurement will be entered into the

database as follows.

Tabular Data

• Mean flow velocity or Mach number; uncertainty

• Gas mix; uncertainty

• Gas temperature; uncertainty

File Data

• Velocity profile records

3.12 Measured/Reported Characteristics: Target and Material Description

3.12.1 Purpose of Specimen Description

A quantitative, accurate, and meaningful description of laser test targets and its

material composition is of critical importance in interpreting the observed results of an

HEL lethality experiment. Without properly documented test specimen descriptions, the

data become significantly less valuable to future users of the data, especially those who

come from indirectly related and unanticipated technical arenas. The purpose of

62

reporting experimental data is to archive technical information in such a way that

someone in the future could duplicate the experiment, obtain comparable results and

arrive at the same conclusions.

One should be able to read the target/test specimen description in a technical

report and be able to determine the composition, structure, dimensions, and chemical,

physical and optical properties of the test target, and as appropriate, identify the system

component and sub-system in which it is used.

As an example, in the case of a flight control surface on a missile, the descriptive

data reported should include the name of the missile, which flight control surface was

tested and a useful description of the material composition. The material description

should Include detailed information regarding coatings and substrate composition,

incorporating such information as composite type, resin or matrix material, composition

of fiber reinforcing, form of reinforcing fibers and MILSPEC numbers (if appropriate). In

the case of actual systems and components, serial numbers, sources and manufactures

and their specs should be included.

Pre-test measured chemical and physical properties such as spectral

absorbance and reflectivity, thermal degradation information, melting point or glass

transition temperature, material density, etc. should be reported. These are all

parameters that describe and characterize the target prior to testing. Without accurate,

quantitative and comprehensive descriptions of the target and the material from which it

is fabricated, the value of laser lethality experiments becomes limited and often subject

to question. In every case It is better to provide more information or even redundant

information than to have an information void that later prevents future investigators from

using the test results to make target response predictions and extrapolations.

Post-test target damage assessment and evaluation is valuable In understanding

both the extent and mechanism of laser induced target damage. Specific quantitative

measurements of damage depend on the form and function of the targeted system.

Measurements of crater depth and shape, the formation and thickness of char and

thermal damage and delamination at both depth and at the perimeter and residual

63

strength relative to the function of the system are all valuable. Post-test measurements

should be as accurate, quantitative and as comprehensive as pre-test measurements.

3.12.2 Examples of Material Description and Characterization Clearly, the reality of laser testing and time and cost prevent all possible

parameters from being measured; however, the more information reported, the more

valuable the experiment may be to future investigators. Reporting comprehensive

quality data prevents the "re-invention of the wheel". Too much reported data is better

than too little data. It should be remembered, that the data may, in the future, be

valuable in technical areas that are not obvious at the time of the experiment. As an

example, the experiment may involve testing the laser lethality of a pressurized motor

case, but the material from which it is made may be the same or similar to an unrelated

system and of great interest to future applications.

Some examples of information that should be reported regarding the description

and characterization of laser target materials are listed below. This list should be

viewed only as examples and should not be considered a comprehensive list of

parameters to be measured and reported.

Target Description and Origin: coupon, lab made or commercial material,

MILSPEC numbers, manufacturer description, system or subsystem component

(missile motor case, UAC tail fin, etc.), name/identification of system or

subsystem, serial numbers, date of fabrication, etc.

Material Form: metal, alloy name/number, composite, MILSPEC numbers, resin

type and chemical family (epoxy, polyester, etc.), fiber type and form (e.g., E-

glass, polyarimide, chopped or woven, etc.), resin thermal damage family group

(high-charring, low-charring, clean-ablator, etc), etc.

Target Physical Properties: thickness, area, mass (before and after testing),

material density, spectral properties of virgin surface, thermal gravimetric

analysis (TGA) and differential scanning calorimetry (DSC) of resin, chemical and

physical composition of paint or coating, etc. Thermal conductivity of the target

material may be measured or found in the literature.

64

Post-Test Measurements: mass, penetration time, mass loss, damage

appearance (descriptive), crater damage profile, observations following cross-

sectioning (char thickness and location, ply separation, damage profile variation

within crater compared to irradiance profile, measured Q* (kJ of incident laser

energy required for the removal of 1 gram of target material) and W (kJ of

incident laser energy required for the removal of 1 cm3 of target material) etc.

3.12.3 Measurements and Accuracy

Weight (mass) measurements of smaller coupon type targets should be made

before and after laser irradiation. Weight should be measured with analytical balances

capable of four to five significant figure accuracy. Target thickness can generally be

measured with a micrometer or caliper. Standard rulers are generally adequate to

measure target areas. This information is useful in determining target material density

in units of mass in grams per unit volume in cubic centimeters. Crater volumes should

be measured by profilimeters if available.

3.12.4 Organization by Materials Family

Target materials may be described according to the material family to which they

belong. In general, a material from which a target is fabricated may be described by the

following "family tree": This material family tree, shown in Table 1, should not be

considered comprehensive and may well have to be modified as new and novel

materials develop and are tested.

3.12.5 Pre-Test Material/Target Description

A detailed description of the test article should be clearly reported, including

information as to its form (i.e., test coupon or system component, etc.), structure (i.e.,

multilayer or single homogeneous material, etc.) and origin. Material composition and

form should be reported, including, as available, material certification certificates,

MILSPEC numbers, manufacturer, system component serial numbers, etc.

Composition of targets should be documented in detail including information, as

available, regarding metal alloy, composite type, form and composition of both

reinforcing fiber and resin (matrix), number of layers or ply, thickness, density and

MILSPEC numbers.

65

Thermal properties such as Thermal Gravimetric Analysis (TGA) graphs should

be included for organic matrix materials. Optical reflectivity of the virgin material prior to

testing should be reported. At a minimum, the chemical family type of polymers and

coating should be documented, such as whether the resin has a high char yield or is a

"cleanly" ablating material, whether its chemical family is best described as a silicone,

epoxy, polyimide, phenolic, etc. Similar information should be provided for any

reinforcing fiber present in the target sample.

Chemical family, color and thickness of any coatings on the exposed surface of

the target should be documented. Where available, MILSPEC numbers should be

provided as well as manufacture's description of any paint or coating. Chemical families

of coatings, such as epoxy or urethane, should be revealed along with pigments

descriptions and spectral properties of the surface coating at the wavelength of laser

testing.

66

Table 1. Material Family Tree

Material Application or Function Structural

Aerodynamic Surface Support Member Container or Vessel'

Pressurized Gas Liquid container (fuel, water, etc.)

Insulation Armor or Protective Component Optical Sensor/Detector Lens Window

Electrical or Electronic Wire Harness Circuit Board Insulation (Electrical or Thermal) Integrated Circuit Power Supply Component Sensor or Guidance or Control System Other

Coating (thin relative to substrate)

Material Form Homogeneous Composition

Molded, Extruded, Foamed, etc. Heterogeneous Composition

Composite Layered Particulate or Chopped Fiber Foamed

Coating (thin relative to substrate) Additives

Fire Retardants Intumsecents

Material Composition (substrate & coatings) Inorganic (sometimes composite reinforcing)

Metal Aluminum (alloy number) (alloy number) Titanium (alloy number) Boron Silicon (chips, detectors, etc.) Other (include composition/assay)

Ceramic Pyroceram Slip Cast Fused Silica Nitride Silica Alumina Carbon/Graphite Other (include composition/assay)

Glass Borosilicate E-glass S-glass Quartz Other (include composition/assay)

Organic (sometimes composite matrix) Linear Ablator Polymethylmethacrylate (PMMA) Polyethylene Polystyrene Polytetrafluoroethylene (Teflon, etc.) Polyvinylchloride Other ((include composition/assay)

Charring Ablator Epoxy (with compositional description) Polyimide Polycarbonate (Lexan, etc.) Polyphthalonitrile Silicone Polycyanurate Polysilosesqutane Polyphenols Polyester (include composition/assay) Polyaramid (Kevlar, etc.) Polysulphone Wood, Cork Other (include composition/assay)

Additives Fire Retardants

Material Properties (measured/fiterature/specs) Density (specific gravity) Flammability Thermal Robustness (TGA data) Thermal Conductivity Specific Heat Heats of melt, boiling, decomposition, etc. IR emissivity Fraction or % Matrix vs. Resin Absorptance at Laser Wavelength Thickness (coating and substrates) Ply lay-up & count (if layered structure) Visual description (color, roughness, etc.) Isotropic vs. Anisotropic Properties

67

3.12.6 Post Test Material Damage Assessment

Post-test examination of the target is critical to determine the nature and extent

of damage. Following target irradiation, the mass of the damaged target coupon should

be determined and from.the weight difference, the mass of material removed by the

laser can be determined. These data, along with information regarding laser energy,

power and time, allow the calculation of Q* (kJ of incident laser energy required for the

removal of 1 gram of target material) and W (kJ of incident laser energy required for the

removal of 1 cm3 of target material). Values of Q* and W are measures of the laser

vulnerability and hardness of the target material and the relationship between them is

W=(density) x (Q*). In the event of laser penetration of the target, mass loss data is not

useful and W is calculated from target thickness, laser irradiance and time to penetrate.

Frequently test targets are too large and heavy to weigh with accuracy and the only

method for determination of W (and Q*) is from penetration time data. Measured values

of W can be used to calculate recession rate (the rate of "hole-drilling" in cm/sec). This

data is valuable for extrapolation to other material thicknesses.

The depth profile of the laser induced damage to a target should be determined

and compared to the irradiance profile of the incident laser beam, which is often

determined by material removal from a standard cleanly (linearly) ablating diagnostic

target such as polymethylmethacrylate (PMMA or Plexiglas™). Various profilimeters

are commercially available or often can be improvised. Laser reflectance profilimeters

are accurate and relatively rapid to use. Mechanical depth probes (similar to those

used to measure remaining tread on automobile tires) are slower to use but are

inexpensive. Burn-depth measurements on an x-y surface coordinate system may be

graphed providing a profile of the laser damage area. The profile of the laser induced

crater provides valuable information as to the extent of damage to a target. Crater

profile data along with laser ¡rradiance profiles obtained from another source may be

used to calculate material damage characteristics such as Q* and W as a function of

irradiance.

In the case where destructive testing of the irradiated target is permitted, cross-

sectioning of the target often provides valuable information as to the nature, extent and

mechanism of target damage. Cross-sectioning of composites is best done using a thin

68

blade diamond saw. Often char formation at depths below the floor of the target crater

may be observed and the char thickness and location (depth or ply number) measured

and correlated with local irradiance. Measurement of electrical properties (conductivity

and/or RF attenuation) of the char itself is useful to determine whether the carbon

residue reached elevated temperatures (>~800C) where amorphous carbon begins to

convert to graphitic carbon. When evaluated in reference to the thermal stability of the

target material information as to in-depth heating may be obtained. Ply separation (de

lamination) should be noted and quantified. This information may be useful in

estimating loss of target strength and integrity and reduced thermal conductivity into

depth. In the case of most fiber reinforced composite or ceramic targets, cross-

sectioning can generally be best achieved with a diamond saw. Metal targets generally

may be cross-sectioned on a metal band saw or abrasive wheel.

In every case, more data is better than too little. Data may in the future be

valuable in technical areas that at time of the experiment may not be immediately

obvious.

69

4 Reporting Uncertainty of Measurements

The standards outlined above list possible contributions to the uncertainty of

each measurement result. These lists are not comprehensive because the conditions

and diagnostic methods employed vary greatly for each lethality test. It is critical that

the experimenter consider all of the sources of uncertainty for each measurement,

estimate values for each uncertainty contribution, and combine them properly to report

an overall uncertainty for the measurement result. This section summarizes the NIST

standard for estimating uncertainty contributions and combining them to yield the best

estimate of the uncertainty for the final result. The experimenter is advised to consult

NIST Technical Note 1297[4] and the NIST Engineering Statistics e-Handbook[51 for

more detail on methodology.

4.1 Definitions

In past assessments of measurement error, the terms accuracy and precision

were often used to describe uncertainty in a measurement due to systematic and

random errors, respectively. NIST recommends an approach that should lead to better

estimates of uncertainty. The following terms are used in the estimate of the uncertainty

of a measurement result.

• Measurand, V, (value sought, e.g. power)

• Measurement result, y (approximation of /obtained in a set of measurements)

• Expanded uncertainty of measurement result, U

• Combined standard uncertainty of measurement result, uc(y)

The result of a measurement and its uncertainty is normally stated as

Y=y±U,

where U = k uc(y) (the expanded uncertainty).

The constant, k, (called the coverage factor) defines the confidence interval for

the measurement result as illustrated in Table 2.

70

Table 2. Coverage Factor Probability

Coverage Factor, k

1

2

3

Probability that /lies in the interval

0.68

0.95

0.99

A coverage factor value of "2" is recommended by NIST and will be used for

lethality test data. This will provide a 95 percent probability that the measurand lies in

the interval defined by the expanded uncertainty. The combined standard uncertainty,

uc(y), then, is the key value that must be extracted from the estimates of uncertainty by

the experimenter.

To calculate the combined standard uncertainty, one must examine the

measurement carefully. For every measurement, there is a measurement function,

Y = f(Xi, X2, ..., Xn),

where fis the functional relationship between the measurand and the quantities used to

compute it. For example, the measurement function for Q* is Q* = E/Am, incident

energy divided by mass loss. Uncertainties in the values of E and Am, both contribute

to the uncertainty in Q*. The estimate of the combined standard uncertainty is made as

follows.

».w=JS 1=1

it dX;

n2

u2(x¡)+A

where u(x¡) is the uncertainty associated with the input quantity, x¡, y = f(xi, x2, ..., xn) is

the measured value and partial derivatives are df/dX, evaluated at X, = x¡. If there are

uncertainties dependent on more than one input quantity, there are additional

covariance terms as discussed in Appendix A of NIST TN-1297. For the example of Q*,

the uncertainty in E and in Am are independent and, therefore, there are just two terms.

In its simplest form, this combination of terms is just the traditional root sum of squares

71

(RSS) method of estimating the propagation of error. It is important to note how the

individual contributions, u(x¡), are estimated as discussed below.

4.2 Evaluation of Uncertainty Contributions The combined standard uncertainty of a measurement result, uc(y), consists of

contributions from "Type A" and "Type B" evaluations of uncertainty as defined by the

following

• Type A Evaluation: values obtained by statistical methods, e.g., standard

deviation

• Type B Evaluation: values obtained by other means

- Previous measurement data

- Experience with or knowledge of materials and instruments

- Manufacturer's specification

- Data in calibration and other reports

- Uncertainties assigned to reference and handbook data

Generally, random error and systematic error may contribute to either category,

however, often Type A evaluations incorporate the random error and Type B

evaluations cover the systematic error.

An example of a Type A evaluation (statistical method) is n measurements of an

input quantity, X¡, (for example ten measurements of the mass loss). The standard

uncertainty in x¡ is given by the estimated standard deviation of the mean

i _ | 1 / 2

( \ s 1

0-1) f̂ T

where s is the standard deviation calculated for the data set.

An example of a Type B evaluation of uncertainty (other methods) is a case

where only a range of values is known for an input quantity (for example, a calorimeter

reading of energy is known to be between 13.3 and 13.5 J). Several probability

72

functions have been generated to cover various cases uncertainty. Three of these are

shown in Figure 7.

Normal Uniform Triangular — a— if—

u(x) = a = a/3 = 0.33 a \ u(x) = a/(6)1'2 = 0.41 a

(a is 99.7% point) "W = al(3)m = 0.58 a

Figure 7. Probability Distribution Functions and Associated Uncertainty Estimates

In the figure, the range from -a to +a covers all or nearly all of the integral of the

distribution function. If no information is available other than a range of possible values

for a quantity, then a uniform probability function provides the best estimate of

uncertainty, u(x¡) = a/(3)1/2. For the energy example, u(E) = 0.058.

73

4.3 Calorimeter Calibration Example

An example of application of the methodology outlined above is the estimation of

the uncertainty associated with the calibration of a custom high-energy calorimeter for

total absorption of multi-kW class laser beams (Black 8 Ball). Figure 8 summarizes

information obtained from the calibration certificate and discussions with a NIST-

qualified calibration service provider.[61

Table 3. Uncertainty Estimate at Calibration

Contribution

Black 8 Ball Calorimeter

BB2 Reference Calorimeter

Digital Voltmeter

Chopper

Estimated Timing

Black 8 Ball Stability

Keithley DMM

Relative Uncertainty (%)

0.885/261/2*

3.00

0.02

2.35

0.50

2.00

0.50

Eval. Type

A

B

B

B

B

B

B

Standard deviation is 0.885% from 26 measurements

"=\2Z i'=l .

u{x.) i 2

+ (0.885)2

26

¿7=5.05%

/c=2

Type B (uniform) Type A (standard deviation of the mean)2

Figure 8. Uncertainty Estimate for Black 8 Ball Calorimeter at Calibration

The calibration process involved use of a mechanical 50/50 beam splitter to send

equal power beams to the Black 8 Ball calorimeter and the NIST-developed BB2

74

reference calorimeter. In the calibration process, 26 calibration measurements were

made and the standard deviation of the data set was determined to be 0.885%. The

standard deviation of the mean is then 0.885%/261/2 = 0.174% (Type A evaluation).

This term is then squared and added to the terms from all other contributions which are

estimated assuming uniform probability distributions (Type B evaluation). The final

reported value was adjusted up to 5.5% to account for uncertainty in the estimate for the

Black 8 Ball device stability between calibrations.

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5 References [1.] Optics and optical instruments - Lasers and laser-related equipment - Test

methods for laser beam power, energy, and temporal characteristics, International

Standard ISO 11554,' International Standards Organization.

[2.] Optics and optical instruments - Lasers and laser-related equipment - Test

Methods for laser beam power (energy) density distribution, International Standard

ISO 13694, International Standards Organization.

[3.] Chuck Lamar, "A Methodology for Directly Characterizing High Energy Laser

Beams," paper presented at DEPS Second Annual High Energy Laser Lethality

Conference, Tampa Florida, March 15-17, 2005.

[4.] Barry N. Taylor and Chris E. Kuyatt, Guidelines for Evaluating and Expressing the

Uncertainty of NIST Measurement Results, NIST Technical Note 1297 1994 Edition,

Physics Laboratory National Institute of Standards and Technology, Gaithersburg,

MD 20899-0001 (Supersedes NIST Technical Note 1297, January 1993)

September 1994.

[5.] NIST/SEMATECH e-Handbook of Statistical Methods,

http://www.itl.nist.gov/div898/handbook/

[6.] Private communication with Bionetics Co., September 20, 2004.

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http://www.itl.nist.gov/div898/handbook/

ABBREVIATIONS AND ACRONYMS

um

A/D

CCD

C02

CW

FWHM

HEL

Hz

InGaAs

IPT

ISO

JTO

kHz

kW

LVT

MILSPEC

NIST

nm

PbS

PbSe

PMMA

micrometers

analog-to-digital

charged-coupled device

carbon dioxide

continuous wave

full width at half maximum power

high energy laser

Hertz

indium gallium arsenide

Integrated Product Team

International Standards Organization

Joint Technology Office

kiloHertz

kilowatt

linear velocity transducer

military specification

National Institute of Standards and Technology

nanometer

phosphate buffered saline

lead selenide

poly methyl methacrylate

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SME

TCC

TGA

TRAM

UV

subject matter experts

thermal coupling coefficient

Thermal Gravimetric Analysis

Thermal Reflectance of Aerospace Materials

ultraviolet

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