OLI-WP-01-20

SPECTRORADIOMETER WORKSTATION

FOR PHOTOBIOLOGICAL SAFETY STANDARD

IEC 62471 COMPLIANCE MEASUREMENTS

A White Paper by

Optronic Laboratories, Inc.

June 2020

1

Table of Contents

1.0 SUMMARY……………………………………….…………………………….………2

2.0 INTRODUCTION……………………………………………………………….………2

3.0 BACKGROUND…………………………………………..…………………….………3

LED Technology…………………………………………..…………………….………3

Photobiological Hazards (PBHs) of LEDs………………………….………………….3

4.0 DEVELOPMENT OF AN INTERNATIONAL PBH STANDARD………….………..5

Table 1: Summaries of IEC 62471 Technical Reports…………………………..……5

5.0 MAKING PHYSIOLOGICALLY RELEVANT PBH MEASUREMENTS……….…..6

Figure 1: Schematic of a Spectroradiometer Workstation .…………………..……..7

Table 2: Required Measurements of IEC 62471 Compliance...…………….………8

6.0 METRICS FOR DETERMINING EXPOSURE LIMITS……………….………..……8

7.0 RISK GROUP CLASSIFICATIONS………….…………………………………..……9

Table 3: Risk Group Categories…..…………………………………….………...……9

8.0 IEC 62471 SPECTRORADIOMETER WORKSTATION..…………….………..……9

OL 750-M-D Double Monochromator…...…………………………………...……….9

Radiance Module – OL 600 Direct Viewing Imaging Optics Module (DVIOM)…..10

Irradiance Module – OL IS-670 Integrating Sphere…...………………..…..……...10

Detection System – OL 750-HSD Hight Sensitivity Detectors…………..………..11

Validation and Compliance……………………………………….……….…………..11

Table 4: IEC 62471 Spectroradiometer Workstation..………………….……….…11

9.0 IEC 62471 COMPUTATIONS SUMMARY………………….…………..…………12

10.0 OL 750 MODULAR WORKSTATION…………………………………….…………13

Figure 2: OL 750 Modular Workstation Platforms…………………….……………13

11.0 PORTABLE SPECTRORADIOMETRIC STSTEMS……..………….………...……14

OL 756 Portable Double Monochromator………………………….………….……14

OL 770 CCD Spectroradiometer…………………………………………….…….…14

12.0 WORKS CITED…………………………………………………………………..……15

2

1.0 SUMMARY

In recent decades, advances in

materials science and technology have

substantially improved the quality of

lighting products. LEDs in particular

have become more rugged, with higher

efficiencies, greater stability, and emit

significantly higher optical power over a

wide range of spectral regions. As a

result, LEDs have found a place in many

aspects of day-to-day life. The growing

concern over the physical repercussions

of increasing human exposure has led

to the creation of an international

standard governing photobiological

hazard assessment of lamps and lamp

systems, IEC 62471 – Photobiological

Safety of Lamps and Lamp

Systems. The spectroradiometric

measurements needed for standard

compliance requires complex

instrumentation, not only for the third

party laboratories, but also LED

manufacturers in order to monitor these

optical properties throughout the

manufacturing process. Optronic

Laboratories, Inc. has developed an IEC

62471 workstation designed using our

modular OL 750 Series

Spectroradiometer that facilitates

straight forward assembly, seamless

measurement scans, and simple

transitions from one measurement

geometry to another. This paper will

describe the IEC 62471 standard and

discuss how the OL 750D IEC 62471

Spectroradiometric Workstation can

facilitate standard compliance for quality

control laboratories and LED

manufacturers alike.

2.0 INTRODUCTION

Historically, lasers and ultraviolet (UV)

lamps were the primary devices

requiring human safety analysis.

However, recent advances in

technology have resulted in light-

emitting diodes (LEDs) of superior

efficiencies and substantially higher

optical outputs, as well as operating at

wavelengths ranging from the UV,

through the visible and near infrared

(NIR), and into the shortwave infrared

(SWIR). These technological advances

have allowed LEDs to permeate most

aspects of daily life, but have also

brought into question the possible

hazards that result from increased

human exposure.

As international standards to assess the

photobiological compatibility of non-

laser lamps and lamp systems became

a reality, LED manufacturers faced new

and daunting responsibilities. While the

quality control laboratories faced

obtaining complex instrumentation

required for full standard compliance,

the manufacturers of LEDs and other

lighting components were now required

to both comprehend the standard

requirements as well as acquire the

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ability to monitor the essential optical

parameters throughout the entire

manufacturing process.

This white paper aims to discuss the

necessity of an international standard on

photobiological safety, clarify the

various requirements of the standard,

and lay out the necessary

spectroradiometric measurements and

instrumentation necessary for full

compliance.

3.0 BACKGROUND

LEDs are semiconducting materials that

emit high-intensity optical radiation

across the UV, visible, and infrared (IR)

spectral regions. They have surpassed

the lighting industry standards in terms

of their efficiencies, lifetimes, and

optical output, resulting in their

ubiquitous presence in most aspects of

daily life. This increase exposure has

brought to the forefront a real concern

for the hazards that may result from

even incidental exposure to LED light.

LED Technology

The artificial lighting industry has long

been dominated by inefficient sources

of radiation such as incandescent and

fluorescent lamps. A review in 20161

estimated that the electrical lighting

industry accounts for 1/6th to 1/5th the

world’s energy consumption.

Incandescent lamps that depend on the

heating of a metal filament are

inherently inefficient, converting less

than 5% of the input energy to light.2

Compact fluorescent lamps (CFLs) use

up to 90% less energy and last up to 15

times longer than incandescent lamps3,

but are not a suitable long-term

replacement as they are toxic and not

environmentally friendly due to the use

of mercury vapor.4 In recent decades,

technical advances in electronics and

material science have allowed the use

of LEDs in residential and commercial

lighting to become more prevalent.

LEDs are much more energy efficient

than both incandescent and CFL lamps,

as well as last much longer.5

LEDs are composed of inorganic or

organic materials that emit light when

electricity flows through them. As a

result of continuing advancements in

their luminous efficacies1 as well as

LEDs that now emit light in the UV,

visible, and IR spectra, they are rapidly

infiltrating applications beyond lighting,

exposing more people to their

emission.6 Optical radiation, as defined

in the photobiological safety standards,

encompasses the wavelength range of

200nm to 3000nm, or UV, visible, near

infrared (NIR), and shortwave infrared

light.

4

Photobiological Hazards (PBHs)

of LEDs

As technological advancements allow

LEDs to be more efficient and more

powerful, the possible consequences of

human exposure to optical radiation,

both in the short-term as well as the

long-term, becomes of more concern.

The potential hazards of human

exposure are well known, particularly to

the skin and eye, and are classified into

two categories: photochemical or

thermal hazards. Photochemical hazards

occur when light of sufficient energy

breaks or rearranges the chemical

bonds in the cellular molecules. This

usually requires light of higher energy,

predominantly UV light. Thermal hazards

involve absorption of radiation,

particularly in the IR, in the form of heat.

This heat increases the temperature of

the surrounding area, which can have

devastating consequences for proteins

or other biochemical macromolecules

that are highly temperature sensitive.

Optical radiation from LEDs typically

affects the skin, the distal surfaces of

the eye, as well as the retina. The

specific hazards include:

• Photokeratitis – photochemical

reactions that induces chains of

biochemical reactions. Light in the

range of 200 – 400 nm causes this

with the peak sensitivity at 270

nm.

• Photoretinisis – often referred to

as “blue light hazards” is

photochemical damage, apparently

to the retinal pigment epithelium

from exposure to blue light,

primarily between 400 – 500 nm.

The peaks sensitivity is

approximately 445 nm.

• UV Cataractogenesis – clouding

of the lens due to prolonged

exposure to UVA light, primarily

from 290 – 325 nm, but possibly all

the way to 400nm. The peak action

is approximately 305 nm.

• IR Cataractogenesis – thermal

damage to the lens of the eye

caused primarily from 700 – 1400

nm, but possibly all the way out to

3000nm. Sometimes referred to as

“industrial heat cataract”,

“furnaceman’s cataract”, or

“glassblower’s cataract”.

• Retinal Thermal Injury – thermal

damage causing denaturing of

proteins and other key biochemical

components resulting in

destruction of the tissue. This is

caused by radiation in the range of

400 – 1400 nm, but peak

5

sensitivity is at 500 nm where the

human eye is most sensitive.

• Erythema – Reddening of the skin

(sunburn) due to exposure to UV

light, predominantly 200 – 320 nm

with a peak action around 290 nm.

Not all LEDs produce radiation that

results in photobiological or thermal

damage. The of the most concern are

those used for illumination and lighting

purposes. By the nature of this use,

long exposure times are likely.

4.0 DEVELOPMENT OF AN

INTERNATIONAL PBH

STANDARD

In the 1990s, LEDs were first evaluated

according to IEC 60825 – Safety of

Laser Products based on the existing

similarities between LEDs and lasers.

However, it is the differences between

the two that makes this an

unsatisfactory long-term solution. The

first attempt to separate LEDs from

lasers came when the Illuminating

Engineering Society of North America

(IESNA) created a standard that

encompassed LEDs as well as other

non-laser lamps and lamps systems

entitled RP-27, Recommended Practice

for Photobiological Safety for Lamps

and Lamp Systems: General

Requirements. The general

requirements in RP-27 were later used

by the International Commission on

Illumination (CIE) to develop an

international standard in the form of

standard CIE S 009/E:2002,

Photobiological Safety of Lamps and

Lamp Systems.

The work done by IESNA and CIE laid

the foundation for what would become

the current international standard on

PBH safety. The International

Electrotechnical Commission (IEC),

along with the IEEE (the world’s largest

technical professional organization)

developed a standard that is largely

based on IESNA RP-27, but also used

some of the updated information

regarding the relevant weighting

functions in CIE S 009, a standard that

became IEC 62471:2006,

Photobiological Safety of Lamps and

Lamp Systems.

Since the initial issuing of IEC 62471 in

2006, the IEC has subsequently

released technical reports to put forth

additional information and clarification

on a variety of subjects. Their contents

are summarized in Table 1.

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5.0 MAKING

PHYSIOLOGICALLY

RELEVANT PBH

MEASUREMENTS One of the purposes of IEC 62471 was

to make standard a set of

measurements including suggestions

for the appropriate spectroradiometric

instrumentation. For the prescribed

measurements to be relevant to the

PBHs being considered, they must be

made in a manner that is consistent

with how the light impacts the body. All

of the PBHs assessed in IEC 62471 can

be classified into two categories, light

that strikes the superficial surfaces of

the body (skin, conjunctiva, cornea, and

lens) and light that enters the eye and

impinges on the retina. The two

spectroradiometric techniques needed

to best assess these phenomena are

irradiance and radiance measurements,

respectively.

• Irradiance – Irradiance is a

measure of the radiant flux (i.e. the

density of light) that impacts a

surface over a given area. This

measure is a good description of

how light coming from many

angles impacts a given area of

superficial surfaces (e.g. skin,

conjunctiva, cornea, and lens).

Irradiance measurements are

performed by collecting the light

using a cosine receptor, or an optic

capable of accepting light from a

180o field-of-view (FOV). This is

typically accomplished using an

integrating sphere. An integrating

sphere has an inner surface that is

coated with a highly reflective

material, typically PTFE or BaSO4 in

the visible and NIR to SWIR. The

light incident on the sphere’s

entrance port is collected from a

full 180o FOV, reflects within the

sphere several times, then exits

the sphere into spectroradiometer.

The spectroradiometer then

separates the collected light into

individual wavelengths, which is

then detected at the exit of the

spectrometer by an appropriate

detector. The schematic of a

spectroradiometer configured for

irradiance measurements is shown

in Fig. 1a:

7

• Radiance – Radiance is a measure

of the radiant flux (i.e. the density

of light) being emitted from a

source over a given area, in a well-

defined FOV, measured using the

3-dimensional angle units of

steradians (sr). The light that enters

the eye and impacts the retina is

determine by many physiological

aspects of the eye, namely the

diameter of the pupil (the aperture

of the eye that limits the light

entering the eye), and the ability of

the lens and other surfaces of the

eye to focus the light onto the

retina. The relevant FOVs used to

assess retinal hazards have been

determined based on these

parameters to be 0.0017 rad for

times shorter than the human blink

reflex (<0.25 s) or for pulsed

sources, 0.011 rad for times

between 10 and 100 s (rapid eye

movements spread the retinal

exposure), and 0.1 rad for times

exceeding 10,000 s (where typical

vision tasks spread the retinal

exposure even further). To achieve

Figure 1: a.) Schematic of a spectroradiometer configured for irradiance measurements with the IS-670 6”

integrating sphere on the entrance of the monochromator, followed by a 750-M-D double monochromator to disperse the light into individual wavelengths, and finally a high sensitivity detector on the exit of the

monochromator to quantify the collected light. b.) Schematic of a spectroradiometer configured for radiance

measurements with the OL 730-9 reflex telescope on the entrance of the monochromator, followed by a 750-M-D double monochromator to disperse the light into individual wavelengths, and finally a high

sensitivity detector to quantify the collected light.

8

these well-defined FOVs, a

telescope is typically used as the

collection optic for radiance

measurements. A telescope uses a

lens or mirror with a particular focal

length, which defines the distance

from the telescope to the light

source. Together with a limiting

aperture of known diameter, a very

well-defined FOV collects the light

to be analyzed by the

spectroradiometer (Fig. 1b).

A summary of all measurements

required for IEC 62471 as well as which

PBH they correspond to can be found in

Table 2.

6.0 METRICS FOR

DETERMINING EXPOSURE

LIMITS

The practicality of the IEC 62471:2006

standard lays in the exposure limits that

are defined for the described PBHs. For

the scope of this standard, the exposure

limit is defined as the maximum amount

of time a person can be continuously

exposed without adverse health effects.

They apply to all continuous sources

with durations not less than 0.01 ms

(i.e. non lasers) but not longer than an 8-

hour period, and pertain to average

people that do not suffer from

photosensitivity or other physiological

conditions causing them to be more

susceptible to PBHs. Each individual

exposure limit is defined based on the

spectral radiance or spectral irradiance

over the pertinent wavelength range

and takes into account the body’s

photosensitivity or response to the

wavelengths being analyzed. Once the

spectroradiometric measurements are

complete, it can be determined whether

an LED passes or fails each individual

PBH. For instance, when assessing the

blue-light hazard of an LED for exposure

time less than 104 s, the product of the

blue-light hazard weighted radiance and

the exposure time has to be at or below

106 J m-2 sr-1:

This equation can be rearranged and

solved for t (exposure time) in order to

determine the maximum permissible

exposure time for that particular LED

9

given its blue-light hazard weighted

radiance:

7.0 RISK GROUP

CLASSIFICATIONS

Once all spectroradiometric

measurements have been completed

and individual exposure limits

determined, the cumulative results can

then be used to assign a radiation

source to a corresponding risk group.

The risk group categories are meant to

communicate only the potential risk

associated with a source and the

general criteria are listed in Table 3.

8.0 IEC 62471

SPECTRORADIOMETER

WORKSTATION

To support IEC 62471 compliance

measurements, Optronic Laboratories,

Inc. has developed a complete

workstation that is based upon our

industry standard OL 750 double

monochromator, combined with

irradiance and radiance modules whose

performance is verified using NIST-

traceable performance verification

systems and standards. What follows is

a description of the system components

and which part(s) of the standard they

are applied to.

OL 750-M-D Double Monochromator

At the heart of the OL 750 platform

configured for IEC 62471:2006

compliance is the OL 750-M-D double

monochromator. Due to the accuracy

required at wavelengths down to

200nm where stray light can be a

significant source of error, the standard

suggests the use of a double

monochromator with superior stray light

rejection. To that end the OL 750-M-D

double monochromator has been

chosen, which has an industry-leading

stray light suppression of 10-8.

At the entrance of the OL 750-M-D is an

optical chopper with lock-in

amplification electronics. This system

helps to differentiate the analytical

10

signal of interest from any existing

background information, a technique

that is crucial in the UV and IR where

signal levels can be low and often

buried in the background signal.

The OL 750-M-D contains a pair of

identical grating turrets that holds up to

three gratings to facilitate seamless

scanning across the entire wavelength

range specified in IEC 62471. The

selected gratings are chosen to attain

the bandpass requirements over the

various wavelength regions using the

various slit configurations provided.

To further increase the reliability of the

measured signal, the OL 750-M-D

contains an 11-position filter wheel that

contains second-order blocking filters to

ensure the light being measured is only

that of interest and not a second-order

harmonic. The filter wheel also contains

a position that shutters the optical path

in order to obtain and accurate dark

current of the detectors and the

corresponding electronics.

Radiance Module - OL 600 Direct

Viewing Imaging Optics Module

(DVIOM)

The collection optics used to make

radiance-based measurements is the

OL 600 DVIOM, specifically configured

for IEC 62471 measurements. The OL

600 has a CCD remote imaging option

to see the telescope FOV in real-time,

as well the portion of the image that is

being spectrally analyzed. A circular

aperture wheel allows effortless

transitions between the various sized

apertures ranging in diameter from 0.3

to 5.0 mm. Accompanying the OL 600

DVIOM are two lenses custom

designed to achieve all of the FOV

requirements for the prescribed

radiance-based measurements. The OL

600 DVIOM can be coupled directly to

the entrance of the OL 750-M-D double

monochromator or via a fiber-optic cable

in order to allow increased

measurement flexibility.

Irradiance Module - OL IS-670

Integrating Sphere

The OL IS-670 is a 6” integrating sphere

that is internally coated with PTFE,

which has remarkable reflectivity over

the wavelength range of interest. The

standard requires that the device under

test be positioned at a distance that

produces an illuminance of 500 lux (but

not less than 200 mm). Therefore, the

sphere developed for IEC 62471 has a

built-in photometer that allows real-time

monitoring of the illuminance from the

source while positioning the device

under test. There is also a FOV adapter

accessory for the entrance port of the

OL IS-670 with apertures selected

provide an alternate method of making

some of the radiance-based

11

measurements.

Detection System - OL 750-HSD High

Sensitivity Detectors

A combination of three high sensitivity

detectors is provided to cover the entire

200 – 3000 nm range. The OL 750-HSD-

310 photomultiplier tube (PMT) provides

outstanding sensitivity in the UV. A

stable silicon detector (OL 750-HSD-

300) provides exceptional stability from

200 – 1100 nm. A thermoelectrically-

cooled PbS detector (OL 750-HSD-340)

overlaps with the top end of the Si

detector’s range and extends out to

3200 nm, completely encompassing the

prescribed wavelength range. There is

an automated detector selector

available that allows all three detectors

to be connecting permitting seamless

scans from one detector to another over

the complete range.

Validation and Compliance

When a spectroradiometer is calibrated,

it is calibrated as an entire system –

meaning the collection optics, the

monochromator, the detector, and all

elements along the optical path. Not

only does IEC 62471 require two

different spectroradiometric

measurement techniques, but it also

requires multiple measurement

geometries within each technique.

Therefore, we offer NIST-traceable

calibration standards and accessories to

facilitate in-house system response

calibration when the platform is

converted from one technique to

another, or the measurement geometry

is altered.

Full system specifications are compiled

in Table 4.

12

9.0 IEC 62471 COMPUTATIONS SUMMARY

1. With a 1200 groove/mm grating

2. Narrower bandwidths obtainable with optional smaller slits 3. Listed respective to aperture sizes

4. Actual size specified at time of order

13

10.0 OL 750 MODULAR

WORKSTATION

The versatility of the OL 750 platform

comes from its modularity. The system

can be easily converted from one

application to another by adding new

accessories or components. Instead of

using a collection optic on the entrance

port of the OL 750 monochromator, our

OL 740-20 or OL 750-20 light sources

can be used to provide a

monochromatic light source as in Fig.

2a-d. The source is configurable with a

deuterium lamp, quartz-tungsten

halogen lamp, and/or IR glower to

produce radiation from 200 nm out to

40 µm. Our collimating optics (either

750-10C shown in Fig. 2a and 2b

or the OL 750-11C) attached to the exit

port of the monochromator allows the

OL 750 to be utilized as a collimated

light source (Fig. 2b), for transmittance

measurements (Fig. 2a), or detector

spectral power/irradiance response

measurements. A wide selection of

accessories for the exit port of the

monochromator also allow the OL 750

spectroradiometric system to be

configured for spectral and/or diffuse

reflectance and transmittance

measurements as well as

internal/external/total quantum

efficiency measurements of detectors

and solar cells.

Figure 2: OL 750 Modular Workstation Platforms a.) OL 750 spectroradiometric system configured for total transmittance measurements. b.) OL 750 spectroradiometric system configured as a collimated monochromatic light source. c.) OL 750 spectroradiometric system configured for diffuse/total reflectance and/or transmittance measurements. d.) 750 spectroradiometric system configured for goniometric specular reflectance measurements.

14

11.0 PORTABLE

SPECTRORADIOMETRIC

SYSTEMS

Optronic Laboratories, Inc. offers two

alternative spectroradiometric platforms

whose small spatial footprints allow

versatile portability for a variety of

applications. These two platforms are

the OL 756 UV/VIS scanning double

monochromator spectroradiometer and

the OL 770 CCD multichannel

spectroradiometer.

OL 756 Portable Double

Monochromator

The OL 756 spectroradiometer is a

scanning double monochromator with a

TE-cooled photomultiplier tube (PMT)

detector with a spectral range from 200

– 800 nm. The dynamic range spans

seven decades with outstanding stray

light rejection of < 10-8 at 285 nm. The

OL 756 is designed for irradiance

measurements of solar radiation and

solar simulators. Its small footprint, light

weight (25 lbs) and quick scan speed

(up to 200 nm/s in quick scan mode)

make the OL 756 uniquely suited for

measurements in the field. A rugged

carrying case designed specifically for

the OL 756 is available to facilitate

transport without compromising

performance.

OL 770 CCD Spectroradiometer

The OL 770 series multichannel CCD

spectroradiometer is a modular system

with a myriad of collection accessories

to be configured for various

measurement types. The OL 770 has an

incredibly small footprint, is light weight

(22.5 lbs), and provides high-precision,

fast, and accurate research-grade

measurements over wavelength ranges

spanning from the UV to the NIR.

Optional accessories allow the OL 770

series spectroradiometric platform to be

configured for applications including

spectral radiance, spectral irradiance,

LED measurements, display testing,

on-line production testing,

reflectance/transmittances

measurements, goniometric

measurements, and NVIS compliance.

15

12.0 WORKS CITED

1. Zissis G. (2016) Energy Consumption and Environmental and Economic Impact of Lighting: The Current Situation. In: Karlicek R., Sun CC., Zissis G., Ma R. (eds) Handbook of Advanced Lighting Technology. Springer, Cham

2. Keefe, T.J. (2007). "The Nature of Light". Archived from the original on 23 April 2012. Retrieved 15 June 2020.

3. "Compact Fluorescent Light Bulbs". Energy Star. Retrieved 15 June 2020.

4. Beth Daley, “Mercury leaks found as new bulbs break: Energy benefits of fluorescents may outweigh risk,” The Boston Globe, February 26, 2008.

5. Pinto, R.A., Cosetin, M.R., Marchesan, T.B., Da Silva, M.F., Denardin, G.W., Fraytag, J., Campos, A. and Do Prado, R.N., “Design procedure for a compact lamp using high-intensity LEDs,” 35th Annual Conference of IEEE Industrial Electronics, pp. 3506-3511, 2009.

6. Opel, D.R., Hagstrom, E., Pace, A.K., Sisto, K., Hirano-Ali, S.A., Desai, S. Swan, J. “Light-emitting Diodes: A Brief Review and Clinical Experience.” J Clin Aesthet Dermatol. 2015 Jun; 8(6): 36–44