Characterisation & Use of Array Spectrometers

Wednesday, 06 February 2008

1

Characterisation & Use of Array Spectrometers

Mike Shaw, Optical Technologies & Scientific Computing Team, National Physical Laboratory, Teddington Middlesex, UK


2

Overview

• Basic design and features of array spectrometers.

• Advantages and disadvantages of array spectrometers over scanning systems.

• Why are NPL using an array spectrometer?

• Some key performance characteristics and how to assess them.

• In system (heterochromatic) stray light: what causes it and how to measure it.

• Stray light reduction using blocking filters.

• Future work and conclusions.


3

Entrance slit

Collimating mirror

Diffraction grating

Focussing mirror

Detector array

Basic optical layout of an array spectrometer


4

Features of array Spectrometers

• Optical layout– Transmissive and reflective diffraction

gratings– Lenses and mirrors– Internal baffles and size of housing

• Detector array– Different photosensitive elements

depending upon wavelength range– Different no. of array elements and

overall size of array– Electronics

•Input optics:–Optical fibre


5

Why use an array spectrometer?

• Usually less accurate than scanning monochromator based instruments.

• Offer three principal benefits over scanning systems:– Speed (multiple wavelengths measured simultaneously).– Size (usually much smaller and lighter than scanning

systems).– Cost (typically much cheaper).


6

• To develop a new capability to measure the spectral output of light sources.

• Need to use an array spectrometer to carry out measurements in a reasonable amount of time.

• Spectral radiant intensity distribution, I(λ)• Total spectral radiant flux,

Why are NPL using an array spectrometer?

∫ Ω=Φπ

λλ4

)()( dI


7

Some key performance characteristics

• Wavelength accuracy• Spectral resolution• Linearity• Responsivity• Stray light

There are also many other important performance parameters including those relating to the detector array itself such as uniformity, well capacity, noise, etc.


8

1. Wavelength accuracy• Measured using emission lamps or laser lines.• Accuracy depends upon calibration method and type of function

used to fit pixel no. to wavelength.

Output of a Mercury Neon Emission Lamp Measured Using an Array Spectrometer

0

5000

10000

15000

20000

25000

30000

250 300 350 400 450 500 550 600 650 700 750 800 850Wavelength (nm)

Sign

al (c

ount

s)

0

2000

4000

6000

8000

10000

12000

14000

16000


9

2. Spectral resolution

Output of a 532 nm Laser Measured Using an Array Spectrometer

-5000

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

525 527 529 531 533 535 537 539

Wavelength (nm)

Sign

al (c

ount

s)• Resolution depends upon the size of the spectrometer entrance slit & pixel bandwidth – dispersion at focal plane x pixel width. May vary across the detector array.


10

3. Linearity

• Best measured using double aperture method or via comparison with a detector of known linearity (e.g. high quality silicon photodiode or trap).

• Although single element photodiodes may be extremely linear the same does not necessary hold for detector arrays made using the same photosensitive material.

• Need to assess linearity with respect to irradiance and exposure time and also look at wavelength dependence.

• Typical non-linearity errors for silicon CCDs are O(10-3) or less for a 2:1 step in irradiance but much larger errors have been observed for other devices.


11

4. Responsivity

• Depends upon many factors including: type of detector array, size of entrance slit, grating efficiency, coupling into spectrograph.

• Spectral variation in responsivity also effects stray light errors.

Responsivity calibration of a linear detector array


12

5. In System (heterochromatic) stray light

• Often the dominant source of uncertainty in measurements made using compact array spectrometers.

• Scattering, and interreflections from the optics, housing and detector array and inadequate blocking of higher diffracted orders cause rays to strike parts of the detector array corresponding to different wavelengths.


13

Stray light signal observed using a laser line

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 200 400 600 800 1000

Pixel no.

Dar

k C

orre

cted

mea

sure

d si

gnal

(nor

mal

ised

to

max

)

Measured SpectraIdeal spectra

Background due to heterochromatic stray light

These results could be used to state that stray light rejection is < 10-5 some distance away from the centre wavelength of the laser line.


14

Stray light errors are source dependent

• Stray light errors tend to be most critical when measuring a broadband spectrum with an intensity varying over several orders of magnitude, e.g. a quartz tungsten lamp.

http://www.promolux.com/english/faq.htmlhttp://www.daviddarling.info/encyclopedia/H/ AE_high-pressure_sodium_lamp.html

http://www.andrew.cmu.edu/user/tlauwers/pr ojects.html http://en.wikipedia.org/?title=Light_bulb

Spectral Total Flux of a Tungsten Halogen Lamp

0

0.005

0.01

0.015

0.02

0.025

0.03

350 400 450 500 550 600 650 700 750 80Wavelength (nm)

Spec

tral

Tot

al F

lux

(arb

. uni

ts)

Relative SPD of Four LEDs

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

1.00E+03

1.20E+03

350 400 450 500 550 600 650 700 750 800Wavelength (nm)

Spec

tral

Tot

al F

lux

(arb

. un

its) LED1

LED2

LED3

LED4

Spectral Total Flux of a Fluorescent Lamp

0

50

100

150

200

250

300

350

400

450

350 400 450 500 550 600 650 700 750 80

Wavelength (nm)

Spec

tral

Tot

al F

lux

(arb

. uni

ts)

Spectral Total Flux of a High Pressure Sodium Lamp

0

0.01

0.02

0.03

0.04

0.05

0.06

350 400 450 500 550 600 650 700 750 80

Wavelength (nm)

Spec

tral

Tot

al F

lux

(arb

. uni

ts)


15


0

0.005

0.01

0.015

0.02

0.025

0.03

350 400 450 500 550 600 650 700 750 80Wavelength (nm)

Spec

tral

Tot

al F

lux

(arb

. uni

ts)

Stray Light Errors for an incandescent source

Relatively high spectral flux at longer visible and NIR wavelengths

Relatively low spectral flux at shorter visible and UV wavelengths


16

Stray Light Errors for an incandescent sourceS p e c tra l T o ta l F lu x o f a T u n g s te n H a lo g e n L a m p

0

0 .0 0 5

0 .0 1

0 .0 1 5

0 .0 2

0 .0 2 5

0 .0 3

3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 7 5 0 8 0W a v e le n g th (n m )

Spec

tral

Tot

al F

lux

(arb

. uni

ts)

Small fraction of radiation inside the spectrometer is measured as

heterochromatic stray light


17

Measurement of lamp signal,

Vlamp (λ)

Measuring stray light signals from a broadband light source

Fibre input to spectrometerBackground corrected Signal Measured from a Quartz

Tungsten Lamp

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

300 400 500 600 700 800

Wavelength (nm)

Bac

kgro

und

corr

ecte

d si

gnal

(cou

nts)


18


Fibre input to spectrometerBackground corrected Signal Measured from a Quartz

Tungsten Lamp Through a GG435 cut on Filter

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

300 400 500 600 700 800

Wavelength (nm)

Bac

kgro

und

corr

ecte

d si

gnal

(cou

nts)

Measurement through cut on filter, Vfilter (λ)

Nominal transmittance of GG435 (3mm thickness) cut on filter

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Tran

smitt

ance

(%)


19


Fibre input to spectrometer

Measurement of background signal, Vbg (λ)

Shutter to block light source from spectrometer field of view

Background Signal

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

sign

al (c

ount

s)


20

Analysis of Stray Light Data

• For an ideal spectrometer:)()()()(

)(λλλλ

λbglamp

bgfilterfilter VV

VVT

−−

=

Stray light signals cause deviations from ideal

behaviour and indicate erroneously high filter

transmittance at wavelengths shorter than

the cut on.

Transmittance of GG435 glass filter (3mm thickness)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

260 300 340 380 420 460 500 540 580

Wavelength (nm)

Tran

smitt

ance

(%)

Measured usingarray spectrometer

Nominal

Stray light error of > 90%!


21

Dark Corrected Signal Measured From a Quartz Tungsten Lamp Through Two Cut on Filters Using an Array Spectrometer

1000

10000

100000

1000000

10000000

300 400 500 600 700 800

Wavelength (nm)

Sign

al (c

ount

s) Dark corrected signal measured through GG435

Dark corrected signal measured through RG610

Analysis of stray light data

Stray light signal level

NSL

By making some simplifying

assumptions, the fractional stray light

error when measuring unfiltered source can be approximated as

NSL /NTotal

Estimated Fractional Stray Light Error in Measurement of the Output of a Quartz Tungsten

Lamp using an Array Spectrometer

0%

20%

40%

60%

80%

100%

300 400 500 600 700 800

Wavelength (nm)

Estim

ated

str

ay li

ght

sign

al /

tota

l m

easu

red

sign

al (%

)


22

Comparison of Different Array Detectors

• The cut on filter method provides a way to compare the performance of different array spectrometers for measuring the spectral irradiance from a broadband light source.

Transmittance of GG435 measured using a quartz Tungsten lamp and different array detectors

1%

10%

100%

200 300 400 500 600 700 800 900

Wavelength (nm)

Mea

sure

d tr

ansm

ittan

ce (%

) spectrometer A

Spectrometer B

Spectrometer C

Spectrometer E

Spectrometer F

Spectrometer G

Spectrometer H

GG435 Nominal


23

How to handle stray light?

•Live with it

•Correct for it

•Reduce it

Determine stray light contribution to measurement uncertainty.

Magnitude of stray light errors is too large for many applications.

Minimise the effect of stray light by calibrating the

detector under conditions as close as possible to those under which it will be used.


0

0.005

0.01

0.015

0.02

0.025

0.03

350 400 450 500 550 600 650 700 750 800Wavelength (nm)

Spec

tral

Tot

al F

lux

(arb

. uni

ts)


24


•Live with it

•Correct for it

•Reduce it

Characterise the stray light rejection of the instrument

and then correct for it.

•S. W. Brown, B. C. Johnson, M. E. Feinholz, M. A. Yarbrough, S. J. Flora, K. R. Lykke, and D. K. Clark, “Stray light correction algorithm for spectrographs”, Metrologia 40, S81-83 (2003).

•Y. Zong, S. W. Brown, B. C. Johnson, K. R. Lykke, and Y. Ohno, “Simple spectral stray light correction method for array spectroradiometers”, Applied Optics, Vol 45 No. 6, 20 Feb 2006.

Input laser radiation at different wavelengths into the array spectrometer to determine amount scattered onto each pixel as a

function of wavelength – stray light contribution to detector responsivity.

Spectrum of HeNe Laser Measured Using Array Spectrometer

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 200 400 600 800 1000

Pixel no.

Dar

k C

orre

cted

m

easu

red

sign

al

(nor

mal

ised

to m

ax)


25


•Live with it

•Correct for it

•Reduce it

Use additional baffles inside spectrometer to block

interreflections – difficult to implement and many detectors are sealed.

Use stray light blocking filters to limit the wavelengths of light reaching the detector array


26


0

0.005

0.01

0.015

0.02

0.025

0.03

350 400 450 500 550 600 650 700 750 80Wavelength (nm)

Spec

tral

Tot

al F

lux

(arb

. uni

ts)

Stray Light Blocking Filters

• Reduce the bandwidth of radiation reaching the spectrometer using bandpass filters.

Measure the spectrum over a reduced wavelength range without influence from stray light caused by scattering of

other wavelengths.

Different blocking filters incorporated into a filter wheel behind the spectrograph entrance slit.


27

Stray light blocking filters

• Target: to reduce the fractional stray light error in the measurement of the irradiance spectrum of a Quartz Tungsten lamp to < 1% over the range 300 nm to 800 nm.

Use data from cut on filter

measurements to estimate stray light

level through theoretical filters.

Gaussian Transmittance Profiles of Four Theoretical Blocking Filters

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

300 400 500 600 700 800Wavelength (nm)

Tran

smitt

ance

(%)

Filter 1Filter 2Filter 3Filter 4

Estimated Fractional Stray Light Errors Through Four Theoretical Stray Light Blocking Filters

0.1%

1.0%

10.0%

100.0%

300 400 500 600 700 800

Wavelength (nm)

Stra

y lig

ht s

igna

l / to

tal m

easu

red

sign

al

filter 1filter 2filter 3filter 4no filter


28

Choice of stray light blocking filters

Theoretical filters with guassian transmittance

Look at effect of filter FWHM and CWL on predicted stray light errorGaussian Transmittance Profiles of Four Theoretical Blocking Filters

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

300 400 500 600 700 800Wavelength (nm)

Tran

smitt

ance

(%)

Filter 1Filter 2Filter 3Filter 4


29

Real blocking filters

Real filter combinations

Measured Transmittance of Four Real Blocking Filter Combinations

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

300 400 500 600 700 800Wavelength (nm)

Tran

smitt

ance

(%)

T Filter 1T Filter 2T Filter 3T Filter 4


30

Real blocking filters

Estimated stray light signals using real filters

Estimated Fractional Stray Light Error Through Four Stray Light Blocking Filters

0.1%

1.0%

10.0%

100.0%

300 400 500 600 700 800

Wavelength (nm)

Stra

y lig

ht /

tota

l mea

sure

d si

gnal

(%)

No filterfilter 1filter 2filter 3filter 4


31

Stray light tests using blocking filters

Transmittance of GG435 (3mm) Measured Without Blocking Filters

-10%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

350 400 450 500 550 600 650 700 750 80

Wavelength (nm)

Tran

smitt

ance

(%)

Nominal

No blocking filter


32

Transmittance of GG435 (3mm) Measured With Blocking Filters

-10%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

350 400 450 500 550 600 650 700 750 80

Wavelength (nm)

Tran

smitt

ance

(%)

Filt1

Filt2

Filt3

Filt4

Nominal

Stray light tests using blocking filters

Small discrepancy between nominal & measured filter transmittance at longer wavelengths.

Significant reduction in stray light signals at short wavelengths.

Increased noise at shorter wavelengths.


33

Limitations of using stray light blocking filters

• Increased measurement time. If using N blocking filters in a filter wheel then N different exposures are necessary + time to move filter wheel.

• Once filters have been selected and tested for suitability it may be possible to incorporate them onto the detector array allowing the spectrum to be acquired in a single exposure. Adding an additional element into the detector may result in additional interreflections and stray light.

• Slightly reduced detector sensitivity (not significant if filters are well chosen).

• Temperature effects (need to be aware of temperature sensitivity of filter transmittance).


34

Future work

• Development & increased automation of goniospectroradiometer facility.

• Validation of array spectroradiometer for measuring different source types.

• Intercomparison with existing integrating sphere facilities.


35

Conclusions

• Array spectrometers are extremely useful instruments, however they can suffer from significant errors if not well understood and characterised.

• In particular, stray light errors from array spectrometers may be several orders of magnitude larger than from traditional scanning double monochromator systems.

• NPL have modified an array spectrometer to incorporate a series of custom designed stray light blocking filters, with the effect that stray light errors have been reduced by 1-2 orders of magnitude for measurements of the irradiance from a quartz tungsten lamp.

• The array spectrometer has been integrated into a new goniospectroradiometric instrument for measurement of the radiant intensity distribution and total spectral flux of a range of different light sources.


36

Acknowledgements

• Thanks to colleagues in the optical technologies and scientific computing team at NPL and Teresa Goodman in particular for her help and advice.


37

Questions?

Mike Shaw, Optical Technologies & Scientific Computing Team, National Physical Laboratory, Teddington, Middlesex, UK

Tel. 02089436646Email. [email protected]

Characterisation & Use of Array Spectrometers

Documents

Transcript of Characterisation & Use of Array Spectrometers