stray light rejection in array spectrometers -...

Thursday, 05 June 2008

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Stray Light Rejection in Array Spectrometers

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


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Overview

• Basic optical design of an array spectrometer• In system (heterochromatic) stray light• Techniques for quantifying stray light rejection• Results: stray light errors in some commercially available

spectrometers.• Methods for reducing stray light errors• Application of a modified array spectrometer:

• New NPL Goniospectroradiometer• Some other important performance parameters for array

spectrometers• Conclusions and future work.


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Entrance slit

Collimating mirror

Diffraction grating

Focussing mirror

Detector array

Basic optical layout of an array spectrometer


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In system (heterochromatic) stray light

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

• Rays strike the wrong part of the detector array causing spurious measured signals at the wrong wavelength.

• Distinguished from ambient (homochromatic) stray light.


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Causes of stray light errors in array spectrometers

Scattering from optical surfaces

Interreflections between surfaces – particularly reflections off detector array

Inadequate blocking of other diffracted orders


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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.

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

Spectral Total Flux of a Tungsten Halogen Lamp

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Relative SPD of Four LEDs

0.00E+00

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tral

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its) LED1

LED2

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LED4

Spectral Total Flux of a Fluorescent Lamp

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Deuterium lamp spectrum

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Spec

tral

Irra

dian

ce (W

/m^2

/nm

)


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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. However this does not tell us what errors to expect when measuring a broadband light source.


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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


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Stray light errors for an incandescent source

S 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

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tral

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Small fraction of radiation inside the spectrometer is measured as

heterochromatic stray light


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Stray light errors for broadband light sources

Problem is often exacerbated by spectral responsivity of detector array.

e.g. Spectral responsivity of a silicon based detectors tends to be higher at longer wavelengths.


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Measurement of lamp signal,

Vlamp (λ)

Quantifying stray light errors

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)

G. R. Hopkinson, T. M. Goodman and S. R. Prince, “A guide to the use and calibration of detector array equipment (SPIE Press Book),” SPIE (2004).


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Fibre input to spectrometerBackground corrected Signal Measured from a Quartz

Tungsten Lamp Through a GG435 cut on Filter

1.E+03

1.E+04

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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

(%)


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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)


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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)

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Wavelength (nm)

Tran

smitt

ance

(%)

Measured usingarray spectrometer

Nominal

Stray light error of > 90%!


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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


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How to handle stray light?

•Live with it

•Correct for it

•Reduce it

Determine stray light contribution to measurement uncertainty.

•Minimise the effect of stray light by calibrating the detector under conditions as close as possible to those under which it will be used.

•Match the F/# of input beam to F/# of spectrometer.

•Stray light errors are too large for many applications.


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•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

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1.E+00

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Pixel no.

Dar

k C

orre

cted

m

easu

red

sign

al

(nor

mal

ised

to m

ax)


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•Live with it

•Correct for it

•Reduce it

Limitations:

•This approach can be time consuming and expensive as it

requires the use of laser radiation over a large wavelength band.

•The resulting correction may also be sensitive to changes in the

spectrometer.

Derive a spectral stray light correction matrix which can be applied to future measurements made with the spectrometer.

measIB

IBIBmeas

YAY

AYYDIY1

][−=

=+=

Has been shown to reduce some stray light errors by 1-2 orders of magnitude.


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•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


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Stray Light Blocking Filters

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


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

other wavelengths.


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Application of stray light blocking filters: NPL Goniospectroradiometer

• An instrument to measure the spectral radiant intensity distribution, Ie (λ, C, γ), of light sources.

• Spectral and luminous flux, chromaticity, CCT etc. derived from Ie (λ, C, γ)

• Array spectrometer used primarily for speed of data acquisition and compact size.

Shaw M J, Goodman T M, “Array based goniospectroradiometer for measurement of spectral radiant intensity and spectral total flux of light sources”, Applied Optics, vol 47 No. 13, 01 May 2008.


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Implementation of stray light blocking filters approach at NPL

Use data from cut on filter

measurements to estimate stray light

level through theoretical filters.

Gaussian Transmittance Profiles of Four Theoretical Blocking Filters

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Filter 1Filter 2Filter 3Filter 4

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

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300 400 500 600 700 800

Wavelength (nm)

Stra

y lig

ht s

igna

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tal m

easu

red

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filter 1filter 2filter 3filter 4no filter


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Choice of stray light blocking filters

Theoretical filters with Gaussian transmittance

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

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ance

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Filter 1Filter 2Filter 3Filter 4


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Real blocking filters

Blocking filters fitted into filter wheel behind spectrograph entrance slit

Measured Transmittance of Four Real Blocking Filter Combinations

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ance

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


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Stray light tests using blocking filters

Transmittance of GG435 (3mm) Measured Without Blocking Filters

-10%

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Wavelength (nm)

Tran

smitt

ance

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Nominal

No blocking filter


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Transmittance of GG435 (3mm) Measured With Blocking Filters

-10%

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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

Significant reduction in stray light signals at short wavelengths.

Increased noise at shorter wavelengths.


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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.

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

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


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Implementation of stray light blocking filters

• Another implementation of the blocking filters is to coat them onto corresponding areas of the detector array, effectively blinding each pixel to radiation at wavelengths other than those which it ‘should’ see.

• Introducing another optical component into the system will change its overall stray light characteristics.

Model the optical system with the filters to determine their effect.


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Results – compact fluorescent lamp

Luminous intensity distribution


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Results – compact fluorescent lamp

Spectral total flux

Scatter plot of chromaticity coordinates


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Results - white LED cluster

Spatially varying correlated colour temperature and

chromaticity


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Some other important performance characteristics of array spectrometers

• Wavelength accuracy• Spectral resolution• Linearity• (Spectral) responsivity

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

Apparatus for measurement of detector linearity.


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Conclusions

• Array spectrometers often suffer from poor stray light rejection which can make them unsuitable for applications requiring a low measurement uncertainty.

• NPL have modified an array spectrometer to incorporate a series of custom designed stray light blocking filters. This instrument has been used to measure the spectral and spatial output characteristics of a variety of different light sources.


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Future work

• Better understanding of uncertainties arising from stray light errors.

• Investigate use of stray light blocking filters further into the UV.

• Investigation into feasibility of monochromator based SL correction measurements at NPL.


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Acknowledgements

• Thanks to colleagues in the optical technologies and scientific computing team at NPL and Teresa

Goodman in particular for her help and advice.


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Questions?

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

Tel. 02089436646Email. [email protected]

mailto:[email protected]

Stray Light Rejection in Array Spectrometers��Mike Shaw, Optical Technologies & Scientific Computing Team, National Physical Laboratory, Teddington, Middlesex, UKOverviewBasic optical layout of an array spectrometerIn system (heterochromatic) stray lightCauses of stray light errors in array spectrometersStray light errors are source dependentStray light signal observed using a laser lineStray light errors for an incandescent sourceStray light errors for an incandescent sourceStray light errors for broadband light sourcesQuantifying stray light errorsQuantifying stray light errorsQuantifying stray light errorsAnalysis of stray light dataComparison of Different Array DetectorsHow to handle stray light?How to handle stray light?How to handle stray light?How to handle stray light?Stray Light Blocking FiltersApplication of stray light blocking filters: �NPL GoniospectroradiometerImplementation of stray light blocking filters approach at NPLChoice of stray light blocking filtersReal blocking filtersStray light tests using blocking filtersStray light tests using blocking filtersLimitations of using stray light blocking filtersImplementation of stray light blocking filtersResults – compact fluorescent lampResults – compact fluorescent lampResults - white LED clusterSome other important performance characteristics of array spectrometersConclusionsFuture workAcknowledgementsSlide Number 36

stray light rejection in array spectrometers -...

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