Stray Light Rejection in Array Spectrometers

Transcription

1 Stray Light Rejection in Array Spectrometers Mike Shaw, Optical Technologies & Scientific Computing Team, National Physical Laboratory, Teddington, Middlesex, UK 1

2 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. 2

3 Basic optical layout of an array spectrometer Entrance slit Collimating mirror Diffraction grating Detector array Focussing mirror 3

4 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. 4

5 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 5

6 Stray light errors are source dependent ojects.html Spectral Total Flux of a Fluorescent Lamp Deuterium lamp spectrum Relative SPD of Four LEDs Spectral Total Flux of a Tungsten Halogen Lamp E E Spectral Total Flux (arb. units) Spectral Irradiance (W/m^2/nm) 7.00E E E E E E E-04 Spectral Total Flux (arb. units) 1.00E E E E E+02 LED1 LED2 LED3 LED4 Spectral Total Flux (arb. units) E E Stray light errors tend to be most critical when measuring a broadband spectrum with an intensity varying over several orders of magnitude. 6

7 Stray light signal observed using a laser line Dark Corrected measured signal (normalised to max) 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E Pixel no. Measured Spectra Ideal 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. 7

8 Stray light errors for an incandescent source 0.03 Spectral Total Flux of a Tungsten Halogen Lamp Spectral Total Flux (arb. units) Relatively low spectral flux at shorter visible and UV wavelengths Relatively high spectral flux at longer visible and NIR wavelengths 8

9 Stray light errors for an incandescent source 0.03 Spectral Total Flux of a Tungsten Halogen Lamp Spectral Total Flux (arb. units) W avelength (nm ) Small fraction of radiation inside the spectrometer is measured as heterochromatic stray light 9

10 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. 10

11 Quantifying stray light errors Measurement of lamp signal, V lamp (λ) Fibre input to spectrometer Background corrected Signal Measured from a Quartz Tungsten Lamp 1.E+07 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). Background corrected signal (counts) 1.E+06 1.E+05 1.E+04 1.E

12 Quantifying stray light errors Measurement through cut on filter, V filter (λ) Fibre input to spectrometer Background corrected Signal Measured from a Quartz Tungsten Lamp Through a GG435 cut on Filter 1.E+07 Transmittance (%) Nominal transmittance of GG435 (3mm thickness) cut on filter 1.E E-01 1.E-02 1.E-03 1.E-04 Background corrected signal (counts) 1.E+06 1.E+05 1.E+04 1.E

13 Quantifying stray light errors Shutter to block light source from spectrometer field of view Fibre input to spectrometer Background Signal 1.E+07 Measurement of background signal, V bg (λ) signal (counts) 1.E+06 1.E+05 1.E+04 1.E

14 Analysis of stray light data For an ideal spectrometer: T filter ( λ) = V V filter lamp ( λ) V ( λ) V bg bg ( λ) ( λ) Stray light signals cause deviations from ideal behaviour and indicate erroneously high filter transmittance at wavelengths shorter than the cut on. Transmittance (%) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Transmittance of GG435 glass filter (3mm thickness) Measured using array spectrometer Nominal Stray light error of > 90%! 14

15 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 100% Measured transmittance (%) 10% spectrometer A Spectrometer B Spectrometer C Spectrometer E Spectrometer F Spectrometer G Spectrometer H GG435 Nominal 1%

16 How to handle stray light? Spectral Total Flux (arb. units) Live with it Correct for it Reduce it Spectral Total Flux of a Tungsten Halogen Lamp 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

17 How to handle stray light? Live with it Correct for it Reduce it 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. Characterise the stray light rejection of the instrument and then correct for it. Dark Corrected measured signal (normalised to max) 1.E+00 Spectrum of HeNe Laser Measured Using Array Spectrometer 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E Pixel no. 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

18 How to handle stray light? 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. Y Y meas IB = = [ I A 1 + D] Y Y meas IB = AY Has been shown to reduce some stray light errors by 1-2 orders of magnitude. IB 18

19 How to handle stray light? 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 19

20 Stray Light Blocking Filters Reduce the bandwidth of radiation reaching the spectrometer using bandpass filters. Spectral Total Flux of a Tungsten Halogen Lamp 0.03 Measure the spectrum over a reduced wavelength range without influence from stray light caused by scattering of other wavelengths. Spectral Total Flux (arb. units)

21 Application of stray light blocking filters: NPL Goniospectroradiometer An instrument to measure the spectral radiant intensity distribution, I e (λ, C, γ), of light sources. Spectral and luminous flux, chromaticity, CCT etc. derived from I e (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

22 Implementation of stray light blocking filters approach at NPL Use data from cut on filter measurements to estimate stray light level through theoretical filters. Stray light signal Transmittance / total measured (%) signal 100% 100.0% 90% 80% 70% 60% 10.0% 50% 40% 30% 1.0% 20% 10% Estimated Gaussian Fractional Transmittance Stray Light Profiles Errors of Through Four Theoretical Four Theoretical Blocking Stray Light Filters Blocking Filters 0% 0.1% filter 1 filter 2 filter 3 filter 4 no filter Filter 1 Filter 2 Filter 3 Filter

23 Choice of stray light blocking filters Look at effect of filter FWHM and CWL on predicted stray light error Gaussian Transmittance Profiles of Four Theoretical Blocking Filters 100% 90% 80% Transmittance (%) 70% 60% 50% 40% 30% 20% 10% Filter 1 Filter 2 Filter 3 Filter 4 0% Theoretical filters with Gaussian transmittance 23

24 Real blocking filters Measured Transmittance of Four Real Blocking Filter Combinations Transmittance (%) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% T Filter 1 T Filter 2 T Filter 3 T Filter 4 0% Blocking filters fitted into filter wheel behind spectrograph entrance slit 24

25 Stray light tests using blocking filters Transmittance of GG435 (3mm) Measured Without Blocking Filters Transmittance (%) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Nominal No blocking filter -10%

26 Stray light tests using blocking filters Significant reduction in 100% stray light signals 90% at short wavelengths. 80% 70% Transmittance of GG435 (3mm) Measured With Blocking Filters Transmittance (%) 60% 50% 40% 30% 20% 10% 0% Filt1 Filt2 Filt3 Filt4 Nominal -10% Increased noise at shorter wavelengths. 26

27 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). 27

28 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. 28

29 Results compact fluorescent lamp Luminous intensity distribution 29

30 Results compact fluorescent lamp Spectral total flux Scatter plot of chromaticity coordinates 30

31 Results - white LED cluster Spatially varying correlated colour temperature and chromaticity 31

32 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. 32

33 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. 33

34 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. 34

35 Acknowledgements Thanks to colleagues in the optical technologies and scientific computing team at NPL and Teresa Goodman in particular for her help and advice. 35

36 Questions? Mike Shaw, Optical Technologies & Scientific Computing Team, National Physical Laboratory, Teddington, Middlesex, UK Tel