Correct for it - National Physical Laboratory

Thursday, 05 June 2008
Stray Light Rejection in Array
Spectrometers
Mike Shaw, Optical Technologies & Scientific Computing Team,
National Physical Laboratory, Teddington, Middlesex, UK
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Thursday, 05 June 2008
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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Thursday, 05 June 2008
Basic optical layout of an array spectrometer
Collimating
mirror
Entrance
slit
Diffraction
grating
Detector
array
Focussing
mirror
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Thursday, 05 June 2008
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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Thursday, 05 June 2008
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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Thursday, 05 June 2008
Stray light errors are source dependent
http://www.andrew.cmu.edu/user/tlauwers/pr
ojects.html
http://www.promolux.com/english/faq.html
Deuteriumlamp spectrum
8.00E-04
400
7.00E-04
300
250
200
150
100
50
0
350
400
450
500
550
600
Wavelength (nm)
•
650
700
750
80
Spectral Total Flux of a Tungsten Halogen Lamp
0.03
0.025
1.00E+03
6.00E-04
Spectral Total Flux (arb. units)
350
Relative SPDof Four LEDs
1.20E+03
Spectral Total Flux (arb.
units)
450
Spectral Irradiance (W/m^2/nm)
Spectral Total Flux (arb. units)
Spectral Total Flux of a Fluorescent Lamp
http://en.wikipedia.org/?title=Light_bulb
8.00E+02
5.00E-04
LED1
LED2
6.00E+02
4.00E-04
3.00E-04
0.015
LED3
LED4
4.00E+02
2.00E-04
2.00E+02
1.00E-04
0.00E+00
200
0.02
0.01
0.005
0.00E+00
220
240
260
280
300
320
340
350
400
450
500
550
600
650
Wavelength(nm)
Wavelength (nm)
700
750
800
0
350
400
450
500
550
600
650
700
750
Wavelength (nm)
Stray light errors tend to be most critical when
measuring a broadband spectrum with an intensity
varying over several orders of magnitude.
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80
Thursday, 05 June 2008
Dark Corrected measured signal (normalised
to max)
Stray light signal observed using a laser line
1.E+00
1.E-01
Background due to
heterochromatic
stray light
1.E-02
Measured Spectra
Ideal spectra
1.E-03
1.E-04
1.E-05
1.E-06
0
200
400
600
800
1000
Pixel no.
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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Thursday, 05 June 2008
Stray light errors for an incandescent source
Spectral Total Flux of a Tungsten Halogen Lam p
0.03
Spectral Total Flux (arb. units)
0.025
0.02
0.015
0.01
0.005
0
350
400
450
500
550
600
650
700
750
80
W avelength (nm )
Relatively low spectral
flux at shorter visible
and UV wavelengths
Relatively high spectral
flux at longer visible
and NIR wavelengths
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Thursday, 05 June 2008
Stray light errors for an incandescent source
S p e c t r a l T o t a l F lu x o f a T u n g s t e n H a lo g e n L a m p
0 .0 3
Spectral Total Flux (arb. units)
0 .0 2 5
0 .0 2
0 .0 1 5
0 .0 1
0 .0 0 5
0
350
400
450
500
550
600
650
700
750
80
W a v e le n g t h ( n m )
Small fraction of radiation inside the
spectrometer is measured as
heterochromatic stray light
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Thursday, 05 June 2008
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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Thursday, 05 June 2008
Quantifying stray light errors
Measurement of
lamp signal,
Vlamp(λ)
Fibre input to spectrometer
Background corrected Signal Measured from a Quartz
Tungsten Lamp
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+07
1.E+06
1.E+05
1.E+04
1.E+03
300
400
500
600
700
800
Wavelength (nm)
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Thursday, 05 June 2008
Quantifying stray light errors
Measurement through
cut on filter, Vfilter(λ)
Fibre input to spectrometer
Background corrected Signal Measured from a Quartz
Tungsten Lamp Through a GG435 cut on Filter
Nominal transmittance of GG435 (3mm
thickness) cut on filter
Transmittance (%)
1.E+00
300
350
400
450
500
550
600
1.E-01
1.E-02
650
700
750
800
Background corrected
signal (counts)
1.E+07
1.E+06
1.E+05
1.E+04
1.E+03
1.E-03
300
400
500
600
700
800
Wavelength (nm)
1.E-04
Wavelength (nm)
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Thursday, 05 June 2008
Quantifying stray light errors
Shutter to block light source
from spectrometer field of view
Fibre input to spectrometer
Background Signal
Measurement
of background
signal, Vbg(λ)
signal (counts)
1.E+07
1.E+06
1.E+05
1.E+04
1.E+03
300
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
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Thursday, 05 June 2008
Analysis of stray light data
• For an ideal spectrometer: T filter (λ ) =
Vlamp (λ ) − Vbg (λ )
Transmittance of GG435 glass filter (3mm
thickness)
100%
90%
Transmittance (%)
Stray light signals cause
deviations from ideal
behaviour and indicate
erroneously high filter
transmittance at
wavelengths shorter than
the cut on.
V filter (λ ) − Vbg (λ )
80%
70%
60%
50%
30%
Measured using
array spectrometer
20%
Nominal
40%
10%
0%
260
300
340
380
420
460
500
540
580
Wavelength (nm)
Stray light error
of > 90%!
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Thursday, 05 June 2008
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 (%)
•
spectrometer A
Spectrometer B
Spectrometer C
Spectrometer E
10%
Spectrometer F
Spectrometer G
Spectrometer H
GG435 Nominal
1%
200
300
400
500
600
700
800
900
Wavelength (nm)
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Thursday, 05 June 2008
How to handle stray light?
•Live with it
Determine stray light contribution
to measurement uncertainty.
•Correct for it
•Minimise the effect of stray
light by calibrating the
detector under conditions as
close as possible to those
under which it will be used.
•Reduce it
Spectral Total Flux of a Tungsten Halogen Lamp
0.03
Spectral Total Flux (arb. units)
0.025
•Match the F/# of input beam
to F/# of spectrometer.
0.02
0.015
0.01
•Stray light errors are too
large for many applications.
0.005
0
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
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Thursday, 05 June 2008
How to handle stray light?
•Live with it
•Correct for it
•Reduce it
Characterise the stray light
rejection of the instrument
and then correct for it.
Spectrum of HeNe Laser Measured Using Array
Spectrometer
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.
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-06
0
200
400
600
800
1000
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 2006.
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Thursday, 05 June 2008
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.
Ymeas = [ I + D]YIB = AYIB
YIB = A −1Ymeas
Has been shown to reduce
some stray light errors by 1-2
orders of magnitude.
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Thursday, 05 June 2008
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
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Thursday, 05 June 2008
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
0.025
Spectral Total Flux (arb. units)
Measure the spectrum over a
reduced wavelength range
without influence from stray
light caused by scattering of
other wavelengths.
0.02
0.015
0.01
0.005
0
350
400
450
500
550
600
650
700
750
80
Wavelength (nm)
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Thursday, 05 June 2008
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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Thursday, 05 June 2008
Implementation of stray light blocking filters
approach at NPL
Estimated
Stray Light
Errorsof
Through
Four Theoretical
Stray Light
Blocking
GaussianFractional
Transmittance
Profiles
Four Theoretical
Blocking
Filters
Filters
100%
90%
Transmittance
(%) signal
Stray light signal
/ total measured
Use data from cut
on filter
measurements to
estimate stray light
level through
theoretical filters.
100.0%
filter 1
filter 2
filter 3
filter 4
no filter
80%
70%
10.0%
60%
50%
Filter 1
Filter 2
Filter 3
Filter 4
40%
30%
1.0%
20%
10%
0%
0.1%
300
300
400
400
500
600
600
Wavelength (nm)
500
700
700
800
800
Wavelength (nm)
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Thursday, 05 June 2008
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%
Transmittance (%)
80%
70%
Filter 1
Filter 2
Filter 3
Filter 4
60%
50%
40%
30%
20%
10%
0%
300
400
500
600
Wavelength (nm)
700
800
Theoretical filters with Gaussian transmittance
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Thursday, 05 June 2008
Real blocking filters
Measured Transmittance of Four Real Blocking Filter Combinations
100%
90%
Transmittance (%)
80%
70%
60%
T Filter 1
T Filter 2
T Filter 3
T Filter 4
50%
40%
30%
20%
10%
0%
300
400
500
600
700
800
Wavelength (nm)
Blocking filters fitted into filter wheel behind
spectrograph entrance slit
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Thursday, 05 June 2008
Stray light tests using blocking filters
Transmittance of GG435 (3mm) Measured Without Blocking Filters
100%
90%
80%
Transmittance (%)
70%
60%
Nominal
50%
40%
30%
No blocking filter
20%
10%
0%
-10%
350
400
450
500
550
600
650
700
750
80
Wavelength (nm)
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Thursday, 05 June 2008
Stray light tests using blocking filters
Transmittance of GG435 (3mm) Measured With Blocking Filters
Significant100%
reduction in
stray light signals
at short
90%
wavelengths.
80%
Transmittance (%)
70%
60%
50%
40%
Filt1
30%
Filt2
20%
Filt3
10%
Filt4
Nominal
0%
-10%
350
400
450
500
550
600
650
700
750
80
Wavelength (nm)
Increased noise at shorter wavelengths.
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Thursday, 05 June 2008
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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Thursday, 05 June 2008
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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Thursday, 05 June 2008
Results – compact fluorescent lamp
Luminous intensity
distribution
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Thursday, 05 June 2008
Results – compact fluorescent lamp
Spectral total flux
Scatter plot of chromaticity
coordinates
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Thursday, 05 June 2008
Results - white LED cluster
Spatially varying correlated
colour temperature and
chromaticity
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Thursday, 05 June 2008
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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Thursday, 05 June 2008
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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Thursday, 05 June 2008
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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Thursday, 05 June 2008
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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Thursday, 05 June 2008
Questions?
Mike Shaw, Optical Technologies & Scientific Computing Team,
National Physical Laboratory, Teddington, Middlesex, UK
Tel. 02089436646
Email. mike.shaw@npl.co.uk
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