Post on 03-Jan-2016
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Correlation between Correlation between visual impression and visual impression and instrumental colour instrumental colour
determination for LEDsdetermination for LEDsJános SchandaJános Schanda
Professor Emeritus of theProfessor Emeritus of the
University of Pannonia, University of Pannonia, HungaryHungary
OverviewOverview
Colour of LEDsColour of LEDs Problems with the colorimetry of LEDsProblems with the colorimetry of LEDs
Photometric and colorimetric Photometric and colorimetric fundamentalsfundamentals
New colorimetric system?New colorimetric system? Objective colorimetry of LEDsObjective colorimetry of LEDs
Instrumental problemsInstrumental problems LED standardsLED standards
RecommendationsRecommendations
Colour of LEDsColour of LEDs LEDs are narrow LEDs are narrow
band emittersband emitters Small errors in Small errors in
colour matching colour matching functions functions produce errors.produce errors. 0
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1
1.2
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wavelength, nm
rel.
in
ten
sity
bl16-03
gr13-06
rd18-07
Visual match differs Visual match differs from colorimetric from colorimetric match match
Photometric and Photometric and colorimetric fundamentalscolorimetric fundamentals
Colorimetry is based Colorimetry is based on a trichromatic on a trichromatic match between test match between test colour stimulus and colour stimulus and three primary three primary (matching) colour (matching) colour stimulistimuli
Transformation from Transformation from real R,G,B primaries real R,G,B primaries to imaginary X,Y,Z to imaginary X,Y,Z primariesprimaries
The CIE 1931 2° The CIE 1931 2° colorimetric system colorimetric system incorporated the CIE incorporated the CIE 1924 photometric 1924 photometric observerobserver
CIE 1924 photometric CIE 1924 photometric observerobserver
Based onBased on Flicker Flicker
photometryphotometry Distinctintness Distinctintness
of boarderof boarder Small step Small step
colour colour differencedifference
Describes Describes Visual acuity Visual acuity
type type observation: observation: reading, reading, observing fine observing fine detailsdetails
NOT NOT BRIGHTNESSBRIGHTNESS
Spectral luminous Spectral luminous efficiency functions efficiency functions and a new proposedand a new proposed yy colour matching colour matching
functionfunction
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400 420 440 460 480 500
wavelength, nm
rel.
resp
onsi
vity
V(l) VM(l) y(Stockman-Sharpe-Fach)
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400 450 500 550 600 650 700
wavelength, nm
rel.
resp
on
sivi
ty
V(l) VM(l) y(Stockman-Sharpe-Fach)
2° standard and proposed, 2° standard and proposed, cone fundamental based, cone fundamental based, colour matching functions colour matching functions
(CMFs)(CMFs)
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0.5
1.0
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wavelength, nm
rel.
re
sp
on
siv
ity
xbar2*(l) ybar2*(l) zbar2*(l)
xbar2(l) ybar2(l) zbar2(l)
Broad-band – RGB-LED visual and Broad-band – RGB-LED visual and instrumental colour matchinstrumental colour match
Error decreased Error decreased by 50 % or moreby 50 % or more
a.)
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0.60
0.00 0.10 0.20 0.30 0.40 0.50 0.60
u'
v'
RGB LEDVisual averageFiltered incandescent
#1
#2
#3
#4
#5
#7
#8#9#6
b.)
0.30
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0.50
0.60
0.00 0.10 0.20 0.30 0.40 0.50 0.60
u'
v'
RGB LEDVisual averageFiltered incandescent
#1
#2
#3
#4#5
#7
#8#9#6
Standard CMFsStandard CMFs
CMFs based on CIE TC CMFs based on CIE TC 1-36 recommendation1-36 recommendation
Enlarged view in the Enlarged view in the vicinity of sample #1vicinity of sample #1
LED colour LED colour characteristicscharacteristics
LEDs are narrow band emittersLEDs are narrow band emitters bandwidth approx. 10 nm – 30 nmbandwidth approx. 10 nm – 30 nm Blue … Green: InGaNBlue … Green: InGaN Yellow … Red: AlInGaPYellow … Red: AlInGaP
Both the absolute intensity and the Both the absolute intensity and the wavelength of the emission maximum is wavelength of the emission maximum is temperature dependenttemperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDslargest changes with Red LEDs
Unusual spatial light characteristicsUnusual spatial light characteristics Solution of measurement problems caused bySolution of measurement problems caused by
Spectral mismatch: spectrometric measurementSpectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetrySpectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignmentGeometric misalignment
Typical LED spectraTypical LED spectra(used in optimization, see (used in optimization, see
later)later)
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wav elength, nm
rel.
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ten
sit
y
TT-LED-11-1
TT-LED-12-1
TT-LED-13-1
TT-LED-9-1
TT-LED-8-1
TT-LED-10-1
TT-LED-OLD2-1
TT-LED-OLD1-1
LED colour characteristicsLED colour characteristics LEDs are narrow band emittersLEDs are narrow band emitters
bandwidth approx. 10 nm – 30 nmbandwidth approx. 10 nm – 30 nm Blue … Green: InGaNBlue … Green: InGaN Yellow … Red: AlInGaPYellow … Red: AlInGaP
Both the absolute intensity and the wavelength of Both the absolute intensity and the wavelength of the emission maximum is temperature dependentthe emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDslargest changes with Red LEDs
Unusual spatial light characteristicsUnusual spatial light characteristics Solution of measurement problems caused bySolution of measurement problems caused by
Spectral mismatch: spectrometric measurementSpectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetrySpectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignmentGeometric misalignment
Temperature dependence Temperature dependence of a blue LEDof a blue LED
Blue
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wavelength, nm
rel.
int.
3 °C
21 °C
34 °C
50 °C
Temperature dependenceTemperature dependenceof a yellow LEDof a yellow LED
YL 3
0
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1
1,5
2
2,5
550 600 650
wavelength, nm
rel.
in
ten
sit
y
22,4°C
36,3°C
46,8°C
56,8°C
Temperature dependence Temperature dependence of a red LEDof a red LED
Temperature dependenceTemperature dependenceof a white LEDof a white LED
WT 2
0
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wavelength, nm
rel.
in
ten
sit
y
22,9°C
33,7°C
54,3°C
67,5°C
LED colour characteristicsLED colour characteristics LEDs are narrow band emittersLEDs are narrow band emitters
bandwidth approx. 10 nm – 30 nmbandwidth approx. 10 nm – 30 nm Blue … Green: InGaNBlue … Green: InGaN Yellow … Red: AlInGaPYellow … Red: AlInGaP
Both the absolute intensity and the wavelength of Both the absolute intensity and the wavelength of the emission maximum is temperature dependentthe emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDslargest changes with Red LEDs
Unusual spatial light characteristicsUnusual spatial light characteristics Solution of measurement problems caused bySolution of measurement problems caused by
Spectral mismatch: spectrometric measurementSpectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetrySpectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignmentGeometric misalignment
Irradiation inhomogeneity in Irradiation inhomogeneity in measurement planemeasurement plane
What should be reported?
Problem of reproducible alignment
LED colour characteristicsLED colour characteristics LEDs are narrow band emittersLEDs are narrow band emitters
bandwidth approx. 10 nm – 30 nmbandwidth approx. 10 nm – 30 nm Blue … Green: InGaNBlue … Green: InGaN Yellow … Red: AlInGaPYellow … Red: AlInGaP
Both the absolute intensity and the wavelength of Both the absolute intensity and the wavelength of the emission maximum is temperature dependentthe emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDslargest changes with Red LEDs
Unusual spatial light characteristicsUnusual spatial light characteristics Solution of measurement problems caused bySolution of measurement problems caused by
Spectral mismatch: spectrometric measurementSpectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetrySpectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignmentGeometric misalignment
Spectrometric measurementsSpectrometric measurements
Critical parameters of spectrometerCritical parameters of spectrometer Sampling interval and bandpass: 10 nm sampling Sampling interval and bandpass: 10 nm sampling
produces produces uu’,’,vv’ errors of several units in 3rd decimal.’ errors of several units in 3rd decimal. Highly over sampling OK (CCD spectrometers)Highly over sampling OK (CCD spectrometers) Wavelength scale error: 0.5 nm error produces Wavelength scale error: 0.5 nm error produces uu’,’,vv’ ’
errors of several units in 3rd decimal.errors of several units in 3rd decimal. Stray light: LED measurement compared to Stray light: LED measurement compared to
incandescent lamp if 10incandescent lamp if 10-4-4 stray light produces stray light produces uu’,’,vv’ ’ errors of several units in 3rd decimal.errors of several units in 3rd decimal.
Experiments showed even larger errors:Experiments showed even larger errors:
Comparison of 5 spectrometersComparison of 5 spectrometersGreen LED chromaticityGreen LED chromaticity
Comparison of 5 spectrometersComparison of 5 spectrometersRed LED chromaticityRed LED chromaticity
0,2940
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0,2948
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0,2954
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x
y
NIST 5 nm
Single mon.
Double mon.
High end CCD
Low end CCD
Comparison of 5 spectrometersComparison of 5 spectrometersBlue LED chromaticityBlue LED chromaticity
0,114
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0,130
0,114 0,115 0,116 0,117 0,118
x
y
NIST 5 nm
Single mon.
Double mon.
High end CCD
Low end CCD
LED colour characteristicsLED colour characteristics
LEDs are narrow band emittersLEDs are narrow band emitters bandwidth approx. 10 nm – 30 nmbandwidth approx. 10 nm – 30 nm Blue … Green: InGaNBlue … Green: InGaN Yellow … Red: AlInGaPYellow … Red: AlInGaP
Both the absolute intensity and the wavelength of Both the absolute intensity and the wavelength of the emission maximum is temperature dependentthe emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDslargest changes with Red LEDs
Unusual spatial light characteristicsUnusual spatial light characteristics Solution of measurement problems caused bySolution of measurement problems caused by
Spectral mismatch: spectrometric measurementSpectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetrySpectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignmentGeometric misalignment
Goodness of fit characterizationGoodness of fit characterization
Modified Modified ff11’ method,’ method, No illuminantNo illuminant Independently forIndependently for
Separately for Red, Separately for Red, Green, Blue LEDsGreen, Blue LEDs Example: Example: VV((ll) channel) channel Use coloured LED Use coloured LED
standardsstandards ff11’ provides estimate of ’ provides estimate of
error to be expectederror to be expected Detector spectral Detector spectral
responsivity measurement, responsivity measurement, not standardized properlynot standardized properly
rel,
01,
0
*( ) ( ) d
' 100%
( ) d
i i
i
i
s t
f
t
s l( ), ( ), ( ), ( )x x y z
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wav elength, nm
rel.
se
ns
itiv
ity
f1'=1.33
f1'=1.97
V(l)
Partial Partial ff11’ error index 1’ error index 1
LED dominant wavelength ranges andLED dominant wavelength ranges andthe dominant wavelength value of the standard LEDs the dominant wavelength value of the standard LEDs
Partial Partial ff11’ error index 2’ error index 2
Maximal photometrical errors to be expected ifMaximal photometrical errors to be expected ifthe the partial fpartial f11’ values are below the given limiting values’ values are below the given limiting values
Value of the Value of the error indexerror index
Largest photometrical Largest photometrical error to be expected:error to be expected:
ff11’’BLBL <2% <2% PE <PE < f f11’’BLBL
ff11’’GNGN <4% <4% PE < PE < ff11’’GNGN
ff11’’YLYL <4% <4% PE < PE < ff11’’YLYL
ff11’’RDRD <10% <10% PE < 2PE < 2∙f∙f11’’RDRD
Correcting tristimulus colour measurement of Correcting tristimulus colour measurement of LEDs by matrix transformationLEDs by matrix transformation
Modern tristimulus colorimeters have four Modern tristimulus colorimeters have four input channels. One caninput channels. One can just add the signals of the xjust add the signals of the xss and x and xll channels channels
(no matrixing)(no matrixing) Use the four channels for improving accuracyUse the four channels for improving accuracy Add a fifth channel Add a fifth channel
Optimization was performed for the LEDs Optimization was performed for the LEDs shown previouslyshown previously
Experimental five filter Experimental five filter colorimeter spectral responsivitycolorimeter spectral responsivity
0
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2
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wav elength, nm
rel.
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x
y
z
xms
xml
ym
zm
km
Average Average colorimetric colorimetric
errors for the errors for the eight LEDseight LEDs
Matrix Matrix typetype
Colorimetric Colorimetric error, Δerror, ΔEEabab**
Without Without matrixingmatrixing
9,679,67
4 filter 4 filter matrixmatrix
3,763,76
5 filter 5 filter matrixmatrix
1,091,09
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wav elength, nm
rel.
in
ten
sit
y
TT-LED-11-1
TT-LED-12-1
TT-LED-13-1
TT-LED-9-1
TT-LED-8-1
TT-LED-10-1
TT-LED-OLD2-1
TT-LED-OLD1-1
LED colour characteristicsLED colour characteristics LEDs are narrow band emittersLEDs are narrow band emitters
bandwidth approx. 10 nm – 30 nmbandwidth approx. 10 nm – 30 nm Blue … Green: InGaNBlue … Green: InGaN Yellow … Red: AlInGaPYellow … Red: AlInGaP
Both the absolute intensity and the wavelength of Both the absolute intensity and the wavelength of the emission maximum is temperature dependentthe emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDslargest changes with Red LEDs
Unusual spatial light characteristicsUnusual spatial light characteristics Solution of measurement problems caused bySolution of measurement problems caused by
Spectral mismatch: spectrometric measurementSpectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetrySpectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignmentGeometric misalignment
Standard LEDStandard LED
Temperature Temperature and current and current stabilized LED stabilized LED for luminous for luminous flux flux measurementmeasurement
Standard LEDStandard LED
Temperature Temperature and current and current stabilized LED stabilized LED for ALI for ALI measurementmeasurement
LED luminance standardLED luminance standard TechnoTeam TechnoTeam
LED based LED based Peltier-cooled Peltier-cooled luminance luminance standardstandard
LED colour characteristicsLED colour characteristics
LEDs are narrow band emittersLEDs are narrow band emitters bandwidth approx. 10 nm – 30 nmbandwidth approx. 10 nm – 30 nm Blue … Green: InGaNBlue … Green: InGaN Yellow … Red: AlInGaPYellow … Red: AlInGaP
Both the absolute intensity and the wavelength of Both the absolute intensity and the wavelength of the emission maximum is temperature dependentthe emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDslargest changes with Red LEDs
Unusual spatial light characteristicsUnusual spatial light characteristics Solution of measurement problems caused bySolution of measurement problems caused by
Spectral mismatch: spectrometric measurementSpectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetrySpectral mismatch: tristimulus colorimetry Temperature dependenceTemperature dependence Geometric misalignmentGeometric misalignment
ALI measurementALI measurement
Input clamp of Input clamp of ALI tubeALI tube
• Clamp for 5 mm LED
ALI-B measuring setupALI-B measuring setup
Flux and thermal measurementFlux and thermal measurementReference LED
Standard LED
Calibration S, YREF-W Measurement of YT and YREF-T ΦT
DUT LED
Detector with different filters
TeraLED complex colorimetric and thermal measuring system
•Radiometric•Photometric•Colorimetric•Thermal measurements
Characteristics of Standard Characteristics of Standard LEDs: SLEDs: Stabilization during the first two tabilization during the first two
minutesminutesLED stabilization
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time, min
rel.
int. RD8stab2
YL3stab2
BL7stab2
SummarySummary
Based on visual observations a new LMS cone Based on visual observations a new LMS cone fundamental based colorimetry is recommendedfundamental based colorimetry is recommended
Careful tristimulus colorimetry can be more Careful tristimulus colorimetry can be more accurate than low cost spectrometric techniquesaccurate than low cost spectrometric techniques
Use temperature stabilized LEDs of similar Use temperature stabilized LEDs of similar colour as the test samplescolour as the test samples
Use good alignment for the LEDs in ALI Use good alignment for the LEDs in ALI measurementsmeasurements
SummarySummary
Current sate of the art in user’s laboratory:Current sate of the art in user’s laboratory:
Spectral mismatch uncertainty: 1 – 2 %Spectral mismatch uncertainty: 1 – 2 % Geometric alignment uncertainty: <+/-0,002%Geometric alignment uncertainty: <+/-0,002% Temperature dependence: +/- 0,2 %Temperature dependence: +/- 0,2 %
Tanks for your kind Tanks for your kind attention!attention!