Correlation between visual impression and instrumental colour determination for LEDs János Schanda...

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

400 450 500 550 600 650 700

wavelength, nm

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

400 420 440 460 480 500

wavelength, nm

V(l) VM(l) y(Stockman-Sharpe-Fach)

400 450 500 550 600 650 700

wavelength, nm

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)

350 400 450 500 550 600 650 700 750 800

wavelength, nm

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

0.00 0.10 0.20 0.30 0.40 0.50 0.60

RGB LEDVisual averageFiltered incandescent

#8#9#6

0.00 0.10 0.20 0.30 0.40 0.50 0.60

RGB LEDVisual averageFiltered incandescent

#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)

400 500 600 700 800

wav elength, nm

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

Temperature dependence Temperature dependence of a blue LEDof a blue LED

450 500 550

wavelength, nm

21 °C

34 °C

50 °C

Temperature dependenceTemperature dependenceof a yellow LEDof a yellow LED

550 600 650

wavelength, nm

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

400 500 600 700 800

wavelength, nm

22,9°C

33,7°C

54,3°C

67,5°C

Irradiation inhomogeneity in Irradiation inhomogeneity in measurement planemeasurement plane

What should be reported?

Problem of reproducible alignment

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

0,2942

0,2944

0,2946

0,2948

0,2950

0,2952

0,2954

0,7030 0,7040 0,7050 0,7060

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 0,115 0,116 0,117 0,118

NIST 5 nm

Single mon.

Double mon.

High end CCD

Low end CCD

LED colour characteristicsLED colour characteristics

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

*( ) ( ) d

' 100%

s l( ), ( ), ( ), ( )x x y z

350 400 450 500 550 600 650 700 750 800

wav elength, nm

f1'=1.33

f1'=1.97

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

350 400 450 500 550 600 650 700 750 800

wav elength, nm

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

400 500 600 700 800

wav elength, nm

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

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

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

0,00 1,00 2,00 3,00

time, min

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!

Correlation between visual impression and instrumental colour determination for LEDs János Schanda...

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