Correlation between visual impression and instrumental colour determination for LEDs János Schanda Professor Emeritus of the University of Pannonia, Hungary

Overview Colour of LEDs Colour of LEDs Problems with the colorimetry of LEDs Problems with the colorimetry of LEDs Photometric and colorimetric fundamentals Photometric and colorimetric fundamentals New colorimetric system? New colorimetric system? Objective colorimetry of LEDs Objective colorimetry of LEDs Instrumental problems Instrumental problems LED standards LED standards Recommendations Recommendations

Colour of LEDs LEDs are narrow band emitters LEDs are narrow band emitters Small errors in colour matching functions produce errors. Small errors in colour matching functions produce errors.  Visual match differs from colorimetric match

Photometric and colorimetric fundamentals Colorimetry is based on a trichromatic match between test colour stimulus and three primary (matching) colour stimuli Colorimetry is based on a trichromatic match between test colour stimulus and three primary (matching) colour stimuli Transformation from real R,G,B primaries to imaginary X,Y,Z primaries Transformation from real R,G,B primaries to imaginary X,Y,Z primaries The CIE ° colorimetric system incorporated the CIE 1924 photometric observer The CIE ° colorimetric system incorporated the CIE 1924 photometric observer

CIE 1924 photometric observer Based on Based on Flicker photometry Flicker photometry Distinctintness of boarder Distinctintness of boarder Small step colour difference Small step colour difference Describes Describes Visual acuity type observation: reading, observing fine details Visual acuity type observation: reading, observing fine details NOT BRIGHTNESS NOT BRIGHTNESS

Spectral luminous efficiency functions and a new proposed y colour matching function

2° standard and proposed, cone fundamental based, colour matching functions (CMFs)

Broad-band – RGB-LED visual and instrumental colour match Error decreased by 50 % or more Error decreased by 50 % or more Standard CMFs CMFs based on CIE TC 1-36 recommendation

Enlarged view in the vicinity of sample #1

LED colour characteristics LEDs are narrow band emitters LEDs are narrow band emitters bandwidth approx. 10 nm – 30 nm bandwidth approx. 10 nm – 30 nm Blue … Green: InGaN Blue … Green: InGaN Yellow … Red: AlInGaP Yellow … Red: AlInGaP Both the absolute intensity and the wavelength of the emission maximum is temperature dependent Both the absolute intensity and the wavelength of the emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDs largest changes with Red LEDs Unusual spatial light characteristics Unusual spatial light characteristics Solution of measurement problems caused by Solution of measurement problems caused by Spectral mismatch: spectrometric measurement Spectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetry Spectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignment Geometric misalignment

Typical LED spectra (used in optimization, see later)

LED colour characteristics LEDs are narrow band emitters LEDs are narrow band emitters bandwidth approx. 10 nm – 30 nm bandwidth approx. 10 nm – 30 nm Blue … Green: InGaN Blue … Green: InGaN Yellow … Red: AlInGaP Yellow … Red: AlInGaP Both the absolute intensity and the wavelength of the emission maximum is temperature dependent Both the absolute intensity and the wavelength of the emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDs largest changes with Red LEDs Unusual spatial light characteristics Unusual spatial light characteristics Solution of measurement problems caused by Solution of measurement problems caused by Spectral mismatch: spectrometric measurement Spectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetry Spectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignment Geometric misalignment

Temperature dependence of a blue LED

Temperature dependence of a yellow LED

Temperature dependence of a red LED

Temperature dependence of a white LED

LED colour characteristics LEDs are narrow band emitters LEDs are narrow band emitters bandwidth approx. 10 nm – 30 nm bandwidth approx. 10 nm – 30 nm Blue … Green: InGaN Blue … Green: InGaN Yellow … Red: AlInGaP Yellow … Red: AlInGaP Both the absolute intensity and the wavelength of the emission maximum is temperature dependent Both the absolute intensity and the wavelength of the emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDs largest changes with Red LEDs Unusual spatial light characteristics Unusual spatial light characteristics Solution of measurement problems caused by Solution of measurement problems caused by Spectral mismatch: spectrometric measurement Spectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetry Spectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignment Geometric misalignment

Irradiation inhomogeneity in measurement plane What should be reported? Problem of reproducible alignment

LED colour characteristics LEDs are narrow band emitters LEDs are narrow band emitters bandwidth approx. 10 nm – 30 nm bandwidth approx. 10 nm – 30 nm Blue … Green: InGaN Blue … Green: InGaN Yellow … Red: AlInGaP Yellow … Red: AlInGaP Both the absolute intensity and the wavelength of the emission maximum is temperature dependent Both the absolute intensity and the wavelength of the emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDs largest changes with Red LEDs Unusual spatial light characteristics Unusual spatial light characteristics Solution of measurement problems caused by Solution of measurement problems caused by Spectral mismatch: spectrometric measurement Spectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetry Spectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignment Geometric misalignment

Spectrometric measurements Critical parameters of spectrometer Critical parameters of spectrometer Sampling interval and bandpass: 10 nm sampling produces u’,v’ errors of several units in 3rd decimal. Sampling interval and bandpass: 10 nm sampling produces u’,v’ 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 u’,v’ errors of several units in 3rd decimal. Wavelength scale error: 0.5 nm error produces u’,v’ errors of several units in 3rd decimal. Stray light: LED measurement compared to incandescent lamp if stray light produces u’,v’ errors of several units in 3rd decimal. Stray light: LED measurement compared to incandescent lamp if stray light produces u’,v’ errors of several units in 3rd decimal. Experiments showed even larger errors: Experiments showed even larger errors:

Comparison of 5 spectrometers Green LED chromaticity

Comparison of 5 spectrometers Red LED chromaticity

Comparison of 5 spectrometers Blue LED chromaticity

LED colour characteristics LEDs are narrow band emitters LEDs are narrow band emitters bandwidth approx. 10 nm – 30 nm bandwidth approx. 10 nm – 30 nm Blue … Green: InGaN Blue … Green: InGaN Yellow … Red: AlInGaP Yellow … Red: AlInGaP Both the absolute intensity and the wavelength of the emission maximum is temperature dependent Both the absolute intensity and the wavelength of the emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDs largest changes with Red LEDs Unusual spatial light characteristics Unusual spatial light characteristics Solution of measurement problems caused by Solution of measurement problems caused by Spectral mismatch: spectrometric measurement Spectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetry Spectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignment Geometric misalignment

Goodness of fit characterization Modified f 1 ’ method, Modified f 1 ’ method, No illuminant No illuminant Independently for Independently for Separately for Red, Green, Blue LEDs Separately for Red, Green, Blue LEDs Example: V(l) channel Example: V(l) channel Use coloured LED standards Use coloured LED standards f 1 ’ provides estimate of error to be expected f 1 ’ provides estimate of error to be expected Detector spectral responsivity measurement, not standardized properly Detector spectral responsivity measurement, not standardized properly

Partial f 1 ’ error index 1 LED dominant wavelength ranges and the dominant wavelength value of the standard LEDs

Partial f 1 ’ error index 2 Maximal photometrical errors to be expected if the partial f 1 ’ values are below the given limiting values Value of the error index Largest photometrical error to be expected: f 1 ’ BL <2% PE < f 1 ’ BL f 1 ’ GN <4% PE < f 1 ’ GN f 1 ’ YL <4% PE < f 1 ’ YL f 1 ’ RD <10% PE < 2∙f 1 ’ RD

Correcting tristimulus colour measurement of LEDs by matrix transformation Modern tristimulus colorimeters have four input channels. One can Modern tristimulus colorimeters have four input channels. One can just add the signals of the x s and x l channels (no matrixing) just add the signals of the x s and x l channels (no matrixing) Use the four channels for improving accuracy Use the four channels for improving accuracy Add a fifth channel Add a fifth channel Optimization was performed for the LEDs shown previously Optimization was performed for the LEDs shown previously

Experimental five filter colorimeter spectral responsivity

Average colorimetric errors for the eight LEDs Matrix type Matrix type Colorimetric error, ΔE ab * Without matrixing 9,67 4 filter matrix 3,76 5 filter matrix 1,09

LED colour characteristics LEDs are narrow band emitters LEDs are narrow band emitters bandwidth approx. 10 nm – 30 nm bandwidth approx. 10 nm – 30 nm Blue … Green: InGaN Blue … Green: InGaN Yellow … Red: AlInGaP Yellow … Red: AlInGaP Both the absolute intensity and the wavelength of the emission maximum is temperature dependent Both the absolute intensity and the wavelength of the emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDs largest changes with Red LEDs Unusual spatial light characteristics Unusual spatial light characteristics Solution of measurement problems caused by Solution of measurement problems caused by Spectral mismatch: spectrometric measurement Spectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetry Spectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignment Geometric misalignment

Standard LED Temperature and current stabilized LED for luminous flux measurement Temperature and current stabilized LED for luminous flux measurement

Standard LED Temperature and current stabilized LED for ALI measurement Temperature and current stabilized LED for ALI measurement

LED luminance standard TechnoTeam LED based Peltier-cooled luminance standard TechnoTeam LED based Peltier-cooled luminance standard

LED colour characteristics LEDs are narrow band emitters LEDs are narrow band emitters bandwidth approx. 10 nm – 30 nm bandwidth approx. 10 nm – 30 nm Blue … Green: InGaN Blue … Green: InGaN Yellow … Red: AlInGaP Yellow … Red: AlInGaP Both the absolute intensity and the wavelength of the emission maximum is temperature dependent Both the absolute intensity and the wavelength of the emission maximum is temperature dependent temperature dependence is composition dependent temperature dependence is composition dependent largest changes with Red LEDs largest changes with Red LEDs Unusual spatial light characteristics Unusual spatial light characteristics Solution of measurement problems caused by Solution of measurement problems caused by Spectral mismatch: spectrometric measurement Spectral mismatch: spectrometric measurement Spectral mismatch: tristimulus colorimetry Spectral mismatch: tristimulus colorimetry Temperature dependence Temperature dependence Geometric misalignment Geometric misalignment

ALI measurement Input clamp of ALI tube Input clamp of ALI tube Clamp for 5 mm LED

ALI-B measuring setup

Flux and thermal measurement Reference LED Standard LED Calibration  S, Y REF-W  Measurement of Y T and Y REF-T  Φ T DUT LED Detector with different filters TeraLED complex colorimetric and thermal measuring system Radiometric Photometric Colorimetric Thermal measurements

Characteristics of Standard LEDs: S tabilization during the first two minutes

Summary Based on visual observations a new LMS cone fundamental based colorimetry is recommended Based on visual observations a new LMS cone fundamental based colorimetry is recommended Careful tristimulus colorimetry can be more accurate than low cost spectrometric techniques Careful tristimulus colorimetry can be more accurate than low cost spectrometric techniques Use temperature stabilized LEDs of similar colour as the test samples Use temperature stabilized LEDs of similar colour as the test samples Use good alignment for the LEDs in ALI measurements Use good alignment for the LEDs in ALI measurements

Summary 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 attention!