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Correlation between visual impression and instrumental colour determination for LEDs

Correlation between visual impression and instrumental colour determination for LEDs. János Schanda Professor Emeritus of the University of Pannonia, Hungary. Overview. Colour of LEDs Problems with the colorimetry of LEDs Photometric and colorimetric fundamentals New colorimetric system?

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Correlation between visual impression and instrumental colour determination for LEDs

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

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

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

  4. Photometric and colorimetric fundamentals • 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 • The CIE 1931 2° colorimetric system incorporated the CIE 1924 photometric observer

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

  6. Spectral luminous efficiency functions and a new proposedy colour matching function

  7. 2° standard and proposed, cone fundamental based, colour matching functions (CMFs)

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

  9. Enlarged view in the vicinity of sample #1

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

  11. Typical LED spectra(used in optimization, see later)

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

  13. Temperature dependence of a blue LED

  14. Temperature dependenceof a yellow LED

  15. Temperature dependence of a red LED

  16. Temperature dependenceof a white LED

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

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

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

  20. Spectrometric measurements • Critical parameters of spectrometer • Sampling interval and bandpass: 10 nm sampling produces u’,v’ errors of several units in 3rd decimal. • Highly over sampling OK (CCD spectrometers) • 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 10-4 stray light produces u’,v’ errors of several units in 3rd decimal. • Experiments showed even larger errors:

  21. Comparison of 5 spectrometersGreen LED chromaticity

  22. Comparison of 5 spectrometersRed LED chromaticity

  23. Comparison of 5 spectrometersBlue LED chromaticity

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

  25. Goodness of fit characterization • Modified f1’ method, • No illuminant • Independently for • Separately for Red, Green, Blue LEDs • Example: V(l) channel • Use coloured LED standards • f1’ provides estimate of error to be expected • Detector spectral responsivity measurement, not standardized properly

  26. Partial f1’ error index 1 LED dominant wavelength ranges andthe dominant wavelength value of the standard LEDs

  27. Partial f1’ error index 2 Maximal photometrical errors to be expected if the partial f1’ values are below the given limiting values

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

  29. Experimental five filter colorimeter spectral responsivity

  30. Average colorimetric errors for the eight LEDs

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

  32. Standard LED • Temperature and current stabilized LED for luminous flux measurement

  33. Standard LED • Temperature and current stabilized LED for ALI measurement

  34. LED luminance standard • TechnoTeam LED based Peltier-cooled luminance standard

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

  36. ALI measurement • Input clamp of ALI tube • Clamp for 5 mm LED

  37. ALI-B measuring setup

  38. Reference LED Standard LED Detector with different filters DUT LED Flux and thermal measurement • TeraLED complex colorimetric and thermal measuring system • Radiometric • Photometric • Colorimetric • Thermal • measurements Calibration S, YREF-W Measurement of YT and YREF-T  ΦT

  39. Characteristics of Standard LEDs: Stabilization during the first two minutes

  40. Summary • 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 • Use temperature stabilized LEDs of similar colour as the test samples • Use good alignment for the LEDs in ALI measurements

  41. Summary • Current sate of the art in user’s laboratory: • Spectral mismatch uncertainty: 1 – 2 % • Geometric alignment uncertainty: <+/-0,002% • Temperature dependence: +/- 0,2 %

  42. Tanks for your kind attention!

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