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[APD] LEDs and color temperature

The mythology is starting to build too fast.

There is no simple correlation between led wavelengths and color temperature (a psychophysical system of measure based on average human observer response).

There is only a difficult and nebulous relationship between color temperature and the spectral properties of a radiating (Plankian) blackbody. It involves a difficult convolution integral that includes the entire Plank curve and the eye's spectral response curve. Actually it is worse, as it involves convolving the three tristimulus spectra of the human eye with a blackbody spectrum!

Color temperature is NOT blackbody radiation and should not be substituted for it in calculations. The results will be absurd.

Color temperature is only a valid measure for spectral distributions of light that approximate white *to the human eye and brain*. The radiation can be a set of monochrome sources (e.g., tri-phosphors) or spread spectra. The human eye cannot tell the difference in white produced by red, green and blue lines of a krypton laser or the continuous spectrum of a 6500K blackbody radiator. LEDs are somewhere in between and cannot easily be calculated to produce a given color temp.

After-the-fact measurements by comparison with a *very expensive* standard lamp illumination will give the resultant color temp for a given set of drive values.

This is not a task for the DIY hobbyist. Most commercial lamp manufacturers at below the mega-international-corp level don't get it right, either. [Trust the "Kelvin" figures on the small mfg. lamps at your LFS if you like. They are generally wrong, though.]


Wright Huntley -- 760 872-3995 -- Rt. 001 Box K36, Bishop CA 93514

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