[APD] LEDs and color temperature

[Wright Huntley <whuntley at verizon_net>]

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

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




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