Re: [APD] LEDs and color temperature

["S. Hieber" <shieber at yahoo_com>]

Not only does mythology build fast but today's myth tends
to be tomorrow's dogma. [Which is one of the things that I
think bothered Feyerabend about conventional science as
practiced and more so, the learning of things scientific.]

Wright's is a *much* better explanation than I have ever
tried to offer. Wright has spents years and years working
on light, how it appears to humans, etc. -- not just an
avocation but a vocation.

Unfortunately, the technical precision of his explanation
goes beyond my abilities to comprehend it all. So correct
me if I don't get this right (Wright?) but I think the gist
of it is that the color temp ratings for LEDs should not be
compared to the color temp ratings that bulb manufaturers
give for fluorescent or MH bulbs. Those Flo and MH bulb
ratings are a general (and usually somewhat reliable) guide
of how bluish a bulb will appear to humans and that's about
all. If you really want to know how good a bulb probably is
for plants, check out the spectral graph, if one is
available, and see how much light is not in the mid green
area -- that rule of thumb works if your thumb is big
enough ;-)

All this periodically present talk about LEDs leads me to
ask this: Has anyone illuminated their tank entirely and
solely with LEDs?  I don't mean nightlights/moonlights; I
mean as the main/sole source of light energy for their
plants? I'd like to hear about that lamp, energy
efficiency, light dispersion, spectral output, etc.

Scott H.
--- Wright Huntley <whuntley at verizon_net> wrote:
> 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.]
> 


=====
S. Hieber

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