Re: Color spectrum Vs Degree Kelvin??

Miles Morrissey asked:

>Is it possible to say I have a 5000k light which corresponds to a certain 
>wavelength of light as measured in nanometers or is degrees Kelvin a 
>combination of spectrums as I would assume it would have to be if a 5000k 
>bulb is considered a full spectrum bulb.

No. None of the above. Degrees Kelvin of Color Temperature can have any kind 
of spectrum that creates the visual appearance, to the "standard observer," 
of a blackbody radiating at that temperature. Three extremely narrow laser 
wavelengths could be combined in the right proportions to "look" exactly 
like a blackbody, but most of that spectrum would be completely empty.

Color Temperature, like lumens, lux, etc. is a purely psychophysical 
measurement that has very little to do with radiometric measurements (like 
Watts per nanometer of spectral power density). It was developed for 
artists, window dressers, photographers, etc. to have a way of saying how 
something "looks," even if the same effect often can be created by widely 
different spectra.

The brain of a "standard observer" can do remarkable things like putting 
three primary colors together to make any color. Plants not only can't do 
that, they "see" a much wider spectrum than we do. For this reason, *any* 
specification intended for human perception should be taken with a huge 
grain of salt before applying it to plants.

Your 5000K bulb looks pretty white, with very little imbalance toward blue 
or red. The other part of the description saying it is a full-spectrum bulb 
is a bit more useful, but that term, unfortunately, lacks any concrete 
technical meaning. It *usually* means that the red and blue that are 
essentially missing in a "cool white" bulb are actually present in this one. 
For a plant, that could be really meaningful.

If the bulb has a considerably *lower* lumens rating than a similar "cool 
white" that is another good clue that your plants will like it. The "cool 
white" is mostly green, which gives it higher lumens/Watt value than a 
full-spectrum bulb that wastes its energy out where our eyes don't respond 
very well. The green is not utilized well (and is even mostly reflected 
away) by most plants, but is weighted heavily in measuring apparent 
brightness (lumens). 

Red and blue may help grow plants, but they are factored very low in 
measuring lumens. Our eye is only about 1/10 as sensitive to those colors as 
it is to green. Therefore, the red and blue ends of the spectrum don't make 
the lamp look much brighter and the Lumens are proportionately reduced for 
any energy "wasted" on them.

Bottom line: Don't put too much reliance on measurements intended for human 
vision if you want good plant growth. Put lots there for how your tank will 
"look," tho.



Wright Huntley (408) 248-5905 Santa Clara, CA USA huntley at ix_netcom.com