Re: Gels to adjust color temperature

> From: gtong at sirius_com (G.Tong)
> In an article in one of the fish magazines recently, I read a
> recommendation for adjusting the color temperature of our lights by using
> photographic gels. The gels are colored plastic sheets generally available.
> The idea is not to worry about the color temperature of the lights when we
> buy them but to adjust them to what we want by placing the gels between the
> lamps and the water in our tanks. Of course, the gels would have to rest on
> glass or some other rigid thing.

	Bad idea.  Any filter of that sort simply absorbs the colors of light
you don't want.  It _doesn't_ make more light of the color you do what.  If
there isn't much of the colors you want to begin with, there won't be much left
in total after the filter.
	You'll see exactly this happening if you stack several colors of
filters -- you get less light out stacking blue over yellow than blue over
blue.  The more perfect the filter the more pronounced the effect (perfection
being defined by the filter letting through less of the colors it is supposed
to filter out).
	You can also see this if you get some of the anaglyph (red/blue)
glasses used for watching those old 3-D movies and looking at your computer
monitor (which has red, green and blue phosphors).  Since the filters are not
perfectly matched you get a bit of bleed over, but it's really amazing how
bright one can get the red and blue stuff before it bleeds into the other eye.

	The only possible use of such filters might be to filter potentially
harmful light wavelengths.. but even so, it's still more efficient to
generate a more desirable wavelength of light in the first place.

	Brian Ferguson, who I ran into over on fishroom one day, is playing
with some dichromatic filters that are supposed to somehow or another convert
light to a different color without loosing intensity.  Personally I don't see
it given the physics of light and photons (of which I only know enough to be
dangerous :-), but I'm willing to wait and see what he finds out.