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Color Perception

By Paul Beckmann. Posted to Reefkeepers emailing list, Saturday 9th October 1999.

Couldn't let this color perception thread pass by without adding my 2 cents (since it's the area of my professional training, I felt a responsibility as well.)

There are four factors (at least) that will affect the perceived color of surfaces and organisms within your aquaria. First is the output spectrum of your lighting; second is the absorption spectrum of the water (including scattering); third is the reflectance spectrum of the surface or organism; fourth is the state of color adaptation of the observer. Notice the first three are SPECTRA, not CRI, degrees K, or any other summary measure of the 'color' of the bulb or surface. While tungsten bulbs do produce close to a 'black body' radiation spectra (hence the temp => spectrum), MH and fluorescent sources have very strong peaks superimposed on a smooth background spectra. Reflectance spectra and scattering can also have some very sharp spectral peaks.

The result of the interaction of the light produced by the source with the water, surface or organism, water (again) plus glass produces a spectrum of light coming to your eye. This spectrum is sampled by the photoreceptors of your eye and is interpreted by your visual perceptual apparatus through adaptation processes to net (in a very complicated way) an approximation of the surface characteristics. In other words, your eye/brain tries to compensate for any shift in the color of the illumination source by taking it out of the equation. It's felt that this is likely designed to give us information about the environment, not about the sun/sky (which often doesn't matter). In an aquarium setting, a surface or organism can be perceived to have different colors just by changing the lighting of the room, even if an appreciable amount of that light doesn't fall on the aquarium. This is due to adaptation. This shift in perceived color can also happen locally. The perceived color of one surface can be dramatically altered by what is next to it or around it.

Film does not have this adaptation process. Remember 'daylight' and 'tungsten' type films? Taking a picture with daylight film under fluorescent lighting yields a blue-green cast. Perception, however, 'filters out' the difference in the illumination spectra. After you've been under fluorescent lighting for a short period of time, everyone's skin tone looks natural, even though the spectrum of light reaching your eyes has changed significantly. This difference between perception and film can make it difficult to take photographs the 'look' correct after processing.

Digital cameras seem to do a pretty good job of compensating or adapting, from my casual observations, but I don't know if they do or how. There are balancing compensations within most '24 hour' film printing machines. These can run you in circles if you're trying to get a 'true' color. My best experiences have been with slide film. However, printing these can be difficult and expensive.

By the way, measuring these quantities as spectra is necessary to make any meaningful predictions and requires expensive equipment to do properly. The end result, however, is a complete understanding of what's going on, at least before the adaptation process of the visual system kicks in.

Created by liquid
Reefs.org
Last modified 2006-11-24 18:41
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