I can think of one reason why the practical advice advice is to use red light over white light. The typical peak sensitivity value for rods is 498 nm, which spectrally is a green-blue. Rod sensitivity is closer to the middle (green) and long (red) wave cones than to the short wave (blue) cones. So why don't we use a blue light to keep our rods at maximum sensitivity? One reason may be the fact that we have fewer (and less densely spaced) short-wave cones. Therefore we may need more absolute light energy to produce activation of the short-wave system to produce the experience of white (assuming simple additive activation). A second variable to consider in the argument is the absolute number (or density) of each type of cone. From this view, the optimal manner to arouse the cone system is to use a spectral wavelength that would simultaneously activate a couple of cones. In this case, a good bet would be a yellow.
And, as a matter of fact, yellow lights are used in photographic darkrooms commonly.
Ken
Stephen Black wrote:
I agree that dark adaptation can take place under weak red light, but I'm not so sure about the further comment that this will stimulate the cones more than the rods. I know this is a widely-believed claim, and was apparently put into effect during the Battle of Britain during WWII to allow off-duty pilots to maintain dark adaptation while still using their cones for useful vision (even have a reference for that, somewhere). I love that story!
The only trouble is I've come to the conclusion that the claim (not the story) is a myth. At long wavelengths of light, the sensitivity of rods falls to a level comparable to that of cones, but not below it, as far as I can determine. The belief that cones are more sensitive to red light than rods comes from the common textbook practice of displaying sensitivity curves for rods and cones each separately normalized so that the maximum rod response is 100% and the maximum cone response is 100%. Then it looks as though cones continue to respond well into the red, while rods have shut down entirely. This is highly misleading. When appropriately plotted, it's clear that both show about the same low-level response, although you have to go back to some pretty early literature to find it displayed this way.
If the point is to retain some degree of useful vision while keeping the eyes at maximum sensitivity in the dark, then the red light trick won't work. Astronomers have debated this point, because they'd like to read star maps while maintaining dark adaptation. The conclusion I find convincing is that it really doesn't matter whether you use red or white light (in fact, reading under red light may be particularly difficult), as long as you keep the intensity down to the minimum that still allows reading.
I've thought about writing up a note on this, but some degree of chutzpah would be involved, given the highly technical nature of such a topic. Safer just to talk about it on TIPS.
Stephen
Stephen Black Department of Psychology Bishop's University [EMAIL PROTECTED]
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