On Jun 9, 2014, at 1:45 PM, John Gnaegy <gnaegy@apple.com> wrote:
Hey list members, is anything perceived by the standard observer as technically, absolutely white?
Superlative question! There are two parts to your question: surfaces and illuminants. There are some substances that, for all pragmatical porpoises, equally and proportionately reflect all visible light that reaches their surfaces. Spectralon is the expensive optical-grade stuff, but Teflon (PTFE) thread seal tape (with enough layers that it's opaque) is close enough that your eyes aren't going to be able to tell the difference. Barium sulfate, from which baryta papers get their name, is another good example. Polystyrene doesn't tend to be 99.9% reflective the way Teflon is; its reflectance is usually in the 80% range. However, its reflectance characteristics are almost perfectly flat as well, making a styrofoam coffee cup a technically better white balance target for a camera than almost any gadget sold as such that you can buy. The other half of the equation, that you allude to, is the illuminant. Our visual systems are incredibly effective at adjusting for different illuminants. As such, something that's equally reflective will appear white regardless of the actual spectral power distribution of the illuminant. That is, it'll appear white until you look critically at it...but, even then, it's not that foam cup doesn't look white, but that _everything_ looks off. Objectively, that foam cup is going to look much yellower in direct sunlight and much bluer in open shade. Subjectively, it's going to look equally white no matter what. An excellent exercise to demonstrate this is to photograph a styrofoam cup in the first light of dawn or last light before sunset, and to compare the sunlit half with the shadowed half. Play around with the white balance, both using the eyedropper to click on each half and to manually adjust to see the changes. Also pay attention to the appearance of everything else in the scene! Now, repeat the exercise, but the next day and without the camera and just your eyes. In the graphic arts industry, the CIE has defined a set of standard illuminants. The D series are designed to be a close approximation of daylight. Though D50 (approx. 5000K) is what's used as the profile connection space, D65 (approx. 6500K) is the closest there officially exists to "generic" light. Widipedia is a good starting place for further research: http://en.wikipedia.org/wiki/Standard_illuminant Cheers, b&