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Message:: [QUOTE="Howard Black, post: 3559055, member: 97119"]One last comment on the subtopic: I understand the process. I was a professional photographer (the real kind -- attended the Germain School of Photography at 225 B'way, a block from what was about three decades later to become known as "Ground Zero," and had a series of B&M studios). I was also a camera repairman -- retail/wholesale. It's the same reason that the antireflective coating on lens elements have varying colors (usually blue, until the multicoating era, at which point they would have a multitude of colors, predominately green). The interaction between the wavelength of light, the depth of each coating layer, and the underlying reflective layer, results in color being produced. Also similar to dichroic interference filters which are used for things like color printing (the traditional type, using an enlarger and chemicals), projection lamps (to reflect visible light, but pass IR (heat) light), and even in digital cameras, the "IR filter" used to among other things prevent cameras from seeing right through people's clothing (IIRC it was Sony that got into a kerfuffle over that issue). So, I understand the process.[/QUOTE]