Quote Originally Posted by Photo Engineer View Post
Interestingly, a grain has to be absorbant to light at the wavelength needed, in order to have imaging take place, so the grain is not 100% transparent. In fact it cannot be. So, if you were to observe holographic emulsions at the wavelength being used, the emulsion must have some absorption, even if it is UV or IR.

Yes, obviously some degree of (visible light) absorption must take place. I am not sure about the AgX crystals, things can get incredibly complicated with grains substantially smaller than the recording wavelength (Raleigh, Mie scattering)!
I guess the main part of (red-green) light absorption is due to the sensitizing dye. But this is barely noticeable. A coating with 10nm grains, 7um thickness, looks completely transparent to the naked eye. And even at the 633nm recording wavelength there's no absorption measurable (at least not with a common lab power meter). By the way the low light absorption is key to the recording of simple reflection (Denisyuk) holograms: the recording light enters the emulsion and hits the object behind the recording plate. There, it will be reflected, part of it will reenter the emulsion and interfere with the incoming light making the holographic recording. The more object light reaching the recording plate in that scheme, the better the condition for high diffraction efficiency of the hologram.