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Some animals have a reflective layer behind their transparent retina, greatly increasing their ability to see in the dark, called tapetum lucidum.
In my 35mm film SLR camera, could I add a thin mirror behind the film, with the same effect? I'm assuming that the film is transparent when unexposed/undeveloped. I know that developed film is indeed transparent.
Film makers avoid a transparent film because: Bright exposing light will penetrate and then hit the pressure plate. The pressure plate has a flat black coat. Nevertheless, highlights are bright and will reflect, re-exposing the film from the rear. This causes a halo like effect surrounding highlights called a halation. To avoid, modern films have an opaque anti-halation coat on their reverse.
The French physicist, Gabriel Lippmann experimented with transparent film plates. He exposed them with a mirror backing. Mostly he used mercury for this reflective surface. He used a reduced exposure. The reflected light re-traversing, completed the exposure. The light waves traced out a chain-like path. At the cross point, at the beginning and end of the chain like links, the exposure is doubled. Thus the intensity of the exposure is at the cross points. The developed film had metallic silver formed at these points. The spacing is exactly the wave length of the exposing light. Because the metallic silver spacing forms a maze that only allows one frequency to pass. This is the exact frequency of the light that made the exposure. The frequency is that feature of light that give it the colors we perceive. Looking at this image via backlighting, we see a full color picture. This is true even though the film was a black and white material.
The Lippmann process, based on a transparent film with mirror backing, is a laboratory curiosity. The process yields beautiful color slides, but the difficulty of viewing and the difficulty of making a copy doomed the popularly of this process.
Tapetum lucidum is not your regular mirror. It's a retroreflector. Or, to be precise, an incredibly numerous array of tiny retroreflectors. It doesn't just shine back, it shines every "ray" of light precisely in the same direction it came from.
To have an effective tapetum for your camera, a single "grain" of reflector would have to be no larger than a single grain of emulsion (intuition says the smaller the better). The tapetum could not be protected with a glass surface, because that would create parasitic reflection off the air-glass boundary. So, you'd get an expensive and delicate micromirror with rough surface, brushing against the film being wound to the next frame. That would damage both the film and the tapetum, soon destroying its ability to reflect light with the required accuracy.
However, it could be (and probably is) done with digital sensors. Because they're more similar to cats' eyes in the aspect that the sensor is permanently tied to the tapetum.
