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.

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    If it were that easy, it would have been standard practice already. =)
    – scottbb
    Oct 23 '17 at 14:10
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    1. Undeveloped film is not transparent. 2. Film has a front side (emulsion side).
    – scottbb
    Oct 23 '17 at 14:26
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    @CamilB Film also has emulsion layers and often several layers respond to the same frequencies but at different rates. Also, as a general rule, reflections in diffracting (dioptric) optical systems is usually to be avoided. For example, light rays passing through a transparent film will diffract and therefore be displaced on their return trip after reflection (angle of incidence = angle of reflection). The returning light rays will also refract again as they pass through the transparent film.
    – user50888
    Oct 23 '17 at 14:35
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    @CamilB, It is an interesting idea. If you are interested in a deeper understanding of photographic science and technology, I recommend Manual of Photography as a research tool. The analogy between eyes and cameras is about as strong as the analogy between brains and clockworks. Each is good as far as it goes, but no further.
    – user50888
    Oct 23 '17 at 14:38
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    Yeah, it's one of those cases where intuition can lead you to the exact opposite conclusion. But if you think about, ideally, the emulsion should not pass any light. If photons went through the emulsion (or were reflected by it), that means they weren't captured as part of the image. Photons not captured in an image might as well not exist, from the standpoint of recording the light.
    – scottbb
    Oct 23 '17 at 14:44

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.

  • 4
    It's nice to see that people actually try pretty crazy stuff :D
    – Mołot
    Oct 24 '17 at 7:42
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    Professor Lippmann and graduate students at Sorbonne, researched methods to make color pictures. His direct color system, using transparent emulsions on mirrored plates did just that. This is an interference process using the same principle as an oil film on water. The images he made are astonishing. The year was 1891. The sad part, he tried and failed to make this method commercially possible. Oct 24 '17 at 15:54

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.

  • Thank you, I didn't realize the distinction.
    – CamilB
    Oct 24 '17 at 7:49
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    According to Wikipedia, non-DSLR camera sensors are retroreflective, but for a different purpose. Museums want to detect people taking photos of the art, so the sensor of a camera appears bright when exposing. I'm not sure where this happens, or what the details are.
    – CamilB
    Oct 24 '17 at 8:24
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    @CamilB Surely that's backwards? I can't believe that camera manufacturers would agree to make sensors retroreflective for this reason. Surely it's actually that the sensors are retroreflective for whatever technical reason, and the museums are exploiting this feature. Oct 24 '17 at 11:06
  • It doesn't make sense for another reason too: the "system" would have to also shine light onto the sensor at a very specific time - how does it know when the sensor is exposed? It wouldn't do it continuously. And it can't see the camera sensor if its own sensor isn't within the work of art being photographed by the tourist.
    – CamilB
    Oct 24 '17 at 13:14
  • @CamilB I think all sensors are slightly retroreflective, but cameras with mechanical shutters (like SLRs) keep sensor hidden.
    – Agent_L
    Oct 24 '17 at 15:08

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