Can I photograph through beam-splitter glass without the subject seeing the camera lens?

Question

I want to shoot a subject through beam-splitter glass or similar glass so the camera can photograph the subject clearly, while from the subject’s side the lens is hidden or not obviously reflected back. Is this possible, and what kind of setup would achieve it?

user76583 · Accepted Answer

Edit: Now that I've already posted the answer, I realized maybe you only need a simple one-way mirror setup: you just put the camera in a dark room or a box and use the beam splitter as a window from which it looks into the brightly lit room with no light directed straight into the window. The brighter the room and darker the box, the better the effect.

It is impossible to make the lens completely disappear, but you can hide it with a much brighter light coming from the same direction. A setup that does this is called coaxial lighting. It is sometimes used in the machine vision industry when you need to illuminate the object from exactly the direction of the lens.

Here is an ugly sketch made by hand.

Full lines represent the beams coming from your light source. Half of them reflect towards the object, half is lost by going straight through the beam splitter to the dark background.

Dashed lines represent beams coming off of your subject (the ones you want to capture with your camera). Half of them go straight towards the camera, half get lost by reflecting towards your light source.

Dotted lines represent the field of view of the camera. Half the image will come from the actual object you are trying to capture, but that image will be mixed in with a half of the dark background. This is why the background needs to be very dark (in the setups I've seen, it's some very black velvety material): it's illuminated by as much light as the object itself, but you want it to still be much darker than the object so the effect on the image is negligible. If it's not dark enough, the image will have a sort of a grey sheen over it.

Some other remarks. As you can see, a lot of light is wasted with this setup. Only 50% of the light will reach the object, and then you will only see 50% of that light that bounces back to the camera. So you're only using 25% of your light source's light. If you're only using this setup to hide the lens, then it's not such a big deal because you'll only lose 50% of the other light source's light (the ones not shining through a beam splitter).

Can I photograph through beam-splitter glass without the subject seeing the camera lens?

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