Fun with IR photography

So, I've recently been playing with digital and computational photography. One mechanism for detecting depth relies on projecting light patterns ("structured light") onto a scene, and using the light pattern in the resultant camera image to deduce the 3d location of objects (the pattern lets you determine at what angle rays from the projector intersected the object, and we know what angle rays hit any given pixel in the camera, so trigonometry gives the distance of the object, given the separation between projector and camera). Of course, lighting a scene with a bright light source is distracting to everyone involved, so you ideally need some way to "invisibly" perform the same task.

It turns out that cheap CMOS sensors, of the kind you get in webcams, are actually rather sensitive to infrared light. Normally, there's a filter built into the camera which filters out IR to prevent it from interfering with the visible light imaging you normally want to do... but it's easy to remove it with a screwdriver and a pair of tweezers. Exposed developed film negatives (amongst other things) are pretty opaque to visible light (since they're black), but they're actually transparent to near-infrared light.
If we stick a piece of film negative over our modified webcam, we can then image only the infrared light in the scene... we've made a cheap IR camera.
(This is basically how every security camera, and many "nightvision" scopes, work.)

So, as I don't have a strong IR source at the moment, I couldn't get on with doing structured light work. I did, however, do some experimenting with imaging things in IR with my modified camera (with comparisons to the same objects imaged in visible wavelengths).

Firstly, it appears that cola is pretty transparent to near-IR:

Secondly, some dyes may be black in visible light, but highly reflective to IR:

Obviously, there's a lot of other things you can discover when imaging things in IR (at least some banknotes reveal interesting watermarks, for example), and I might get around to making some more posts in the future.

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