Infrared (IR) Light
Updated February 18, 2009
Infrared light is a part of the electromagnetic spectrum just below the visible light (the lowest frequency of which is red: thus infra+red=below red) that behaves essentially like visible light, though reflectivity and transparency differs somewhat. It is not visible to the human eye, but the majority of electronic image sensors (CCD, CMOS sensors) are highly sensitive to it, sometimes even more sensitive than to visible light. These sensors (in digital cameras and web cams) thus have an IR filter added to them to reduce the amount of IR that makes it to the sensor so that the colors are as expected and the camera sees roughly what a human eye sees.
The fact that IR works like normal light, but is invisible has some benefits. The principles of Diffuse Illumination, Frustrated Total Internal Reflection, and LED Light Planes work just as well with IR as they do with visible light, but the IR light used does not interfere with seeing a visible image below the IR interactions. Often, we can easily remove webcam IR filters, and even substitute a visible-light filter (the black part of a bad, developed film negative or the inside of a floppy disk) so that our camera only sees IR. This then separates what a user of a display sees (visible light) and the computer-vision technology enabling optical multi-touch (infrared) effectively by frequency. This is the principle behind the vast majority of computer-vision-based multi-touch displays.
Filter Removal from a Logitech QuickCam Pro 4000
This camera, already owned by the computer science discipline, has a high quality sensor, lens, and drivers, good compatibility with Linux, Windows, and Mac OS X, and importantly, an easily accessible (and removable) IR filter. When I completed this modification on February 13, I documented the brief process below. Note: the small, odd-colored (reddish in real life) square is the IR filter.
Click on any photo for a larger version.
 A single screw in the external housing |
 Camera casing opens |
 The important bits |
 Lens assembly removed to reveal image sensor |
 Bottom of lens assembly holds IR filter |
 Unscrewing the two halves and a gentle push with a cotton swab is all it takes |
 Nearly reassembled, without the IR filter |
 Camera back together and working |
Webcam Images Before and After IR Filter Removal
I took some representative and interesting images before and after the removal of the IR filter. Note that I haven't yet installed the visible-light filter, so these images are of both the visible and IR spectra. Pure IR photography can be pretty unique and haunting: take a peek at some artistic examples [external link] of what can be done with infrared photography (and doubtless a little retouching).
Click on any photo for a larger version.
 Before: Even with the filter on, a remote can be used as a dim light in a dark room |
 After: Webcam showing image in both visible and IR light |
 After: Infrared light and camera reveal some security features on currency |
 After: With the filter removed, a remote is blinding even in full light |
 After: In a dark room with a remote shining into my plexiglass desk, you can see the white spot at the point of contact, due to the FTIR effect |