All auto focus assist lights are either red or white. Why do DSLRs not use infrared instead? It seems it would be beneficial to a photographer to not blind their subject before taking a picture.
I've done some google searching, with unsatisfactory results. Some say that the lens "isn't optimized for the wavelengths of IR", I don't know how legitimate that claim is.
Lenses certainly are not optimised for the infra-red spectrum. I know this from pursuing infra-red photography with a converted camera. Chromatic aberration (well wavelength specific aberration, infrared light has no colour) is much worse, resolution is lower and some lenses exhibit "hot spots" a curious type of flair that occurs in the centre of the image.
There is also the fact that IR light focusses at a different distance to visible light. In theory you could model this affect and account for it, but the semi closed-loop system of phase detection AF would have to be modified, and the system wouldn't work with contrast-detect AF.
Finally there's not a huge advantage of a pure IR system. The AF assist beam of most cameras is not that intrusive, it's low intensity red light and is only on for a short amount of time.
Why do DSLRs not use infrared instead?
If you used an infrared sensor to focus the lens, the image would be out of focus in the visible spectrum. The refractive index of a lens depends in part on the wavelength -- that's why a triangular prism breaks white light up into its component colors, and it's also the cause of chromatic aberration. Lens designers correct for chromatic aberration in the visible spectrum by using multiple elements with different materials and coatings, but to correct for aberration in the IR region would surely add a lot of cost and weight to a lens (if it's even possible).
If you look at your lens, you'll see a (usually) white line that indicates the distance at which the lens is currently focussed. You may also see a smaller red line a bit to one side -- this is the infrared focus distance indicator. If you want to take a photo in the IR range, you first focus on the subject. Next, look at the white line to find the focal distance. Finally, adjust the focus so that that same focal distance lines up at the red line instead of the white one.
Here's a Luminous Landscape post that's mostly about taking IR photos with an IR converted body, but which touches a few times (especially at the end) on focusing issues. You may find it interesting.
Most of the information provided is from personal experience so please take it with a grain of salt.
There are at least 3 reasons I can think of why they wouldn't use IR, in most cases.
First is the hot mirror. CCDs in your camera probably ARE very sensitive to IR light, however all cameras come with a preinstalled hot mirror that reduces IR light from reaching the sensor. This is because IR is outside the normal visible spectrum and basically if it's in a normal photo it gives the edges a soft look (check out full spectrum photos for that effect) As this is considered 'undesired' data they block it. And as someone posted it appears as Chromatic Aberration. So using an IR light to help focus would have reduced feasibility unless it was really 'bright'.
Second, the IR spectrum is a very long wavelength, and so the focusing mechanisms in your camera, which are calibrated to the visible spectrum, will be a bit inaccurate. I believe, the closer the focal length the further off it is. This page describes it best http://www.lifepixel.com/focus-calibration-options so I'll leave the science to them.
Third, is interference. Many speed lights/remote triggers/etc use IR to send information from each other or to the camera, with specific encoded messages in the IR range. If you were to use a focus light while, say pointing an IR remote trigger at your camera while standing in front of your camera. Well you'd potentially drown out that remote's message, and the camera wouldn't be able to accurately tell when to actually take the photo.
Finally, I believe the reason they use red light as opposed to blue, is that it's the least invasive of the visible light bands. I guess that's subjective somewhat though.
In addition to some of the good answers already provided, there is the fact that modern cameras are not very good at detecting infrared light. If you point a television remote at your camera you will only see a very small spec of light. So if we used infrared for focusing it would not be very effective.
Infrared illuminators are mostly used in security systems, surveillance systems etc. Its main objective is to improve image quality and not to improve focus. Other typical application of ir illuminator is cctv cameras along with infrared illuminators. Auto Focus is mostly used for focusing which helps in focusing on the target image even when it is poorly lit. Infrared illuminators thus not used for focusing but to improve image quality.
