New Scientist has this article that discusses a new invention by computer-science student Dilip Krishnan and assistant professor Rob Fergus. Their invention is something they call dark flash photography.
Dark flash photography involves a flash that will produce a burst of light outside the visible spectrum and a camera that will be able to both read that information then be able to accurately interpret the visible colour in a scene. The result will be the ability to use a camera flash that will be invisible to the human eye. This invisible flash won’t dazzle, disturb or be intrusive to photographic subject.
The flash in their system has been modified to produce light in a wider spectrum than a normal flash. It emits both infrared and UV light, but at the same time, visible light is filtered out. Normal cameras have filters that prevent infrared and UV light from hitting the sensor, but the pair have modified a camera to be able to pick up both of these forms of light. The resultant image will have strange, unnatural colour, but Krishnan and Fergus have a way around this problem.
Their modified camera will take two exposures in quick succession to get one shot. The first exposure will record the luminosity generated by the invisible IR/UV flash. The second exposure will be used to record colour information using only ambient light. This second image, taken immediately after the first, will lack the detail of the first and will inevitably be unusably dark. But between the two images, all the necessary information. The first has the luminosity information, the second has the colour information.
Using software, the two exposures are combined and the result is a normal-looking photograph produced using the invisible flash.
The New Scientist story says there are still some bugs left eliminate. Certain objects and materials (freckles, for example) absorb the IR/UV light burst. Based on this article at sportsshooter.com on taking infrared photos of basketball players, I suspect there may also be some issues with certain materials reflecting IR/UV light more than they should as well.
I would guess, also, that there would be limitations to what you can photograph based simply on the fact that you are taking two exposures. Fast-moving subjects will be problematic because they will have changed position from one exposure to the next.
There is, however, a variety of applications for this invention. What immediately jumped to my mind was to be able to take flash-lit photos in venues where it is not permitted (usually because it is distracting to other visitors). Perhaps wedding photographers will soon have the option to flash light a ceremony and not disturb the bride and groom with flash pops during their big moment.