With the increasing popularity of devices such as night vision goggles, digital still cameras and video cameras with night vision features, IR illuminators are quickly becoming more popular as well. Night vision devices are sensitive to the IR (infrared) light spectrum and an illuminator functions as a flashlight that shines light that is only visible through night vision devices. Commercial IR illuminators can cost up to £650. Fortunately, a homemade IR illuminator can be built for as little as £22.
Unscrew the front end of the flashlight. This will give you access to the light bulb and reflector.
Remove the reflector from the lens assembly. Depending on the flashlight, you may have to remove the light bulb first.
Place the reflector face down on top of each sheet of gel filter and carefully trace around the reflector with the razor blade.
Place the cut-out circles of gel filter on the inside of the front end of the flashlight, between the reflector and the lens. The order of the filters is not important. It is only necessary that they completely cover the lens so that no light shows through.
Reassemble the flashlight and turn it on. You should see a reddish purple glow coming from the flashlight. When you turn off the lights, you should see a small red light being projected from your flashlight.
Gel light filters are used by photographers, filmmakers and stage productions to give lights a specific tint. They can be found at photography shops or online.