This neat DIY video shows how you can convert an ordinary digital camera into a night vision camera. The video uses a digital video camera, but the same concept can be applied to still cameras as well.
Digital camera sensors are sensitive to both visible and infrared (IR) light. However, there’s a special IR filter used to block IR light from the sensor, keeping images from being washed out. If this special filter is removed, the camera can be made sensitive to IR light. The hack in this video involves replacing the filter with the black end of a film negative, and then using red and blue lighting gels on a flashlight to have it give off mostly IR light. The result is a camera/flashlight setup that can be used to take stills and videos in the dark where ordinary cameras can’t.
This is similar to the pricey modifications you can have done to your DSLR to use it as an infrared camera.