IR Light Primer
What we perceive with our eyes as visible light is really a small portion of the electromagnetic spectrum. Radio waves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays are all types of electromagnetic radiation that differ in one major respect... wavelength. At the very low end, with wavelengths measured in meters, are the radio waves. At the high end of the energy spectrum are gamma rays with very short wavelengths that can only be generated by high energy processes like those present in stars and atomic explosions.
Our eyes, while very capable receivers of electromagnetic radiation, are sensitive to only a very narrow region of this spectrum, the range from about 400 to 750 nm wavelength. This is true of almost all mammals, reptiles, and amphibians, but is not true of birds and insects, whose vision often extends into the ultraviolet.
The diagram above shows a very small portion of the electromagnetic spectrum near that of visible light. Within this range we can measure, with our eyes, the wavelength of the light, which we perceive as the colors of the rainbow. Red is a longer wavelength than yellow, which is longer than green and so forth; with the shortest wavelengths we can detect are referred to as violet. Beyond the violet is the ultraviolet, responsible for sunburns and faded carpets. At the other end of our visual spectrum, beyond red, is the infrared, a broad section of the spectrum that can have many properties depending on wavelength. This region is broad enough that we divide it up into sections of near and far infrared.
Detecting wavelengths beyond the visual range requires special instruments. Inventing and building these instruments represents much of the advancement of human science and engineering across the centuries. Photographing with light far into the infrared (3000 to 10,000 nm), as in thermal imaging, usually takes lenses made of exotic materials like germanium or beryllium and detectors just as unusual.
Photographing with light in the near infrared does not require specialized equipment. Using wavelengths as close to visible light as possible (800-900 nm) is desirable. Wavelengths this close to the visible spectrum will behave much as visible light, allowing the use of commonly available lenses and cameras. By using a carefully chosen wavelength, we can illuminate a scene for filming, while keeping the scene in apparent darkness.
For more information on the electromagnetic spectrum you can try the following links...