Light sources are crucial in infrared photography. Your choice of light source can have a big effect on the pictures you take.
As a light source, the sun is the primary source of infrared light. It emits visible light, infrared light and ultraviolet light. However, daylight can be unpredictable due to changing weather conditions and clouds tend to block most of the infrared spectrum. The best infrared photographs therefore tend to be captured in direct sunlight.
Normal tungsten bulbs emit more infrared radiation than visible light; a 100W tungsten bulb emits only 1W of visible light, but 99W of infrared. Unfortunately most of the infrared light is deep IR and falls outside of the sensitivity of digital camera sensors. You would need higher ISO setting and longer exposure to capture infrared photographs under tungsten illumination.
Fluorescent light bulbs are designed to save energy and reduce heat by only emitting visible light. There is still just a bit of near IR that is emitted but this is so low of an intensity that infrared photography under fluorescent lighting is impractical.
HID (high intensity discharge) lamps produce light by surging electricity through pressurized gas. Normally, these lights are used in locations such as shopping malls, sports stadiums and street lighting. Like fluorescent light, HID is a discontinuous light source, meaning that it doesn’t burn (continuous light sources are the opposite – the sun, candles etc. all burn). In common with other discontinuous light sources, HID emits mainly visible light and quite a bit of far infrared light but just like fluorescent lights lacks useful output in the near IR range that is used for infrared photography.
Electronic flash light
Electronic flash units emit quite a bit of near IR light and other than the sun is the most useful light source for IR photography. In fact, flash units emit just as much and in some cases even more infrared light than visible.