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An infrared (IR) light source is any object capable of emitting light with wavelengths between about 1 and 400 terahertz (THz). Many sources emit infrared light in addition to visible and ultraviolet light, though some are designed or modified to only release wavelengths that fall within the IR band. Most sources of white light can be modified into exclusively infrared light sources using optical filters, though an IR light emitting diode (LED) requires no such modifications. The other type of IR light source is thermal radiation, which is created by the thermal motion of matter. Infrared cameras that make use of thermography can use the differing intensity of these IR light sources to determine temperature.
Most conventional light sources emit both ultraviolet and infrared light in addition to visible light. Over half of the energy that reaches Earth from the sun is invisible infrared light, a little less than half is visible and a small amount of ultraviolet accounts for the remainder. Conventional light bulbs, flashlights and other sources of light typically reach into the infrared band as well. In order to turn this type of broad spectrum light into an IR light source, an optical filter can be used.
Optical filters are designed to only allow the transmission of certain wavelengths of light. A red optical filter will appear red to the naked eye, as it is designed to only allow the transmission of wavelengths between about 400 and 480 THz. These filters can use a number of different methods to affect the passage of light, such as either absorbing or reflecting the unwanted wavelengths. When a properly designed optical filter is applied to a flashlight or light bulb, it can result in a practical IR light source.
Another type of IR light source emits only infrared light without any modifications or filters. The infrared LEDs that are commonly used in remote controls are one example of this type of IR light source. There is no optical filter involved, and the surface of these LEDs appear colorless. When electricity is applied to this type of LED, it can emit invisible infrared light.
Every object that emits heat also emits light, and in the case of normal matter at room temperature that light typically falls within the infrared spectrum. This is the basis of thermography, which involves the photography of thermal images. These images can be displayed in false-color, which assigns darker hues to cooler areas and lighter colors to warmer areas. Thermography is sometimes used for night vision, though it is also possible to use an invisible IR light source to illuminate a dark area.
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