Infrared cut-off filter

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Infrared cut-off filters, sometimes called IR filters or heat-absorbing filters, are designed to reflect or block mid-infrared wavelengths while passing visible light. They are often used in devices with bright incandescent light bulbs (such as slide and overhead projectors) to prevent unwanted heating. There are also filters which are used in solid state (CCD or CMOS) video cameras to block IR due to the high sensitivity of many camera sensors to near-infrared light. These filters typically have a blue hue to them as they also sometimes block some of the light from the longer red wavelengths.

IR transmitting/passing filters in photography

IR transmitting filters, used in photography.

In contrast to the naming convention of filters where the name of the filter denotes the wavelengths that are blocked, photographic filters are counter-intuitively named after the color of light they pass. This is likely due to the fact that they do not attenuate the blocked wavelengths as strongly. A blue filter marginally allows more light in the blue wavelength to pass resulting in a slight shift of the color temperature of the photo to a cooler color. Because of this, the term "IR filters" is commonly used to refer to filters that pass infrared light while completely blocking other wavelengths. However, in some applications the term "IR filter" still can be used as a synonym of infrared cut-off filter.

IR transmitting (passing) filters are commonly used in infrared photography to pass infrared light and block visible and ultraviolet light. Dark black sections of processed film works to pass infrared light and is commonly used as a cheap alternative to expensive glass filters.

See also

• UV filter
• Cold mirror and Hot mirror
• Interference filter

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