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Infrared cut-off filters, sometimes called IR filters or heat-absorbing filters, are designed to reflect or block near-
infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
wavelengths while passing visible light. They are often used in devices with bright
incandescent light bulb An incandescent light bulb, also known as an incandescent lamp or incandescent light globe, is an electric light that produces illumination by Joule heating a #Filament, filament until it incandescence, glows. The filament is enclosed in a ...
s (such as slide and
overhead projector An overhead projector (often abbreviated to OHP), like a Movie projector, film or slide projector, uses light to Projector, project an enlarged image on a Projection screen, screen, allowing the view of a small document or picture to be shared ...
s) to prevent unwanted heating. There are also filters which are used in solid state ( CCD or
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss ", , ) is a type of MOSFET, metal–oxide–semiconductor field-effect transistor (MOSFET) semiconductor device fabrication, fabrication process that uses complementary an ...
) video cameras to block IR due to the high sensitivity of many camera
sensors A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
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
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s.


IR transmitting/passing filters in photography

In contrast to the naming convention of
optical filters Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
where the name of the filter denotes the wavelengths that are blocked, and in line with the convention for air filters and
oil filter An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) and lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsatur ...
s,
photographic filter In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a ...
s are named for the color of light they ''pass''. Thus a blue filter makes the picture look blue. 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. Unlike the eye, sensors based on silicon (including CCDs and
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss ", , ) is a type of MOSFET, metal–oxide–semiconductor field-effect transistor (MOSFET) semiconductor device fabrication, fabrication process that uses complementary an ...
sensors) have sensitivities extending into the near-infrared. Such sensors may extend to 1000 nm. Digital cameras are usually equipped with IR-blocking filters to prevent unnatural-looking images. IR-transmitting (passing) filters, or removal of factory IR-blocking filters, are commonly used in infrared photography to ''pass'' infrared light and ''block'' visible and
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
light. Such filters appear black to the eye, but are transparent when viewed with an IR sensitive device. Since the dyes in processed film block various part of visible light but are all fairly transparent to infrared, dark black sections of any processed film (where all visible colors are blocked) pass only infrared light and are commonly used (layering one over another if necessary for better visual light filtering) as a cheap alternative to expensive glass-backed filters. Such filters can be used both over color camera lenses, and to filter visible light from IR illumination sources. Such filter stock is most easily made available most simply by having any commercial color negative film developed after being fully exposed to light. The leaders of 35mm film are ideal for this, without wasting an entire roll of film. (Some special communication may be necessary in such submission, to ensure that all of the "black" negative film thus produced is indeed returned, and that there is no need to print the color-negative results on photographic paper). In the same way, visually opaque "black" color-positive film emulsions mounted in cardboard, as for routine slide projection, provide inexpensive cardboard-mounted infrared filters. Film sizes larger than 35 mm may be handled in the same way for larger filter production. For
astrophotography Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1839, but it was no ...
, many photogenic targets (such as
emission nebula An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. Among the several different types of emission n ...
e) are bright in the far red and near infrared. Removal of factory filters increases sensitivity to such targets, and may also increase sharpness, as such filters may also include anti-aliasing filters.


See also

* UV filter * Cold mirror and Hot mirror * Interference filter {{DEFAULTSORT:Infrared Cut-Off Filter Optical filters