![Gandhi Commons](https://upload.wikimedia.org/wikipedia/commons/3/3a/Gandhi_Commons.jpg)
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 those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
wavelengths while passing
visible
Visibility, in meteorology, is a measure of the distance at which an object or light can be seen.
Visibility may also refer to:
* A measure of turbidity in water quality control
* Interferometric visibility, which quantifies interference contrast ...
light. They are often used in devices with bright
incandescent light bulb
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxida ...
s (such as
slide
Slide or Slides may refer to:
Places
* Slide, California, former name of Fortuna, California
Arts, entertainment, and media Music Albums
* ''Slide'' (Lisa Germano album), 1998
* ''Slide'' (George Clanton album), 2018
*''Slide'', by Patrick Glees ...
and
overhead projector
An overhead projector (often abbreviated to OHP), like a film or slide projector, uses light to project an enlarged image on a screen, allowing the view of a small document or picture to be shared with a large audience.
In the overhead project ...
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 metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
) video cameras to block IR due to the high sensitivity of many camera
sensors
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
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, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s.
IR transmitting/passing filters in photography
![IR filters 01 Pengo](https://upload.wikimedia.org/wikipedia/commons/6/62/IR_filters_01_Pengo.jpg)
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, ultraviol ...
where the name of the filter denotes the wavelengths that are blocked, and in line with the convention for
air filter
A particulate air filter is a device composed of fibrous, or porous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. Filters containing an adsorbent or catalyst such as charcoal (carbon) may als ...
s and
oil filter
An oil filter is a filter designed to remove contaminants from engine oil, transmission oil, lubricating oil, or hydraulic oil. Their chief use is in internal-combustion engines for motor vehicles (both on- and off-road ), powered aircraft, r ...
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 gl ...
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
Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
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 metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
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
''Top:'' tree photographed in the near infrared range. ''Bottom:'' same tree in the visible part of the spectrum.
In infrared photography, the film or image sensor used is sensitive to infrared light. The part of the spectrum used is r ...
to ''pass'' infrared light and ''block''
visible
Visibility, in meteorology, is a measure of the distance at which an object or light can be seen.
Visibility may also refer to:
* A measure of turbidity in water quality control
* Interferometric visibility, which quantifies interference contrast ...
and
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
light. Such filters appear black to the eye, but are transparent when viewed with an IR sensitive device.
![Auditorio,Obelisco,Marquize](https://upload.wikimedia.org/wikipedia/commons/9/9f/Auditorio%2CObelisco%2CMarquize.jpg)
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 1840, 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 emissio ...
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 filter
An anti-aliasing filter (AAF) is a filter used before a signal sampler to restrict the bandwidth of a signal to satisfy the Nyquist–Shannon sampling theorem over the band of interest. Since the theorem states that unambiguous reconstruction ...
s.
See also
*
UV filter
UV filters are compounds, mixtures, or materials that block or absorb ultraviolet (UV) light. One of the major applications of UV filters is their use as sunscreens to protect skin from sunburn and other sun/UV related damage. After the inventio ...
*
Cold mirror
A cold mirror is a specialized dielectric mirror, a dichroic filter, that reflects the entire visible light spectrum while very efficiently transmitting infrared wavelengths. Similar to hot mirrors, cold mirrors can be designed for an incidence ...
and
Hot mirror
A hot mirror is a specialized dielectric mirror, a dichroic filter, often employed to protect optical systems by reflecting infrared light back into a light source, while allowing visible light to pass. Hot mirrors can be designed to be inserte ...
*
Interference filter
An interference filter or dichroic filter is an optical filter that reflects one or more spectral bands or lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest. An interference filte ...
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Optical filters