Filter (optics)

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Coloured and Neutral Density filters

Optical filters, generally, belong to one of two categories. The simplest, physically, is the absorptive filter, while the latter category, that of interference or dichroic filters, can be quite complex. Optical filters selectively transmits light having certain properties (often, a particular range of wavelengths, that is, range of colours of light), while blocking the remainder. They are commonly used in photography, in many optical instruments, and to colour stage lighting. In astronomy, optical filters can be used to eliminate light from the Sun or from a star much brighter than the target object^[1]. Even seeing Mercury can necessitate using optical filters from extreme northern latitudes like Scandinavia or Alaska where the sky is invariably bright whenever the planet is above the horizon due to the low angle at far elongations^[2].

[edit] Absorptive

Absorptive filters are usually made from glass to which various inorganic or organic compounds have been added. These compounds absorb some wavelengths of light while transmitting others. The compounds can also be added to plastic (often polycarbonate or acrylic) to produce gel filters, which are lighter and cheaper than glass-based filters.

[edit] Dichroic filter

Alternately, dichroic filters (also called "reflective" or "thin film" or "interference" filters) can be made by coating a glass substrate with a series of optical coatings. Dichroic filters usually reflect the unwanted portion of the light and transmit the remainder.

Dichroic filters use the principle of interference. Their layers form a sequential series of reflective cavities that resonate with the desired wave lengths. Other wavelengths destructively cancel or reflect as the peaks and troughs of the waves overlap.

Dichroic filters are particularly suited for precise scientific work, since their exact colour range can be controlled by the thickness and sequence of the coatings. They are usually much more expensive and delicate than absorption filters.

They can be used in devices such as the dichroic prism of a camera to separate a beam of light into different coloured components.

The basic scientific instrument of this type is a Fabry-Pérot interferometer. It uses two mirrors to establish a resonating cavity. It passes wavelengths that are a multiple of the cavity's resonance frequency.

Etalons are another variation: transparent cubes or fibers whose polished ends form mirrors tuned to resonate with specific wavelengths. These are often used to separate channels in telecommunications networks that use wavelength division multiplexing on long-haul optic fibers.

[edit] Monochromatic

Monochromatic filters only allow a narrow range of wavelengths (that is, a single colour) to pass.

[edit] Infrared

Infrared (IR) or heat-absorbing filters are designed to block or reflect mid-infrared wavelengths but pass 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 video cameras to block IR due to the high sensitivity of many camera sensors to near-infrared light.

[edit] Ultraviolet

Advanced Image Quality Space-Qualified Ultraviolet Interference Filter (Courtesy-NASA/JPL/Caltech)

Ultraviolet (UV) filters block ultraviolet radiation, but let visible light through. Because photographic film and digital sensors are sensitive to ultraviolet (which is abundant in skylight) but the human eye is not, such light would, if not filtered out, make photographs look different from the scene that the photographer saw. This causes images of distant mountains to appear hazy. By attaching a filter to remove ultraviolet, photographers can produce pictures that more closely resemble the scene as seen by a human eye.

[edit] Neutral density

Neutral density (ND) filters have a constant attenuation across the range of visible wavelengths, and are used to reduce the intensity of light by reflecting or absorbing a portion of it. They are specified by the optical density (OD) of the filter, which is the negative of the common logarithm of the transmission coefficient. They are useful for making photographic exposures longer. A practical example is making a waterfall look blurry when it's photographed in bright light. Alternatively, the photographer might want to use a larger aperture (so as to limit the depth of field); adding an ND filter permits this. ND filters can be reflective (in which case they look like partially-reflective mirrors) or absorptive (appearing grey or black).

[edit] Longpass

A longpass (LP) Filter is an optical interference or coloured glass filter that attenuates shorter wavelengths and transmits (passes) longer wavelengths over the active range of the target spectrum (ultraviolet, visible, or infrared). Longpass filters, which can have a very sharp slope (referred to as edge filters), are described by the cut-on wavelength at 50 percent of peak transmission. In fluorescence microscopy, longpass filters are frequently utilized in dichroic mirrors and barrier (emission) filters. Use of the older term of highpass to describe longpass filters is now discouraged because it more accurately refers to frequency rather than wavelength.

[edit] Guided-mode resonance filters

A relatively new class of filters introduced around 1990. These filters are normally filters in reflection, that is they are notch filters in transmition. They consist in their most basic form of a substrate waveguide and a subwavelength grating or 2D hole array. Such filters are normally transparent, but when a leaky guided mode of the waveguide is excited they become highly reflective (A record of over 99% experimentally) for a particular polarization, angular orientations, and wavelength range. The parameters of the filters are designed by proper choice of the grating parameters. The advantage of such filters are the few layers needed for ultra-narrow bandwidth filters (in contrast to dichroic filters), and the potential decoupling between spectral bandwidth and angular tolerance when more than 1 mode is excited.

[edit] Shortpass

A shortpass (SP) Filter is an optical interference or coloured glass filter that attenuates longer wavelengths and transmits (passes) shorter wavelengths over the active range of the target spectrum (usually the ultraviolet and visible region). In fluorescence microscopy, shortpass filters are frequently employed in dichromatic mirrors and excitation filters.

[edit] Bandpass

Combining an LP filter and an SP filter produces a Bandpass (BP) filter. These filters usually have lower transmittance values than SP and LP filters, and block all wavelengths outside of a selected interval, which can be wide or narrow, depending on the number of layers of the filter.

[edit] Polarizer

Another kind of optical filter is a polarizer or polarization filter, which blocks or transmits light according to its polarization. They are often made of materials such as Polaroid and are used for sunglasses and photography. Reflections, especially from water and wet road surfaces, are partially polarized, and polarized sunglasses will block some of this reflected light, allowing an angler to better view below the surface of the water and better vision for a driver. Light from a clear blue sky is also polarized, and adjustable filters are used in colour photography to darken the appearance of the sky without introducing colours to other objects, and in both colour and black-and-white photography to control specular reflections from objects and water.

Polarized filters are also used to view certain types of stereograms, so that each eye will see a distinct image from a single source.

[edit] See also

• Filter (photography)
• Filter (signal processing)
• Filter fluorometer
• Lyot filter
• Dichroic filter
• Dichroic prism
• Atomic line filter
• Warm filter
• Machine vision optical filters

[edit] References

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