Optical filter

Optical filters selectively transmit light in a particular range of wavelengths, that is, colours, while absorbing the remainder.

The passband may be narrower or wider; the transition or cutoff between maximal and minimal transmission can be sharp or gradual.

The widest range of color-selection is now available as colored-film filters, originally made from animal gelatin but now usually a thermoplastic such as acetate, acrylic, polycarbonate, or polyester depending upon the application.

There are now many absorptive filters made from glass to which various inorganic or organic compounds[citation needed] have been added.

Colored glass optical filters, although harder to make to precise transmittance specifications, are more durable and stable once manufactured.

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.

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.

Monochromatic filters only allow a narrow range of wavelengths (essentially a single color) to pass.

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 visible to people, for example making images of distant mountains appear unnaturally hazy.

[4] 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.

Alternatively, the photographer might want to use a larger aperture (so as to limit the depth of field); adding an ND filter permits this.

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).

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).

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.

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

An arc source puts out visible, infrared and ultraviolet light that may be harmful to human eyes.

Therefore, optical filters on welding helmets must meet ANSI Z87:1 (a safety glasses specification) in order to protect human vision.