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Photonics Dictionary

APD
avalanche photodiode
APD detector
An APD detector, or avalanche photodiode detector, is a type of photodetector used to detect light or optical signals by converting incoming photons into electrical current. It operates based on the...
apertometer
An instrument designed to measure the numerical aperture of an objective.
aperture
An opening or hole through which radiation or matter may pass.
aperture card
A combination 80-column computer card containing a 35-mm microfilm frame. Reference data can be punched onto the card to facilitate retrieval of it on film.
aperture diaphragm
The second adjustable iris diaphragm in an optical system. In the common microscope condenser system, it usually is located beneath the substage condenser on a transmitted light microscope. When the...
aperture distortion
A loss of resolution or detail in a television signal caused by the size of the electron scanning beam.
aperture illumination
The amplitude, polarization and phase contained in the field distribution over the aperture.
aperture imaging
The formation of an image by a pinhole aperture that transmits radiation, such as gamma radiation.
aperture mask
Also known as a shadow mask, a perforated plate placed between the focusing and accelerating electrodes, and the tricolor phosphor screen in a color-television tube.
aperture ratio
The ratio of the lens aperture to its focal length (1/f/#).
aperture stop
A physical constraint, often a lens retainer, that limits the diameter of the axial light bundle allowed to pass through a lens.
aphelion
The point of a planet or comet's orbit that is farthest from the sun.
API
application programming interface
APL
average picture level
aplanat
Also known as aplanatic lens. A lens corrected for spherical aberration and coma.
aplanatic lens system
A system that satisfies the Abbe sine condition, and is free from spherical aberration and coma.
aplanatic points
Aplanatic points refer to specific points in an optical system where certain aberrations, such as spherical aberration or coma, are minimized or eliminated. In an optical system, aberrations are...
aplanatic surface
An aplanatic surface is an optical surface that is specifically designed or shaped to minimize spherical aberration and coma. Spherical aberration is an optical aberration that occurs when light rays...
apochromat
An apochromat, often referred to as an apochromatic lens or apochromatic objective, is a type of optical system designed to minimize chromatic aberration. Chromatic aberration occurs when different...
apochromatic system
An optical system that is corrected chromatically for three colors simultaneously.
apodization
The use of a variable transmission filter at the aperture stop of a lens to modify its diffraction pattern. Reduced transmission at the center of the aperture will favor performance at high...
apogee
The point on an elliptical orbit about the Earth that is the farthest distance from the Earth.
APOMA
American Precision Optics Manufacturers Association
apostilb
A unit of luminance equal to 1/p candela per square meter.
apparent contrast
The perceived brightness difference between light and dark areas on a target.
apparent field
The angular subtense of the field of view in the image space of a telescope, as differentiated from that in the object space (the true field).
apparent luminance
The perceived brightness of an object being viewed at some distance, especially through an optical instrument.
apparent movement
The visual perception of motion when fixed stimuli are exposed in rapid temporal and spatial succession.
apparent visual angle
The angle subtended by an object, determined by the size of the object and its distance from the viewer.
APS
active pixel sensor; active pixel system; Advanced Photon Source; American Physical Society; atom probe spectroscopy; autocorrelated photon spectroscopy
APT
automatic picture transmission
aptamer
An aptamer is a short, single-stranded DNA or RNA molecule that can bind to a specific target molecule with high affinity and specificity. Aptamers are often referred to as chemical antibodies due to...
aqueous humor
The transparent fluid that fills the portion of the eye between the cornea and the eye lens (the anterior chamber).
AR
Abbe refractometer; acoustic radiometer; acoustic resonance; active region; alpha ray; amplitude ratio; antireflective; aperture ratio; applied research; argon; aspect ratio; atomic reactor; augmented reality
Arago spot
A bright spot or point, due to Fresnel diffraction, that appears at the center of the shadow of a circular object in light originating from a point source. Also referred to as a Fresnel spot or...
arc discharge
The electric arc that is a particular discharge between two electrodes in a gas or vapor which is characterized by high cathode densities and a low voltage drop.
arc flash
An arc flash is a sudden release of electrical energy due to an electric arc — a continuous, high-current discharge of electricity between conductors or from a conductor to ground. Arc flashes...
arc light source
In present usage, especially for spectroscopic identification, an arc between electrodes that serves as a radiation source. Gas tubes that contain mercury vapor and mixtures of helium, neon and argon...
arc spectrum
The spectrum of the light produced by vaporizing an element in an electric arc.
arcuate displacement
Displacement in a direction perpendicular to that of the intended displacement, as in a translation stage with simple flexures that experiences vertical displacement during horizontal motion; it is...
ARE
activated reactive evaporation
area concentration
The ratio of aperture area over receiving area for a specific lens. Also called geometric concentration.
area image sensor -> mosaic detector array
A group of photosensors arranged in a grid-like pattern covering the entire field of view, enabling them to record it all at once (as a camera records an image on film) rather than by scanning parts...
area scan
Area scan, in the context of imaging and cameras, refers to a method of capturing an entire two-dimensional image in a single snapshot or exposure. Unlike line scan, which captures images one line at...
argon-fluoride excimer laser
An argon-fluoride (ArF) excimer laser is a type of ultraviolet laser that operates using a mixture of argon and fluorine gases. Excimer lasers are a class of gas lasers that emit light in the UV...
argon-ion laser
gas laser using ionized argon as the active medium and applying electronic excitation in order to produce the laser light
ARL
acceptable reliability level
ARM
antiradiation missile
armor
A protective jacket added to an optical fiber to facilitate use in harsh environments. Armor usually consists of steel or aluminum and is placed between the fiber and its outer jacket.

Photonics Dictionary

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