Glossary - A

A 

absolute luminance threshold luminance threshold for a bright object like a disk on a totally dark background. 

absorption a general term for the process by which incident flux is converted to another form of energy, usually and ultimately to heat.  All of the incident flux is accounted for by the processes of reflection, transmission, and absorption. 

accent lighting directional lighting to emphasize a particular object or surface feature or to draw attention to a part of the field of view. See directional lighting. 

acceptance half-angle† the acceptance half-angle is set by the refractive indices of the core and cladding of fiber optics, and it is expressed in terms of the numerical aperture (NA). The numerical aperture is given as NA = n0 sin p.† 

accommodation the process by which the eye changes focus from one distance to another. 

actinic a term meaning photochemically active. 

action spectrum the quantitative actinic response of a chemical or biological substance or living organism as a function of an appropriate spectral parameter such as wavelength or photon energy. 

adaptation the process by which the retina becomes accustomed to more or less light than it was exposed to during an immediately preceding period. It results in a change in the sensitivity to light. See scotopic vision, photopic vision, and chromatic adaptation. 

Note Adaptation is also used to refer to the final state of the process, such as reaching a condition of adaptation to a specific luminance level. 

adaptive color shift the change in the perceived object color caused solely by change of the state of chromatic adaptation. 

ambient lighting lighting throughout an area that produces general illumination.  

ampere† the intensity of electrical current flow. The symbol often used in equations is I, although A is also acceptable. 

angstrom,ņ a unit of wavelength equal to 10−10 m (one ten-billionth of a meter).

aperture color† the perc0eived color of the sky or of a patch seen through an aperture and not identifiable as belonging to a specific object. 

apostilb (asb) a lambertian unit of luminance equal to 1/π = 0.3183 cd/m2. This term is obsolete, and its use is deprecated. 

arc discharge an electric discharge characterized by high cathode current densities and a low voltage drop at the cathode. 

Note The cathode voltage drop is small compared with that in a glow discharge, and secondary emission plays only a small part in electron emission from the cathode. 

arc lamp a discharge lamp in which the light is emitted by an arc discharge or by its electrodes. 

Note The electrodes can be either of carbon (operating in air) or of metal. 

artificial pupil a device or arrangement for confining the light passing through the pupil of the eye to an area smaller than the natural pupil. 

average luminance luminance is a property of a geometric ray. Luminance as measured by conventional meters is averaged with respect to two independent variables, area and solid angle; both must be defined for a complete description of a luminance measurement.