Glossary - A

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

absorptance α = ①a/①I the ratio of the absorbed flux to the incident flux. See *absorption*.
Note The sum of the hemispherical reflectance, the hemispherical transmittance, and the absorptance is one.

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

Note 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.

adverse weather lamp† See *fog lamp*.

aerodrome beacon an aeronautical beacon used to indicate the location of an aerodrome.

Note An aerodrome is any defined area on land or water--including any buildings, installations, and equipment--intended to be used either wholly or in part for the arrival, departure, and movement of aircraft.

aeronautical beacon an aeronautical ground light visible at all azimuths, either continuously or intermittently, to designate a particular location on the surface of the earth. See *aerodrome beacon, airway beacon, hazard or obstruction beacon,* and *landmark beacon*.

aeronautical ground light any light specially provided as an aid to air navigation, other than a light displayed on an aircraft. See *aeronautical beacon, angle-of-approach lights, approach lights, approach- light beacon, bar (of lights), boundary lights, circling guidance lights, course light, channel lights, obstruction lights, perimeter lights, runway alignment indicator, runway end identification light, runway lights, taxi-channel lights,* and *taxiway lights*.

aeronautical light any luminous sign or signal that is specially provided as an aid to air navigation.

after image a visual response that occurs after the stimulus causing it has ceased.

aircraft aeronautical light any aeronautical light specially provided on an aircraft. See *anticollision light, ice detection light, fuselage lights, landing light, navigation light system, position lights,* and *taxi light.*

airway beacon an aeronautical beacon used to indicate a point on the airway.

alphanumeric display (digital display) an electrically operated display of letters and/or digits. Tungsten filaments, gas discharges, light-emitting diodes, liquid crystals, projected numerals, illuminated numbers, fluorescent screens, and other principles of operation can be used.

altitude (in daylighting) the angular distance of a heavenly body measured on the great circle that passes, perpendicular to the plane of the horizon, through the body and through the zenith. It is measured positively from the horizon to the zenith, from 0° to 90°.

ambient lighting lighting throughout an area that produces general illumination.

anchor light (aircraft) an aircraft light designed for use on a seaplane or amphibian to indicate its position when at anchor or moored.

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

angle-of-approach lights aeronautical ground lights arranged so as to indicate a desired angle of descent during an approach to an aerodrome runway. (Also called *optical glide path lights*.)

angle of collimation the angle subtended by a light source at a point on an irradiated surface.
angstrom,Å† a unit of wavelength equal to 10−10 m (one ten-billionth of a meter).

anticollision light a flashing aircraft aeronautical light or system of lights designed to provide a red signal throughout 360° of azimuth for the purpose of giving long-range indication of an aircraft's location to pilots of other aircraft.
aperture color† the perceived 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/n = 0.3183 cd/m2. This term is obsolete, and its use is deprecated.

approach-light beacon an aeronautical ground light placed on the extended centerline of the runway at a fixed distance from the runway threshold to provide an early indication of position during an approach to a runway.
Note The runway threshold is the beginning of the runway usable for landing.
approach lights a configuration of aeronautical ground lights located in extension of a runway or channel before the threshold to provide visual approach and landing guidance to pilots. See *angle-of- approach lights, approach-light beacon,* and *VASIS*.

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.
atmospheric transmissivity the ratio of the directly transmitted flux incident on a surface after passing through unit thickness of the atmosphere to the flux that would be incident on the same surface if the flux had passed through a vacuum.
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.
azimuth the angular distance between the vertical plane containing a given line or celestial body and the plane of the meridian.