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 This term is obsolete, and its use is deprecated.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.
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