Beam-spread: the angular cone of light created
by the distribution of the lamp or luminaire,
in the plane of the beam axis. See alsobeam
angleandfield angle.
Borosilicate: a silicate glass containing oxide of
boron that has high heat-resistance.
Brightness: a subjective experience that occurs
in the consciousness of a human observer, a
result of the intensity of light falling on a
given region of the retina at a certain time,
the intensity of light that the retina has been
subject to in the recent past, and the intensi-
ties of light falling on other regions of the
retina. See alsoluminance.
Bulb: the outer hard, soft, or quartz glass enve-
lope of an electric lamp, which may contain a
vacuum, elemental inert gas, or metal, and a
means of light generation. Also, the lay-
man’s term for an electric lamp.
Candela (cd): the unit of luminous intensity
emitted in a specific direction by a source,
equal to one lumen per steradian.
Candlepower: a vernacular term for luminous
intensity expressed in candelas.
Candlepower distribution curve: properly called
a luminous intensity distribution curve,it
depicts the amount of luminous intensity
(expressed in candelas) generated in each
direction by a light source in a plane through
the center of the source.
Capacitive: the part of an electric circuit that
exhibits the ability to store charge.
Capacitor: an electric circuit element included in
some magnetic ballasts that consists of two
metallic plates, separated and insulated
from each other, used for storing charge
temporarily.
Cathode: one of two electron-emitting elec-
trodes hermetically sealed into a fluorescent
lamp, consisting of metal cylinders (cold
cathode) or coiled tungsten wire (hot cath-
ode), and usually coated with an electron-
emissive material.
Clerestory: a window located in the upper por-
tion of a wall that admits natural light into the
center of a room.
Coefficient of utilization: the ratio of the
number of lumens expected to reach the
work plane divided by the number of lumens
generated by the bare lamps of a specific
luminaire in a specific room.
Color constancy: knowledge of the normal color
of objects. We tend to see surface and
object colors as the same despite changes
to the color of light illuminating the surface
or object.
Color rendering index (CRI): the comparison
between the color-rendering ability of a given
light source and a reference light source,
expressed as an Rafactor on a scale of 1 to
100.
Color rendition: how surface and object colors
appear under a given light source, in com-
parison with their color appearance under a
reference light source.
Color spectrum: the continuum of color formed
when a beam of white light is dispersed (as
by a prism) so that its component wave-
lengths are arranged in order.
Color temperature: the color appearance of the
light that emanates from a source, mea-
sured in Kelvin (K). Not a measure of the sur-
face temperature of a lamp, it is the absolute
temperature of a laboratory blackbody radia-
tor when its visible radiation matches the
color of the light source.
Contrast: the relationship between the intensi-
ties of an object and its surrounding areas;
the degree of difference between light and
dark.See alsoluminance contrast.
Correlated color temperature (CCT): the color
appearance of the light that emanates from
an electric light source with a discontinuous
spectrum, measured in Kelvin (K).
Cove: a concave or canted interior corner or
molding, especially at the transition from
wall to ceiling; sometimes used to shield
light sources that distribute light across the
ceiling plane.
Current: the flow of electricity in a circuit; the rate
of flow of an electric current is measured in
amperes(amps, A).INTERIOR LIGHTING FOR DESIGNERS