Cutoff: measured up fromnadir, the angle of the
first line of sight at which the bare light
source is not visible.
Daylighting: the illumination of indoor spaces by
natural light.
Dichroic: the property of transmitting certain
wavelengths of light while reflecting those
not transmitted, usually with little absorp-
tion.
Diffraction grating: a glass or polished metal
surface that has a large number of very fine
parallel grooves or slits, used to change the
directions and intensities of a group of wave-
lengths of reflected or transmitted light.
Diffuse light: a distribution of light that is dis-
persed in a wide pattern and not incident
from any particular direction.
Diffuser: a glass or plastic material that dis-
perses light from a source in all directions,
eliminating the directional quality of the
beam; in fluorescent downlights, used to
redirect light from the glare zone down
toward work surfaces.
Dimmer: a device that provides variation in the
intensity of a luminaire by controlling the
voltage or current available.
Direct glare: excessive light misdirected toward
the eye.It refers to glare from the direct view
of luminaries or bare lamps. See also
reflected glare.
Direct/indirect: light emitted in a downward and
upward direction, with little or no light emit-
ted at angles near the horizontal.
Direct (light): a distribution of light emitted in a
downward direction.
Downlight: a recessed, surface-mounted, or
pendant-mounted luminaire which emits
light in a downward direction with no upward
component of light.
Downlighting: a distribution of light emitted in a
downward direction.
Discharge source: a lamp that produces light by
the passage of an electric current through a
vapor or gas, rather than through a tungsten
wire as in incandescent lamps. (These
include cold-cathode, fluorescent, high-
pressure sodium, low-pressure sodium, mer-
cury vapor, metal halide, and neon lamps.)
Efficacy: the ratio of lumens produced to elec-
tricity consumed when referring to a light
source, expressed in lumens per watt.
Efficiency: the ratio of lumens emitted by a
luminaire to that emitted by the lamp(s) con-
tained within it, expressed as a percentage.
Ellipsoidal: in the shape of an ellipse, which has
two focal points. In a reflecting contour, a ray
of light originating at one focal point is
reflected through the second focus.
Emissive material: in electric lamps, a sub-
stance that discharges electrons, usually
applied as a coating to the cathode of a dis-
charge source.
Energy-effective: when referring to electric
lighting, using the available watts to supply
light where it is needed and when it is
needed, and limit light from where it is
unwanted.
Exitance: the total quantity of light emitted,
reflected, or transmitted in all directions
from a surface. Properly defined asdensity of
flux leaving a surface, it is measured in
lumens per square foot (lm/ft^2 ) or lumens per
square meter (lm/m^2 ).
Fenestration: The arrangement, proportioning,
and design of windows and doors in a build-
ing for the admission of daylight.
Fiber optics: thin, flexible fibers of glass or plas-
tic that are enclosed by a material of a lower
index of refraction, transmitting light
throughout their length by internal reflection.
Field angle: the angle between the two direc-
tions for which the intensity is 10% of the
maximum, as measured in a plane through
the beam centerline. See alsobeam angle.
Filament: the fine tungsten wire in an electric
lamp, which acts as a conductor and
becomes incandescent by the passage of an
electric current.
Filter: a transparent material that modifies the
color or quantity of light by transmission or
reflection.GLOSSARY