“Fixture”: layman’s term for a luminaire. See
luminaire.
“Floor lamp”: layman’s term for a torchère. See
torchère.
Fluoresce: the emission of visible light caused by
the absorption of radiation of shorter wave-
lengths followed by a nearly immediate re-
radiation at a longer wavelength.
Fluorescent lamp: a low-pressure, mercury-
vapor, electric-discharge lamp having a
phosphor coating on its inner surface that
transforms the ultraviolet energy generated
by the discharge into visible light.
Flux: seeluminous flux.
Focused light or focal glow: concentrated light
of greater intensity on a particular area or
object, compared to its background
illuminance, intentionally establishing a hier-
archy between foreground and background.
Footcandle (fc): the unit measurement of
illuminance equal to one lumen per square
foot, originally defined with reference to a
standardized candle burning at one foot from
a given surface.
Footlambert (fL): unit measurement of reflected
light from a perfectly diffusing surface that
emits or reflects one lumen per square foot;
equal to 1/πcandela per square foot; now
deprecated.
Fresnel (pronounced fra-nel/) lens: a thin, opti-
cal lens that consists of a series of concen-
tric lens sections regressed into a planar
array; in luminaires, it produces a concen-
trated beam of light while also reducing the
brightness of the source.
General diffuse: a multidirectional lighting distri-
bution produced by luminaires that deliver
both upward and downward components of
light.
Glare: the sensation produced by an extreme
luminance within the normal field of view
that is sufficiently greater than the lumi-
nance to which the eyes are adapted.
High-bay lighting: interior lighting where mount-
ing height is greater than approximately 25 ft
above the floor.
High contrast: a lighting condition characterized
by a large proportion of focused light (on an
object or the foreground) and a small
amount of diffuse light (the background).
High load: an elevated degree of psychological
stimulation or arousal, caused by activities
or environments that are complex, crowded,
asymmetrical, novel, unfamiliar, surprising,
or random.
High-pressure sodium: a high-intensity dis-
charge lamp in which light is produced by
radiation from the combined vapors of mer-
cury and sodium, with the latter dominating
the yellow-amber color.
Hue: spectral color; light of a specific wavelength.
Hydroform: a method of machine-forming metal
that consists of a draw ring, flexible die,
punch, and a pressurized forming cavity. (In
luminaires, it is used to form reflectors from
aluminum.)
Illuminance: the density of light received at a
point on a surface. Properly defined asden-
sity of flux incident on a surface measured
perpendicular to the surface, it is measured
in footcandles (fc).
Incandescence:the emission of light from an
object as a result of its being heated.
Incandescent lamp: an electric lamp in which a
filament produces light when heated to
incandescence by an electric current.
Incident: light rays falling upon or striking a sur-
face.
Included angle: formed between or within two
intersecting straight lines.
Indirect: an upward distribution of light which
produces illumination on the horizontal
workplane via reflection from the ceiling and
upper walls.
Inductive: the property of an electric circuit in
which an electromagnetic charge is induced
in it as the result of a changing magnetic flux.
Infrared: wavelengths in the region of the elec-
tromagnetic spectrum immediately above
the visible spectrum, from 770 to 10^6 nm.
Intensity: the physical energy of light emitted in a
specific direction by a source. Properly calledINTERIOR LIGHTING FOR DESIGNERS