Brightness
Specifying the direction and distribution of light in a space yields the desired
brightness contrast.
Brightness versus luminance
Brightnessis the subjective sensation that
occurs in the consciousness of a human
observer.Luminanceis the objective mea-
surement of intensity per unit of projected
area.
Direction and Distribution of Light
Aluminaire(lighting fixture) emits light in one
of three directions—downward, upward, or
multidirectional—and in one of two distribu-
tions—concentrated or diffuse (figure 3.1).
Downwardlight from a properly designed
luminaire has a restricted angular spread;
direct glare is prevented by both this
restricted spread and the shape of the human
eyebrow.Upwardlight usually covers a large
area of the ceiling; the light reflected from the
ceiling is of low luminance and is unlikely to
cause distracting glare.Multidirectionallight
is emitted in all directions, but it cannot emit
much of its output sideways without causing
objectionable glare.
Upward and downward light is emitted in
patterns that vary from narrow to wide.Con-
centrated distribution focuses light in a
narrow pattern;diffusedistribution disperses
light in a wide pattern.
Luminaires with narrowbeam-spreads
that lack an upward component of light pro-
duce aconcentrated downward(also called
direct) distribution (figure 3.2). When
located in low ceilings, concentrated down-
ward beams—with spreads of 30° or less—
create areas of high luminance on the floor
with dark areas in between. To avoid this
unevenness, luminaires would need to be
placed inordinately close to each other. Low
ceilings require the use of diffuse downward
luminaires.
When located in high ceilings, concen-
trated downward beams overlap and avoid
such light and dark areas, yet only horizontal
surfaces and the tops of objects are lighted;
faces and walls receive little light and appear
in shadow. This yields a high-contrast space,
one of low ambient brightness with high
brightness accents (figure 3.3).
Luminaires with diffuse beam-spreads
and a downward distribution producediffuse
downward(direct) light (figure 3.4). Diffuse
downward beams—with spreads from 80° to