Light Control
Directional sources, commonly called reflector lamps, such as AR, MR, PAR,
and R lamps, have built-in optical systems. All other electric light sources
require external devices to modify their distributions in order to be useful in
architectural applications.
These modifications have two purposes: (1)
to direct light to where it is wanted and (2) to
block light from where it is unwanted—to
shield the lamp from viewing angles that
would otherwise cause glare. The control of
light direction is accomplished by three meth-
ods: reflection, transmission, and refraction.
Reflection
Reflectionis the return of light from a sur-
face; it occurs when a portion of the light fall-
ing on the surface is thrown back by that
surface just as a ball bounces back from the
floor. Three kinds of reflection are involved in
the control of light: specular, semi-specular,
and diffuse.
Specular reflection
A smooth, highly polished surface, such as a
mirror, alters the direction of a beam of light
without changing its form. The angle of
reflection is equal to the angle of inci-
dence—a property that makes specular
materials ideal where precise beam control
is desired (figure 9.1).
Becausespecularsurfaces are virtually
mirrors, their own surfaces are almost invisi-
ble; they may appear dark or bright, depend-
ing on the observer’s position and on the
luminance of the reflected image.Semi-specular (spread) reflection
Irregular surfaces, such as those that are cor-
rugated, hammered, brushed, sandblasted,
or etched, partially disperse or “spread” the
reflected beam. The greatest intensity, how-
ever, is still reflected at an angle near the
angle of incidence (figure 9.2).
Semi-specular materials appear with
highlights or streaks of higher brightness on
a background of lower brightness. In interi-
ors, they are often used as elements of spar-
kle. In luminaires, semi-specular materials
produce a moderately controlled beam that
is smooth and free from striations.Diffuse reflection
Rough or matte surfaces neutralize the
directional nature of the incident beam.