Interior Lighting for Designers

Specular reflection is a primary tech-
nique for modifying and controlling the direc-
tion and distribution emitted by a light
source. Specular reflection takes light that
would otherwise be lost or wasted within a
luminaire or emitted at glaring angles and
conserves it by redirecting the light into a
room or onto a surface at useful angles.
In addition to this more efficient use of
light, the application of properly contoured
reflectors produces predictable luminaire
distributions and controlled room brightness
patterns. Specular reflector contours com-
monly used in luminaires include actual or
modified ellipses, parabolas, and circles.

Elliptical contour
Ellipses have two focal points; a ray of light
originating at one focal point is reflected
through the second focus. This produces a
divergent beam; its spread depends on the
distance between the two foci (figure 9.4).
Most downlights use reflectors with exact or
modified elliptical shapes. A special use of
this contour produces a beam that passes
through a small, inconspicuous opening
flush with the ceiling plane.

Parabolic contour

The parabola is a special form of the ellipse,

in which the two foci are far apart. A ray of

light originating at the exact focal point of a

perfectly shaped parabolic contour is redi-

rected in a direction parallel to the axis of the

reflector, producing a beam of parallel rays

(figure 9.5). The filament or source is never

an actual point; this results in a degree of

beam-spread that depends on the size of the

source and the diameter of the reflector.

In its pure form, the parabolic contour is

used for searchlights, spotlights, and direc-

tional equipment where a concentrated beam

and a limited spread of light are desired. The

beam is often given additional spread by

passing it through a diffusing or refracting

lens, as is done with many reflector lamps.

Circular contour

The circle is also a special form of the

ellipse, one where both foci are coincident; it

is the opposite of the parabola. A ray of light

originating at the focal point of a circular

contour is reflected back through the same

point (figure 9.6). It is used separately or in

combination reflectors called compound

LIGHT CONTROL

Figure 9.5Parabolic contour. Figure 9.6Circular contour.