The optical principle of a spherical
reflector
is simple: When a
light source
is placed at the center of a sphere, i.e. at its focal point, each ray of light hits the
reflector
at an angle of 90° to the surface of the
reflector
at that point. Thus, both the angle of incidence and the angle of
reflection
are 0°, and the ray is reflected back on the same course, passing once more through the center of the sphere .
Some theatre luminaires use spherical reflectors to increase the efficiency of the luminaire by collecting light from the
lamp
which would otherwise have been directed to the back of the instrument and never reached the stage. A spherical
reflector
can increase the efficiency of the luminaire by as much as 40%.
Several conditions are necessary for a spherical
reflector
to be effective:
The centers of the reflector, of the filament and of the
lens
must be aligned on the
optical axis
of the luminaire.
The distance from the center of the filament to the
reflector
must be equivalent to the radius of the sphere.
The
reflector
must be large enough to collect light rays from as large a part of the circumference as possible.
Spherical reflectors are found in most focusable
lens
spotlights and are also used in some types of profile spots.
