Figure 7-5.—Overhead projector.
blower motor; the third position is the OFF
reflector mounted
position. The purpose of this arrangement is to
provide blower motor operation for cooling
after the projection lamp has been turned off. If
the heat was not removed by the blower, the pro-
jection lamp could be damaged. Also the blower
motor maintains a constant temperature in the
vicinity of the lamp when the lamp is on, pro-
longing the life of the lamp.
The following information is quite technical
but is presented to help the RP understand how
the projector works. This, in turn, may help RPs
in the operation and maintenance of the equip-
ment.
The source of illumination for the projector
is a 1,000-watt projection lamp. A concave
142.338
behind the lamp reflects the
light rays. The glass condenser lens gathers the
light rays onto a 45° mirror which deflects the
light upward toward the projection stage. Imme-
diately beneath this projection stage is the
specially designed Fresnel lens.
The Fresnel lens is used in projection equip-
ment to minimize the effects of spherical aberra-
tions (faulty focusing). One surface of the lens
consists of a number of stepped facets. These
facets are circular (similar to a phonograph
record), concentric grooves that extend from the
center of the lens to the edges. The slope of each
facet is independent of the slope of all the other
facets. These slopes are designed to provide a
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