Figure 220. Overhead projector
a. Overhead Projector. You would use an overhead projector to
project colored or opaque images from a transparency or viewgraph. An
advantage of the overhead projector is you can use it in a room without
turning off all the lights (when shielding the screen from direct light
sources). Because of the relatively large size of the viewgraph, the
speaker has the capability of working directly on the viewgraph while
talking. You can use this type of projector for front and rear
projection.
(1) Parts of an overhead projector. An overhead projector weighs
about 26 pounds, has a 12inch triplet lens system with a front surface
mirror, and a 10 1/2 inch aperture.
(a) The older projectors have a threeposition switch that
controls the operation of the blower and lamp. In the first position,
the switch only provides power to the blower for cooling the lamp after
operation. If you do not properly cool the lamp after operation, the
excess heat damages the lamp. In the second position, the switch
provides power to both the lamp and the blower.
(b) The newer projectors have an onoff switch and a
thermostatic switch that controls the blower operation. In the ON
position, the switch provides power to the lamp. As long you have the
projector connected to an electrical outlet, the thermostatic switch
controls the blower operation. When the heat around the lamp reaches a
predetermined point, the
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