for transparencies to be projected. The optical system is designed so
that the projected image appears on a screen behind and above the
head of the operator or behind and slightly to one side of the operator.
51. Housing and Projection Head
a. Optical System. The optical system in the housing consists of a
projection lamp, a reflector, a condenser lens, a rear surface mirror,
a Fresnel condenser lens, and a projection stage. Light from the
lamp is concentrated by the reflector and projected through the con-
denser lens to the rear surface mirror, which is installed at a 45 angle
with reference to the Fresnel condenser lens. Light is reflected from
the mirror through the Fresnel condenser lens, the plate-glass pro-
jection stage, and the transparency to be projected.
Note. The Fresnel condenser lens increases the brightness of the corners of
the projected image. It is a thin sheet of molded transparent plastic on which
the required convex thickness has been reduced by cutting concentric circular
steps into the raised surface. The resultant saw-tooth ridges bend the oblique
light rays so that they are directed towards the image field instead of cut off.
b. Blower. A blower system provides for cooling the projection
lump and housing. In Projector PH-637A/PFP, air from the blower
is directed towards the lamp by a metal duct. The blower system
consists of a motor, a fan, and a nozzle.
120-volt, 60-cycle, ac power source. On the front of the projector are
a two-way ON-OFF switch and a male receptacle connector for the
power cable. The receptacle and the switch are connected in series
with the projection lamp, and the blower motor is connected in
parallel.
Figure 25. Schematic diagram, projector PH-637(*)/PFP.
d. Roll Attachment. The roll attachment consists of a supply unit
and a take-up unit. A roll of cellophane or plastic is inserted between
rotating pivots on the supply unit, and the cellophane or plastic sheet
is unrolled over the projection stage and attached to a roll on the
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