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Chapter 2—Functional Descriptions
2-76 HJT Model 330, 340SC, and 370SC Service Manual
Optical Subassemblies
Cold Mirror
The Cold Mirror lets most of the infrared light pass through and reflects the rest of
the light toward the prism through an ultraviolet filter/condensing lens, dichroic
mirrors and down-steering mirrors.
The infrared light is absorbed in a series of fan-cooled screens.
CAUTION!
Cold mirrors absorb IR light and can get very
hot!
Ultraviolet Filter and Condensing Lens
The arc lamp light beam reflected off the cold mirror passes through the
Ultraviolet Filter/Condensing Lens that removes most of the Ultraviolet light and
condenses the light beam. Therefore, most of the infrared and ultraviolet light is
filtered out before the beam enters the more sensitive portions of the optics,
leaving only the visible portion. Without these filters, the infrared light would
overheat the prisms and the ILA
®
assemblies, and the ultraviolet light would
damage the ILA
®
assemblies and be hazardous to personnel.
Dichroic Mirrors and Down-Steering Mirrors
The condensed light beam strikes the first dichroic mirror that is designed to pass
red and green light but reflect blue light. The blue light is reflected to a down-
steering mirror which reflects it again directly to the prism in the blue system. The
red and green light travel on to the next dichroic mirror that passes the red light
and reflects the green light to the down-steering mirror and prism in the green
system. The red light travels on to the last Dichroic mirror which reflects the
remaining red light to the last down-steering mirror and prism in the red system.
Each of these three (3) light beams independently combines with the video image
in their own (red, green or blue) color systems at the ILA
®
assemblies as described
above.
CAUTION!
Do not attempt to realign any mirrors. They require
the use of a complex laser beam alignment fixture.
2.8 Image Light Amplifier
A closer examination of the output side of the ILA
®
assembly as illustrated in
Figure 2-20 helps in understanding its operation.
Visible light from the arc lamp passes through dichroic mirror assemblies and is
then reflected into a prism assembly that polarizes the light horizontally. The