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Optics
The EM27 uses a sophisticated optical system. An improperly designed internal housing or improper selection of window
material can degrade the engine's performance.
Window Placement
The window should be positioned properly to let the illumination and aiming beams pass through as much as possible and
no reflections back into the engine (reflections can degrade the reading performance).
The window should be mounted directly against the front of the engine (a=0).
Window Material and Color
Window material must be clear. Use only cell-cast plastics or optical glass. PMMA and chemically tempered glass are
recommended. Window material selected for the engine should meet or exceed the specifications specified in Table 2-2.
When using a clear plastic window, it is recommended to apply anti-reflection (AR) coating on it.
• PMMA (Cell-cast acrylic): When fabricated by cell-casting, has very good optical quality and low initial cost, but surface
must be protected from the environment due to its susceptibility to attack by chemcials, mechanical stresses, and UV
light. Reasonably good impact resistance.
• Chemically tempered glass: Glass is a hard material which provides excellent scratch and abrasion resistance. But
unannealed glass is brittle. Increased flexibility strength with minimal optical distortion requires chemical tempering.
Glass is hard to be cut into odd shapes and can not be ultrasonically welded.
Figure 2-2
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