Symbol SE 923, Integration Manual

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2-9
Installation
Avoiding Scratched Windows
Scratches on the window can greatly reduce the performance of the scan engine. We suggest
you either recess the window into the housing, or apply a scratch resistance coating.
Window Material
Many window materials that look perfectly clear to the eye can contain stresses and
distortions that can reduce scan engine performance. For this reason cell-cast acrylic with
an anti-reflection coating is highly recommended. Following is a description of acrylic,
and CR-39, another popular window material. Table 2-3 outlines the suggested window
properties.
Note: Symbol recommends that polycarbonate NOT be used.
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 chemicals,
mechanical stresses, and UV light. Reasonably good impact resistance. Acrylic can be
ultrasonically welded.
CR-39
A thermal-setting plastic produced by the cell-casting process. Excellent chemical and
environmental resistance. Quite good surface hardness, and therefore does not have to be
hard-coated. But may be coated for severe environments. Reasonably good impact resistance.
Most plastic eye glasses sold today are uncoated, cell-cast CR-39. This material cannot be
ultrasonically welded.
Table 2-3. Suggested Window Properties
Material Red cell-cast acrylic.
Spectral Transmission 85% minimum from 640 to 690 nanometers.
Thickness 0.059 ± 0.005
Wavefront Distortion (transmission) 0.2 wavelengths peak-to-valley maximum over any
0.08 in. diameter within the clear aperture.
Clear Aperture To extend to within 0.04 in. of the edges all around.
Surface Quality 60-20 scratch/dig