Window Material - Motorola SE4710 Integration Manual

2 - 8 SE4710 Integration Guide

Window Material

Many window materials that look clear can contain stresses and distortions that reduce performance. For this
reason, use only cell-cast plastics or optical glass (with or without an anti reflection coating, depending on the
application). Following are descriptions of three popular window materials: PMMA, ADC (CR-39
chemically tempered glass.
Suggested Window Properties
Table 2-1
Property
Thickness
Wavefront Distortion (transmission)
Clear Aperture
Surface Quality
When using plastic materials pay extra attention to the wave front distortion recommendation specified above.
Plastic materials are not recommended for tilted windows since surface scratches cause image artifacts. Colored
windows are not recommended if motion detection mode is required since it reduces engine sensitivity to the
moving target.
Cell Cast Acrylic (ASTM: PMMA)
Cell Cast Acrylic, or Poly-methyl Methacrylic (PMMA) is fabricated by casting acrylic between two precision sheets
of glass. This material has very good optical quality, reasonably good impact resistance and low initial cost, but is
relatively soft and susceptible to attack by chemicals, mechanical stresses, and UV light. Therefore polysiloxane
coating is strongly recommended. Acrylic can be laser cut into odd shapes and ultrasonically welded.
Cell Cast ADC (ASTM: ADC)
Also known as CR-39
plastic eyeglasses sold today are uncoated, cell-cast CR-39. This material has excellent chemical and
environmental resistance, and reasonably good impact resistance. It also has quite good surface hardness, and
therefore does not have to be hard-coated, but may be coated for severe environments. This material cannot be
ultrasonically welded.
Chemically Tempered Glass
Glass is a hard material that provides excellent scratch and abrasion resistance. However, unannealed glass is
brittle. Increasing flexibility strength with minimal optical distortion requires chemical tempering. Glass cannot be
ultrasonically welded and is difficult to cut into odd shapes.
Table 2-1 outlines the suggested window properties.
Typically 0.03 - 0.06 in. (0.7 - 1.5 mm)
0.2 wavelengths peak-to-valley maximum and 0.04 λ maximum rms over
any 0.08 in. diameter within the clear aperture
To extend to within 0.04 in. of the edges all around
60-20 scratch/dig
TM
, Allyl Diglycol Carbonate (ADC) is a thermal-setting plastic produced by cell-casting. Most
Description
TM
), and