• SM72 Integration Guide
Window Material
Window Material

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).
The following are descriptions of three popular window materials: PMMA, ADC (CR-39TM), and chemically tempered glass. The following table outlines the suggested window properties.
Suggested Window Properties
Property
Description
Thickness
Typically 0.03 - 0.06 in. (0.7 - 1.5 mm)
Wavefront distortion (transmission)
0.2 wavelengths peak-to-valley maximum and 0.04 maximum rms 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
When using plastic materials, pay extra attention to the wavefront distortion recommendation specified above. Colored windows are not recommended if motion detection mode is required since it reduces engine sensitivity to the moving target.
The use of a tilted window will likely have a negative impact on scan performance.

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
TM
, Allyl Diglycol Carbonate (ADC) is a thermal-setting plastic produced by cell-casting. Most 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.