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or without an anti reflection coating, depending on the application). Following are
descriptions of three popular window materials: PMMA, ADC (CR-39
tempered float glass.
Material
Thickness
Wavefront Distortion (transmission)
Clear Aperture
Surface Quality
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
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 Float 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.
Table 2-1. Suggested Window Properties
TM
, Allyl Diglycol Carbonate (ADC) is a thermal-setting plastic
Clear cell-cast acrylic
0.06 in. (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
Installation
TM
), and chemically
2-5
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