15
ensure that no reflections back into the lens.
Window Material and Color
Many window materials include stresses and distortions, which leads to poor performance. So use only cell-cast plastics or
optical glass. There are three common window materials, chemically tempered glass, PMMA and ADC. Below are the
recommended window features.
Table 2-3
Feature
Description
Thickness
Generally 0.8-2.0mm
Wavefront Distortion
PV m
aximum: 0.2λ
RMS maximum: 0.04λ
Clear Aperture
To extend the area within 1.0mm
Surface
60-20 scratch/dig
Consider the wavefront distortion when using plastic materials.
Plastic materials are not recommended if the window is designed as tilted, because scratches will reduce the performance.
If the motion detection mode is needed, colored windows are not recommended as well.
Wavelength of the illumination and aiming light should be taken into consideration when choosing window material and
color, in order to achieve the possible highest spectral transmission, lowest haze level and homogeneous refractive index.
It is suggested to use PMMA or optical glass with spectral transmittance of red light over 90% and haze less than 1%.
Whether to use an anti-reflection coating or not depends on the material and application needs.
PMMA
PMMA is made by casting acrylic between two sheets of glass. It obtains the advtanges of high quality, good impact
resistance and low cost, while it is soft and brittle. Polysiloxane coating is recommended to avoid attack from stresses.
Acrylic is enabled to be cut into kinds of shapes and welded ultrasonically.
ADC
ADC obtain good chemical and environmental resistance and impact resistance. Because of its hardness, coating is not
required except some terrible circumstances. Ultrasonical welding is not allowed..
