Windows are optical glass with ground and polished faces that are relatively parallel.They are used to isolate two physical environments while allowing light to pass.Windows made of BK7 material have high transmission in the visible spectral region, do not scratch easily, do not degrade under normal operating environment and hence do not need any special precaution or treatment. Anti-Reflective Coating will provide higher transmission.
When selecting windows, you should consider the following: material,transmission,scattering,wave front distortion,parallelism and resistance to certain environment. 
China Star Optics offers all types of windows made from different materials. Such as N-BK7, Fused silica, sapphire,CaF2, Silicon, Borofloat, high borosilicate glass and other optical glass supplied by Schott and Chinese CDGM are available upon request. 
Windows at shape of  round,quadrate, triangular or other polygonal are also available upon request
.In addition, we provide a variety of  single layer or multiplayer anti-reflecting coating,HR,PR coatings on optical windows.  

Specification of our optical windows as follow:
*Material:high borosilicate glass, BK7,UV-grade fused silica (JGS1) or other optical crystals materials
*Length tolerance:+/-0.01
*Width tolerance:+/-0.01
*Center thickness tolerance:0.005mm
*Surface quality:80-50 to 10-5 Scratch/Dig
*Surface flatness:<lambda to lambda/20
*Parallelism<=1min -- 2sec
*Clear Apenture:80% -- 100%