Borosilicate

Low cost floatglass solidifies on a tin cast with a smooth surface and does not need to be polished. The other side is not equally smooth, that's why they should be used as viewport only.

Transmission range: 0.35 - 2 µm
Limited transmission in the UV-spectrum
Neutronabsorber
Extremely temperature-consistent, small thermal expansion coefficient
Good chemical resistance
Bakeable up to 350 °C (662 °F)