Meller Optics, Inc. has introduced custom fabricated sapphire viewports and sight windows for builders of UHV systems and process machinery that permit viewing in hostile environments.
Meller Sapphire Viewport Windows are suitable for applications from UHV to 10,000 psi and from cryogenic to 1,000°C, depending upon configuration.
Meller Sapphire Viewport Windows provide Moh 9 hardness which is second only to diamond and are suitable for applications from UHV to 10,000 psi and from cryogenic to 1,000°C, depending upon configuration. Manufactured to precise customer specifications, these sight windows resist chemicals including fluorine to 300°C, most acids, and are impervious to fast moving water and particulates such as blowing dirt, and sand.
Available in a variety of configurations and sizes from 0.25" to 10" dia., and 0.5mm to 1" thick, Meller Sapphire Viewport Windows can be custom fabricated with stepped edges and elliptical edge shaping, holes, slots, and wedges for mounting, and include A/R coatings. They exhibit flatness to 0.5 fringes HeNe and parallelism from 20 to 2 arc/secs.
Source: http://www.melleroptics.com/