The sapphire step window is a double-plane circular or rectangular optical sapphire glass, which has a step surface to be used with flanges or other concealed installations, and is usually used as a protective window for sensors and detectors. The windows do not change the magnification of the system.
Uncoated sapphire has excellent surface hardness and transmittance extending from the UV to the mid-infrared wavelength region. Sapphire can only be scratched by a few substances other than it. Uncoated substrates are chemically inert and insoluble in water, common acids or bases at temperatures up to about 1000°C.