Window (optics)

Not to be confused with Optical window.
A flatness test of a float-glass optical window on top of a λ/10 optical flat. The window measures a flatness of 4–6 λ per inch.

In optics, a window is an optical element that is transparent to a range of wavelengths, and that has no optical power. Windows may be flat or curved. They are used to block the flow of air or other fluids while allowing light to pass into or out of an optical system.