A viewport, sometimes referred to as a window, is an optical component that provides a means for visual, optical or broad-band energy transmission in and out of a vacuum system.

There are two types of viewports offered, zero-length and tubulated. A zero-length viewport has a thickness no larger than that of its mounting flange. This makes it easier to protect from accidental damage as there are no parts protruding from the mounting flange. It also allows for the best angle of view.