Viewports, also known as windows or sight glasses, are commonly used to observe or illuminate processes inside a vacuum chamber. In optical applications, viewports transmit electromagnetic radiation into or out of a vacuum chamber. Other applications benefitting from the equipment of a vacuum viewport to a system include general viewing, precision lasers, infrared imaging, and research utilizing harsh environments and thermal shock.

Viewport selection is typically based on requirements for flange type, transmission range, thermal stability, durability, and other optical and physical properties of the window material.