What is a Sight Glass

Typically, a sight glass, a highly-engineered sight glass used in industry, is a kind of window or view port on a processing tank or pipeline. Sight glasses are used in industries ranging from refineries to breweries. Perhaps because of their widely-varied applications, sight glasses are called by different names.

Sight Glass Window.

Broadly speaking think of a sight glass window or “sight window” a highly-engineered glass fused to a metal frame to create a single-piece component. This is the way to make the glass stronger, because the metal frame places the glass under compression (glass is weak under tension, but strong under compression) The metal frame also takes the stress of mounting, making the glass immune to the forces of uneven bolting and gasket residue making sight windows re-usable.

View port.

This is a term that distinguishes a sight glass used for viewing from a sight glass used for lighting. Often two sight glasses are installed on a tank: one on the top is dedicated to a light, a “light port” and one on the side is dedicated to viewing, a “view port”. For cost savings, some sight glasses are combined with a light called Luminaires.

Sight port.

The term “sight port” is a general term interchangeable with “sight glass.”


It is not uncommon to encounter the term Lumiglas® as a general term interchangeable with “sight glass.”

Sight flow indicator.

Pipelines are different from tanks and vessels in that the sight glass is used to observe flow. The sight flow indicator is an assembly consisting of two opposed sight glasses mounted to a housing. This arrangement allows ambient light to illuminate the flow from behind. There are also versions using a tubular glass that admits more light and permits viewing from any angle.  Another name for sight flow indicator is visual flow indicator.

Gauge glass.

Often tanks will have a vertical sight glass assembly used to observe the level of fluid inside the tank.

Boiler sight glass.

Boilers often have a tubular glass that makes it easy to observe water level.


Lumiglas® Sight Glass, Luminaires and Visulex Camera Systems can be found in the Pharmaceutical, Oil Refining, Food & Beverage Industries, Mining, Water Treatment, Power Generation and Port Operations. The applications are endless.