The typical pressure-bearing industrial sight glass looks like a port window on a ship. It is installed directly onto the sidewalls of tanks and vessels to allow its interior and contents to be visually inspected. An opening must be on the pressure-bearing shell of the tank, pipe, or vessel that will host the window. Most common of such openings are industry standard flanges – ANSI size flanges, for example – and reinforcing pads that have been welded onto the opening to compensate for the loss of structural strength caused by the opening; this reinforcement essentially decouples the attached sight glass from flexing with the pressure shell. Highest-pressure sight glasses are almost universally circular.

When the purpose of the opening is to inspect the interior of a process vessel rather than to verify a liquid level, an elongated or oblong window shape is hardly ideal. Typical general-purpose sight windows are circular. Distribution of stresses from pressure onto homogeneous materials results in most even stress profiles when pressure-bearing geometries are circular. This gives rise to the fact that virtually all pressure-bearing vessels from gas cylinders to aircraft fuselages have a circular cross-section. Likewise, industry-standard piping components are all of circular cross sections.