"Leveltrol" Reflex Level Gauges allow accurate reading of liquid levels at a glance. These glasses have a prism glass window that uses the basic optical principle of light refraction and reflection to enhance the indication of the liquid level. This lets the level of even colorless liquids be seen without close inspection. When rays of light enters the liquid space behind the groove it is virtually absorbed by the gauge body, so the liquid space behind the reflex glass appears black. Light striking the open space above the liquid level is reflected back to the observer, so that area appears silvery.

The distinct line that separates the dark and silvery areas clearly shows the actual liquid level in the system.