Colour port type direct water level gauges operate on the principle of the differences in refractive index of different mediums (steam and water) for level indication. The gauge consists of a vertically oriented metal body with a toughened glass to the front and the rear. The front and rear body surfaces are in non-parallel vertical planes. Behind the gauge body are light sources - red and green in the illuminator.

When light passes through water, it gets refracted at an angle of approximately 10o. The convex lenses converges the light sources such that for ports submerged in water, a GREEN indication is observed in the Illuminator viewing panel and for ports that are not submerged in water, a RED indication is observed.