Reflex Level Gauge for the Measurement of liquid level at a glance in a tank. This is particularly used for High-pressure High Temperature OR Toxic Liquid Storage Tanks. These gauges have a prism glass window that uses the basic optical principles of light refraction & reflection to enhance the indication of the liquid level. In brief, when the light strikes the area of glass covered by the liquid is allow passing to the Interior of the gauge. So that the area appears dark (Black). While the light strikes the open space, above the liquid level is reflected back to the observer so that the area appears silver-white. We can clearly see the actually liquid level in the systems.