Klinger Gauge Glasses, KLINGER reflex glasses are provided with moulded prismatic grooves on the side in contact with the medium. As a result of the difference in refractive index, light rays entering the gauge from outside are reflected or absorbed. The light ray entering the steam or gas space strikes the groove face at an angle of 45o, being thereby refracted into the opposite groove face from which it is reflected back in direction of observation. The steam or gas space thereby appears silver-white. The liquid space behind the reflex glass appears black.