are also name Reflexion Gauge Glass which are made of specialized formular tempered borosilicate glass 4.2, distinguished by the grooves down the face.
There is 1,4 and 5 grooves for different market.The grooves increase the angle of refraction which makes the visibility of the liquid level is much better than when using Transparent Plain glass. Mica are not suitable for reflex gauge glass, and so without this protection they are not suitable for alkaline steam applications.
Reflex glasses may be used in pressure up to 35 bar. In this area are sufficiently resistant to the environment. The reading is clear and contrasting. In the case of factors other than steam reflex glasses may be used up to 400 bar or 400˚C.