The mirror hygrometer is an instrument for the direct measurement of dew-point or frost-point. It is based on the formation of a thin film of water (or ice) on a small reflecting surface (called a mirror) which is cooled appropriately. An optical system checks that the layer of water has a constant thickness by measuring the reflectivity of the mirror and adjusting the mirror cooling system accordingly.
From the measurement of the dew point and air temperature it is possible to calculate with good accuracy the other humidity values (e.g. relative and absolute humidity).
This type of instrument guarantees excellent accuracy and stability of measurement and is widely used in laboratory practice.