An instrument for determining the dewpoint. The most widely used systems employ a small polished-metal reflecting surface, cooled and heated electrically using a Peltier-effect device, and a small temperature sensor embedded on the underside of the mirror. An electro-optical system is used to detect the formation of condensate and to provide the input to the servo-control system to regulate the temperature of the mirror. The specular reflectance of the bare mirror decreases with dew layer thickness; a preset threshold defines a specific constant dew layer thickness that is maintained by the optical servo-control. The mirror temperature equals the dewpoint.