ultrasonic vibrators , attached to the back of a mirror , are vibrated according to an a . c . signal pattern which is selected from various a . c . signal patterns for the removal of water , frost and / or ice from the surface of the mirror . further included is a temperature detecting device attached to the back of the mirror for inhibiting operation of the ultrasonic vibrators when the temperature of the mirror exceeds a predetermined temperature , to thus avoid thermally - induced damage to the mirror .