A rearview mirror system comprises an electrically controlled optical transmission plate disposed over a reflecting surface. The optical transmission plate is selectively operable to any of a number of different states of opaqueness between a miximum light transmission and a maximum opaqueness. The opaqueness of the optical transmission plate is controlled by an electronic circuit which includes two sensors, one for sensing the brightness of incident light from the field of view and the other for sensing ambient light. The control operates to cause the light which is incident on the reflecting surface to be closed loop regulated to a desired degree of brightness which is also correlated with ambient light conditions. A manually operable adjustment is also provided forthe observer to set a desired brightness characteristic.