A twilight sensor for controlling the headlamps of vehicles employs a photodiode having a predetermined spectral response and a diffuser which also has a predetermined spectral response, the spectral responses being combined to achieve an overall spectral response which corresponds to that of the human eye. With this arrangement, the headlamps of a motor vehicle are turned on and off under different ambient light conditions, in a manner which corresponds to the perception of the driver. In addition, the invention employs a control system which turns the lights on rapidly after the vehicle enters a tunnel.