Abstract: An apparatus for controlling the light intensity output from an arc lamp for use with an arc lamp-illuminated projector includes a light valve to control the projector output light intensity by the selective positioning of the light valve in all or a portion of the light path between the arc lamp and the projector output. The light valve is operatively coupled to the output of a magnetically coupled clutch, the input of the clutch being continuously driven by a motor. A programmer applies a voltage proportional to the desired light intensity to the clutch for controlling the degree of magnetic coupling, and therefore the position of the light valve. The variable torque applied through the clutch to the light valve may be countered by a variable counter-torque produced as by a spring to establish the desired continuous range of equilibrium positions of the light valve.