An improved spatial light modulation display system includes light sources for providing red, green, and blue light. A system controller includes functionality for controlling a color balance of the red, green, and blue light. A sensor is provided for sensing light from each of the light sources. The controller can detect a shift in color balance based on the intensity of light sensed by the sensor. If the sensor output indicates that the sensor is operating out of a desirable range, the spatial light modulator can modulate the light in order change the brightness of light sensed by the sensor. The modulation pattern can be varied until the sensor output is a specified value or within a specified range of values. In a preferred embodiment, the sensor is located to receive off-state light from the spatial light modulator so as to avoid obstruction of light used for displaying images.