An augmented reality display system can include a partially transparent eyepiece, an ambient light sensor, and an image input device. The image input device can include a light source and a spatial light modulator. The system can also include a controller that determines that no virtual content is being displayed to the user and compares the ambient light to display light to determine if a light source shutoff criterion is satisfied. The light source shutoff criterion can be indicative of whether shutting off the light source that illuminates the spatial light modulator will result in a change in the display light that will be user-perceptible. If the light source shutoff criterion is satisfied, the controller can turn off the light source.