A system controls a brightness of an augmented reality (AR) eye-mounted device. The system includes an eye-mounted display, a photodetector system, and a controller. The eye-mounted display includes a contact lens and a femtoprojector. The femtoprojector is contained in the contact lens and is configured to project an AR image to a user's retina. The AR image is overlaid on an external scene viewed by the user through the contact lens. The photodetector system detects a brightness level of the external scene. Based on the brightness level of the external scene, the controller adjusts a brightness level of the AR image projected to the user's retina. In some embodiments, the eye-mounted display receives image data defining the AR image and the controller adjusts a bit depth of the image data based on the brightness level of the AR image.