The auto dimming of our mirror works automatically when you start the engine, and the indicator will light up . The light sensor in the back of the rearview mirror will always detect the ambient light. The auto-dimming doesn't work in the broad day for the adequate daylight. However, when night falls and the ambient light is weak, the auto-dimming starts to work and automatically eliminate the glare of rearward-approaching vehicles. Furthermore, the degree of brightness of the auto-dimming glass depends on the level of the light. The stronger the light from the back of the car is, the darker the glass is. Hence it protects drivers from becoming dizzy and avoiding car accident.