A system of adjusting power of a light source includes an image sensor for capturing an image and obtaining gain and exposure; a target object detection unit that detects a target object in the image and finds a size of the target object; a distance determination unit that determines distance of the target object according to the size of the target object; a light energy measuring unit that measures light energy according to the gain and the exposure; and a controller that controllably adjusts power of the light source according to the distance and the light energy.