A light source transmits detecting light toward an object. The object reflects the detecting light and forms a reflected light. A sensor is used for sensing the reflected light. Then, an exposure control unit coupled to the sensor performs luminance convergence on the reflected light according to luminance of the reflected light sensed by the sensor. And a distance measurement device coupled to the sensor detects a distance between the object and the light source and/or the sensor according to an image position of the reflected light on the sensor.