There is provided an optical sensor that acquires a first image frame corresponding to a first flicker period and acquires a second image frame corresponding to a second flicker period. The optical sensor adds the first image frame to the second image frame to generate a sum of image frames for the motion detection. Or, the optical sensor respectively adds pixel data of every two pixels in neighboring rows of the first image frame and the second image frame to generate a low-resolution image frame for the motion detection.