1. An object tracking device, comprising: a light source configured to emit light at a lighting frequency; an image sensor configured to output an operating image when the light source is being turned on and output a background image when the light source is being turned off; and a processing unit configured to obtain background information from the background image, calculate a differential image of the operating image and the background image, identify at least one object in the differential image, and remove an invalid object, which is associated with the background information and smaller than a size threshold, in the differential image.