1. An optical distance measurement system comprising: a light source configured to emit light at an operating frequency; an image sensor configured to capture, with a first exposure time, a first bright image corresponding to activation of the light source, capture, with the first exposure time, a first dark image corresponding to deactivation of the light source, capture, with a second exposure time, a second bright image corresponding to activation of the light source, and capture, with the second exposure time, a second dark image corresponding to deactivation of the light source, wherein the second exposure time is longer than the first exposure time; and a processing unit configured to receive the first bright image, the first dark image, the second bright image and the second dark image, adjust the first exposure time according to a first average brightness value of the first dark image, and adjust the second exposure time according to a second average brightness value of the second dark image.