1. An optical touch system, comprising: a touch surface; a plurality of reflection light bars respectively disposed at a plurality of edges of the touch surface; a first image sensor configured to capture image frames looking across the touch surface; a first active light source disposed adjacent to the first image sensor; and a second active light source disposed at a corner of the touch surface and within a field of view of the first image sensor, wherein during an interval that the first image sensor captures a first image frame, the first active light source emits light with a first brightness value and the second active light source emits light with a second brightness value smaller than the first brightness value.