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; a second active light source disposed at a corner of the touch surface and within a field of view of the first image sensor; and a second image sensor disposed at the corner of the touch surface where the second active light source is disposed, and configured to capture image frames looking across the touch surface; 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, the first image sensor and the first active light source are within a field of view of the second image sensor, and during an interval that the second image sensor captures a second image frame, the first active light source emits light with a third brightness value and the second active light source emits light with a fourth brightness value larger than the third brightness value.