1. A line-image-forming optical system for thermally annealing a semiconductor wafer having a surface, comprising: a primary optical system configured to form from a primary laser beam a line image having a long axis and a first amount of intensity non-uniformity along the long axis; a secondary laser system that generates a secondary laser beam having a secondary laser beam wavelength and a secondary laser beam intensity; a scanning optical system having a first field of view configured to receive only the secondary laser beam and form therefrom a secondary image at the wafer surface, wherein the secondary image at least partially overlaps the line image and is scanned over at least a portion of the line image to form a time-averaged modified line image having a second amount of intensity non-uniformity that is less than the first amount; a thermal emission detection system configured to detect thermal emission light from the wafer surface through the scanning optical system and over a second field of view substantially the same as the first field of view of the scanning optical system, the thermal emission detection system configured to generate an electrical signal corresponding to the detected thermal emission light; and a controller configured to receive the electrical signal and in response thereto adjust at least one of the secondary laser beam intensity and a scanning speed of the secondary laser beam.