Patent ID: 7538868

Claim:
In a bright field pattern recognition system having an objective lens proximate a stage, a beam splitter optically coupled to the objective lens, an illuminator optically coupled to the beam splitter along a first optical path, beam shaping and relay optics optically coupled to the beam splitter and the illuminator, and an imaging detector optically coupled to the beam splitter along a second optical path, a method for calibrating the system for imaging low contrast substrates, the method comprising: bringing a target pattern on a substrate on the stage into focus to form an image at the imaging detector; scanning the image in a first direction and obtaining from the image a first edge scattering pattern from a first feature of the target pattern, wherein the edge scattering pattern is characterized by first and second peaks; and adjusting a position of the illuminator or beam shaping and relay optics along one or more directions perpendicular to the first optical path until the first and second peaks are approximately equal in height.