Patent ID: 8280172

Claim:
A method for correcting coaxial light edge position skewing in operations of a machine vision inspection system which comprises an imaging system, a movable workpiece stage, a coaxial illumination portion which projects coaxial light from an objective lens of the imaging system toward the workpiece stage, a stage light illumination portion which projects stage light from the workpiece stage toward the imaging system, and a control system, the method comprising: (a) positioning a workpiece on the workpiece stage with an edge feature of the workpiece in a field of view of the imaging system, wherein the edge feature comprises a boundary between a region which reflects coaxial light to the imaging system and a region which transmits stage light to the imaging system; (b) holding the edge feature steady at a first position in the field of view and acquiring a first image of the edge using one of the coaxial light and the stage light; (c) holding the edge feature steady at the first position in the field of view and acquiring a second image of the edge using the other of the coaxial light and the stage light; (d) determining a first edge position measurement for a defined portion of the edge feature in the first image; (e) determining a second edge position measurement for the defined portion of the edge feature in the second image; (f) determining a coaxial light edge position correction factor based on a difference between the first edge position measurement and the second edge position measurement; and (g) storing the coaxial light edge position correction factor for correcting subsequent edge position measurements that are based on images acquired using coaxial light.