Optical comparator system to separate unacceptable defects from acceptable edge aberrations

Apparatus for comparing a light affecting, patterned workpiece with a similar reference workpiece to detect unacceptable defects in the patterned workpiece to the exclusion of acceptable edge aberrations, the apparatus comprising:

• means for generating first and second synchronously scanning light signals;
• means for maintaining said workpieces in optically equivalent positions, said position maintaining means being movable in a plane orthogonal to said scanning light signals;
• means for detecting said light signals affected by said workpieces, said signals being intensity modulated by the patterns on said workpieces;
• and
• gating means, including means for producing a first set of amplitude weighted replicas of said first and second modulated signals and means for generating a second set of predetermined, defect defining threshold levels, said gating means producing an output indication when at least one of said amplitude weighted replicas of said first and second modulated signals in said first set exceeds a corresponding defect defining threshold level in said second set thereby enabling the separation of unacceptable pattern defects to the exclusion of acceptable edge aberrations.