Patent ID: 7633041

Claim:
Apparatus for determining the position of optimum focus of an imaging system of an object inspection system, comprising: an objective element projecting a single image of an object onto an imaging plane; a detector disposed at said imaging plane and tilted at an angle with respect to said imaging plane, such that the sharpness of focus of said image varies across said detector; a source of illumination illuminating said single image; and a focal position calculator operative to determine the point of optimum image sharpness on said detector based on said single image, wherein said single image of said object is generated using illumination of a different wavelength from that used for inspection of said object and wherein said object inspection is performed with illumination of a first wavelength in a dark field mode, and said focal position calculator is operative to determine said position of optimum focus of said imaging system from data obtained by using illumination of a second wavelength in a bright field mode.