Patent ID: 8004655

Claim:
An apparatus for acquiring an optical image, comprising: a movable table on which a test sample having a patterned surface is mounted; a light source configured to apply first and second light beams on the patterned surface to form first and second areas, respectively, the first area being defined as an image pickup area on the patterned surface and the second area being formed as a focus adjusting area to have a substantially rectangular shape with long and short sides; an image acquiring unit configured to detect a third light beam which is reflected from the first area or is transmitted through the first area, and acquire an image of the image pick up area from the third light beam when the patterned surface is located in an image pickup position; a focusing optical system including a splitting element configured to split a fourth light beam into first and second segment light beams, which is reflected from the second area, wherein the first and second segment light beams are directed to first and second optical paths and focused on first and second image forming planes, respectively, when the patterned surface is positioned at the image pickup position; a first aperture stop having a first aperture, which is located in the first optical path between the first image forming plane and the splitting element, wherein the first aperture has first and second axes crossing on a first origin point and corresponding to the short and long sides, respectively, and has a first aperture length along the second axis, and the first aperture length is maximum on the first origin point and is decreased along the first axis depending on a first distance from the first origin point along the first axis; a second aperture stop having a second aperture, which is located in the second optical path such that the second image forming plane is arranged between the splitting element and the second aperture, wherein the first aperture has third and fourth axes crossing on a second origin point and corresponding to the short and long sides, respectively, and has a second aperture length along the third axis, and the second aperture length is maximum on the second origin point and is decreased along the third axis depending on a second distance from the second origin point along the third axis; a detection unit configured to detect the first and second segment light beams passing through the first and second apertures to output first and second detection signals, respectively; and a control unit configured to control a position of the movable table based on the first and second detection signals.