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A novel focusing method and apparatus for determining the position of the image plane of a coherently illuminated object is presented. The principle of this method is based on the light distribution in the defocused image plane of a luminous knife-edge object. The radiation energy extending beyond the geometrical image is a function of the relative position of the projection objective and its minimum indicates the best location of the image plane. The photodetected energy can be used as a feedback signal to focus various optical systems in applications such as high-resolution laser patterning. Preliminary results obtained by means of a 193 nm excimer laser radiation and a reflecting objective are also reported.
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Popovici et al. (2001) studied this question.
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