| 摘要 |
A method for evaluating an optical member for photolithography composed of a fluoride crystal comprises a step of measuring a crystal plane orientation of the optical member, and a step of specifying a twin region on the basis of a result of the measurement. The twin region in an effective region of an optical element is specified as a total area of a region obtained by projection onto a plane perpendicular to an optical axis of the optical element. It is judged that the optical member is usable when the total area is not more than 10% of an effective diametric area or a partial diametric area of the optical element. The obtained optical element has satisfactory image formation characteristics because of less birefringence.
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