| 摘要 |
PROBLEM TO BE SOLVED: To uniformly illuminate a surface to be irradiated at a high illumination efficiency, without shading an annular secondary light source by guiding an annular light flux to a reticle with the outer diameter and the annulus ratio changed, based on information of the reticle. SOLUTION: A collimated light flux from a light source 1 passes through an annular flux converter 2 having replaceable prism members 20, 21 to convert it to an annular collimated flux. The prism member 21 has a thinner on-axis thickness (distance between vertexes) than that of the prism member 20. The annular collimated flux converted by the prism member 20 has a larger outer diameter than that of the annular collimated flux converted by the prism member 21, though the annulus width is constant. As the result, the annular collimated flux converted by the prism member 20 has a lower annulus ratio than that of the annular collimated flux converted by the prism member 21. Hence the annulus ratio can be varied by replacing the prism members 20, 21.
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