| 摘要 |
PURPOSE: To obtain a large numerical aperture on the image side, to secure a sufficient working distance on the image side and to miniaturize a beam splitter by locating the rear principal point of a second lens group closer to the image side than a light entrance surface on the side of the beam splitter of the second lens group and satisfying a specified condition. CONSTITUTION: A reflection/refraction optical system makes a light beam from a first surface R pass through a first lens group G1 and a beam splitter BS in order, introduces it to a concave mirror M, makes the light beam from the beam splitter BS reflected by the concave mirror M pass through the beam splitter BS and a second lens group G2 in order and introduces it to a second surface W. The rear principal point of the second lens group G2 is located closer to the image side than a light entrance surface on the side of the beam splitter BS and the conditions:-1<1/βM<0.5. 0.85<L1 /f2 are satisfied, where,βM: the image forming magnification of the concave mirror M, L1 : a distance between the rear principal point and the light entrance surface and f2 : the focal distance of the second lens group G2 .
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