| 摘要 |
PURPOSE:To decrease the optical length of a detection system while a beam diameter on a four-division detector is kept to a prescribed size by providing an astigmatic difference lens where one face has a spherical lens face and the other has a cylindrical lens face in an optical path of a focused luminous flux. CONSTITUTION:The one face of the astigmatic difference lens 30 is a concave spherical fase 30A and the other face has a convex cylindrical face 30B. Since the astigmatic difference lens 30 is arranged in the optical path, the focused angle of the focus luminous flux 32 is decreased by the concave spherical face 30A and the flux is irradiated on the four-division detector 31 as the focus luminous flux 33 having an astigmatic difference in combination with the convex cylindrical face 30B. The optical length of the detection system is reduced by providing the condenser lens 29 and the astigmatic difference lens 30 without reducing the beam diameter on the four-division detector 31 in the focusing state, that is, while the focusing angle is kept to a conventional angle. |
