| 摘要 |
A projection lens comprises four coaxial, spherical surfaces with lenses (1-4 of which, in the direction from the object side (14) to the picture side (15), the first lens (1) has a convex surface facing the object side, the second lens (2) and the third lens (3) are located close together and spaced from the first lens (1) and the fourth lens (4), and the fourth lens (4) has a convex surface (41) facing the picture side. The surface (11) of the first lens (1) facing the picture side is planar. The surfaces (21, 31) of the second (2) and third (3) lenses facing the picture side are also planar. The surfaces (20, 30) of the second (2) and third (3) lenses facing the object side are concave. The surface (40) of the fourth lens (4) facing the object side is preferably slightly convex but may even be planar. The lens is used eccentrically in relation to the optical axis (70) of a projector, preferably an overhead projector, in which the lens is located in the region above the end edge of an object window (14) situated in the projection direction, and in focus (61) of an elliptical mirror (6) illuminating the object window, in the other focus (60) of which a light source is located, the lens thus being slightly eccentric from said adjacent mirror focus (61) in the direction of the central axis of the object window (14). |
