| 摘要 |
PURPOSE:To provide the performance equiv. to the performance in the central part of a lens up to the peripheral part while having a half angle of view as large as about 20 deg. and about -1% distortion aberration by providing the 1st, 2nd, and 3rd lenses successively from an object side to said lens, using germanium to form the respective lens and constituting the lens in such a manner as to satisfy specific conditions. CONSTITUTION:This lens for IR rays of 3-group 3-element constitution is arranged with the 1st, 2nd, and 3rd lenses successively from the object side and satisfies the conditions expressed by the formulas (I)-(III) when the distances between the lens faces are designated as d1-d5 successively from the object side, the radii of curvature of the respective lens faces on the object side of the 1st, 2nd, and 3rd lenses as r1, r3, r5, and the combined focal length of the entire system is designated as f. The respective lenses are made of germanium. For example, the left side of the figure is the object side and this lens is constituted of the 1st lens (convex meniscus lens) 10A, 2nd lens (concave meniscus lens) 12A and 3rd lens (convex meniscus lens) 14A.
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