| 摘要 |
PURPOSE:To obtain a collimator lens having a large numerical aperture and a short length on the axis by arranging a positive lens, a positive lens, and a negative lens in order from the luminous flux exit side and joining the second lens and the third lens to constitute a lens system consisting of three lenses in two groups and satisfying specific conditions. CONSTITUTION:The lens system consists of the first lens group and the second lens group in order from the luminous flux exit side, and the first lens group consists of the positive lens provided with a face having a high curvature in the luminous flux exit side, and the second lens group consists of a cemented positive lens of the positive lens having a high curvature in the luminous flux exit side and the negative lens having a high curvature in the opposite side, and conditions of formulas are satisfied where (f) and f1 are the resultant focal length of the whole of the system and the focal length of the first lens group respectively and n2 and n3 are the refractive index of the positive lens in the second lens group and that of the negative lens of the second lens group respectively and r3, r4, and r5 are the radius of curvature of the luminous flux exit-side face of the positive lens in the second lens group, that of the joint surface in the second lens group, and that of the luminous flux incidence-side face of the negative lens in the second lens group respectively. Thus, the length on the axis of this lens system is shortened while keeping good spherical aberration though it has a large numerical aperture.
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