| 摘要 |
A collimator lens comprises, successively from a parallel luminous flux side, a positive lens, a cemented lens made of negative and positive lenses, a positive lens, and a negative lens, and satisfies a predetermined conditional expression, whereby, when carrying out multi-beam scanning, various kinds of aberration, such as image surface curvature amount in particular, are greatly reduced with respect to the respective light beams from individual light sources. Successively disposed from the parallel luminous flux side are a first lens L1 made of a biconvex lens having a surface with a stronger curvature directed onto the parallel luminous flux side, a second lens L2 made of a biconcave lens having a surface with a stronger curvature directed onto the parallel luminous flux side, a third lens L3 made of a biconvex lens having a surface with a stronger curvature directed onto the light source side, a fourth lens L4 made of a biconvex lens having a surface with a stronger curvature directed onto the parallel luminous flux side, and a fifth lens L5 made of a negative meniscus lens having a concave surface directed onto the parallel luminous flux side and a plane directed onto the light source side. The second lens L2 and the third lens L3 are cemented together. These lenses satisfy a conditional expression of 0.9<|F5/FT|<2.5, where F5 is the focal length of the fifth lens L5, and FT is the focal length of the whole lens system.
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