PURPOSE: To improve the offaxial performance of a collimator lens of 4-element constitution which consists, successively from a parallel luminous flux side, a biconvex positive lens, a negative lens having a concave face on the parallel luminous flux side, a positive lens having a convex face on the parallel luminous flux side and a positive lens having a convex face on the parallel luminous flux and to facilitate the production thereof while assuring the size of the numerical aperture by condition equations.
CONSTITUTION: The collimator lens composed of the four elements in four groups is so constituted as to satisfy the conditions of the equations (1) 0.9<f1/f3<1.6, (2) 0.5<f3/f4<2.0, (3) 1.4<{r2(n2-1)}/{r3(n1-1)}<6.3, (4) 1.0<-r3/{f(n2-1)}<4.8, (5) 1.1<{r5(n4-1)}/{r7(n3-1)}<2.5, (6) 0.7<r7/{f(n4-1)}<1.8 where a second lens L2 which are the 2nd lens from the parallel luminous flux side is composed of a negative lens, the focal length of the entire system is defined as f, the focal length of the i-th lens as fi, the radius of curvature of the i-th lens face from the parallel luminous flux side as ri and the refractive index of the i-th lens as ni.
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