PURPOSE: To simplify the constitution of a collimator lens of three-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 and a positive lens having a convex face on the parallel luminous flux side while assuring the size of the numerical aperture, lens performance and the case of production by constituting this collimator lens in such a manner that the lens satisfies condition equations.
CONSTITUTION: The collimator lens composed of three elements L1 to L3 in three groups is so constituted as to satisfy the conditions of the equations (1) 0.8<f1/f3<2.0, (2) 1.5<{r2(n2-1)}/{r3(n1-1)}<2.7, (3) 0.8<r5/{f(n3-1)}<1.4, (4) 0.8<-r1/r2<2.4, (5)0.5<-f2/f<6.0 where the focal length of the entire system is defined as f, the focal length of the i-th lens is defined 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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