| 摘要 |
PROBLEM TO BE SOLVED: To obtain a good transmittance even in a UV ray region and to preferably correct aberrations by forming a lens having a spherical face as a first face from the light source side and having an aspherical face as a second face satisfying specified conditional expressions. SOLUTION: The lens consists of, from the light source S side, successively, a spherical first face and an aspherical second face satisfying the expressions of (1)|R2|<f<|R1|, (2) 1.5<f/d<2.5, (3) NA>0.45, and (4) 2.48<nF/|R2|<2.52. In the expressions, R1 is the radius of curvature of the first face (a convex to the light source side is regarded as positive), R2 is the radius of curvature at the top of the second face (a convex to the light source is regarded as positive), f is the focal length for the F line (ray at 486 nm wavelength) of the aspherical collimating lens L, d is the center thickness of the aspherical collimating lens L, AN is the numerical aperture in the light source side of the aspherical collimating lens L, and nF is the refractive index of the aspherical collimating lens L for the F line (ray at 486 nm wavelength).
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