【発明の名称】読取用スキャナレンズ

Japanese Patent JP2790919

B2

A scanner lens for reading has first to fourth lens groups sequentially arranged from an object side to an image side. The first to fourth lens groups are constructed by six lenses. Each of the six lenses is constructed by a positive or negative meniscus lens having a convex or concave face directed on the object side. The scanner lens further has a diaphragm arranged between the second and third lens groups. Refractive indices Ng, NF, Nd and NC at respective wavelengths on lines g, F, d and C, an Abbe's number (ny) on line d, and a partial dispersion ratio (theta)=(Ng-Nd)/(NF-NC) satisfy the following condition, (-0.0020793*(ny)+1.3649)-(theta)>0.004.(1) with respect to one or both of third and fourth lenses adjacent to the diaphragm. A combined focal length f of an entire lens system and a focal length f1 of the first lens group satisfy the following condition, 0.95