Patent ID: 6295169
Filing Date: 2001-09-25
Classification: G02B

Abstract:
An image readout lens comprising, successively from an object side, a first lens which is a lens closest to the object side made of a positive meniscus lens which is convex toward the object side, second and third lenses each having a surface on an image side concave toward an image side and a negative refracting power, and a fourth lens made of a biconvex lens, andwherein at least one of said second and third lenses comprises a glass material and wherein said lens glass material of at least one of said second and third lenses satisfies the following conditional expression (1):.THETA..sub.g.multidot.F +0.0019.upsilon..sub.d <0.650 (1)where.THETA..sub.g.multidot.F is the relative partial dispersion of the lens glass material defined by .THETA..sub.g.multidot.F =(N.sub.g -N.sub.F /N.sub.F -N.sub.c); and.upsilon..sub.d is the Abbe number of the lens glass material defined by .upsilon..sub.d =(N.sub.d -1)/N.sub.F -N.sub.c)whereN.sub.g is the refractive index of the glass material at a wavelength of 435.8 nm;N.sub.F is the refractive index of the glass material at a wavelength of 486.1 nm;N.sub.c is the refractive index of the glass material at a wavelength of 656.3 nm; andN.sub.d is the refractive index of the glass material at a wavelength of 587.6 nm.