Reading lens system

A reading lens system comprises, in order from the object side, a positive meniscus first lens element having a convex surface directed toward the object, a positive meniscus second lens element having a convex surface directed toward the object, a negative meniscus third lens element having a convex surface directed toward the object, a negative meniscus fourth lens element having a concave surface directed toward the object, a positive meniscus fifth lens element having a concave surface directed toward the object, and a positive sixth lens element. The second and third lens elements are cemented and the fourth and fifth lens elements are also cemented to provide an overall system of four-unit-six-element composition, with the lens system satisfying a number of conditions to obtain a lens having an F number of about 1:4.0 and a half viewing angle of not greater than 20.degree. and which is capable of efficient compensation for aberrations, in particular astigmatism and chromatic aberrations.

BACKGROUND OF THE INVENTION 
The present invention relates to a reading lens system suitable for use 
with facsimiles, image scanners and like apparatus. 
Lens systems used with facsimiles, image scanners and like apparatus must 
have a resolving power of approximately 7 .mu.m on the image side and 
hence are required to provide an extremely good contrast at high 
frequencies. Further, in order to minimize the loss of light in an inner 
peripheral portion of an image circle formed by the reading lens system, a 
nearly 100% aperture efficiency is needed. In addition, it is necessary to 
achieve satisfactory compensation for distortion. To meet these 
requirements, Gauss type lenses are conventionally used, as described, 
e.g., in Unexamined Published Japanese Patent Application Nos. 50-62631, 
59-90810, 60-181716 and 63-75720. 
A problem with the conventional Gauss type lenses, which are used with an F 
number on the order of 1:4.0 and a half viewing angle of about 20.degree., 
is that their performance in a sagittal direction is not given as much 
consideration as in a meridional direction. If one wants to improve the 
overall image quality by a suitable method such as increasing the reading 
density in a sagittal direction, good lens performance is required in both 
meridional and sagittal directions. However, this requirement has not been 
completely met by prior art lens systems. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to solve the above-mentioned 
problem, and it is a principal object to provide a reading lens system of 
high performance that has an F number of about 1:4.0 and a half viewing 
angle of not greater than 20.degree. and which is capable of efficient 
compensation for aberrations, in particular astigmatism and chromatic 
aberrations. 
The reading lens system of the present invention comprises, in order from 
the object side, a positive meniscus first lens element having a convex 
surface directed toward the object, a positive meniscus second lens 
element having a convex surface directed toward the object, a negative 
meniscus third lens element having a convex surface directed toward the 
object, a negative meniscus fourth lens element having a concave surface 
directed toward the object, a positive meniscus fifth lens element having 
a concave surface directed toward the object, and a positive sixth lens 
element. The second and third lens elements are cemented and the fourth 
and fifth lens elements are also cemented to provide an overall system of 
four-unit-six-element composition, with the lens system satisfying the 
following conditions: 
(1) 0.28&lt;d.sub.3 &lt;0.35 
##EQU1## 
where r.sub.i is the radius of curvature of the ith surface, and d.sub.i is 
the distance between the ith and the (i+1)th surfaces.