Projection lens

A projection lens for projecting onto a screen an enlargement of an image appearing on a CRT wherein comma and chromatic aberration, which degrades imaging quality, can be properly corrected comprises 4 lens units. The projection lens comprises, in the order from the screen end, a first lens unit of positive optical power having a strong convex surface facing the screen end, a second lens unit comprising a biconvex lens element of positive optical power and a lens element of negative optical power having a concave surface facing the screen end, a third lens unit of positive optical power at an optical axis having a convex surface facing the screen end, and a fourth lens unit of negative optical power having a concave surface facing the screen end.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a projection lens, and more particularly 
to a projection lens used in a video projector for obtaining an enlarged 
picture on a screen by projecting an image appearing on a cathode ray tube 
(CRT). 
2. Description of the Prior Art 
U.S. Pat. No. 4,300,817 or U.S. Pat. No. 4,348,081 shows a basic 
arrangement of a projection lens, which comprises three lens units 
(groups). However, correction of chromatic aberration is not considered in 
these U.S. Patents. 
A projection lens used in a video projector of the three-tube type is 
required for obtaining a high quality color image to have a capability of 
correcting chromatic aberration. A projection lens having the chromatic 
aberration correction capability is disclosed Japanese Laid-Open Patent 
Application No. 58-198016. This lens is composed of, from the screen end, 
a first convex lens element of bi-convex type, a meniscus convex lens 
elements having a concave surface directed to the screen end, and a second 
concave lens element. The first, second and meniscus convex lens element 
is made so as to have an Abbe number of 50 to 65 and the first concave 
lens element is made so as to have an Abbe number of 20 to 40, thereby 
reducing the chromatic aberration. The lens elements except for the second 
concave lens element are made of plastic. 
Another projection lens having the chromatic aberration correction 
capability is disclosed in Japanese Laid-Open Patent Application No. 
58-139111 or its corresponding U.S. Pat. No. 4,530,575. This lens 
comprises, from the screen end, a first positive lens element having a 
convex surface directed to the screen end, a second positive lens element, 
a third negative lens element, a fourth biconvex lens element made of a 
glass material, and a fifth negative lens element. The lens satisfies 
specific conditions to correct chromatic aberration. 
SUMMARY OF THE INVENTION 
It is an object of the invention is to provide a high-performance 
projection lens which can effectively correct comma and chromatic 
aberration to obtain a high quality projected image. 
In order to achieve this object, the projection lens according to the 
present invention comprises, in the order from the screen end, a first 
lens unit of positive optical power having a strong by convex surface 
facing the screen end, a second lens unit comprising a biconvex lens 
element of positive optical power and a lens element of negative optical 
power having a concave surface facing the screen end, a third lens unit of 
positive optical power and at an optical axis having a convex surface 
facing the screen end, and a fourth lens unit of negative optical power 
having a concave surface facing the screen end. The first lens unit has at 
least one aspherical surface for correcting aberration depending on the 
aperture of the lens. The second lens unit corrects spherical aberration 
and chromatic aberration along the optical axis. The third lens unit has 
at least one aspherical surface for correcting comma and astigmatism. The 
fourth lens unit corrects aberration depending on the view angle, 
especially curvature of field and distortion.