Projector lens system having three lens groups for image projector

Abstract

This specification discloses an image projector for projecting a tri-color composite optical image on a screen as a magnified optical image. A reading light beam emitted from a light source is separated into three color beams, e.g., Red, Green and Blue color beams through a tri-color separation and composition optical system. The separated color beams are irradiated on image forming devices provided correspondingly with Red, Green and Blue color images to read out color images respectively therefrom. Each of the reflected light beams from the image forming device carrying color image information impinges again on the tri-color separation and composition optical system to compose a composite image beam. The composite image beam is projected on the screen through an optical lens system comprising a first lens group, a second lens group and a third lens group disposed along an optical axis in order. The first lens group forms a composite color image on the second lens group by focusing the composite image beam from the tri-color separation and composition optical system. The second lens group converges the composite image beam therefrom to the third lens group. The third lens group projects the composite image beam on the screen as a magnified image. A focal length of the lens system is arranged shorter than a back-focal length defined as a distance between the front surface of the first lens group and a object focal point thereof.