In picture image readers such as prior art facsimiles and scanners, a solid state image sensor is generally used having a CCD array of detectors arranged in the image plane. In recent years, the trend towards higher definition of pixels in CCD arrays has been rapid. Accompanying this trend, it has become necessary to use higher resolution lenses in picture image readers.
Moreover, the development of color picture image readers that read color manuscripts by splitting white light into light of three primary colors (e.g., red, blue and green) has been rapid. Thus, lenses have become necessary which have small aberrations over a wide range of wavelengths as compared to a lens required by a monochrome picture image reader.
As for prior art lenses that are used for picture image reading there are, for example, deformed Gauss-type, six lens element, lenses disclosed in Japanese Laid Open Patent Publication H04-31368 (Japanese Patent Application No. Sho 59-246132) and Japanese Laid Open Patent Publication H07-140383.
In the lens described in Japanese Laid Open Patent Publication H04-31368, there is a problem in that the distortion is large. In the lens described in Japanese Laid Open Patent Publication H07-140383, there is the problem that the manufacturing costs are high because expensive glass material is necessary for the high refractive index material that is used.