1. Field of the Invention
The present invention relates to image-reading apparatuses such as optical scanners.
2. Description of the Related Art
Optical scanners serving as image reading apparatuses include those of the so-called contact image sensor (CIS) type using unit-magnification optical systems and those of the so-called charge-coupled device (CCD) sensor type using reduction optical systems. The scanners of the CCD-sensor type read original images using reading units including CCD line sensors and reduction optical systems by raster scanning of the sensors, convert signals from each pixel into image data, and output the data. The scanners read two-dimensional images by a combination of the raster scanning of the line sensors in a main scanning direction and relative scanning of the line sensors to originals in a sub-scanning direction. Japanese Patent Laid-Open No. 2000-244717, for example, describes a scanner of this type.
In order to reduce the size of the scanners of the CCD-sensor type, the reduction optical systems embedded in the scanners are desirably made more compact. However, as the optical path lengths of the optical systems are reduced for the compact optical systems, various aberrations such as distortion and chromatic aberration of the optical systems are increased in degree due to lack of margins in optical design.
Increases in the degrees of aberrations cause the following problems. First, scaling factors of images formed on the line sensors in central portions and those in peripheral portions differ from each other due to distortion. Accordingly, the resolutions of the read image data in the central portions and those in the peripheral portions differ from each other. Furthermore, differences in the resolutions between the central portions and the peripheral portions vary in terms of color due to chromatic aberration.