Image reading apparatus with A/D conversion and adjustment thereof

摘要

An image reading apparatus such as a color copying apparatus has a plurality of sensing CCD chips for converting an image of an object into an electrical signal, each sensing CCD chip having a plurality of sensing elements. The apparatus also has a circuit for converting the electrical signal into digital data. A first adjusting circuit is provided for conducting either an operation for obtaining level matching of the electrical signal converted by the sensing chips, or an operation for controlling the gain of the electrical signal converted by the sensing chips. The apparatus further has a second adjusting circuit for effecting compensation for fluctuation of characteristics of the sensing elements of the sensing chips, by making use of the digital data converted by the converting circuit, the second adjusting circuit being adapted for effecting the compensation on the basis of electrical signals which are produced as a result of conversion by the sensing chips from two objects having different levels of density from each other, such as white and black levels. An analog-to-digital converter may also be provided to convert analog data into digital data. In that case, first adjusting means will adjust the analog data discretely, through a step smaller than the quantizing step used in the A/D conversion, and a second adjusting means may preferably further adjust the analog data in accordance with digital data obtained through the A/D conversion.