Patent ID: 7999215

Claim:
An image reading apparatus provided with a light source and an image sensor having a plurality of light sensing elements arranged in a main scanning direction, and operable to irradiate an object to be read with light emitted from the light source and read reflected light from the object by the image sensor, the image reading apparatus comprising: a white reference plate having a length which is no shorter than a main scanning width of the image sensor; an auxiliary white reference plate which is smaller than the white reference plate; a storing unit operable to store differential profile data indicative of a time-dependent change amount of a light intensity on the white reference plate detected by each of the light sensing elements corresponding to a time-dependent change amount of the light intensity of the light source, and differential profile auxiliary data indicative of a time-dependent change amount of a light intensity on the auxiliary white reference plate read by the image sensor as a representative light intensity of the light source corresponding to the time-dependent change amount of the light intensity of the light source; and a shading compensation unit operable to perform shading compensation, when a plurality of document sheets are successively read, such that in advance of reading a first document sheet, the white reference plate is read to acquire light intensity reference data indicative of the light intensities detected by the light sensing elements of the image sensor respectively, and the auxiliary white reference plate is read to acquire light intensity auxiliary reference data indicative of a representative light intensity of the light source, that after reading the first document sheet, the shading compensation is performed to the read data of the first document by the use of the light intensity reference data, that in advance of reading a second or subsequent document sheet, the auxiliary white reference plate is read to acquire latest light intensity auxiliary data indicative of the representative light intensity of the light source, and that after reading the second or subsequent document sheet, the shading compensation is performed to the read data of the second or subsequent document by calculating correction values on the basis of the differential profile data and a ratio of the difference between the light intensity auxiliary reference data and the latest light intensity auxiliary data to the differential profile auxiliary data, and correcting the light intensity reference data with the correction values.