Patent ID: 8310742

Claim:
An image reading apparatus comprising: a reading unit that performs a reading operation whenever a line start signal is input thereto, to read a reading target document line by line in a main scanning direction; a transport unit that transports any one of the reading unit and the reading target document as a transport target in a sub-scanning direction; an encoder that outputs an encoder signal whenever the transport target is moved by a predetermined distance; a control unit that transmits the line start signal to the reading unit; a time measuring unit that measures a time elapsed from a time point when the line start signal was last transmitted; a cycle measuring unit that measures an output time interval of the encoder signal output from the encoder, as an encoder cycle; and a storage unit that stores the latest encoder cycle measured by the cycle measuring unit, wherein the control unit transmits the line start signal to the reading unit whenever the elapsed time measured by the time measuring unit is equal to or greater than a time obtained by multiplying a value of the encoder cycle stored in the storage unit by a correction coefficient, the correction coefficient being determined by a ratio R 2 /R 1 of a reading resolution R 1 that is the number of reading lines per unit length in the sub-scanning direction and an encoder resolution R 2 that is the number of outputs of encoder signals per unit length in the sub-scanning direction, the encoder resolution R 2 corresponding to a reciprocal of the predetermined distance.