An information processing system includes: a light source that illuminates a document; an image sensor that reads an image of the document; and at least one processor, wherein when incidence of external light is detected from output data of the image sensor, the at least one processor moves the image sensor in a direction opposite to a reading direction from a first position for starting reading of the document in a stationary state, and corrects a black level of the image sensor at a second position where an entire light receiving surface of the image sensor is hidden by a housing.

An image sensor includes a pixel array including at least one light-shielded area where no light enters and an imaging area where light enters, wherein each pixel includes a photoelectric conversion element, a black level processing unit that corrects an output of each pixel in the imaging area, and a memory that stores a predetermined black level reference for each pixel in the imaging area. The processing unit calculates a Slope, which is determined by an average output value at imaging of pixels in the at least one light-shielded area taken during imaging and a reference average output value of pixels in the at least one light-shielded area under certain conditions taken prior to imaging, and correct an output of each pixel in the imaging area using the predetermined black level reference and the Slope.

A method for black level correction, including determining whether the first image sensor and/or the second image sensor triggers a calibration mode based on a calibration signal, performing, in response to the first image sensor triggering the calibration mode and the second image sensor not triggering the calibration mode, first image acquisition through the first image sensor based on a first automatic exposure configuration, and obtaining a calibration result by performing black level calibration, obtaining, in response to the first image sensor not triggering the calibration mode and the second image sensor triggering the calibration mode, an image to be processed by performing second image acquisition through the first image sensor based on a second automatic exposure configuration, and performing black level correction on the image to be processed based on the second automatic exposure configuration and the calibration result.