Patent ID: 8054510
Filing Date: 2011-11-08
Classification: H04N

Abstract:
1. A document reading method comprising: providing a document reader comprising a plurality of line sensors arranged in a zigzag manner to form a first row of line sensors having photoelectric conversion elements that read a document in advance and a second row of line sensors having a plurality of photoelectric conversion elements that read the read areas of the document following reading thereof by the photoelectric conversion elements of the first row of line sensors; a transparent document support plate that supports the document and covers the line sensors so that the line sensors are disposed on one side of the document support plate at preselected variable distances from a surface of the document support plate; a first white reference plate spaced apart from the document support plate and disposed on a side of the document support plate opposite to the side thereof on which the line sensors are disposed, the first reference plate forming a reference of gray gradation; and a second white reference white plate disposed on the document support plate; using the photoelectric conversion element of each of the line sensors, reading the first white reference plate while illuminating the first white reference plate to obtain a white reference read value A of the photoelectric conversion element for each of the line sensors corresponding to a read value at a focal position, and reading the second white reference plate while illuminating the second white reference plate to obtain a white reference read value B of the photoelectric conversion element for each of the line sensors corresponding to a read value at a focal position shifted from the focal position corresponding to the white reference read value A, the white reference read value A being smaller than the white reference read value B; averaging the white reference read value A of the photoelectric conversion element obtained for each of the line sensors to obtain a white reference read average value AA for each of the line sensors, and averaging the white reference read value B of the photoelectric conversion element for each of the line sensors to obtain a white reference read average value BB for each of the line sensors; dividing the white reference read average values BB obtained for the line sensors by the respective white reference read average values AA obtained for the line sensors to obtain a sensitivity offset coefficient C for each of the line sensors for offsetting variations in sensitivities of the photoelectric conversion elements of the line sensors due to the variation in distances from the line sensors to the surface of the document support plate; and for each of the line sensors, correcting shading using the expression N×[(E−D)/(Aa×C)−D], where N represents the number of gradation levels for image regions corresponding to read areas of the document read by the photoelectric conversion elements of the line sensors; D represents a black reference read value of the photoelectric conversion element of each of the line sensors obtained by reading the first white reference plate using the photoelectric conversion element of each of the line sensors under a condition that illumination is turned off immediately before reading of the document; Aa represents a white reference read value of the photoelectric conversion element of each of the line sensors obtained by reading the illuminated white first reference plate using the photoelectric conversion element of each of the line sensors immediately before reading of the document; C represents the sensitivity offset coefficient obtained for each of the line sensors; and E represents a document read value of the photoelectric conversion element of each of the line sensors obtained by reading the illuminated document using the photoelectric conversion element of each of the line sensor.