Image processing apparatus

An image processing apparatus for obtaining image information by a photoelectric conversion element which outputs, as an image signal, an electrical signal corresponding to a reflectance or transmittance of an original, said image processing apparatus comprising:

• an A/D converting circuit for converting an analog signal outputted from the photoelectric conversion element into an n-bit digital signal;
• a white memory and a dark memory which store, respectively, a high m bits and a low m bits of the n-bit digital signal outputted from said A/D converting circuit in accordance with the photoelectric conversion element;
• control means for processing the n-bit digital signal outputted from said A/D converting circuit to store the high m bits of the n-bit digital signal as effective bits in said white memory when white shading data is obtained, and to store the low m bits of the n-bit digital signal as effective bits in said dark memory when black shading data is obtained;
• and
• a shading correcting portion for correcting an output characteristic of the photoelectric conversion element, by performing a shading calculation based on the white shading data from said white memory and the black shading data read from said dark memory, said shading correcting portion adding (n-m) bits as low bits to the m-bit white shading data read from said white memory into an n-bit white shading data, and adding (n-m) bits as high bits to the black shading data read from said dark memory into an n-bit black shading data, and said shading calculation being performed by a linear operation in which a division is performed between the image signal from which is subtracted the n-bit black shading data, and the n-bit white shading data from which is subtracted the n-bit black shading data.