Image signal processing apparatus

An image signal processing apparatus for correcting an image signal at Nyquist limit, the apparatus comprising;

• an image signal generation means for sequentially generating analog image signals representing information on light and shade of an original image per every picture element, said image signal generation means including a lens for receiving light from an original and an image reading device for converting light passing through said lens into said analog image signals;
• an analog to digital converting means for converting generated image signals into essentially proportional digital image signals representing the same information of said generated image signals, respectively;
• an extreme portion detection means for comparing said converted digital image signal of a picture element of interest and each of said converted digital image signals of a plurality of peripheral picture elements which are positioned in the vicinity of said picture element of interest, and judging whether said picture element of interest is an extreme portion or not thereby detecting a picture element of which a level of a digital image signal is deteriorated due to an influence of the Nyquist limit;
• a correction means for correcting a level of said digital image signal of said picture element of interest, if the picture element of interest is judged to be said extreme portion under said influence of the Nyquist limit by said extreme portion detection means, by adding a value proportional to said level of said digital image signal, said correction being accomplished in accord with the expression:
• M1h=M1+K*M1*(1/MTF-1)
• wherein M1 represents an original level of said digital image signal of said picture element of interest, M1h is a corrected level thereof, K is a correction coefficient and MTF is a modulation transfer function, K and MTF being peculiar values determined by the characteristic of the combination of said lens and said image reading device and the MTF being constant at the Nyquist limit;
• and
• output means for outputting a corrected digital image signal in case that the picture element of interest is the extreme portion, and outputting an original digital image signal without correction in the case that the picture element of interest is other than the extreme portion.