Patent ID: 7724951

Claim:
A signal processing method for processing a visible light image signal and an infrared image signal obtained by illuminating a transparent document with light beams respectively coming from a visible light source for mainly emitting visible light and an infrared light source for mainly emitting infrared light, and photoelectrically converting optical images of the transparent document, comprising: a detection step of detecting signal components corresponding to a holder for holding the transparent document from the infrared image signal; a replacement step of replacing, when the signal components corresponding to the holder are detected in the detection step, the signal components of the infrared image signal with a predetermined signal value; a generation step of generating a histogram on the basis of the infrared image signal that has undergone the replacement step; a calculation step of calculating a threshold value on the basis of the histogram generated in the generation step; an extraction step of comparing the threshold value calculated in the calculation step with the infrared image signal that has not undergone the replacement step, and extracting infrared image signal components not more than the threshold value; and an interpolation step of executing an interpolation process of the visible light image signal on the basis of the infrared image signal components extracted in the extraction step.