A CMOS optical sensor comprises spare readout channels to replace readout channels found defective at the end of the manufacturing process. These spare readout channels are dispatched over the width of the optical sensor (corresponding to the row direction) in the form of spare groups Gm1, Gm2, Gm3 of m spare readout channels each, m integer at least equal to 1. Each spare group is inserted between two successive default groups Gn1 and Gn2 of n default readout channels each and coupling means SW1 are configured to replace a defective default readout channel in a default group as well as any default readout channels of the group between the defective one and the spare group next to the default group of concern. Advantageously, for a row Rowi being currently selected for CDS reading each pixel in the row, a row noise level VRNi is obtained from the A spare readout channels that are not used in the implemented repairing scheme, by sampling an analogic DC reference signal by each of the A spare readout channels and averaging the A values Spk obtained. The row reference value VRNi is then subtracted from each of the pixel digital signal Si,j outputs for the current selected row, to finally obtain a signal value di,j with row noise suppression.