1. A method of correcting an image signal generated by a charge coupled device (CCD) image sensor of an imaging system, the charge coupled device (CCD) image sensor comprising a plurality of effective pixels for producing the image signal and a plurality of optical black (OB) pixels for determining a dark reference of the image signal, the imaging system comprising a memory for storing a plurality of gamma correction curves, each of which has a respective correction factor to increase contrast in a dark portion of the image signal, the method comprising: measuring a plurality of gray scale values of the plurality of effective pixels and the plurality of optical black (OB) pixels in the CCD image sensor; estimating a contrast level of an object scene to be imaged using the plurality of measured gray scale values; and correcting the image signal using a corresponding gamma correction curve depending on the estimated contrast level of the object scene.