An electronic calibration scheme for logarithmic CMOS pixels

Logarithmic cameras have the wide dynamic range required to image natural scenes and encode the important contrast information within the scene. However, the images from these cameras are severely degraded by fixed pattern noise. Previous attempts to improve the quality of images from these cameras by removing additive fixed pattern noise have led to disappointing results. Using a three parameter model for the response of logarithmic pixels, it is concluded that the residual fixed pattern noise in these images is caused by gain variations between pixels. In order to reduce the effects of these variations a new type of readout circuit has been designed. However, even with this readout circuit high quality images will only be obtained if each image is corrected to remove the effects of both gain and offset variations. Measurement results are presented that show that the quality of the output from logarithmic pixels is significantly improved if a procedure is used which corrects for both types of variations. In fact with this procedure the contrast sensitivity of the logarithmic pixels becomes comparable to that of the eye over five decades of input illumination intensity.