1. Apparatus for correcting an image, wherein said image is an array of rows and columns of pixels each having a brightness represented by a digital value, comprising: first means for estimating a gross brightness of each pixel in said image from a brightness of at least one of a first set of pixels surrounding said pixel to produce a gross brightness map of said image; second means for estimating a smoothed brightness of each pixel in said gross brightness map from a weighted brightness of a second set of pixels surrounding said each pixel to produce a smoothed brightness map; and means for multiplying each pixel in said image by an inverse of its corresponding pixel in said smoothed brightness map to produce a corrected image from which artifacts of an imaging system are removed.