Flat field correction for vignetting

I am using grayscale images of apples that suffer from vignetting- uneven illumination that results in having darker the surroundings of the apple.

Dark areas are also the defects on the surface of the apple that I am trying to find out so, I would like to deal with this problem. The flat field correction technique seems to be used for such purposes.

Does anyone have any MATLAB code for this technique or steps on how to perform it? Any hint is welcome