No paper off the top of my head. I got the idea from a Matlab blog: http://blogs.mathworks.com/steve/2010/12/23/two-dimensional-histograms/ Empirically, when I applied an equalization to the lena image on each RGB channel, the result was a distorted image (color-wise). Applying the LAB method shed the original image of the soft focus the photographer had used, and presented a harsher lighting that showed more detail, while preserving the colors.