Balance brightness and contrast between two images
The images you are working with are probably with gamma compensation, you should remove it first by applying inverse gamma.
More accurately, each camera has its own tone curve mapping. You can build a lookup table that converts one from another.
After you removed the non-linearity source, (whether you used method 1 or method 2), as a first order approximation, you can map colors from different cameras by using a 3x3 matrix applied on RGB. 


Now you can compare your images.