Histogram evaluation for image quality concerning illumination
I understand that what you are looking for is a contrast measure. Since, I am not sure as to which different research papers you have looked at, I am giving here links to some important and good papers in this area. Refer to them. If you are unable to make sense of them, reply back, and I would try my best to help 

Measuring pereptual contrast in color images

Measuring the perceived contrast of natural images

Hope this helps