gray scale level of an image

i all, i took a rgb image and changed it to gray scale (with matlab). i want to ask,if someone knows ,how can i know how many gray levels i have in my image? and how can i calculated the dispersion constant of the image as seen in this link on page 2. http://sethares.engr.wisc.edu/paperspdf/icassp02.pdf

thanks, matan