[radiance-imageproc] calculate medium value in a picture

i m still pillo (the beginner)
and now i m wondering if there s a way to calculate the medium value for illuminance (or luminance) on a calculated picture (that isn t but a matrix, is it?)

may i use pvalue? how?

excuse me for the question...
i need a seedy answer, so i must ask without trying to solve by myself...

thanks again