Determining value of intensity L from cdf of 0.1 ?

Dear all, I want to do illumination processing for effective face detection from unconstrained video. for i that i have decided to use the nonlinear transfer function based illumination processing. To perform dynamic range compression, i need to determine the image dependent parameter z, which can be determined from the intensity value L which is determined from cdf of 0.1. My question is how to determine the value of L from the CDF in matlab. if possible plz provide the piece of code.

[Torsten]

help icdf

Best wishes

Torsten.