De Vaucouleurs' law

The de Vaucouleurs profile describes ( according to Gérard Henri de Vaucouleurs ), how it changes the surface brightness of an elliptical galaxy or the bulge of a spiral galaxy with distance from the center. In a simple form it can be written as:

In which case, the surface brightness in the effective radius (ie the radius of the disc, which contributes to the brightness of the half Galaxy ).