In this chapter we have considered the surface brightness distribution of different types of galaxies. We have noted that the surface brightness distribution of almost all galaxies can be well described as being a combination of a spheroidal bulge, a flat disk and a point source.
>From the point of view of understanding the galaxy morphology, it is very
important for us to be able to separate these components. Early
observations and rudimentary fits indicated that the bulge follows
the de Vaucouleurs 
law while the disk has an exponential
distribution. Both these
laws are empirical in nature. We have also described some other laws that
are in use.
To actually bring about the decomposition of the galaxy brightness distribution, a radial profile of the galaxy is first obtained by fitting ellipses to the isophotes of the galaxy. Minimization routines are then used to obtain the different parameters. Alternately one can use the 2D galaxy image directly to obtain the parameters. The 2D fitting has been found to be very promising in locating assymmetric features.
In the next chapter we provide the actual scale lengths obtained and discuss the trends.