Before going into that this is how the distribution would look like the top figure shows the distribution of energy. If you take one single if you take one single wave I mean spectrum say this can be as a function of frequency for a particular direction or vice versa on the y axis, it will be the energy or of amplitude square which is nothing referred to as spectral density. So, if you superpose all these things then you get a three dimensional view and that three dimensional view is called as the directional spectrum is that clear. So, you will have a long expression, but I will just show some of these expressions and only thing is after having a look at the lecture patiently go through all the equations and try to understand