$$\require{cancel}$$
Not all bodies radiate isotropically, and a word is needed to describe how much energy is radiated in different directions. One can imagine, for example, that a rapidly-rotating star might be nonspherical in shape, and will not radiate isotropically. The intensity of a source towards a particular direction specified by spherical coordinates ($$\theta$$ , $$\phi$$) is the radiant flux radiated per unit solid angle in that direction. It is expressed in $$\text{W sr}^{-1}$$, and the standard symbol is $$I$$. In astronomical custom, the word "intensity" and the symbol $$I$$ are commonly used to describe a very different concept, to which we shall return later.