$$\require{cancel}$$
Instead of the word "radiance", for which we have hitherto used the symbol $$L$$, the name commonly used in the context of stellar atmosphere theory is specific intensity and the symbol used is $$I$$. In most branches of physics, the word "specific" is used to mean "per unit mass", but that meaning is not intended in the present context. It might be noted that the word "specific" is often omitted, so that what we have hitherto called "radiance" $$L$$ is now "intensity" $$I$$. Note also that this in not the same as what is meant in "standard" usage by "intensity", for which the symbol, in standard usage, is also $$I$$. In this and subsequent chapters, I shall always include the word "specific".