The hollow-cathode lamp has to provide radiation characteristic of the element being analysed. Its name comes from its cathode cylindrical cup-shaped form. Just click here if you want me to describe for you how the lamp works.
The thing you must keep in mind is that the lamp emits electromagnetic radiation characteristic of the element the cathode is made of. There are some lamps which can be used to analyse more than one element because their cathodes contain atoms from two or more atomic species.
You might say: "Well, but why do we need a monochromator if the lamp emits characteristic radiation anyway ?". The answer is very simple: The lamp emits all the lines of that element and using the monochromator we select only one of them, usually the most sensitive one.
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