Hollow-cathode lamp
A HCL consists of an anode and a cathode inside of a glass tube. The glass tube is filled with some type of inert gas such as argon or neon with a pressure of around 5 torr (666 Pascal). It is important that the gas is inert in order to minimize interferences in the output of the data. The anode is an electrode in which loss of electrons by ionization takes place.  The anode is typically constructed of an inert conducting metal to ionize the inert gas.