Atomic absorption spectroscopy (AAS) relies on the Beer-Lambert law, which requires that the radiation source emits a narrow range of wavelengths to match the absorption characteristics of the analyte atom. The primary criteria for choosing an appropriate radiation source in AAS is to provide a precise and intense emission at specific wavelengths that will allow accurate detection of the analyte.

Two common narrow-range 'line' sources used in AAS are hollow-cathode lamps (HCLs) and electrodeless-discharge lamps (EDLs), which were chosen based on the element being analyzed and the intensity required.

Hollow-Cathode Lamps (HCLs):
HCLs are widely used for elements that can be detected with moderate intensity. The lamp consists of a cylindrical hollow cathode coated with the analyte element and a tungsten or zirconium anode. These are housed in a glass tube filled with an inert gas at low pressure. When a voltage is applied across the electrodes, the inert gas atoms are ionized, causing them to bombard the cathode. This knocks out analyte atoms in a process called 'sputtering,' which leads to the emission of radiation specific to the element. HCLs are ideal for many routine AAS measurements due to their reliability and specificity.

Electrodeless-Discharge Lamps (EDLs):
EDLs are used for elements that require more precise or intense radiation. These lamps contain a sealed quartz tube containing an inert gas and the analyte element or its salt. By applying a strong radio-frequency or microwave field, the analyte is vaporized and excited, emitting its characteristic spectrum. EDLs provide more intense emissions than HCLs, making them suitable for elements that require higher sensitivity and precision.

Both HCLs and EDLs are selected based on the specific requirements of the AAS analysis, ensuring that the radiation emitted matches the analyte's absorption characteristics for accurate measurements.