Hollow-cathode lamp
A hollow-cathode lamp (HCL) is type of cold cathode lamp used in physics and chemistry as a spectral line source (for example, in atomic absorption spectrometers) and as a frequency tuner for light sources such as lasers. An HCL takes advantage of the hollow cathode effect, which causes conduction at a lower voltage and with more current than a cold cathode lamp that does not have a hollow cathode.