Hollow Cathode Spectral Lamp

These are used to produce spectral lines for the metals within the cathode. This particular one has a copper cathode. In this case the lamp also contains argon which allows the discharge to occur between the anode and cathode. Where the discharge occurs inside the hollow cathode sputtering of the metal produces the metal spectral lines. The sputtered metal condenses on the outer glass wall and darkens it, as seen here. However, the top viewing window remains clear for light to pass. The insert image shows the discharge emission occurring inside the hollow cathode. The anode, formed by the metal tab, can be seen on the outside of the cathode. Light output from these lamps is very low and used purely for spectroscopy emission and absorption research. The lamp current is limited to 5mA in this case. The voltage is around over 300V to strike with a ballast resistor to limit the current. They have old style valve bases for connections.