As mentioned above medium pressure UV lamps are gas discharge lamps. In operation the plasma is in the quartz glass tube, which generates the desired spectrum. The plasma has a temperature of approximately 5,000 to 7,000 K. This is made possible by the fact that a high current goes through the electrodes. Due to the small dimensions of the electrodes the current density is very high at their tips. Combined with the small dimensions this means that the electrodes are heavily loaded. After several hours of operation often the inner surface of the quartz glass tube begins to blacken in the electrode area. This is denoted as the so-called blackening-effect and can be significantly reduced by the addition of halogen.
By filling the lamp additionally with halogen, a cycle process is initiated which works as described: Tungsten, which is sputtered from the electrode deposits on the quartz glass tube (coldest place) and combines with the halogen. This combination breaks apart again near the electrode, depositing tungsten on it. Through this the depositing of tungsten on the quartz glass tube is prevented. A precondition for the optimal functioning of the halogen cycle is a sufficient high temperature of the quartz glass tube.
For this reason, we recommend that the tube wall temperature of a UV medium pressure lamp should be 700 – 900 °C.