Metal halides lamps have a finite life and deteriorate with use. While the lamp may be rated for several tens of thousands of hours of use, in typical reef applications the lamps� output may drop by 30% or more in a year, necessitating a change of lamp. Several effects take place in these arc tubes that ultimately affect the lamp�s light output: deposits of electrode material build up on the arc tube�s wall, changes occur in the arc stream�s chemical composition, the quartz deteriorates to a more crystalline form that is opaque to light, etc. Each time a MH lamp is turned on; tungsten sputters from its electrodes and, over time, blackens the arc tube�s wall. Figure 4 compares a new lamp (left) to a lamp used for over a year (right). The deterioration in the lamp�s arc tube manifests itself as a change in its spectrum, shifting the lamp�s color temperature toward lower Kelvin values. This is often referred to in the hobby as a spectrum shift, and results from decreased output at different wavelengths, with larger reductions at smaller wavelengths i.e. towards the blue end of the spectrum
