The impact of dysprosium halide additives on a mercury arc

The picture above show how the aspect of a high-pressure mercury arc (5 mm at 30 bars) changes upon the addition of dysprosium halides. The hot plasma core shrinks due to radiative cooling of the plasma (more energy is being converted into light, which affects the plasma's thermal balance), while a reddish mantle appears as a result of the formation of dysprosium monohalide molecules at lower temperatures. While the latter species are instrumental in the color improvement of the emitted light, the resulting mantle causes color segregation in projected light beams. It is for this reason that artificial daylight lamps heavily dosed with such additive, such as the standard Osram HMI and Philips MSR, are used primarily in floodlighting applications. Short-arc light sources intended for projection applications use different fill chemistries that mitigate that problem while increasing the arc's brightness. The drawbacks, however, are that lumen efficacy and color quality are not as good.