| Because the particular halides do not have high enough vapour pressure to attain the desired spectral performance at the available arc tube wall temperature. So more halide is added to raise the total vapour pressure, and improve performance.
It is maybe a rather old lamp, over time all manufacturers optimised their thermal designs to be able to reduce the salt dose. And in Sylvania’s case changed from dosing with pure mercury to 3% cadmium in mercury to improve colour rendering without need for halides.
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