A major difficulty with long linear metal halide type lamps is nonuniformity of light output along the axis of the lamp. The extent of nonuniformity increases with the number of metals included in the lamp filling and the factors primarily responsible for it are cataphoresis and thermal gradients. The cataphoretic effect is the stronger and the nonuniformity may be overcome by supplying a DC component to the discharge current and controlling its polarity and magnitude. In one system, the lamp is operated from a DC power supply through a control circuit using semi-conductors for polarity reversing switches wherein the time of current flow in one polarity is determined by the direction and degree of unbalance of the spectral output.
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