On the importance of running Iwasaki's ColorArc lamps properly

Iwasaki's ColorArc range of low-wattage HID lamps (70-150 W) have the highest light color rendering of any quartz metal halide lamps (96 Ra8). This feat of engineering is achieved through the use of rare earth halides at high vapor pressure in burners operating at high temperature and whose thermal balance is finely tune for maximum performances and life. Interestingly, in order to ensure a good color consistency between lamps, the ColorArcs were specified for an operation at constant wattage on special ballasts. Moreover, their electrical characteristics are slightly different from those of other standard 70-150 W metal halide lamps.

The unfortunate consequence of such design characteristics is a noticeable shift of their light-technical properties when those lamps are run on standard European series-choke ballasts as this results in a reduction of the dissipated power. This affects the lamp's thermal balance and the burner cold-spot temperature, which are no longer optimal. To illustrate this issue, I took pictures of the three main color types (SDW= 3500 K top, SW = 4500 K middle, and FD = 6500 K bottom) under two modes of operation: at the nominal power level (right, using a variac to elevate the system's input voltage), and at the exact 230 V mains input voltage of the 70 W series-inductor ballast (left, resulting in a power dissipation reduced by about 5-10 W). The camera settings, including the white balance, were kept constant in order to ensure a proper comparison between cases (see the composite image above).

While the reduction in input power doesn't cause any significant color change in the daylight lamp (FD), the light color of the other ones acquires a noticeable greenish hue. This is caused by the presence of thallium in SDW and SW lamps, a green-emitting additive which is used for the optical absorption in the blue from its iodide vapor (one of the factors permitting a decrease of the light color temperature in those lamps). Lowering the burner temperature results in a faster decrease of the rare earth halide vapor pressures than that of thallium iodide, which is more volatile, hence causing a rapid degradation of the light color quality. Since thallium is not present in the FD lamp, its light color is therefore much more stable over the input power range considered here.

This color-vs-power characteristic certainly needs to be taken into account if one wants to run ColorArc lamps on a system other than that specified by Iwasaki. This is particularly important if the full color quality is expected from those lamps.