Graphic art metal halide lamp at two power settings

Based on Sylvania's Metalarc, the MP500T16 is a special metal halide lamp filled with gallium iodide emitting mainly in the 360–450 nm range for reprographic and photochemical applications. The lamp is designed to start and run on standard M59 400 W CWA transformer ballasts with a different series series capacitor so as to let in more current in the discharge (4.04 vs. 3.25 A) for the dissipated power to reach 500 W. In order to avoid any risk of ballast overheating, to conserve energy, and to extend service life, the lamp power was usually reduced by half to 250 W between exposures. This was achieved by replacing the capacitive impedance between the lamp and the ballast by two capacitors connected in parallel, one of which was disconnected to reduce the discharge current. The picture above shows the lamp at high (top) and low (bottom) power settings.