HMI bulbs (like all arc bulbs) need a current limiting unit to function. Two ways of doing this are described in the ballast section below. The problem of "flickering" exists only when using the bulb in combination with magnetic ballast (electronic ballasts produce flicker-free light). HMI bulbs (running with magnetic ballast) present an inherent problem of possibly producing light on film or video with a noticeable "flicker". This is caused by the method by which the unit produces light. An HMI, like an incandescent lighting unit, runs on mains power, which means that the lamp cycles on and off 100 or 120 times per second (twice for every line voltage cycle). Although not visible to the human eye, a film or video camera must be properly synchronized to this cycle or each frame recorded will show different light output. Although incandescent lamps also run off mains power, they don't exhibit perceptible flicker because their filaments don't cool down enough between cycles for their light output to decrease very much. For HMI lamps, flicker can be avoided by the use of electronic ballasts that cycle at frequencies thousands of times faster than the mains frequency.