Hydrargyrum Medium-arc Iodide Lamp - Flicker and Color Temperature

Flicker and Color Temperature

Similar to fluorescent lights, HMIs present problems with color temperature when used for film or video lighting applications. Unlike incandescent-lighting units, which are blackbody radiators limited to a theoretical maximum of 3680 K (the melting point of tungsten), HMI lamps, like all gas discharge lighting, emit the emission spectral lines of its constituent elements, specifically chosen so that combined, they resemble the blackbody spectrum of a 6000 K source. This closely matches the color of sunlight (but not skylight), because the sun's surface is a 6000 K blackbody radiator.

With HMI bulbs, color temperature varies significantly with lamp age. A new bulb generally will output at a color temperature close to 15,000 K during its first few hours. As the bulb ages, the color temperature reaches its nominal value of around 5600 K or 6000 K. With age, the arc length becomes larger as more of the electrodes burn away. This requires greater voltage to sustain the arc, and as voltage increases, color temperature decreases proportionately at a rate of approximately 0.5–1 kelvin for every hour burnt. For this reason, and other safety reasons, HMI bulbs are not recommended to be used past half their lifetime.

HMI bulbs (like all arc bulbs) need a current limiting unit to function. Two possibilities to do that 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 flickerfree 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.