Hydrargyrum medium-arc iodide lamp
Hydrargyrum medium-arc iodide (HMI) is the trademark name of Osram's brand of metal-halide gas discharge medium arc-length lamp,[1] made specifically for film and entertainment applications.
An HMI lamp uses mercury vapour mixed with metal halides in a quartz-glass envelope, with two tungsten electrodes of medium arc separation.
In the late 1960s German television producers requested lamp developer OSRAM to create a safer and cleaner replacement for carbon arc lighting used by the film industry.
Several other bulb variations exist, including GEMI (General Electric metal iodide), CID (compact iodide daylight;[3][4] Thorn EMI, UK, since 1990 GE), CSI (compact source iodine; Thorn EMI, UK), DAYMAX (made by ILC), and BRITE ARC (|[Sylvania Lighting Inc. (SLI)]).
Much research was invested into making HMI lamps smaller because of their use in moving light fixtures such as those manufactured by Vari-Lite, Martin, Robe, and Highend.
Philips' main contribution after this was the invention of a phosphor coating on the weld of the filament to the molybdenum foil that reduces oxidization and early failures at that point, making that area capable of withstanding extreme heat.
HMI bulbs (running with magnetic ballast) present an inherent problem of possibly producing light on film or video with a noticeable flicker.
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 do not exhibit perceptible flicker because their filaments do not cool down enough between cycles for their light output to decrease very much.
Essentially, a magnetic ballast is a large, heavy transformer coil that uses a simple principle to generate the high startup voltages needed to create an arc in a cold lamp.
Then, rectifiers and capacitors charge and discharge to invert the negative half of the AC cycle and convert the line to positive DC voltage.
This control board carefully adjusts the high frequency duty cycle of its transistors to maintain optimum color and light output as the lamp ages.
Since the IGBTs switch on and off at a regulated cycle rate, a generator can be slightly off-speed and the lamp will still be flicker-free, which is not the case with a standard magnetic ballast.
The lamp housing does not help this, acting as a resonating chamber that amplifies the noise and presents a problem for sync-sound recording for film and video.
To correct this, most electronic ballasts are equipped with a silent mode that eliminates the higher frequencies, but rounds off the voltage transition, causing the same flicker issue with magnetics, though to a lesser extent.
With a square wave voltage, the cathodes spend much more time emitting electrons and exciting the plasma, creating a gain of 5–10% in lumen output.
Nearly all present-day header cables are equipped with 'twist-lock' fittings at either end to ensure a safe and precise connection, both at the ballast and the lamp housing.
If the tip is oriented down, the rare earth metals will collect in it over time and the color of the lamp will shift as they are no longer included in the plasma arc.
Most lamp housing designs are inherently tougher and thicker than traditional tungsten units so that in the event of a bulb explosion, those nearby are protected from flying debris.
In 2014, journalist Kerry Sanders reported that he had been blinded for 36 hours as a result of overexposure to ultraviolet light from a malfunctioning HMI lamp.
