Hydrargyrum medium-arc iodide

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Hydrargyrum medium-arc iodide, or HMI, is a mercury-halide gas discharge medium arc-length lamp with a multi-line spectra emission. The name implies that hydrargyrum, an archaic term for Mercury (Hg), is held as a vapour mixed with other rare halides in a quartz-glass envelope with two tungsten-coated electrodes of medium arc separation.

Unlike traditional tungsten lighting units, HMIs use ballasts to regulate and supply electricity to the lamp head via a header cable. The lamp operates not by heating a tungsten-based filament, but rather by creating an electrical arc between two electrodes within the bulb that excites the pressurized mercury vapour and provides phenomenal light output with greater efficiency than tungsten-based lighting units. The efficiency advantage is near four-fold, with approximately 85-108 lumens per watt of electricity. Unlike tungsten bulbs where the gas is inert and solely for filler and recirculation, HMI bulbs rely heavily on the mercury vapour for light output, and the other metal halides mixed with the mercury to give it the spectral peaks in output wavelengths that bring it to approximately 5600K, or the color temperature of noon sunlight.

Contents

1 History
2 Flicker & Color Temperature
3 Ballast Operation
4 Safety
5 See also
6 References

History

In the late 1960s German television producers sought out lamp developer OSRAM to create a less expensive replacement for incandescent lights for the film industry. Osram developed and began producing HMI bulbs at their request.

Philips produced a variation on the HMI, a single-ended version called MSR (Medium Source Rare-Earth). It uses a standard two-prong lampbase. In order to avoid the colour shift over use they added a secondary envelope around the gas-chamber. Several other bulb variations exist, including GEMI (General Electric Metal Iodide), CID (Compact Indium Discharge; Thorne, UK), CSI (Compact Source Iodine; Thorne, UK), DAYMAX (made by ICL), and BRITE ARC (Sylvania). All are variations and different names for essentially the same concept.

Within the last ten years, a lot of research has gone into making HMI bulbs smaller because of their use in moving light fixtures such as the Vari-Lite, Martin and Highend. Philips' main contribution after this was the invention of new ceramic bases that are capable of withstanding extreme heat. Osram developed a new shape to their bases and added gold-coating to it.

Multi-kilowatt HMI lights are used in the film industry because of their daylight-balanced light output, as well as their efficiency.

Flicker & Color Temperature

Similar to fluorescent lights, HMI's present problems with color temperature when used for film or video lighting applications. Unlike tungsten units, HMI units do not emit a continuous spectrum of wavelengths with a gradual peak at 3200K, but rather emit various lines, or peaks, of wavelengths that combined appear to be a color temperature closely resembling 5600K on the traditional color temperature scale. This spectrum of wavelengths is inherent in gas discharge sources (such as the sun) and does not occur in blackbody radiator sources, such as a tungsten lamp. The HMI, due to its composition of rare earth halides, produces a rather large green spike in its emission spectrum. This green spike, although slight, tends to result in much harsher and unpleasing color reproduction on film or video. As a result, this green "curse" is often corrected with a 1/8 or 1/4 Color Temperature Blue (CTB) gel.

With HMI bulbs, color temperature varies significantly with lamp age. A brand new bulb generally will output at a color temperature close to 15,000 K during its first few hours burnt. As the bulb burns in, the color temperature reaches its nominal value of around 5600K or 6000K. With age, the arc length becomes larger and larger as more of the electrodes burn away. This results in more voltage being needed 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 present an inherent problem of possibly producing light on film or video with a noticeable pulse or flicker. This is caused by the method by which the unit produces light. An HMI, like a tungsten-based lighting unit, runs on 60Hz 120V line voltage (in North America), which means that the lamp cycles on and off 120 times in a second (twice for every line cycle). Although not visible to the human eye, a film or video camera must be properly synchronized to this cycle, or else each frame recorded will show different light output. Tungsten-based lights do not have this problem due to the nature of their operation. Tungsten is a natural blackbody radiator that emits lights in increasing wavelengths proportionate to its temperature. As the filament heats up as electrons pass through it, it emits the correct wavelength of light that it is calibrated for, and although the line voltage is still cycling at the same 60Hz, the filament does not have enough time to significantly cool in between cycles to cause a shift in light output. Therefore the light appears consistent on film and to the human eye. HMI units are more complicated, unfortunately, since they employ an arc that cycles on and off with the line.

Ballast Operation

In order to power an HMI bulb, special ballasts act as an ignitor to initially start the arc, and then regulate it by acting as a choke. Two types of ballasts exist - magnetic and electronic (square-wave or flicker-free). Magnetic ballasts are generally much heavier and bulkier than electronic ballasts, yet can usually be obtained for much cheaper. Standard magnetic ballasts exhibit the previous mentioned problems of flicker on film or video unless the camera being used is crystal-controlled, the camera is run at a specific frame rate that evenly divides into 120, and the line voltage is crystal-controlled at 60Hz. If all three of these requirements are not met, a noticeable pulsing will be seen on the final image. Magnetic ballasts, however, are very simple devices compared to electronic ballasts. Essentially, a magnetic ballast is a large, heavy transformer coil that uses a very simple principle to generate the high startup voltages needed to arc a cold bulb. Input power is routed to a choke coil connected between the main input and the lamp. The coil may be tapped in several places to provide for various input voltages (120V or 240V) and a high start-up voltage. Capacitors are also included to compensate for the inductance of the coil and restore a unity power factor. Because of the high amount of current running through the ballast, a dull humming is often heard as the resin holding the coils together weakens by magnetic force fluctuations from induction.

Within the last ten years, electronic flicker-free (or Square-Wave) ballasts have become increasingly more popular and affordable as an alternative to magnetic ballasts that eliminates most of the problems associated with HMI flicker. Unfortunately, their operation is not as simple as a magnetic ballast. Electronic ballasts can be thought of as operating in three stages - a DC intermediate converter, a power module, and an AC inverter. Power initially flows through the main breakers into an RF mains filter that prevents the flow of noise back onto the supply service. 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 is called the DC intermediate. In the second stage, a buck converter draws from the DC intermediate and regulates current flow to the final power electronics via an electronic control board. This control board carefully adjusts the high frequency duty cycle of its transistors to maintain optimum color and light output as the lamp ages. Finally, the regulated current flow is inverted by an LF-converter board that uses four Insulated Gate Bipolar Transistors (IGBTs) to switch the DC signal on and off at precisely 60Hz in a square-wave pattern (unlike the sinusoidal pattern of line AC).

By using a square-wave output that is not referenced to the line cycle rate, a flicker-free output can be produced. 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. As well, the square wave nature of the output results in a straight-line power output from the lamp. The lamp switches on and off almost instantaneously, which means that safe (flicker-free) filming can occur at camera framerates up to 10,000fps on most electronic ballasts. By analyzing power output, which can be considered the product of the current and voltage waveforms, the negative portions of the waveform multiply together to produce a positive straight-line output.

Unfortunately, this very sharp switching on and off inherent to the square-waveform causes extremely high frequency vibrations in the bulb. The rising and falling edges of the waveform can be thought of as having an extremely high frequency, while the straight-line portions of the waveform can be thought of as having an extremely low frequency (or long wavelength). As a result, the bulb emits a high-pitched whistle when in flicker-free mode with an electronic 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 rounds off the corners of the square-waveform to make a softer transition from such a high frequency to such a low frequency. This mode provides safe, flicker-free filming at framerates up to 34fps on most electronic ballasts.

In addition to solving the problems of flicker, electronic ballasts also provide other advantages over magnetic ballasts. Because light output is carefully regulated by the ballast, a 5% increase in light output from the bulb is possible, making electronic ballasts more efficient than magnetic ballasts. As well, the square-wave nature of the power flow allows bulb life to be extended by as much as 20%. Most modern ballasts are now also equipped with a dimmer, which allows current to be controlled to allow the lamp to be dimmed to up to 50%, or as much as one stop of light. Just as is the case with dimming a tungsten-based light, however, color temperature will shift, though in the opposite direction (approximately 200K bluer at 50% output).

Safety

HMI bulbs are roughly the same color temperature as the sun, and like most other mercury-based high intensity discharge lamps, generate ultra-violet light. Each HMI light has a UV safety glass cover that should be used to protect any people that may be in front of the light. Exposure to an unprotected bulb can cause retina damage and severe skin burns.

HMI lamps can reach ignition voltages of up to 17,000V when striking hot, and are therefore considered very dangerous if miswired. It is generally good practice to strike the light from the ballast and not the head, in the event that there is a short in the lamp head. Proper striking procedures should be followed as well, such as calling out whenever a light is turned on to warn those around, and ensuring that the header cable is properly and securely connected (most header cables will twist and click into place).

In addition to these concerns, HMI bulbs have been known to explode violently at the end of their lifetime or if stressed enough. While not as violent as the explosion of a xenon short-arc bulb, it is still something to be cautious of. As a result, HMI bulbs should not be used past half their rated lifetime, and care should be taken with larger bulbs when striking, as a bulb will most likely explode within the first five minutes of striking. Care should also be taken transporting the bulb and replacing bulbs. The gasses in an HMI bulb are held under pressure, which increases dramatically with bulb temperature, and dropping the bulb could result in an explosion, sending hot quartz glass flying. Like quartz-halogen bulbs, care should be taken not to touch the glass directly as skin oils can attract heat and cause a weak point on the bulb. 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. Yet there is still the possibility of the front lens element on the lamp head cracking from thermal shock. Once again, proper safety procedures should be followed when using HMI units, as they can be quite dangerous if misused.

References

[Short Arc/Compact Source Lamps]
[Working with HMI lights]
Box, Harry. Set Lighting Technician's Handbook 3rd Ed., Focal Press, 2003.

L i g h t i n g and L a m p s
Incandescent:	Incandescent light bulb>Conventional - Halogen - Parabolic Aluminized Reflector (PAR)
Fluorescent:	Compact fluorescent lamp>Compact Fluorescent (CFL) - Linear fluorescent - Induction lamp
Gas discharge:	High-intensity discharge lamp>High-Intensity Discharge (HID) - Mercury-vapor - Metal-halide - Neon - Sodium vapor
Electric arc:	Arc lamp - Hydrargyrum medium-arc iodide>HMI - Xenon arc - Yablochkov candle
Combustion:	Carbide lamp>Acetylene/Carbide - Candle - Gas lighting - Kerosene lamp - Limelight - Oil lamp - Safety lamp
Other types:	Sulfur lamp - Light-emitting diode>Light-Emitting Diode (LED) - Optical fiber - Plasma