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A HMI on a stand.


Hydrargyrum medium-arc iodide, or HMI®, is a mercury-halide gas discharge medium arc-length lamp with a multi-line emission spectrum. The "H" in HMI stands for hydrargyrum, latin for mercury (Hg). An HMI lamp uses mercury vapour mixed with metal halides in a quartz-glass envelope, with two tungsten-coated electrodes of medium arc separation.

Unlike traditional lighting units using incandescent light bulbs, HMIs use ballast 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 very high light output with greater efficiency than incandescence-based lighting units. The efficiency advantage is near fourfold, with approximately 85-108 lumen per watt of electricity. Unlike incandescent lightbulbs where the gas is inert and solely for filler and recirculation purposes, HMI bulbs rely heavily on the mercury vapour and the metal halides to give them the spectral peaks in their light output to approximate the sunlight.

History

In the late 1960s Germanmarker 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 during 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 ILC), 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 those manufactured by Vari-Lite, Martin, and Highend. Philips' main contribution after this was the invention of a phosphor coating on the weld of the filament to the molybednum foil that reduces oxidization and early failures at that point, making that area capable of withstanding extreme heat.

Multi-kilowatt HMI lights are used in the film industry and for large-screen slide projection because of their daylight-balanced light output, as well as their efficiency.

Flicker & Color Temperature

Similar to fluorescent lights, HMIs present problems with color temperature when used for film or video lighting applications. Unlike incandescent-lighting units, HMI units do not emit a continuous spectrum with a gradual peak at 3200 K, but rather emit various lines, or peaks, of wavelengths that, combined, resemble the 6000 K blackbody spectrum. This discontinuous spectrum is inherent to most gas discharge.

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 5600 K or 6000 K. 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 (like all arc bulbs) need a current limiting unit in order 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 else each frame recorded will show different light output. Although incandescent lamps also run off of mains power, they don't exhibit 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.

Ballast operation

A HMI ballast.
In order to power a 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 at lower cost. Standard magnetic ballasts exhibit the previously 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 60 Hz. 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 create an arc in a cold lamp. 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 (120 V or 240 V) and a high start-up voltage. Capacitors are also included to compensate for the inductance of the coil and improve the power factor. Because of the high amount of current through the ballast, a low humming sound is often heard due to magnetostriction of the ballast iron laminations. Some magnetic ballasts have internal insulation around the coil allowing silent operation.

Within the last ten years, electronic flicker-free (or Square-Wave) ballasts have become increasingly popular and affordable as an alternative to magnetic ballasts by eliminating 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 incoming power line. 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 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 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 60 Hz in a square wave pattern (unlike the sinusoidal pattern of line AC). Leaders in this field include Power Gems Corp, B&S, & Mytronic.

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. 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 10,000 frame/s 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 lamp. 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 24 frame/s 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. The square-wave nature of the power flow allows lamp 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 200 K bluer at 50% output).

Safety

HMI lamps are approximately the same color temperature as the sun, and as with 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 persons that may be in front of the light. Exposure to an unprotected lamp can cause retinal damage and severe skin burns.

HMI lamps can reach ignition voltages of up to 70,000 V when striking hot, and are considered very dangerous if miswired. It is good practice to strike the light from the ballast and not the head, in the event that there is a short circuit in the lamp head. Proper striking procedures should be followed as well, such as calling out a vocal warning whenever a light is turned on to warn persons in the area. Also, the header cable should be properly and securely connected (most header cables will twist and click into place).

In addition to these concerns, HMI lamps 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, they still require caution. As a result, HMI lamps should not be used past half their rated lifetime, and care should be taken with larger lamps when striking (turning on the lamp), as a lamp is most likely to explode within the first five minutes of striking. Care should also be taken transporting the lamp and replacing lamps. The gasses in an HMI lamp are under pressure, which increases with temperature. Dropping the lamp could result in an explosion, sending hot quartz glass flying. As with 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. There is the possibility of the front lens element on the lamp head cracking from thermal shock. Proper safety procedures should be followed when using HMI units, as they can be quite dangerous if misused.

See also



References




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