Hydrargyrum medium-arc iodide (HMI) is the trademark name of
Osram
Osram Licht AG is a German company that makes electric lights, headquartered in Munich and Premstätten (Austria). Osram positions itself as a high-tech photonics company that is increasingly focusing on sensor technology, visualization and tre ...
's
brand
A brand is a name, term, design, symbol or any other feature that distinguishes one seller's good or service from those of other sellers. Brands are used in business, marketing, and advertising for recognition and, importantly, to create an ...
gas discharge Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionization of the gas. Depending on several factors, the discharge may radiate visible light. The properties of electric discharges in gases are studied i ...
medium arc-length lamp, made specifically for film and entertainment applications. ''Hydrargyrum'' comes from the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
name for the element
mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
.
An HMI lamp uses mercury vapour mixed with metal
halide
In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...
s in a
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
-glass envelope, with two tungsten electrodes of medium arc separation. Unlike traditional lighting units using
incandescent light bulbs
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxidat ...
, HMIs need electrical ballasts, which are separated from the head via a header cable, to limit current and supply the proper voltage. The lamp operates by creating an electrical arc between two electrodes within the bulb that excites the pressurized mercury vapour and metal halides, and provides very high light output with greater
efficiency
Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
than incandescent lighting units. The efficiency advantage is near fourfold, with approximately 85–108 lumens per
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...
of electricity. Unlike regular incandescent halogen lamps where a halide gas is used to regenerate the filament and keep the evaporated tungsten from darkening the glass, the mercury vapour and the metal halides in HMI lamps are what emit the light. The high
color rendering index
A color rendering index (CRI) is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with a natural or standard light source. Light sources with a high CRI are desirable in ...
(CRI) and
color temperature
Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
are due to the specific lamp chemistry.
History
In the late 1960s
German
German(s) may refer to:
* Germany (of or related to)
**Germania (historical use)
* Germans, citizens of Germany, people of German ancestry, or native speakers of the German language
** For citizens of Germany, see also German nationality law
**Ger ...
television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
producers requested lamp developer
OSRAM
Osram Licht AG is a German company that makes electric lights, headquartered in Munich and Premstätten (Austria). Osram positions itself as a high-tech photonics company that is increasingly focusing on sensor technology, visualization and tre ...
to create a safer and cleaner replacement for carbon arc lighting used by the
film industry
The film industry or motion picture industry comprises the technological and commercial institutions of filmmaking, i.e., film production companies, film studios, cinematography, animation, film production, screenwriting, pre-production, post ...
. Osram developed and began producing HMI lamps at their request.
Philips
Koninklijke Philips N.V. (), commonly shortened to Philips, is a Dutch multinational conglomerate corporation that was founded in Eindhoven in 1891. Since 1997, it has been mostly headquartered in Amsterdam, though the Benelux headquarters i ...
produced a variation on the HMI, a single-ended version called MSR/HR (medium source rare-earth hot-restrike). It uses a standard two-prong lampbase. 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 iodide daylight; Thorn EMI, UK, since 1990 GE), CSI (compact source iodine; Thorn EMI, UK), DAYMAX (made by ILC), and BRITE ARC (, ylvania Lighting Inc. (SLI). All are variations on, and different names for, essentially the same concept.
Much research was invested into making HMI lamps smaller because of their use in moving light fixtures such as those manufactured by
Vari-Lite
Vari-Lite is a brand of automated, variable-colour stage lighting systems. Their intelligent lighting fixtures are commonly used in theatre, concerts, television, film and corporate events.
History
Pre-history
The origins of Vari-Lite date to ...
, 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.
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.
The lamp is a favorite among film enthusiasts, with widespread coverage of the lamps and their history featuring in educational institutions.
Flicker and color temperature
Similar to
fluorescent lights
A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet lig ...
, HMIs present problems with color temperature when used for film or video lighting applications. Unlike incandescent-lighting units, which are
blackbody
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
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
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
's surface is a 6000 K blackbody radiator.
With HMI bulbs,
color temperature
Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
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 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.
Ballast operation
To power an HMI bulb, special ballasts act as an ignitor to 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, as they consist primarily of a network of large inductors. They are usually cheaper than electronic ballasts. Since the magnetic type of ballast does not maintain the discharge continuously, the lamp actually extinguishes at zero-crossings of the mains waveform; unless the camera is locked to the mains waveform, the difference in frequency between the lamp and the shutter will produce a
beat frequency
In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, ''perceived'' as a periodic variation in volume whose rate is the difference of the two frequencies.
With tuning instruments that can produce su ...
that is visible in the resulting recording. This is why TV standards typically use the power grid frequency as their basic frame rate. Magnetic ballasts are simple devices compared to electronic ballasts. Essentially, a magnetic ballast is a large, heavy
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
coil that uses a 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
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of a ...
are also included to compensate for the inductance of the coil and improve the
power factor
In electrical engineering, the power factor of an AC power system is defined as the ratio of the '' real power'' absorbed by the load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of v ...
. Because of the high amount of
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
through the ballast, a low humming sound is often heard due to magnetostriction of the ballast iron laminations. Some magnetic ballasts have insulation around the coil for silent operation.
Since the early 1990s, 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
A buck converter (step-down converter) is a DC-to-DC power converter which steps down voltage (while stepping up current) from its input (supply) to its output (load). It is a class of switched-mode power supply (SMPS) typically containing at ...
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
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
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 (
IGBT
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch, which, as it was developed, came to combine high efficiency and fast switching. It consists of four alternating lay ...
s) to switch the DC at precisely 60 Hz into a
square wave
A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions b ...
AC (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 time where cathodes aren't emitting electrons of high enough energy is very short, meaning that safe (flicker-free) filming can occur at camera framerates up to 10,000 frame/s on most electronic ballasts.
Unfortunately, this very sharp switching on and off inherent to the square-waveform causes extremely high frequency vibrations in the lamp. A
square wave
A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions b ...
can be thought of as an infinite sum of odd-numbered harmonics, which will include frequencies at the
resonant frequency
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
of the bulb, causing it to vibrate at that frequency like a bell or whistle. 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. 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. 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. 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 uses
pulse-width modulation
Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed ...
to dim the lamp up to 50%, or as much as one
stop
Stop may refer to:
Places
* Stop, Kentucky, an unincorporated community in the United States
* Stop (Rogatica), a village in Rogatica, Republika Srpska, Bosnia and Herzegovina
Facilities
* Bus stop
* Truck stop, a type of rest stop for truck d ...
of light. Unlike a tungsten-based light, which has a negative color temperature shift with a drop in power, the mercury emission spectra takes over with a drop in power (approximately 200 K bluer at 50% output).
Safety
HMI lamps are approximately the same color temperature as the sun at noon (6000 K), and like other high intensity discharge lamps which contain mercury, generate
ultra-violet light
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
. Each HMI fixture has a UV safety glass cover that should be used to protect people who 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. 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.
In all quartz lamps which have a gas fill, there is a small area that looks like a nipple. It is in fact the location of the exhaust tube where the lamp was filled with its gas mixture. The position of the nipple is very important and if it's facing the wrong way, it may show up as a shadow in the optical path. Whenever possible, the exhaust tip should be oriented straight up, or up to 45 degrees from vertical. This will keep the tip in the hottest location, and prevent the iodides and rare earth metals from collecting in it as the lamp cools. 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.
In addition to these concerns, HMI lamps have been known to shatter at the end of their lifetime or if stressed enough. While not as violent as the explosion of a xenon short-arc bulb, they require caution. As a result, HMI lamps should not be used beyond half of their rated life, 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. For this reason, each HMI lamp will usually carry with it a detailed log of the number of strikes and number of hours it has been used. Care should be taken transporting the lamp and replacing lamps. The gases in an HMI lamp are under a very small pressure, but it does increase with temperature. As with quartz-halogen bulbs, care should be taken not to touch the glass directly as skin oils left behind on the envelope can heat beyond the working temperature of the envelope and cause bubbling and/or a weak point. For this reason, anytime a lamp is handled it should be cleaned with an isopropyl alcohol wipe. 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 (though not completely blowing out or shattering). Proper safety procedures should always be followed when using HMI units, as they can be dangerous if misused.
In 2014, journalist
Huffington Post
''HuffPost'' (formerly ''The Huffington Post'' until 2017 and sometimes abbreviated ''HuffPo'') is an American progressive news website, with localized and international editions. The site offers news, satire, blogs, and original content, and ...
; by Katherine Fung; published March 7, 2014; retrieved March 7, 2014
See also
*
High-intensity discharge lamp
High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tu ...
(HID)
*
List of light sources
This is a list of sources of light, the visible part of the electromagnetic spectrum. Light sources produce photons from another energy source, such as heat, chemical reactions, or conversion of mass or a different frequency of electromagnetic ener ...