High-intensity discharge lamp
High-intensity discharge (HID) lamps include these types of electrical lamps: mercury vapor, metal halide (also HQI), high-pressure sodium, low-pressure sodium and less common, xenon short-arc lamps. The light-producing element of these lamp types is a well-stabilized arc discharge contained within a refractory envelope (arc tube) with wall loading in excess of 3 W/cm² (19.4 W/in.²).
Compared with fluorescent and incandescent lamps, HID lamps produce a far higher quantity of light per unit area of lamp package.
Construction
HID lamps produce light by striking an electrical arc across tungsten electrodes housed inside a specially designed inner fused quartz or fused alumina tube. This tube is filled with both gas and metals. The gas aids in the starting of the lamps. Then, the metals produce the light once they are heated to a point of evaporation, forming a plasma.
Types of HID lamps include:
- Mercury vapor (CRI range 15-55)
- Metal halide (CRI range 65-80, ceramic MH can go to 90s)
- Low-pressure sodium (CRI 0 owing to their monochromatic light)
- High-pressure sodium (CRI range 22-75).
Mercury vapor lamps, which originally produced a bluish-green light, were the first commercially available HID lamps. Today, they are also available in a color corrected, whiter light. But they are still often being replaced by the newer, more efficient high-pressure sodium and metal halide lamps. Standard low-pressure sodium lamps have the highest efficiency of all HID lamps, but they produce a yellowish light. High-pressure sodium lamps that produce a whiter light are now available, but efficiency is somewhat sacrificed. Metal halide lamps are less efficient but produce an even whiter, more natural light. Colored metal halide lamps are also available.
Auxiliary devices
Like fluorescent lamps, HID lamps require a ballast to start and maintain their arcs. The method used to initially strike the arc varies: mercury vapor lamps and some metal halide lamps are usually started using a third electrode near one of the main electrodes while other lamp styles are usually started using pulses of high voltage.
Applications
HID lamps are typically used when high levels of light over large areas are required, and when energy efficiency and/or light intensity are desired. These areas include gymnasiums, large public areas, warehouses, movie theaters, outdoor activity areas, roadways, parking lots, and pathways. More recently, HID lamps, especially metal halide, have been used in small retail and residential environments. HID lamps have made indoor gardening practical, especially for plants that require a good deal of high intensity sunlight, like vegetables and flowers. They are also used to reproduce tropical intensity sunlight for indoor aquaria.
Some HID lamps such as Mercury Vapor Discharge produce large amounts of UV radiation and therefore need diffusers to block that radiation. In the last few years there have been several cases of faulty diffusers, causing people to suffer severe sunburn and Arc eye. Regulations may now require guarded lamps or lamps which will quickly burn out if their outer envelope is broken.
Recently, HID lamps have gained use in motor-vehicle headlamps. This application has met with mixed responses from motorists, mainly in response to the amount of glare that HID lights can cause. They often have an automatic self-leveling system to minimise this issue and as such are usually an expensive optional extra on most cars. However, many motorists still prefer these lights as they emit a clearer, brighter, more natural appearing light than normal headlamps.
HID lamps are used in high-end bicycle headlamps. They are desirable because they produce much more light than a halogen lamp of the same wattage. Halogen lights appear somewhat yellow in color; HID bicycle lights look faintly blue-violet.
HID lamps are also being used on many general aviation aircraft for landing and taxi lights.
Mercury-vapor lamp
A mercury-vapor lamp is a gas discharge lamp that uses mercury in an excited state to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb. The outer bulb may be clear or coated with a phosphor; in either case, the outer bulb provides thermal insulation, protection from ultraviolet radiation, and a convenient mounting for the fused quartz arc tube.
Mercury vapor lamps (and their relatives) are often used because they are relatively efficient. Phosphor coated bulbs offer better color rendition than either high- or low-pressure sodium vapor lamps. They also offer a very long lifetime, as well as intense lighting for several applications.
Theory and relations
The mercury vapor lamp is a negative resistance device and requires auxiliary components (for example, a ballast) to prevent it from taking excessive current. The auxiliary components are substantially similar to the ballasts used with fluorescent lamps. It is used often for outside lighting (signs) and for auditoriums and stages.
Also like fluorescent lamps, mercury vapor lamps usually require a starter, which is usually contained within the mercury vapor lamp itself. A third electrode is mounted near one of the main electrodes and connected through a resistor to the other main electrode. When power is applied, there is sufficient voltage to strike an arc between the starting electrode and the adjacent main electrode. This arc discharge eventually provides enough ionized mercury to strike an arc between the main electrodes. Occasionally, a thermal switch will also be installed to short the starting electrode to the adjacent main electrode, completely suppressing the starting arc once the main arc strikes.
Variation: Metal halide
A closely related lamp design, called the metal halide lamp, uses various other elements in an amalgam with the mercury. Sodium iodide and scandium iodide are commonly used. These lamps can produce much better quality light without resorting to phosphors. If they use a starting electrode, there is always a thermal shorting switch to eliminate any electrical potential between the main electrode and the starting electrode once the lamp is lit. (This electrical potential in the presence of the halides can cause the failure of the glass/metal seal). More modern metal halide systems do not use a separate starting electrode; instead, the lamp is started using high-voltage pulses, as with high-pressure sodium vapor lamps. "MetalArc" is Osram Sylvania's trademark for their metal halide lamps; "Arcstream" and "MultiVapor" are General Electric's trademark. Besides their use in traditional outdoor lighting, these lamps now appear in most computer and video projectors.
Operation
When the lamp is first turned on, mercury-vapor lamps will produce a dark blue glow because only a small amount of the mercury is ionized and the gas pressure in the arc tube is very low (so much of the light is produced in the ultraviolet mercury bands). As the main arc strikes and the gas heats up and increases in pressure, the light shifts into the visible range and the high gas pressure causes the mercury emission bands to broaden somewhat, producing a light that appears more-white to the human eye (although it is still not a continuous spectrum). Even at full intensity, the light from a mercury vapor lamp with no phosphors is distinctly bluish in color.
Color considerations
To correct the bluish tinge, many mercury vapor lamps are coated on the inside of the outer bulb with a phosphor that converts some portion of the ultraviolet emissions into red light. This helps fill in the otherwise very-deficient red end of the electromagnetic spectrum. These lamps are generally called "color corrected" lamps. Most modern mercury vapor lamps have this coating. One of the original complaints against mercury lights was they tended to make people look like "bloodless corpses" because of the lack of light from the red end of the spectrum. There is also an increase in red color (e.g., due to the continuous radiation) in ultra-high pressure mercury vapor lamps (usually greater than 200 atm.) which has found application in modern compact projection devices.
Emits Wavelengths - 253.7, 365.4, 404.7, 435.8, 546.1, and 578.0 nm.
Ultraviolet hazards
All mercury vapor lamps (including metal halide lamps) must contain a feature (or be installed in a fixture that contains a feature) that prevents ultraviolet radiation from escaping. Usually, the borosilicate glass outer bulb of the lamp performs this function but special care must be taken if the lamp is installed in a situation where this outer envelope can become damaged.[1] There have been documented cases of lamps being damaged in gymnasiums and sun burns and eye inflammation have resulted.[2] When used in locations like gyms, the fixture should contain a strong outer guard or an outer lens to protect the lamp's outer bulb. Also, special "safety" lamps are made which will deliberately burn out if the outer glass is broken. This is usually achieved by a thin carbon strip used to connect one of the electrodes, which will burn up in the presence of air.
Even with these methods, some UV radiation can still pass through the outer bulb of the lamp. This causes the aging process of some plastics used in the construction of luminaires to be sped up, leaving them horribly discoloured after only a few years' service. Polycarbonate suffers particularly from this problem; and it is not uncommon to see fairly new polycarbonate surfaces positioned near the lamp to have turned a dull, 'ear-wax'-like colour after only a short time. Certain polishes, such as Brasso, can be used to remove some of the yellowing, but usually only with a limited success.
Light pollution considerations
For placements where light pollution is of prime importance (for example, an observatory parking lot), low pressure sodium is preferred. As it emits light on only one wavelength, it is the easiest to filter out. Mercury vapor lamps without any phosphor are second best; they produce only a few distinct mercury lines that need to be filtered out.
End of life
At the end of life, many types of high-intensity discharge lamps exhibit a phenomenon known as cycling. These lamps can be started at a relatively low voltage but as they heat up during operation, the internal gas pressure within the arc tube rises and more and more voltage is required to maintain the arc discharge. As a lamp gets older, the maintaining voltage for the arc eventually rises to exceed the voltage provided by the electrical ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast can once again cause the arc to strike. The effect of this is that the lamp glows for a while and then goes out, repeatedly.
More-sophisticated ballast designs detect cycling and give up attempting to start the lamp after a few cycles. If power is removed and reapplied, the ballast will make a new series of startup attempts.
See also
ReferencesISBN links support NWE through referral fees
- Waymouth, John (1971). Electric Discharge Lamps. Cambridge, MA: The M.I.T. Press. ISBN 0-262-23048-8.
- National Highway Traffic Safety Administration. Glare from headlamps and other front mounted lamps. Federal Motor Vehicle Safety Standard No. 108. US Department of Transportation. Retrieved 2006-01-23.
External links
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Natural/prehistoric light sources: |
Bioluminescence | Celestial objects | Lightning |
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Combustion-based light sources: |
Acetylene/Carbide lamps | Candles | Davy lamps | Fire | Gas lighting | Kerosene lamps | Lanterns | Limelights | Oil lamps | Rushlights | |
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Nuclear/direct chemical light sources: |
Betalights/Trasers | Chemoluminescence (Lightsticks) | |
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Electric light sources: |
Arc lamps | Incandescent light bulbs | Fluorescent lamps | |
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High-intensity discharge light sources: |
Ceramic Discharge Metal Halide lamps | HMI lamps | Mercury-vapor lamps | Metal halide lamps | Sodium vapor lamps | Xenon arc lamps | |
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Other electric light sources: |
Electroluminescent (EL) lamps | Globar | Inductive lighting | Discrete LEDs/Solid State Lighting (LEDs) | Neon and argon lamps | Nernst lamp | Sulfur lamp | Xenon flash lamps | Yablochkov candles | |
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- ↑ http://www.sylvania.com/content/display.scfx?id=003690196
- ↑ Thun, MJ and Altman R, Ellingson O, Mills LF, Talansky ML (Nov 1982). Ocular complications of malfunctioning mercury vapor lamps. Ann Ophthalmol. 14 (11): 1017-20. PMID: 7181332.
