Mercury-vapor lamp
Encyclopedia
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
through vaporized mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
to produce light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
. The arc discharge is generally confined to a small fused quartz
Fused quartz
Fused quartz and fused silica are types of glass containing primarily silica in amorphous form. They are manufactured using several different processes...
arc tube mounted within a larger borosilicate glass
Borosilicate glass
Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
bulb. The outer bulb may be clear or coated with a phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
; in either case, the outer bulb provides thermal insulation
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
, protection from the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation the light produces, and a convenient mounting for the fused quartz arc tube.
Mercury vapor lamps (and their relatives) are more energy efficient than incandescent and most fluorescent lights, with luminous efficacies
Luminous efficacy
Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power. Depending on context, the power can be either the radiant flux of the source's output, or it can be the total electric power consumed by the source.Which sense of the term is...
of 35 to 65 lumens/watt. Their other advantages are a long bulb lifetime in the range of 24,000 hours and a high intensity, clear white light output. For these reasons, they are used for large area overhead lighting, such as in factories, warehouses, and sports arenas as well as for streetlights. Clear mercury lamps produce white light with a bluish-green tint due to mercury's combination of spectral lines. This is not flattering to human skin color, so such lamps are not used in retail stores. "Color corrected" mercury bulbs overcome this problem with a phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
on the inside of the outer bulb that emits white light. They offer better color rendition than the more efficient high or low-pressure sodium vapor lamp
Sodium vapor lamp
A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure...
s.
Origins
Charles WheatstoneCharles Wheatstone
Sir Charles Wheatstone FRS , was an English scientist and inventor of many scientific breakthroughs of the Victorian era, including the English concertina, the stereoscope , and the Playfair cipher...
observed the spectrum of an electric discharge in mercury vapor in 1835, and noted the ultraviolet lines in that spectrum. In 1860, John Thomas Way used arc lamps operated in a mixture of air and mercury vapor at atmospheric pressure for lighting. The German physicist Leo Arons (1860–1919) studied mercury discharges in 1892 and developed a lamp based on a mercury arc.
According to Encyclopædia Britannica
Encyclopædia Britannica
The Encyclopædia Britannica , published by Encyclopædia Britannica, Inc., is a general knowledge English-language encyclopaedia that is available in print, as a DVD, and on the Internet. It is written and continuously updated by about 100 full-time editors and more than 4,000 expert...
, the first mercury vapor lamp was invented in 1901 by American engineer Peter Cooper Hewitt
Peter Cooper Hewitt
Peter Cooper Hewitt was an American electrical engineer and inventor, who invented the first mercury-vapor lamp in 1901. Hewitt was issued U.S. patent #682692 on September 17, 1901. In 1903, Hewitt created an improved version that possessed higher colour qualities which eventually found widespread...
. Hewitt was issued on September 17, 1901. In 1903, Hewitt created an improved version that possessed higher colour qualities which eventually found widespread industrial use. The ultraviolet light from mercury vapor lamps was applied to water treatment by 1910. The Hewitt lamps used a large amount of mercury. In the 1930s, improved lamps of the modern form, developed by the Osram-GEC company, General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
company and others lead to widespread use of mercury vapor lamps for general lighting.
How it works
The mercury in the tube is a liquid at normal temperatures. It needs to be vaporized and ionizedIonization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
before the tube will conduct electricity and the arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
can start. So, like fluorescent tubes, mercury vapor lamps require a starter
Starter
- Science and technology :* Starter motors, used to start internal-combustion engines* Motor starters, used to start large electric motors* Fluorescent lamp starters, used to initiate operation of fluorescent lights- Food and meals :...
, which is usually contained within the mercury vapor lamp itself. A third electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
is mounted near one of the main electrodes and connected through a resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
to the other main electrode. In addition to the mercury, the tube is filled with argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
gas at low pressure. When power is applied, there is sufficient voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
to ionize the argon and strike a small arc between the starting electrode and the adjacent main electrode. This starting arc discharge heats the mercury and eventually provides enough ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ized mercury to strike an arc between the main electrodes. This process takes from 4 to 7 minutes, so mercury lamps are slow starting. Some bulbs include a thermal switch which shorts the starting electrode to the adjacent main electrode, extinguishing the starting arc once the main arc strikes.
The mercury vapor lamp is a negative resistance
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
device. This means its resistance decreases as the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
through the tube increases. So if the lamp is connected directly to a constant-voltage source like the power lines, the current through it will increase until it destroys itself. Therefore it requires a ballast to limit the current through it. Mercury vapor lamp ballasts are similar to the ballasts used with fluorescent lamp
Fluorescent lamp
A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power into useful...
s. In fact, the first British fluorescent lamps were designed to operate from 80-watt mercury vapor ballasts.
Variation: Metal halide
A closely related lamp design called the metal halide lampMetal halide lamp
Metal-halide lamps, a member of the high-intensity discharge family of lamps, produce high light output for their size, making them a compact, powerful, and efficient light source. By adding rare earth metal salts to the mercury vapor lamp, improved luminous efficacy and light color is obtained...
uses various compounds in an amalgam
Amalgam (chemistry)
An amalgam is a substance formed by the reaction of mercury with another metal. Almost all metals can form amalgams with mercury, notable exceptions being iron and platinum. Silver-mercury amalgams are important in dentistry, and gold-mercury amalgam is used in the extraction of gold from ore.The...
with the mercury. Sodium iodide
Sodium iodide
Sodium iodide is a white, crystalline salt with chemical formula NaI used in radiation detection, treatment of iodine deficiency, and as a reactant in the Finkelstein reaction.-Uses:Sodium iodide is commonly used to treat and prevent iodine deficiency....
and Scandium
Scandium
Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element, together with yttrium and the lanthanoids...
iodide
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
are commonly in use. 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
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
pulses as with high-pressure sodium vapor lamps. "MetalArc" is Osram Sylvania
Osram Sylvania
Osram Sylvania Inc. is the North American operation of lighting manufacturer Osram GmbH, which is owned by Siemens AG. It was established in January 1993, with the acquisition of GTE’s Sylvania lighting division by Osram GmbH....
's trademark for their metal halide lamps; "Arcstream" and "MultiVapor" are General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
's trademark. Besides their use in traditional outdoor lighting, these lamps now appear in most computer and video projector
Video projector
A video projector is an image projector that receives a video signal and projects the corresponding image on a projection screen using a lens system. All video projectors use a very bright light to project the image, and most modern ones can correct any curves, blurriness, and other...
s. However, Philips
Philips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....
' UHP
UHP (lamp)
The UHP, or Ultra High Performance, mercury arc lamp was developed by Philips in 1995 for use in commercial projection systems, home theatre projectors, MD-PTVs and video walls. Unlike other common mercury vapor lamps used in projection systems, it is not a metal halide lamp, but uses only mercury....
lamp, introduced in 1995, contains only mercury. As an example of application and efficiency of mercury lamps, the 61" Samsung DLP rear projection TV (HL-S6187W) uses a 132-watt Philips UHP lamp.
Self-ballasted (SB) lamps
There are mercury vapor lamps with a filament inside connected in series with the arc tube that functions as an electrical ballast. This is the only kind of mercury vapor lamp that should be connected directly to the mains without an external ballast. These lamps have only the same or slightly higher efficiency than incandescent lamps of similar size, but have a longer life. They give light immediately on startup, but usually needs a few minutes to restrike if power has been interrupted. Because of the light emitted by the filament, they have slightly better color rendering properties than mercury vapor lamps. The color temperature is higher than incandescent lamps.Operation
When a mercury vapor lamp is first turned on, it will produce a dark blueBlue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
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
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
mercury bands. As the main arc strikes and the gas heats up and increases in pressure, the light shifts into the visible range
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
and the high gas pressure causes the mercury emission bands to broaden somewhat, producing a light that appears more nearly white to the human eye, although it is still not a continuous spectrum
Continuous spectrum
The spectrum of a linear operator is commonly divided into three parts: point spectrum, continuous spectrum, and residual spectrum.If H is a topological vector space and A:H \to H is a linear map, the spectrum of A is the set of complex numbers \lambda such that A - \lambda I : H \to H is not...
. Even at full intensity, the light from a mercury vapor lamp with no phosphors is distinctly bluish in color. The pressure in the silica glass tube rises to approximately one atmosphere once the bulb has reached its working temperature. If the discharge should be interrupted (e.g. by interruption of the electric supply), it is not possible for the lamp to restrike until the bulb cools enough for the pressure to fall considerably.
Color considerations
To correct the bluish tinge, many mercury vapor lamps are coated on the inside of the outer bulb with a phosphorPhosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
that converts some portion of the ultraviolet emissions into red light. This helps to fill in the otherwise very-deficient red end of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. 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. A common method of correcting this problem before phosphors were used was to operate the mercury lamp in conjunction with an incandescent lamp. 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. When outside, coated or color corrected lamps can usually be identified by a blue "halo" around the light being given off.
Emission line spectrum
The strongest peaks of the emission line spectrum are| Wavelength (nm) | Name (see photoresist Photoresist A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists.... ) | Color |
|---|---|---|
| 184.45 | ultraviolet (UVC) | |
| 253.7 | ultraviolet (UVC) | |
| 365.4 | I-line | ultraviolet (UVA) |
| 404.7 | H-line | violet |
| 435.8 | G-line | blue |
| 546.1 | green | |
| 578.2 | yellow-orange |
In low-pressure mercury-vapor lamps only the lines at 184 nm and 253 nm are present. Only the light at 253 nm is usable unless synthetic quartz is used to manufacture the tube as the line is otherwise absorbed. In medium-pressure mercury-vapor lamps, the lines from 200-600 nm are present. The lamps can be constructed to emit primarily in the UV-A (around 400 nm)or UV-C (around 250 nm). High-pressure mercury-vapor lamps are those lamp commonly used for general lighting purposes. They emit primarily in the blue and green.
Usage of low-pressure lamps for surface cleaning
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
bulb in order to allow the transmission of short wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
light. If synthetic quartz is used, then the transparency of the quartz is increased further and an emission line
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
at 185 nm is observed also. Such a lamp can then be used for the cleaning or modification of surfaces. The line 185 nm will create ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
in an oxygen containing atmosphere, which helps in the cleaning process, but is also a health hazard.
Light pollution considerations
For placements where light pollutionLight pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
is of prime importance (for example, an observatory
Observatory
An observatory is a location used for observing terrestrial or celestial events. Astronomy, climatology/meteorology, geology, oceanography and volcanology are examples of disciplines for which observatories have been constructed...
parking lot
Parking lot
A parking lot , also known as car lot, is a cleared area that is intended for parking vehicles. Usually, the term refers to a dedicated area that has been provided with a durable or semi-durable surface....
), 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.
Bans
The use of mercury vapor lamps for lighting purposes will be banned in the EU in 2015. As this ban is designed to phase out less efficient lamps it does not affect the use of mercury in compact fluorescent lampCompact fluorescent lamp
A compact fluorescent lamp , also called compact fluorescent light, energy-saving light, and compact fluorescent tube, is a fluorescent lamp designed to replace an incandescent lamp; some types fit into light fixtures formerly used for incandescent lamps...
nor the use of mercury lamps for purposes other than lighting. In the USA, ballasts and fixtures were banned in 2008. Because of this, several manufacturers have begun selling replacement compact fluorescent lamp
Compact fluorescent lamp
A compact fluorescent lamp , also called compact fluorescent light, energy-saving light, and compact fluorescent tube, is a fluorescent lamp designed to replace an incandescent lamp; some types fit into light fixtures formerly used for incandescent lamps...
s for mercury vapor fixtures, which do not require modifications to the existing fixture.
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 ultravioletUltraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation from escaping. Usually, the borosilicate glass
Borosilicate glass
Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common 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. There have been documented cases of lamps being damaged in gym
Gym
The word γυμνάσιον was used in Ancient Greece, that mean a locality for both physical and intellectual education of young men...
nasiums by a ball hitting it. Sun burns and eye inflammation have resulted. 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 that will deliberately burn out if the outer glass is broken. This is usually achieved by using a thin carbon strip, which will burn up
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
in the presence of air, to connect one of the electrodes.
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 accelerated, leaving them significantly discolored after only a few years' service. Polycarbonate
Polycarbonate
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...
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 color after only a short time. Certain polishes, such as Brasso
Brasso
Brasso is a metal polish designed to remove tarnish from brass, copper, chrome and stainless steel. It is available either directly as a liquid or as an impregnated wadding pad.-History:...
, can be used to remove some of the yellowing, but usually only with limited success.
End of life
Mercury vapor lamps rarely burn out completely but suffer from lumen depreciation. The lamp produces 50% less light every five years, to the point of becoming ineffective while still drawing the same amount of power it drew when it was new. This comes about because the emitter is deposited as a film darkening the arctube wall and reducing light output.Photoresist exposure
Ultra high pressure mercury vapor lamps are used in the area of photolithographyPhotolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
to expose various photoresists. The unique spectral emission characteristics of mercury vapor lamps are ideal for photoresists, the most common of which are generally photosensitive between 350 and 500 nm wavelengths.
See also
- History of street lighting in the United StatesHistory of street lighting in the United StatesThe use of street lighting was first recorded in the city of Antioch from the 4th century, later in the Arab Empire from the 9th–10th centuries, especially in Cordova, and then in London from 1417 when Henry Barton, the mayor, ordered "lanterns with lights to be hanged out on the winter evenings...
- List of light sources