low-pressure sodium lamp
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A sodium-vapor lamp is a gas-discharge lamp that uses
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
in an excited state to produce
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
at a characteristic wavelength near 589  nm. Two varieties of such lamps exist: low pressure and high pressure. Low-pressure sodium lamps are highly efficient electrical light sources, but their yellow light restricts applications to outdoor lighting, such as
street lamps A street light, light pole, lamp pole, lamppost, street lamp, light standard, or lamp standard is a raised source of light on the edge of a road or path. Similar lights may be found on a railway platform. When urban electric power distribution ...
, where they are widely used. High-pressure sodium lamps emit a broader
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...
of light than the low-pressure lamps, but they still have poorer color rendering than other types of lamps. Low-pressure sodium lamps only give monochromatic yellow light and so inhibit color vision at night. Single ended self-starting lamps are insulated with a mica disc and contained in a
borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
gas discharge tube (arc tube) and a metal cap. They include the sodium-vapor lamp that is the gas-discharge lamp in street lighting.


Development

The low-pressure sodium arc discharge lamp was first made practical around 1920 owing to the development of a type of glass that could resist the corrosive effects of sodium vapor. These operated at pressures of less than 1 Pa and produced a near monochromatic light spectrum around the sodium emission lines at 589.0 and 589.56 nanometres wavelength. The yellow light produced by these limited the range of applications to those where color vision was not required. Research into high-pressure sodium lamps occurred in both the UK and the US. Increasing the pressure of the sodium vapor broadened the sodium emission spectrum so that the light produced had more energy emitted at wavelengths above and below the 589 nm region. The quartz material used in mercury discharge lamps was corroded by high pressure sodium vapor. A laboratory demonstration of a high pressure lamp was carried out in 1959. The development by General Electric of a sintered aluminum oxide material (with magnesium oxide added to improve light transmission) was an important step in construction of a commercial lamp. The material was available in the form of tubing by 1962, but additional techniques were required to seal the tubes and add the necessary electrodes—the material could not be fused like quartz. The end caps of the arc tube would get as hot as 800 degrees C in operation, then cool to room temperature when the lamp was turned off, so the electrode terminations and arc tube seal had to tolerate repeated temperature cycles. This problem was solved by Michael ArendashUSA patent US3737717A, Arendash, Michael, "High intensity lamp containing thermal shorting fuse", published 1972-03-13, issued 1973-06-05, assigned to General Electric Co. at the GE Nela Park plant. The first commercial high-pressure sodium lamps were available in 1965 from companies in the United States, the United Kingdom, and the Netherlands; at introduction a 400 watt lamp would produce around 100 lumens per watt.Raymond Kane, Heinz Sell, ''Revolution in Lamps: A Chronicle of 50 Years of Progress, Second Edition'', Fairmont Press, 2001. pp. 238-241.J. J. de Groot, J. A. J. M. van Vliet, ''The High-Pressure Sodium Lamp'', Macmillan International Higher Education, 1986, . pp. 13-17. Single-crystal artificial sapphire tubes were also manufactured and used for HPS lamps in the early 1970s, with a slight improvement in efficacy, but production costs were higher than for polycrystalline alumina tubes.


Low-pressure sodium

Low-pressure sodium (LPS) lamps have a
borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
gas discharge tube (arc tube) containing solid
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
and a small amount of neon and
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...
gas in a Penning mixture to start the gas discharge. The discharge tube may be linear (SLI lamp) or U-shaped. When the lamp is first started, it emits a dim red/pink light to warm the sodium metal; within a few minutes as the sodium metal vaporizes, the emission becomes the common bright
yellow Yellow is the color between green and orange on the spectrum of light. It is evoked by light with a dominant wavelength of roughly 575585 nm. It is a primary color in subtractive color systems, used in painting or color printing. In the ...
. These lamps produce a virtually monochromatic light averaging a 589.3 nm wavelength (actually two dominant spectral lines very close together at 589.0 and 589.6 nm). The colors of objects illuminated by only this narrow bandwidth are difficult to distinguish. LPS lamps have an outer glass vacuum envelope around the inner discharge tube for thermal insulation, which improves their efficiency. Earlier LPS lamps had a detachable dewar jacket (SO lamps). Lamps with a permanent vacuum envelope (SOI lamps) were developed to improve thermal insulation. Further improvement was attained by coating the glass envelope with an infrared reflecting layer of
indium tin oxide Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy. Indium tin oxide is typically encountered as an oxygen-saturated ...
, resulting in SOX lamps. LPS lamps are among the most efficient electrical light sources when measured in
photopic Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108  cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visua ...
lighting conditions, producing above 100 and up to 206 lm/ W. This high efficiency is partly due to the light emitted being at a wavelength near the peak sensitivity of the human eye. They are used mainly for outdoor lighting (such as street lights and security lighting) where faithful color rendition is not important. Recent studies show that under typical nighttime mesopic driving conditions, whiter light can provide better results at a lower level of illumination. (Comparison is with HPS and MH lamps) LPS lamps are similar to fluorescent lamps in that they are a low-intensity light source with a linear lamp shape. They do not exhibit a bright arc as do
High-intensity discharge 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 tub ...
(HID) lamps; they emit a softer luminous glow, resulting in less glare. Unlike HID lamps, during a voltage dip low-pressure sodium lamps return to full brightness rapidly. LPS lamps are available with
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
ratings from 10 to 180 W; longer lamp lengths can, however, suffer design and engineering problems. Modern LPS lamps have a service life of about 18,000 hours and do not decline in lumen output with age, though they do increase in energy consumption by about 10% towards end of life. This property contrasts with mercury vapor HID lamps, which become dimmer towards the end of life to the point of being ineffective, while consuming undiminished electrical power. In 2017 Philips Lighting, the last manufacturer of LPS lamps, announced they were discontinuing production of the lamps due to falling demand. Initially, production was due to be phased out in the course of 2020, however this date was brought forward and the last lamps were produced at the Hamilton factory on Tues the 31st of December 2019.


Light pollution considerations

For locations where
light pollution Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...
is a consideration, such as near
astronomical observatories An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been constructed. His ...
or
sea turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhe ...
nesting beaches, low-pressure sodium is preferred (as formerly in San Jose and Flagstaff, Arizona). Such lamps emit light on just two dominant spectral lines (with other much weaker lines), and therefore have the least spectral interference with astronomical observation. (Now that production of LPS lamps has ceased, consideration is being given into the use of narrow-band amber LEDs, which are on a similar color spectrum to LPS.) The yellow color of low-pressure sodium lamps also leads to the least visual sky glow, due primarily to the Purkinje shift of dark-adapted human vision, causing the eye to be relatively insensitive to the yellow light scattered at low luminance levels in the clear atmosphere. One consequence of widespread public lighting is that on cloudy nights, cities with enough lighting are illuminated by light reflected off the clouds. Where sodium vapor lights are the source of urban illumination, the night sky is tinged with orange.


Film special effects

Sodium vapor process (occasionally referred to as yellowscreen) is a film technique that relies on narrowband characteristics of LPS lamp. Color negative film is typically not sensitive to the yellow light from an LPS lamp, but special black-and-white film is able to record it. Using a special camera, scenes are recorded on two spools simultaneously, one with actors (or other foreground objects) and another that becomes a mask for later combination with different
background Background may refer to: Performing arts and stagecraft * Background actor * Background artist * Background light * Background music * Background story * Background vocals * ''Background'' (play), a 1950 play by Warren Chetham-Strode Record ...
. This technique originally yielded results superior to blue-screen technology, and was used in years 1956 to 1990, mostly by
Disney Studios The Walt Disney Studios is an American film and entertainment studio, and is the Studios Content segment of the Walt Disney Company. Based mainly at the namesake studio lot in Burbank, California, the studio is best known for its multifacete ...
. Notable examples of films using this technique include Alfred Hitchcock's '' The Birds'' and the Disney films '' Mary Poppins'' and ''
Bedknobs and Broomsticks ''Bedknobs and Broomsticks'' is a 1971 American live-action animated musical fantasy film directed by Robert Stevenson and produced by Bill Walsh for Walt Disney Productions. It is loosely based upon the books '' The Magic Bedknob; or, How t ...
''. Later advancements in blue- and green-screen techniques and computer imagery closed that gap, leaving SVP economically impractical.


High-pressure sodium

High-pressure sodium (HPS) lamps have been widely used in industrial lighting, especially in large manufacturing facilities, and are commonly used as plant grow lights. They contain mercury. They have also been widely used for outdoor area lighting, such as on roadways, parking lots, and security areas. Understanding the change in human color vision sensitivity from
photopic Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108  cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visua ...
to mesopic and
scotopic In the study of human visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. The term comes from Greek ''skotos'', meaning "darkness", and ''-opia'', meaning "a condition of sight". In the human eye, ...
is essential for proper planning when designing lighting for roadways. High-pressure sodium lamps are quite efficient—about 100 lumens per watt, when measured for
photopic Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108  cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visua ...
lighting conditions. Some higher-power lamps (e.g. 600 watt) have efficacies of about 150 lumens per watt. Since the high-pressure sodium arc is extremely chemically reactive, the
arc tube An arc lamp or arc light is a lamp that produces light by an electric arc (also called a voltaic arc). The carbon arc light, which consists of an arc between carbon electrodes in air, invented by Humphry Davy in the first decade of the 1800s, ...
is typically made of translucent aluminum oxide. This construction led the General Electric Company to use the tradename "Lucalox" for its line of high-pressure sodium lamps.
Xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
at a low pressure is used as a "starter gas" in the HPS lamp. It has the lowest
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
and lowest
ionization potential Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
of all the stable
noble gases The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...
. As a noble gas, it does not interfere with the chemical reactions occurring in the operating lamp. The low thermal conductivity minimizes thermal losses in the lamp while in the operating state, and the low ionization potential causes the
breakdown voltage The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to experience electrical breakdown and become electrically conductive. For diodes, the breakdown voltage is the minimum reverse voltage that mak ...
of the gas to be relatively low in the cold state, which allows the lamp to be easily started.


"White" high pressure sodium lamp

A variation of the high-pressure sodium introduced in 1986, the White HPS has a higher pressure than the typical HPS lamp, producing a
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 ...
of around 2700 kelvins with a
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) of about 85, greatly resembling the color of an incandescent light. These lamps are often used indoors in cafes and restaurants for aesthetic effect. However, white HPS lamps have higher cost, shorter service lives, and lower light efficiency, and so they cannot compete with HPS at this time.


Theory of operation

An amalgam of metallic sodium and mercury lies at the coolest part of the lamp and provides the sodium and mercury vapor that is needed to draw an arc. The temperature of the amalgam is determined to a great extent by lamp power. The higher the lamp power, the higher will be the amalgam temperature. The higher the temperature of the amalgam, the higher will be the mercury and sodium vapor pressures in the lamp and the higher will be the terminal voltage. As the temperature rises, the constant current and increasing voltage consumes increasing energy until the operating level of power is reached. For a given voltage, there are generally three modes of operation: # The lamp is extinguished and no current flows. # The lamp is operating with liquid amalgam in the tube. # The lamp is operating with all amalgam evaporated. The first and last states are stable, because the lamp resistance is weakly related to the voltage, but the second state is unstable. Any anomalous increase in current will cause an increase in power, causing an increase in amalgam temperature, which will cause a decrease in resistance, which will cause a further increase in current. This will create a runaway effect, and the lamp will jump to the high-current state (#3). Because actual lamps are not designed to handle this much power, this would result in catastrophic failure. Similarly, an anomalous drop in current will drive the lamp to extinction. It is the second state that is the desired operating state of the lamp, because a slow loss of the amalgam over time from a reservoir will have less effect on the characteristics of the lamp than a fully evaporated amalgam. The result is an average lamp life in excess of 20,000 hours. In practical use, the lamp is powered by an AC voltage source in series with an inductive "
ballast Ballast is material that is used to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship ...
" in order to supply a nearly constant current to the lamp, rather than a constant voltage, thus assuring stable operation. The ballast is usually inductive rather than simply being resistive to minimize energy waste from resistance losses. Because the lamp effectively extinguishes at each zero-current point in the AC cycle, the inductive ballast assists in the reignition by providing a voltage spike at the zero-current point. The light from the lamp consists of atomic emission lines of mercury and sodium, but is dominated by the sodium D-line emission. This line is extremely pressure (resonance) broadened and is also self-reversed because of absorption in the cooler outer layers of the arc, giving the lamp its improved color rendering characteristics. In addition, the red wing of the D-line emission is further pressure broadened by the
Van der Waals forces In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...
from the mercury atoms in the arc.


End of life

At end of life, high-pressure sodium (HPS) lamps exhibit a phenomenon known as ''cycling'', caused by a loss of sodium in the arc. Sodium is a highly reactive element and is lost in a reaction with the aluminum oxide of the arc tube. The
products Product may refer to: Business * Product (business), an item that serves as a solution to a specific consumer problem. * Product (project management), a deliverable or set of deliverables that contribute to a business solution Mathematics * Produ ...
are
sodium oxide Sodium oxide is a chemical compound with the formula Na2 O. It is used in ceramics and glasses. It is a white solid but the compound is rarely encountered. Instead "sodium oxide" is used to describe components of various materials such as glass ...
and
aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
: :6 Na + Al2O3 → 3 Na2O + 2 Al As a result, 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 An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. An ...
. As a lamp gets older, the maintaining voltage for the arc eventually rises to exceed the maximum voltage output 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, typically starting at a pure or bluish white then moving to a red-orange before going out. More sophisticated ballast designs detect cycling and give up attempting to start the lamp after a few cycles, as the repeated high-voltage ignitions needed to restart the arc reduce the lifetime of the ballast. If power is removed and reapplied, the ballast will make a new series of startup attempts. LPS lamp failure does not result in cycling; rather, the lamp will simply not strike or will maintain the dull red glow of the start-up phase. In another failure mode, a tiny puncture of the arc tube leaks some of the sodium vapor into the outer vacuum bulb. The sodium condenses and creates a mirror on the outer glass, partially obscuring the arc tube. The lamp often continues operating normally, but much of the light generated is obscured by the sodium coating, providing no illumination.


See also

* * (HID) * * * * * * * *


References


Sources

* * *


External links


Museum of Electric Discharge Lamps
{{Artificial light sources Gas discharge lamps Light pollution Sodium Street lighting