Sodium-vapor lamp

A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light 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-orange light restricts applications to outdoor lighting, such as street lamps, where they are widely used. High-pressure sodium lamps emit a broader spectrum of light than the low-pressure lamps, but they still have poorer color rendering than other types of lamps. Low-pressure sodium lamps give only monochromatic yellow-orange light, inhibiting color vision at night.
Single ended self-starting lamps are insulated with a mica disc and contained in a borosilicate glass gas discharge tube with a metal cap. They include the sodium-vapor lamp that is the gas-discharge lamp used 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 589.6 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 United Kingdom and the United States. 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 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 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 Arendash 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.
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 lamps have a borosilicate glass gas discharge tube containing solid sodium and a small amount of neon and argon gas in a Penning mixture to start the gas discharge. The discharge tube may be linear 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. These lamps produce a virtually monochromatic light averaging a 589.3 nm wavelength. 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. Lamps with a permanent vacuum envelope were developed to improve thermal insulation. Further improvement was attained by coating the glass envelope with an infrared reflecting layer of indium tin oxide, resulting in SOX lamps, the current, standard type of Low pressure sodium lamps.
Following the development of the Philips Mini-SOX 18W, some of the design features that contributed to its high efficacy were applied to the existing high power SOX lamps. The result was the SOX-E range, that was more energy-efficient and operated on less current than before. They are easily distinguished by their black bayonet caps instead of red on standard SOX lamps. Lamp power was significantly reduced compared to that of the traditional SOX range, lowering discharge current density and increasing luminous efficacy. This was achieved through improved thermal insulation and a modified infrared-reflective coating.
LPS lamps are among the most efficient electrical light sources when measured in photopic 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 where faithful color rendition is not important.
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 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 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, a major 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, but this date was brought forward and the last lamps were produced at the Hamilton, Scotland factory on December31, 2019.
LPS lamps, ballasts, ignitors and sockets are still being manufactured and sold by Qian Shun Lighting Company in Taizhou, China.

Light pollution considerations

For locations where light pollution is a consideration, such as near astronomical observatories or sea turtle nesting beaches, low-pressure sodium is preferred. Such lamps emit light on just two dominant spectral 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 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

is a film technique that relies on narrowband characteristics of LPS lamp. Color negative film typically is insensitive to the yellow light of 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, and another that becomes a mask for later combination with a different background. This technique originally yielded results superior to blue-screen technology, and was used in years 1956 to 1990, mostly by Disney Studios. Notable examples of films using this technique include Alfred Hitchcock's The Birds and the Disney films Mary Poppins and Bedknobs and Broomsticks. Later advancements in blue- and green-screen techniques and computer imagery closed that gap, leaving SVP economically impractical.

Electrical parameters

Lamp type	Rated power	Lamp power	Nominal lamp voltage	Lamp current
SOX-E	18	18	57	0.35
SOX-E	26	27	69	0.45
SOX-E	36	35	120	0.35
SOX-E	66	65	123	0.62
SOX-E	91	90	173	0.62
SOX-E	131	127	250	0.62
SOX	35	37	70	0.60
SOX	55	56	109	0.59
SOX	90	91	112	0.94
SOX	135	135	164	0.95
SOX	180	185	240	0.91

High-pressure sodium

High-pressure sodium 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 to mesopic and scotopic 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 lighting conditions. Some higher-power lamps have efficacies of about 150 lumens per watt.
Since the high-pressure sodium arc is extremely chemically reactive, the arc tube 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 at a low pressure is used as a "starter gas" in the HPS lamp. It has the lowest thermal conductivity and lowest ionization potential of all the stable noble gases. 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 of the gas to be relatively low in the cold state, which allows the lamp to be easily started.