Low Pressure Sodium Lighting

Low-pressure sodium lightingLight at night brings benefits to human users, but it also has clear unintended and often detrimental impacts on not only the natural night environment but even humans. Low-pressure sodium lighting causes, overall, the lowest environmental impacts of any currently available outdoor lighting source – it causes by far the lowest visible sky glow; uses the lowest or nearly the lowest amount of energy; disturbs the human circadian rhythm the least; and generally disturbs other nocturnal organisms less. Yet there is more “everybody knows” mis-information about this technology than balanced information; as a result it is usually discounted without balanced consideration and is too rarely used.

Even in this age of LED lighting, LPS energy use / efficiency is comparable to the best white LED, while it’s sky glow and other environmental impacts (see LPS and Living Systems, below) are dramatically lower.

The following discussions and links concern issues in low-pressure sodium lighting. Readers aware of other information, particularly reliable information documenting any of the many serious claims often heard against LPS, are encouraged to contact us.


General References

Information on the history and technology of low-pressure sodium lighting, including manufacturing techniques, luminous efficacy, lumen maintenance, lamp lifetimes, color characteristics, etc.


Designing with LPS

(See also LPS Costs) Low-pressure sodium, like all lighting types, has its own pecularities, advantages, and disadvantages. Some of these are discussed in these links.

  • LPS light provides essentially no color rendition, so is inappropriate for any use where 1) color perception is important, and 2) LPS will be the only light source illuminating the area. This drawback should not be minimized (nor exagerrated), and is intimately related to the high efficiency (lumens per watt) and low environmental impact of LPS. However, color perception is not required for every lighting or visual task, and even so it is very common that several light sources will contribute to the illumination of a given area, intentionally or inadvertently, thus providing the ability to see colors (see also below under LPS, Security, and Crime).
  • The large size of LPS lamps leads to generally greater difficulty in controlling the distribution of light as it exits the fixture towards the ground. For applications where such control is unusually critical (such as for narrow roadways or pedestrian walkways) this leads to lower application efficiency than for lamp technologies with smaller lamps (such as HPS or metal halide) or with LED. For other applications where such control is less critical, such as parking lots or wider roadways, this is less of an issue.
  • GARDCO Form 10 LPS Brochure. LPS’s unique lumen maintenence means lighting systems will always be close to the design level, instead of half the time being brighter than necessary and half fainter than designed.
  • Fully Shielded LPS Fixtures Though it is certainly true that the variety of hardware using LPS is limited compared to other lamp types, there are nonetheless quite a number of manufacturers that carry LPS products. This link includes only fully shielded products, and as of June 2003 contains over a dozen manufacturers and two dozen fixtures.

Disposal and Lamp Safety

The comment is often heard that LPS lamps contain sodium metal, and therefore require special disposal. It is also often claimed that the lamps are therefore dangerous, and that they cause fires. While it is true that the lamps require special disposal (see the MSDS below), it is also true that other lamp types require special disposal: HPS, metal halide and fluorescent lamps contain mercury , which is a hazardous material (HAZMAT) according to the U.S. EPA, and they require special disposal considerations. Metal halide lamps also contain various heavy metals, some classified as HAZMATs, also requiring special consideration. We have been unable to locate any material documenting actual LPS disposal problems such as fires, though we do have copies of tests and investigations reporting negative results, linked below.


LPS, Security, and Crime

LPS light, being nearly monochromatic, provides essentially no color perception. This is clearly a drawback for visibility, though it is also essential to its low environmental impact, low sky glow, and advantage for professional astronomy (see below under LPS and Astronomy). In application in the real world, however, LPS light is in most situations supplemented by other light sources such as roadside commercial lighting or automobile headlights, which provides for quite adequate color perception. In many applications, particularly in the Flagstaff and Tucson, AZ areas, fixtures or poles combining LPS with another lamp type (fluorescent or metal halide) have been used. An investigation by R.M. Boynton and K.F. Purl (“Categorical colour perception under low-pressure sodium lighting with small amounts of added incandescent illumination,” Lighting Res. Technol. 21(1) 23-27 (1989)) shows that categorical color perception (placing colors accurately into categories, such as “red,” “yellow,” “green,” etc.) is restored by the addition of 5-10% broad-spectrum light (such as incandescent or LED) to LPS light.


LPS and Scotopic/Peripheral Visibility

Yellow light, LPS light in particular at 589nm wavelength, is relatively inefficient at stimulating the rods in the human eye (relative to its efficiency at photopic or light-adapted conditions, where the lumen and conventional measures of light and lighting level are defined). Since rods dominate the peripheral retina, and since at low light levels rods increase in sensitivity, it is expected that visibility under LPS light will decrease relative to white or blue-rich sources as light levels fall and when we are perceiving peripheral stimuli. Research by Lewis, Rea, and others shows the effect clearly under laboratory conditions. How much this might affect actual visual performance under the more complex conditions such as night driving is uncertain. It is important to recognize that, except for color perception, the research (and theoretical expectation) shows that all light types can give equal visibility or allow equal visual task performance, providing light levels are adjusted accordingly. Also, the adaptation level of the eye is clearly not scotopic under artificial outdoor lighting, but rather mesopic, where both rods and cones are active. Thus the more extreme divergence in the visual performances of the different lamp types shown under the most dark-adapted (laboratory) conditions (often uncritically promoted in literature from the lighting industry) has limited relevance to visual performance under artificial outdoor lighting.

  • IDA Outdoor Lighting Code Handbook Section 4.11 Lighting and the Eye
  • IDA Information Sheet 136: Some Issues in Low Light Level Vision
  • A. Lewis, “Visual Performance as a Function of Spectral Power Distribution of Light Sources at Luminances Used for General Outdoor Lighting,” JIES Winter 1999 (Brief summary)
  • S. Berman and R. Clear, “Some Vision and Lighting Issues at Mesopic Lighting Levels,” Proceedings: Vision at Low Light Levels, EPRI/LRO Fourth International Lighting Research Symposium, pg. 123, 1999 (available from EPRI)
  • W. Adrian, “The Influence of the Spectral Power Distribution for Equal Visual performance in Roadway Lighting Levels,” Proceedings: Vision at Low Light Levels, EPRI/LRO Fourth International Lighting Research Symposium, pg. 85, 1999 (available from EPRI) (Brief summary)

LPS Overall Ownership Costs

The overall costs of installing and operating LPS lighting systems vary depending on details of the application. LPS is the most efficacious light source when measured conventionally, using lumens per watt. Because of this, energy costs tend to be lowest. But other factors reduce this advantage in practice. Luminaire optical control is generally poorer, leading to decreased efficiency of getting the light into the needed area (such as a roadway); in some applications more fixtures are needed because of the poorer optical control and because of the limited luminous output (max. is 33,000 lumens), leading to higher initial costs; shorter lamp lifetimes (18,000 hours compared to high-pressure sodium at 30,000 hours and LED at 50,000 hours) mean that lamps are replaced more frequently, and lamp prices are much higher, so maintenance costs are higher. A general comment may be made that the overall differences are not as large as often claimed; in some situations the overall costs are likely to be lower for LPS. Energy use is almost always lower, even when comparing to LED. As references detailing careful cost comparisons come to our attention, we will add them here.

  • Roadway Lighting: An Investigation and Evaluation of Three Different Light Sources,” I. Lewin, P. Box and R. Stark, Final Report 522. (Also available from the National Technical Information Service, Springfield, Virginia, 22161.  Using  their document search facility, search for “PB2004100097”)
    • As a part of this study, total costs were calculated for an HPS, LPS, and metal halide roadway lighting system. The study included initial, power, and maintenance costs, and assumed fully shielded or full cutoff luminaires and a three lane roadway with 12 ft. lanes. The final 30-year “cost of ownership” comparison, based on HPS at 100%, showed the LPS system to cost 117% and the metal halide system 107%.  Energy costs, again assuming HPS at 100%, were 76% for LPS and 100% for metal halide.
  • “San Jose: Study and report on low-pressure sodium lighting,” Section 6. Cost Comparisons of HPS Versus LPS Systems
    • Total costs for installation, energy and maintenance were calculated for several streetlighting systems, from 100% LPS to 100% HPS and several mixed systems.  Maintenance costs for the LPS systems were somewhat higher, but more than compensated for by lower installation and energy costs.

LPS and Astronomy


LPS and Public Acceptance

One frequently hears, generally from lighting industry representatives who are not familiar with LPS or whose firms do not sell LPS, that the public dislikes lighting provided by low-pressure sodium lamps, that the lighting is “dingy,” “ugly orange,” or appears much darker than would be indicated by the measured illuminance levels.  We have been unable to find any published documentation to support these contentions.  Communities where LPS is actually used generally appear to be either indifferent to or to be positive toward LPS.

  • Flagstaff, Arizona
    • As of April, 2002, Flagstaff streetlighting consisted of 1616 low-pressure sodium and 1108 high pressure sodium or mercury vapor luminaires.  The city continues a steady conversion of all streetlights to low-pressure sodium.  The following articles describe the results of surveys of local businesses and citizens when low-pressure sodium streetlights were first installed.
      • Residents warming up to yellow-lit road (Arizona Daily Sun, 16 Sept. 1987). Newspaper article reporting on the results of a newspaper survey of three local businesses on their response to a test installation of low-pressure sodium streetlighting.  The response is uniformly positive.  This article was developed in anticipation of a much more extensive survey undertaken by the local power utility that had installed the lights, the results of which are described in the following article.
      • Romantics, stargazers make case for adding yellow lights (Arizona Daily Sun, 23 October 1987). Newspaper article summarizing the results of a survey of 400 Flagstaff residents on their response to the test installation of low-pressure sodium streetlighting.  Residents approved of LPS lighting being installed citywide nine to one over those that disapproved.
  • San Diego, California
    • In the mid-1980s, the city of San Diego made a bold move to change the majority of their municipally owned streetlighting system to low-pressure sodium.  The reasons for the change were based on the astronomer friendly nature of the light and the the high energy efficiency of the lighting leading to a large, multimillion-dollar-per-year energy savings for the city.  Unfortunately, since that time, San Diego’s commitment to low-pressure sodium has deteriorated, though it has not collapsed as is commonly said.  Many who are unfamiliar with the details of this complex story are fond of misusing it as an example illustrating the poor public acceptance for low-pressure sodium, when in fact the story is much more complex and has little if anything to do with the public acceptance or nonacceptance of low-pressure sodium lighting.  These newspaper articles give some of the details of this story.
  • San Jose, California
    • A door-to-door survey of 500 persons in both commercial and residential areas was conducted under contract to the city of San Jose by Public Response Associates of San Francisco.  An excerpt of the San Jose: Study and report on low-pressure sodium lighting, 1980
  • California
    • Pierce, S., “Outdoor Lighting Baseline Assessment, Final Report,” California Energy Commission, Public Interest Energy Research Program, 2002 (PDF, 1.3 MB)
      • The goal of this report was to understand the amount of energy used by outdoor lighting in California, and not to investigate low-pressure less sodium lighting per se.  However, since a fair amount of low-pressure sodium lighting is used in the State of California, this report did gather some information relevant to the acceptance of low-pressure sodium lighting.  As part of the evaluation done for each lighting installation, data gatherers and members of the public using the lighting were asked if they found parking lots illuminated with various kinds of lighting “comfortable.”  Ratings ranged from a low of 56% for compact fluorescent, to 86% for standard fluorescent.  Low-pressure sodium was judged “comfortable” 64% of the time.  The author comments “Surprisingly, the percentage of respondents who considered the lighting quality at these [LPS] sites to be about the same as similar areas, is roughly the same as for other lamp types.” (see page 50 and Table 69).

LPS and Living Systems

To celebrate, promote, and protect the glorious dark skies of Flagstaff and northern Arizona.