Streetlights

There have been many different types of lights that have lit up our streets at night. From the very early days, long before the invention of the lightbulb, oil lamps were used to light up the night. Oil lamps could be made from fuels ranging from olive oil to fish oil to beeswax (6). They were not very bright and eventually gas lighting became available in the early 1800s for commercial use (6). Eventually light bulbs became commonplace and soon our nights were light up incredibly bright. There are three main types of lighting that are used in streetlights today. These are sodium vapor lights which include low pressure and high pressure sodium and light-emitting diode lights.

Low Pressure Sodium Lights

Low pressure sodium lights were among the earliest to be used for outdoor applications soon after their invention. Invented in the 1920s by Arthur H. Compton, they didn’t become commercially available until the 1930s (3). These were used all the way up until the late 1960s, when the high pressure sodium light was invented. They are still in use around parts of the world today, however they are a lot less popular than high pressure sodium lights and now the rapidly growing use of LEDs.

The Technical Info

A low pressure sodium lights works by using a mixture of neon, argon, and sodium that is ignited by an electric spark (4). Low pressure sodium lights do not last very long- only about 18,000 hours (5). The light then gives off a yellow hue that can cause poor color rendering (3). Color rendering is what allows a light to accurately show the colors of the object that it is lighting up (7). Because these lights can only emit this yellow hue, objects may not appear true to color when seen with our eyes as seen below.

Image credit: https://www.lighting-gallery.net/gallery/displayimage.php?album=2302&pos=7&pid=63050

Why do we use them?

LPS lights are actually very efficient lights- they usually produce 183 lumens per watt (8). They were used when they first became commercially available as a much brighter source of light in the 1930s, however they were slowly replaced when high pressure sodium lights became available. Although they are still in use in some places today, the main manufacturer of LPS lights, Phillips Lighting, stopped producing these lights in November of 2019 (4). Despite having a higher efficiency than a high pressure sodium light because of its poor color rendering, they are not widely used.

The Replacement- High Pressure Sodium Lights

High pressure sodium (HPS) lights quickly becamse popular after their invention and are the most commonly used lighting for streetlights across the world. This type of lighting was invented in 1968 for use in outdoor lighting (1) and quickly replaced low pressure sodium lights. While they are slightly less efficient than an LPS light, they offer better color rendering and last longer.

A high pressure sodium streetlight, Image credit: Alexandra Nachman

The Technical Info

Despite having the word sodium in their name, HPS lights actually contain a mixture of gas with xenon, sodium, and mercury (1). They are a type of gas-discharge lamp which works by sending an electrical charge through ionized gas (2). Xenon vaporizes first, followed by the mercury, and then finally the sodium (3). This produces the distinctive orange glow of an HPS light. The orange coloring from this type of light is from the sodium; the xenon and mercury actually emit a blue color but it isn’t until the sodium is vaporized that it becomes orange (3).

Why do we use them?

Although these lights are actually less efficient than low pressure sodium lights, usually producing about 100 lumens a watt (4), they are widely used. They last longer than an LPS light, usually about 24,000 hours (5). Because of its better color rendering, it quickly became popular for outdoor lighting applications. They remained this way from its invention in the late ’60s until today, where they are slowly being phased out for the much more energy efficient and brighter LED light.

Our Current Lighting-Light-Emitting Diode Lights

Light-Emitting diodes (LED) have taken the world by storm. They are quite different than the LPS and HPS lights. They are an interesting technology that uses a semiconductor material in which electrons flow through and emit light (11). LED lights have a huge variety of uses, ranging from the screen on a touchscreen phone to digital clocks and watches to streetlights. They can be used in a many applications and because they use less energy to run, they are a more popular choice.

A light-emitting diode streetlight. Image credit: Alexandra Nachman

The Technical Info

LED lights were unique in that they relied on a semiconductor material, rather than heated filaments or gas. LED lights got their start in 1968, the same year as the HPS light. However it did not catch on as quickly because they were inefficient and only produced a red color (9). By 1994, a blue LED light was invented and this allowed for breakthroughs to make white LED lights, which is the color that was preferred (9). Slowly, LED lights became more efficient and the cost to produce them commercially went down. As of today, energy consumption in the US has actually decreased due to replacing lights with LED lights (12). While they are more efficient there have been problems noted with the scattering effects of the wavelength of light used for LED lights and how they affect our eyes and endocrine systems (10)

Why do we use them?

LED lights are quite long lasting and are much more efficient than LPS lights and HPS lights. They have a life of anywhere from 25,000 all the way to 200,000 hours (5). Their peak efficiency is 200 lumens per watt (9)! And although they might seem like they would use more energy to shine brighter, they actually use much less energy to operate. A good example of the cost comparison can be found here: https://unpluggedpowersystems.ca/compare-hps-lights-vs-led-lights/

While LED lights can be much more energy efficient and save a lot of money, they might not be the best for our eyes at night. They emit in the blue-end of the spectrum of visible light, this color scatters more in our eyes and can cause us to have a harder time adapting to low light conditions (10). There might also be correlation between LED lighting and breast cancer (10). Despite these drawbacks, LEDs are the best type of lighting for our streets at night. LEDs that are more orange than white (warm than cool) will be even better for us in the long run.

And of course, the best course of action is minimal lighting! Our eyes are naturally adapted to a day-night cycle. While we should not do away completely with artificial night at light, we do not need as much lighting as we currently have in our world. And in 50-100 years, that will only continue to grow as the human population increases. Interested in finding out how you can reduce your nighttime light? Contact us now!

Sources

  1. https://www.lightingassociates.org/i/u/2127806/f/tech_sheets/High_Pressure_Sodium_Lamps.pdf
  2. https://en.wikipedia.org/wiki/Gas-discharge_lamp
  3. https://edisontechcenter.org/SodiumLamps.html
  4. https://en.wikipedia.org/wiki/Sodium-vapor_lamp
  5. https://www.stouchlighting.com/blog/led-vs-hps-lps-high-and-low-pressure-sodium
  6. https://en.wikipedia.org/wiki/Gas_lighting
  7. https://en.wikipedia.org/wiki/Color_rendering_index
  8. https://www.georgiapower.com/business/save-money-and-energy/customer-resource-center/equipment/lighting/low-pressure-sodium-systems.html
  9. https://en.wikipedia.org/wiki/LED_lamp
  10. https://www.darksky.org/wp-content/uploads/bsk-pdf-manager/8_IDA-BLUE-RICH-LIGHT-WHITE-PAPER.PDF
  11. https://electronics.howstuffworks.com/led.htm
  12. https://www.wsj.com/articles/americans-are-no-longer-gluttons-for-electricitythank-the-led-bulb-11570791602