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What is a Sodium Vapour Lamp?

This is an image representation of an advancement in the technology of lighting.

Well jeez, considering how long light bulbs have been around (since 1879!) the complexity of how they work is entirely underappreciated. There are far more complex things, like say, the internet, the CRISPR method, the human ego, octopuses… Alas, inventors started tinkering around with electric light in 1835, and to give you an idea of how revolutionary this was, here’s a few other things that were going on in the early 1830s:

  • Charles Darwin was exploring the Galapagos
  • the man who invented dynamite was born
  • slavery was abolished in the British Empire
  • Samuel Colt patented the revolver

After nearly 100 years of development with lightbulbs, the sodium vapor lamp was invented. Good chances are these bulbs are what light up the street when you’re on a midnight stroll, or what startles your dog when he goes outside for a late-night poop. Thank you for gathering folks, and read slowly as we explore the universe inside the sodium light bulb.

Related: 56 Different Types of Light Bulbs (Illustrated Charts & Buying Guide) | Corn Bulb | Smart Light Bulbs | Types of LED Lights | Types of Lighting

How Sodium Vapour Lamps Work!

Before we begin, the terms bulb and lamp are used interchangeably when living in this bulba-verse.

There are high-pressure sodium lamps (HPS), and low-pressure sodium lamps (LPS), but they function in a similar fashion. They are both gas discharge lamps that generate light by sending an electrical discharge through plasma. Plasma itself is the lesser-known fourth state of matter: an ionized gas in which electrons become free from atoms or molecules, where electrons and ions can coexist.

This is a close look at a light bulb with glowing blue vapor inside.

A borosilicate glass tube (inside the bulb itself) will contain a cocktail of solid sodium metal, argon (a noble gas), and neon. What the heck is borosilicate you may ask? It’s made of silica and boron trioxide, which are two major components of glass. This kind of glass is more resistant to thermal shock and can withstand temperatures up to 329 degrees Fahrenheit.

When the lamp is turned on, the solid sodium metal found inside the borosilicate tube vaporizes. While the sodium is slowly heating up, the bulb will emit a reddish/pinkish glow. Once it’s all the way warmed up it becomes that familiar yellow you’re familiar with in most outdoor settings. This is because neon vaporizes first, and is responsible for the red glow. Sodium vaporizes last and is responsible for the yellow glow.

A foggy morning at the park with lit lamp posts.

In order to remain lit, sodium vapor lamps require additional voltage by a ballast to control the electrical current. The ballast is the two-pronged portion of the bulb that connects the arc of electrical impulse.

The Light Emitted

The currents encompassed by a sodium vapor lamp creates light at the most sensitive color frequency to the human eye. It is seen as a monochromatic yellow hue, but this restricts the bulbs’ purposes to night lighting. These bulbs create an odd and eerie effect, where objects illuminated at night appear shadowy and slightly menacing. Those afraid of the dark share this bias.

Stone steps at a park lit by the bright yellow lights of the lamp posts.

As a reference point to try and understand the frequencies of light; incandescent light bulbs create light at frequencies from infrared all the way to UV. The issue with this kind of bulb is that much of the light emitted is not even visible to the human eye, and so it ends up being a waste of energy since only certain frequencies can be seen. The colors it allows us to see are more varied than the sodium vapor lamp, but it is not energy efficient.

Fun and Slightly Unrelated Fact: Bees are capable of seeing UV light, and flowers reflect patterns of UV light to attract them. JUST IMAGINE how overwhelming those colors must be. Okay, thank you for this slight pause, moving on.

This is an illustrative representation of the visible light wave spectrum for the human eyes.

Differences Between HPS and LPS

  • High-pressure sodium lamps emit a greater spectrum of light, whereas low-pressure sodium lamps can only emit monochromatic yellow light.
  • LPS lamps are more energy-efficient. For thermal insulation purposes, they have an outer glass vacuum around the inner discharge tube. This temperature regulation results in less energy required to keep it lit.
  • HPS lamps contain a small amount of mercury which is a toxic and hazardous material. This is not harmful while they are in use, but they make waste disposal more complicated once it reaches the end of its life.
  • HPS lamps contain mercury because it burns with more white and light blue emissions, which counteract the yellow glow that comes from burning sodium.
  • HPS lamps are omnidirectional (meaning they produce light in all 360-degree angles). This is why they are less efficient than LPS lamps because HPS lamps require more accessory parts in the fixture in order to properly reflect and redirect the desired areas for illumination.

A close look at a high pressure sodium vapour light bulb emitting a warm orange light.

Where Sodium Vapour Lamps Are Used

Both HPS and LPS lamps emit a limited spectrum of light, and so their uses are restricted to outdoor settings. They are most used in backyard lighting, highway lighting, street lights, and tunnels. Although they impede the human eye’s ability to see the true color of the objects underneath their glow, they are the most energy-efficient option for large areas that require lighting.

A massive park with a canal lit by the multiple rows of lamp posts with a warm glow.

Benefits of HPS & LPS Lamps

  • highly energy efficient
  • the lifespan of up to 50,000 hours
  • warm color temperature
  • their emitted light can penetrate the fog


What are sodium vapor lamps used for?

They are mainly restricted to outdoor settings. Airports, seaports, highways, street lamps, tunnels, backyards, etc.

How do sodium vapor lamps work?

They emit light when sodium, argon, and neon are charged with electricity and vaporized. The first hue emitted is usually reddish/pink that eventually fades into yellow.

Why do sodium vapor lamps start out glowing red?

This is due to the fact that the neon in the bulb is the first of the elements to vaporize, and this emits a red hue. The sodium is responsible for the yellow hue.

Are sodium lamps expensive? 

The bulbs themselves are at a similar price to other bulbs but do to their energy efficiency they will save money in the long run.

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