The Sodium Lamp - How it works and history (2024)

Sodium Lamp
High lumen output at high efficiency
(1920 - Today)

The Sodium Lamp - How it works and history (1)

The Sodium Lamp - How it works and history (2) Introduction

The Sodium Lamp - How it works and history (3) How They Work LPS

The Sodium Lamp - How it works and history (4) HPS Lamps

The Sodium Lamp - How it works and history (5) Inventors and Developments LPS

The Sodium Lamp - How it works and history (6) Inventors of the HPS

Sodium Lamps were first produced commercially by Philips in Holland in 1932. There are two kinds of sodium lights: Low Pressure (LPS) and High Pressure (HPS). These lamps are mostly used for street lighting as well as industrial uses.

The lamp works by creating an electric arc through vaporized sodium metal. Other materials and gases are used to help start the lamp or control its color. See the photos lower on this page for more details.

All credits and sources are located at the bottom of each lighting page

LPS Lamp

HPS Lamp


The LPS lamp was the first sodium lamp to be developed. It is known by its signature monochromatic yellow color. It is mostly used in Europe since it did not appeal in other markets due to its poor CRI or color rendering. It is among the most efficient lamps in the world because it uses all the current it gets to create light at the most sensitive color (frequency) to the human eye. An incandescent lamp in contrast creates light at all frequencies from Infrared (non-visible) to UV at the other end of the spectrum. The energy used to make non-visible light is a waste of energy since it does not help do the principle job of an electric light. The LPS lamp is also called a SOX lamp (SO for sodium)

- Very efficient lamp
- Powerful lamp for use of large areas
- Despite a warm up time of 5-10 minutes it restarts immediately if there is a brownout
- Lumen output does not drop with age (such as in LEDs or incandescents)

- Worst color rendering of any lamp
- Sodium is a hazardous material which can combust when exposed to air (such as if the bulb is broken in the trash)

CRI -44
Color Temperature: 1800
100-190 lumens per watt
Bulb life: 18,000 hrs

Common Uses: Outdoor lighting, security lighting, long tunnel lighting (the light seems to give less fatigue in tunnel driving than white lights flashing by at close proximity).

1. How a modern LPS lamp works
2. Inventors and Developments

The Sodium Lamp - How it works and history (7) 1. How the LPS lamp WorksThe LPS Lamp is mostly in Europe for outdoor lighting. They create a monochromatic yellow light. In the diagrams below you will see how as it starts it creates a red glow due to the neon gas. The neon gas lights at a lower temperature. As the temperature increases the sodium begins to vaporize and the lamp turns to a pure yellow.

The Sodium Lamp - How it works and history (8)

The Sodium Lamp - How it works and history (9)
The Sodium Lamp - How it works and history (10)
An LPS with it's yellow glow

The Sodium Lamp - How it works and history (11)

The Sodium Lamp - How it works and history (12)

Argon has a lower glow voltage, argon helps the smaller lamps start at a lower voltage. The larger LPS lamps used in street lighting for the most part do not use argon.

The Sodium Lamp - How it works and history (13) 2. LPS Development and Inventors:

The Sodium Lamp - How it works and history (14)The Sodium Lamp - How it works and history (15)

Low pressure sodium lamps were invented first in 1920 by Arthur H. Compton at Westinghouse. The first lamp was a round bulb with two electrodes on each side. The solid sodium metal remained on the bottom center of the bulb. When heated up the metal would vaporize and the lamp would glow yellow. The lamp had to be designed in a sphere because after the metal cooled when the lamp was turned off, the sodium has a property of migrating to the coolest part of the bulb where it solidifies. A tube design would be more particle, similar to the neon lamp which had already been developed by 1920, but it was found that the sodium would migrate to the outer ends of the tube, and there the sodium would destroy the electrodes over time as well as not get hot enough to vaporize. The problem with Compton's models is that the highly corrosive sodium would attack and blacken regular silica glass.

The Sodium Lamp - How it works and history (16)

Marcello Pirani led the next major advancement in the low pressure sodium lamp. He worked for Osram in Germany where he developed a sodium-resistant glass in 1931. He used an oven to heat the lamp, which vaporized the sodium and started the lamp.

1932 - The first sodium lamps for commercial sale were made by Philips. Philips has not released the names of the individuals who did the monumental work of developing a reliable sodium lamp ready for widespread use. The first lamps had a removable outer jacket with a vacuum between glass to insulate the bulb to keep it hot enough to keep the sodium in vapor form.

Later developments include:

-Integrating the outer vacuum jacket as a bulb with the inner discharge tube inside, therefore eliminating the separate outer jacket and improving insulation properties.

-Indium tin coating on the inside of the outer jacket to reflect infrared (heat) waves back to the bulb, keeping it warmer and improving reliability in cold weather.

High Pressure Sodium Lamp (HPS Lamp)

The Sodium Lamp - How it works and history (17)

The HPS lamp is the most ubiquitous lamp for street lighting on the planet. The lamp is an improvement over the LPS lamp in that it has more acceptable color with the great efficiency of the sodium lamp. The better color rendering comes with a bit of sacrifice, it has less efficiency than the LPS. General Electric first developed the lamp in Schenectady, New York and Nela Park, Ohio. The first lamp came on the market in 1964.

The Sodium Lamp - How it works and history (18)An HPS lamp with a starting strip, it uses a xenon starting gas.

-Good efficiency (lumens per watt)
-Smaller size than LPS or fluorescent, the HPS fits into many fixture types
-Can be retrofitted into older Mercury Vapor fixtures
-Better bulb life than LPS lamps

-Still has a bad color rendering compared to metal halide and halogen lamps
-Requires a lossy ballast (inefficient) that operates a low arc voltage of 52-100V. This reduces the actual efficiency of the lamp when you count the whole system together.

CRI 20-30

80-140 lumens per watt
Bulb Life: 24,000

Uses: Outdoor lighting, municipal lighting, home yard lighting, high bay lighting

1. How it Works
2. Inventors/Developments

The Sodium Lamp - How it works and history (19) 1. HPS: How it Works

The Sodium Lamp - How it works and history (20)

The HPS lamp consists of a narrow arc tube supported by a frame in a bulb. The arc tube has a high pressure inside for higher efficiency. Sodium, mercury and xenon are usually used inside the arc tube. The arc tube is made of aluminum oxide ceramic which is resistant to the corrosive effects of alkalis like sodium.

The Sodium Lamp - How it works and history (21)

The lamp comes in variations, but the most common way to start the lamp is with a pulse start. There is an ignitor built into the ballast which sends a pulse of high voltage energy through the arc tube. This pulse starts an arc through the xenon gas. The lamp turns sky blue as the xenon lights. The arc then heats up the mercury and the mercury vapor then lights, giving the lamp a bluish color. The lamp heats and the sodium is the last material to vaporize. The sodium vapor strikes an arc over 240 C. The sodium is mixed with other impurities to create a more "white" light. The mercury helps add a blue spectrum light to the pure yellow of the sodium.

Maintaining a vacuum is difficult, oxygen and other gasses can seep in over time. The getter keeps a stable vacuum by sucking out remaining oxygen and unwanted gasses. The sodium is stored often stored in the amalgam reservoirs on the ends of the arc tube when it is cool unlike the LPS lamp where the sodium is stored in the bumps on the side of the tube (see LPS diagrams)


The Sodium in these lamps is a highly volatile substance. When exposed to air the sodium may explode. The sodium lamp should not be disposed of in normal the normal garbage disposal. There have been many cases of garbage trucks catching fire when the bulbs in the back broke. Sodium lamps also contain mercury. The newer LPS lamps contain less mercury than before, but this has effected performance negatively.

The Sodium Lamp - How it works and history (22) Inventors of HPS Lamps

Early inventors of the sodium lamp knew that with a higher pressure in the arc tube that better efficiency could be achieved. The problem was that there was no material that could stand the high pressure, high temperature, and corrosive properties of sodium.

The Sodium Lamp - How it works and history (23)

Finally, 35 years after Compton invented the sodium lamp an ideal material was discovered for making an HPS lamp. A researcher named Robert L. Coble working at the General Electric Research Lab near Schenectady, New York developed a material called Lucalox. Lucalox was a commercial term for aluminum oxide ceramic.

His work helped pave the way for William Louden, Kurt Schmidt, and Elmer hom*onnay to invent the HPS lamp.

Louden, Schmidt and hom*onnay worked to create a HPS lamp at Nela Park, Ohio (A General Electric research park in Cleveland, OH). Using the new Lucalox material they figured out how to create an arc tube, evacuate the tube, and insert electrodes that would withstand the hostile conditions inside the tube. The first commercial release of the lamp was in 1964. In the 1980s GE engineers further improved the lamp life and efficiency.

The Sodium Lamp - How it works and history (24)
Louden, Schmidt and hom*onnay with a prototype light

Below: video of probably the only remaining 1960s HPS prototypes in the world. It was saved from being disposed of at the GE research lab in Niskayuna, New York.

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Lamps are presented in the order of chronological development

Previous: Mercury Vapor Lamps 1901

Next: Fluorescent Lamps 1934

Arc - Incandescent - Nernst - Neon - Mercury Vapor - Sodium Lamp - Fluorescent - Halogen - EL - LED - MH - Induction

The Electric Light Introduction Page

The Sodium Lamp - How it works and history (29)

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Written by M.Whelan with additional research by Rick DeLair
Please contact us if you are a historian and wish to correct or improve this document.

The Subdivision of the Light by Unknown
"A History of Electric Light and Power" by B. Bowers
Westinghouse lamp catalogues from 1901-1903

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