MATTHEWEAGLES.CO.UK: The low-pressure sodium (SOX) lamp.

The information on this page is also available on my new website,

Low-pressure sodium lamps (SOX lamps) have several applications but are most commonly used in streetlighting - in fact they have been used in streetlighting in the United Kingdom and in various places around the world since the 1930s. Many areas of the UK still use SOX lighting with its distinctive deep yellow light, which is caused by the sodium that is used within the lamp. When the light switches on, it glows red, and slowly warms up to yellow over the course of a few minutes.

This part of the website contains several sub-pages; use the navigation bar to find out more information.

Left: A SOX lamp warming up from 0 to 7 mins. Each picture frame is taken 20 seconds apart. The lantern is a 90-watt GEC Z9454. Below: a SOX lamp (Thorn Beta 5, 35-watt) in Wrexham, U.K., 2004.

The reason I created this part of the website is that I found it very difficult to find information from elsewhere on the Internet, and so researched the information myself for others to use. Most people in the UK will be familiar by sight with the type of streetlight pictured above but will have no idea how these lights work. This part of the website was originally launched in November 2005.

SOX streetlight in Wrexham, U.K.I have always been intrigued by all the golden yellow, almost orange, lights in the street. And when they first lit up they shone red, changing colour to deep yellow only after a few minutes. If you have several on them on different streets, lighting up at slightly different times after sunset, you have a multi-coloured light show of red, orange and yellow as they warm up. The lights I am describing are referred to as sodium lamps (strictly speaking, low-pressure sodium lamps — they are also commonly referred to as SOX lamps in the lighting industry, or LPS lamps in the USA and Australia).

If you hold a CD up to sodium light you will not see the usual rainbow-coloured pattern at all, but instead shimmering bands of yellow and black, owing to the monochromatic light from the lamp. The sodium lamp has had a huge impact on streetlighting in the UK and most probably no other country has used it to such an extent. However, I have heard from other people who have told me that they have been popular in countries such as Belgium and Australia, and that even the USA gave them a go.

SOX installations therefore have low energy consumption costs which is of crucial importance when thousands of miles of roads must be lit and the electricity bills must be kept as low as possible. The main reason why they are effective as a light source is because the colour of the light is close to the maximum sensitivity of the human eye in normal viewing conditions.

These lamps exist in a variety of wattages from 18W to 180W (10W can be found on rare occasions), but a typical side-road lantern of 35W is less than even a domestic light bulb of 60W-100W. Therefore, using a SOX lamp, part of a street can be illuminated with less energy than is normally used for a single room in a house.

Although it looks as if the low-pressure sodium lamp has not changed much over the years, apart from the designs of lanterns, there have been decades of research and development that have gone into making the SOX lamp what it is today. Its tale begins commercially in the 1930s, but even before this time there were years of research by top scientists into light sources, as indicated in the history section, that led to the introduction of these lamps. Most of the technology has gone into the lamp tubes themselves.

Resources: book and video

I have brought together my knowledge on SOX lamps into a book, "Why Is He Interested In Streetlighting?" (1st edition 2005, 2nd edition 2007). From 2017 this is being offered as a free download. It includes the information displayed on this website, plus much more. To access it, please click here or on the picture below on the left (file size: 14MB)

Also available is my 28-minute video documentary, "SOX Lighting - A Twilight Saga", which I filmed in January/February 2018. It is available in the viewing window below, and is also available on YouTube at

Please feel free to browse the other pages relating to SOX lamps on this website: these are Advantages, Colour, Data, Diagnostics, History, How Do They Work?, Lamp-posts, Lighting-Up, Models, Soxtown and UK/Overseas.

Finally, below are a few links to other websites ... (Claire Pendrous) (Martyn Hicks) (Simon Cornwell) (French-language site)

All content © Matthew Eagles 2005-2018 unless otherwise specified