Sodium Lighting and LEDs

Introduction

My friend Atila wrote a great blogpost on how his local area's lampposts suddenly changed in colour, and he even linked to my blog. So it's only kind if I were to do the same: https://ani.bearblog.dev/blog/

The post is curious because it reminds me of how we're seeing increasingly fewer orange streetlights, instead giving us harsher white light. Specifically, the orange colour is generated from sodium, where its electrons are excited into a higher energy state.

Physics

Sodium is an alkali metal, and as such has one valence electron. When it is heated up, this valence electron will be excited and move up to a higher energy level, before it juts back, and this change in position emits a photon. You can calculate the energy emitted by using the formula E = hc/λ, where h is the very minute Planck constant. This formula also shows that the energy emitted corresponds to a given wavelength - so all elements, when excited, will emit light at certain wavelengths. For sodium, the valence electron will emit a photon at a wavelength of 589nm, which corresponds to a yellow colour. The same principle applies in flame tests, where sodium will burn a lovely yellow. As a result, a sodium lamp will burn yellow, which is a warm and ambient colour, perfect for outdoor lighting.

The flame test image is courtesy of Wikipedia user Swn.

Effectiveness in light

In streetlights, sodium lamps come in two categories - low pressure, and high pressure. They work in different ways: low pressure, or LPS, lamps contain neon (or rarely argon) with sodium metal in a discharge tube. At low temperatures, the neon will heat up and the lamp will shine red. As the temperature increases, the sodium will start to be excited and the yellow will begin to shine through. 

High pressure, or HPS, lamps contain sodium, xenon and mercury in an aluminium oxide arc tube. A high voltage ionises the xenon, which vapourises the mercury and sodium and makes them emit light. The main issue with these lamps, aside from mercury being rather toxic, is that they can completely go out if the sodium completely runs out; it is reactive, and so reacts with the aluminium oxide.

Most streetlights are low pressure, and they are great since they are very efficient and not too costly. Along with this, their invention effectively opened up the night to easier access as well, and these lamps have become rather ubiquitous. Despite this, sodium lamps remained the main way to get outdoor lighting, at least until LEDs arrived on the scene.

LEDs are diodes which, when a current passes through them, emit light, and are both more efficient than sodium lamps and also shine brighter over a longer life cycle. Many councils have thus pivoted to them, removing the warm ambient glow that we've all become so attuned to. There is one key disadvantage to LEDs, however - they cause greater light pollution, which was already a problem with sodium lamps, yet as the brightness increases, so does the pollution which greatly impacts sleep cycles. This BBC article discusses those issues rather effectively. It does seem like they're not going away, though - Oxford Street's Christmas lights this year use LEDs too.

Millwall use LEDs to light their stadium...

But most council streetlights settle for sodium

Conclusion

Both sodium lamps and LEDs have clear pros and cons, and I'm not entirely sure myself which one is more favourable. I suppose it depends on context - if it's especially dark, you'd like a brighter light source if you're outside, but would prefer a calmer light if asleep. Councils and the public will have different opinions on what is favourable too. 

Atila's blogpost didn't focus on the light used in the streetlights, rather the height of the lampposts, and this is also possibly a factor. If the lamppost is taller, it's more likely to obstruct your sleep, but it's also likely to be caught out by a tree branch. There are thus many compromises that need to be made, which will likely still displease other people.

Bus anecdote

White text blinded bus

This colour contrast also reminds me of bus blinds. Growing up in  London, I have been surrounded by buses all my life, and I distinctly remember when they shifted from a "day-glo" neon yellow to white. The yellow supposedly made the blind (containing the route number and destination) clearer to see in the dark, however TfL changed this as supposedly the white on black provides a better contrast, as well as LEDs enabling better lighting of the blinds. That hasn't stopped several buses from still having the older yellow blinds, however, though they're few and far between.