AskUs: Why do we use Sodium for street lighting ?

So one of our readers asked us this question the other day. Why do we use Sodium for street lighting? I know about how every atoms has its own emission spectrum -and absorption- and that sodium has a double spectral line around 589nm, which is in the ‘orange part’ of the visible spectrum, but I was wondering, why did we pick sodium and orange lightning specifically. Was it for economical reasons? Because Orange was a better choice for a reason I don’t really get ?

ANSWER

I can’t speak for all of the reasons why high-pressure sodium lights took over, but I can say why industrial environments still use them. It’s pretty much based on cost and availability. They are efficient, cheap to replace, and easy/quick to get.

If you’ve ever passed a chemical plant at night, you’ve seen the thousands of orange lights that are pretty much everywhere. Those are likely 100W HPS fixtures. They put out ~9500 lumens (rough amount of light output) and use about 100W of power (usually more due to inefficiencies through the ballast). An incandescent bulb that pulls 100W is only going to output around 1600 lumens. A 100W HPS bulb costs around $10 and lasts around 25,000 hours. That’s almost 6 years of running 12 hours a day. The ballasts cost $40 or so and they last 20+ years. Both the ballast kit and the bulb are easily replaceable within a fixture.

The main downsides of a HPS is that it’s not the most environmentally friendly (contains mercury) and has a terrible CRI. Color-rendering-index basically represents how well the human eyes can see proper colors when lit up by the light source. In areas where you just need to have light in order to navigate, HPS lights have been great solutions. When you want better light quality, you move toward metal halide, fluorescent lights, or LED. I could go on for pages about what I know about the different types of lighting, but I’m going to try and not ramble on.

Bottom line, sodium lighting is well established and very effective. As much as I personally love LEDs, it will still be a while before they are truly cost-effective enough to replace everything. Certain types of lights are better for certain applications, and sodium lighting is thought to be effective for street lighting.