Why are some streetlights deliberately manufactured to provide illumination with a reddish color?
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Street lights use a ton of power so a lot of effort was put into making the most efficient and longest lasting lamps possible.
Prior to the invention of blue and white LEDs, the most efficient and longest lasting lamps were a technology called “discharge lamps”. Essentially controlled spark is put across a special gas.
For example, a spark in xenon gas produces a crisp, bright pure white light. This was widely used in cinema projectors. However, xenon gas is super rare and expensive. It would cost nearly $10 of xenon to fill a small toy balloon. A projector bulb might need $50 of xenon.
Alternative gases were searched for. Neon gas produces a deep red, which looks stylish and is great for signs, but useless for lighting.
Mercury is cheap (it’s a liquid but it evaporates easily) and works really well. However it produces ultraviolet light,useless for lighting, so you need to add a UV reactive coating which glows white. Mercury lights were widely called fluorescent lights because of the UV fluorescent coating.
The problem with the Mercury light is that they are two stage. Produce UV light, then convert it to white. This drastically reduces energy efficiency.
Another cheap metal which evaporates easily is sodium. Sodium lamps work well, are cheap and extremely efficient. When a low intensity spark is used, a very deep yellow light is produced with incredible efficiency. These were widely used for street lighting because they were so good, and the yellow light also helped cut down glare. But they had a problem, you couldn’t see color at all. Your vision under these lights was basically black and white (well, black and yellow). A green car would look deep black, so would a blue car, so would a red car.
An upgraded version of the sodium lamp was developed. By making the spark more powerful and making the lamp smaller it would get hotter and the heat would alter the sodium color changing it from pure yellow, into a pinky-orange color. Some efficiency was lost, but the smaller bulbs could be focused better on the road, and you could see color, although with some distortion.
These enhanced sodium lamps were the most efficient and long reliable lighting technology for a long time. It’s only been in the last few years that LEDs have improved enough to beat it.
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