Compare these day and night shots of the Seattle monorail:
Why are US cities lit by all the colors of the rainbow – i.e., white – during the day but the color of grapefruit flesh at night?
The answer is the sodium vapor lights we use to light our cities. They are relatively inexpensive in terms of the amount of light per unit of electricity, but the light they cast is a hideous yellow-orange. It’s like one of those box-of-64 crayons we had as kids where there were always at least a couple of crayons whose names didn’t make much sense (burnt umber?) and for which we could never find a use. Maybe all those unused crayons are secretly used to power these lights?
There is a project afoot in Seattle to replace the sodium vapor lights on utility poles with bright-white LED lights.
Wow, does it make a difference! Suddenly, nights are full of color again. In theory, we don’t see colors as well in dim light; the color-seeing cones in our eyes aren’t as sensitive as the light-gathering rods. But streetlights aren’t dim. We do see color.
As a society we seem to have become inured to the yellow light. We’ve in effect been told, “Nights are yellow; deal with it.”
Sickly yellow light at night clearly isn’t one of the top-three problems facing us as a society.
Still, little things matter. Here are two Seattle residential streets showing the difference in lighting:
Thanks, Seattle City Light. At least one customer notices, and is grateful.
By the way, here’s a night shot from the Big Island of Hawaii. They use monochromatic yellow lights for all public lighting. These low-pressure sodium vapor units emit light at a single wavelength that can be filtered out by the telescopes atop Mauna Kea. (The weirdest thing is that they’re almost exactly the same yellow as used in traffic signals, making it almost impossible, at least for visiting drivers, to spot a traffic light that’s gone to yellow!)