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Why is light from a low pressure sodium vapor lamp unpleasant?
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Low pressure sodium (LPS) lamps give off a very distinctive, low bandwidth yellow glow sometimes considered unpleasant. However, that very coloration makes them highly efficient and remarkably useful.
The Yellow-Light Road
LPS lamps glow yellow because sodium atoms emit light on one dominant yellow spectral line. The light is monochromatic---single colored---consisting almost entirely of yellow wavelengths. Other colors from the electromagnetic spectrum are not present in sufficient quantities to be detectable.
History
The first LPS lamp was patented by Arthur H. Compton of Westinghouse Electric in 1919. Noting the high luminosity of heated sodium, he sought to develop a workable device that could contain it. However, due to its highly corrosive nature, an effective, commercial model wasn't manufactured until 1932.
Efficiency
LPS lamps are the most efficient electrically powered light source available, providing approximately two hundred lumens per watt of power. They are so efficient because the yellow color of the light is at a wavelength in the range where the eye is most responsive.
Safety Yellow
Yellow LPS light is also the safest to drive under: its monochromatic nature makes distinction of other colors nearly impossible. This reduces glare, increases perception of contrast and makes it easier for that light to penetrate fog and rain with little dispersion---image blurring caused by air moisture acting like tiny prisms, breaking light up into disruptive multi-colored images.
Curious Fact
Yellow and black are the colors that show up best under LPS lamps. They also contrast each other best in nature. That is why many creatures have evolved yellow and black patterning, and why many essential road signs are painted those colors.
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