In article , John Grabowski wrote:

Personally I think mercury vapor lights are outdated technology. I never
liked the fact that the bulbs don't burn out. They just get progressively
dimmer with age.

They do eventually burn out. Eventually. And now they're making them
cheaper, and they don't last like they used to.

Metal halide is more efficient as is high pressure sodium.
For maximum lumens per watt there is always low pressure sodium, but you may
have a difficult time finding cheap fixtures.

Most effectively efficient lighting for outdoors at night among these is
metal halide. Lumens aren't all the same when things are dim enough for
scotopic vision to make a significant contribution to sensation of
illumination.
Furthermore, sodium light (both low and high pressure, excepting shorter
life lower efficiency higher cost higher color rendering index HPS) tends
to make reds, greens, blues and purples darker. Low pressure sodium has
the ultimate bad color rendition - everything is an orange-yellow version
of black-and-white.

- Don Klipstein )