But Central Maine Power (CMP) wanted to modernize the town's lighting from incandescent, platter-type fixtures to unshielded
mercury-vapor lamps. After its first proposal was rejected (because the number of fixtures and their cost would have quadrupled), CMP introduced a reasonably priced, high-pressure-sodium lamp in 1979.
After the energy shocks of the 1970s, high-pressure sodium lights gradually took over the night, Following the economic imperative to use the most cost-effective lighting--high-pressure sodium lights consume half as much energy as
mercury-vapor lamps and can last up to 16,000 hours longer--transportation departments and cities embraced sodium light.
Incidentally, light pollution itself is becoming more broadband, as white metal-halide lamps add to the mix of older sodium-and
mercury-vapor lamps. So filters designed to block the sodium and mercury wavelengths are less effective than they used to be.