Low-pressure sodium-vapor lamp

The low-pressure sodium lamp achieved the highest luminous efficacy of any electrical light source—200 lumens per watt—by sacrificing everything except efficiency. Its monochromatic orange glow at precisely 589 nanometers, the sodium D-line, falls near the peak sensitivity of human vision. The price of this efficiency is a world rendered in shades of yellow, gray, and black—a color rendering index of essentially zero.

The adjacent possible assembled over decades. Heinrich Geissler's 1857 tubes demonstrated that rarefied gases could emit light when electrified. Peter Cooper Hewitt commercialized mercury vapor lamps in 1901, proving gas discharge could be practical. But sodium's extreme reactivity defeated early attempts. Arthur Compton at Westinghouse demonstrated a working sodium lamp in 1920, but the metal corroded electrodes and blackened glass tubes within hours.

The breakthrough came from materials science. In 1931, Marcello Pirani at Osram in Germany invented a borosilicate glass resistant to sodium attack—the critical enabling technology. Within a year, three companies moved simultaneously: Osram installed trial lamps outside its Berlin factory in August 1931, Philips lit roads in the Netherlands by June 1932, and GEC deployed lamps in Britain. The first American installation illuminated a rural highway near Port Jervis, New York in 1933.

Low-pressure sodium found its niche in applications where efficiency trumped color: highway interchanges, bridge approaches, tunnel lighting. The yellow light penetrates fog better than white light, scattering less in moisture. By the late 1940s, lamps reached 10,000 lumens with 4,000-hour lifespans. Astronomical observatories adopted sodium lighting because the narrow 589nm emission can be easily filtered from telescope observations.

Philips announced discontinuation in 2017, producing the last lamp at its Hamilton, Scotland factory on December 31, 2019. LED technology had finally matched sodium's efficiency while offering full color rendering. But for nearly ninety years, the low-pressure sodium lamp proved that sometimes the most efficient solution is also the most uncompromising.