The latter were introduced by General Electric for commercial use around the time this photograph was taken. The lamps used within these luminaires were nearly three times more energy-efficient as the incandescent ones commonly in street lighting practice at that time. Low pressure sodium (LPS) lamps with fixtures like this one were used along the Golden State Bridge until about 1970 and emitted a yellow-appearing light, similar to a yellow caution traffic signal. With the introduction of mercury vapor lamps after WW2, LPS lamps and their fixtures slowly began to disappear on account of the economy, longevity, simplicity and better illuminated appearance of mercury vapor light sources. The luminaire in this photograph might have been installed for trial practice by its utility (believed to be Southern California Edison).