The 120 years since Edison's first commercial light bulb have
seen tremendous improvements in the efficacy of light sources. That bamboo-filament lamp of 1880 gave about 1.6
lumens per watt (lpw). By contrast, today's common tungsten lamps are about ten times more
efficient, and many discharge lamps give over 100 lpw.
Much of the motivation
for increasing efficacy stemmed from the low power of early lamps. From 1880
until the late 1910s, most common lamps gave a dim eight or sixteen candlepower
(about the same amount of light as a modern 25-watt lamp). As the gas-filled
tungsten lamp became more common in the 1920s, light levels began to rise in
many homes. Gas lamps and candles became obsolete or were used only on special occasions.
users however, found even tungsten lamps too dim for many applications. They
either had to install large numbers of fixtures or use higher power light
sources like arc lamps or discharge tubes. These lamps were not easy to install
or maintain and generally did not give the efficacy of the tungsten lamp, so
they cost more to operate.
In the 1930s several efficient discharge lamps
became available. Low pressure sodium (LPS), mercury vapor, and fluorescent
lamps gave 40-50 lpw, lasted for thousands of hours, and were easy to maintain.
Today, LPS lamps approach 200 lumens per watt while fluorescent tubes and
metal-halide lamps both provide over 100 lpw.
Sometimes there are trade-offs.
LPS, though efficient, gives a stark yellow light. High pressure sodium (HPS or,
as it's known in the industry "SON") gives a somewhat better color,
but at a lower efficacy. In the 1970s, an even better color HPS lamp (known in
the industry as "white-SON") was developed, but the lamps' efficacy
suffered still more.