the advance of lighting -- 11/23/22

Today's selection -- from Numbers Don't Lie by Vaclav Smil. Outdoor light -- the sun -- has a luminous efficacy of 683 lm/W. It is technically difficult to produce indoor light greater than 100 lm/W, though certain breakthroughs that may eventually be commercially available can now produce 200 lm/W:

"You can roughly track the advance of civilization by the state of its lighting -- above all, its power, cost, and lumi­nous efficacy. That last measure refers to the ability of a light source to produce a meaningful response in the eye, and it is the total luminous flux (in lumens) divided by the rated power (in watts).

"Under photopic conditions (that is, under bright light, which allows color perception), the luminous efficacy of visible light peaks at 683 lm/W at a wavelength of 555 nanometers. That's in the green part of the spectrum­ -- the color that seems, at any given level of power, to be the brightest.

"For millennia, our sources of artificial light lagged three orders of magnitude behind this theoretical peak. Candles had a luminous efficacy of just 0.2 to 0.3 lm/W; coal gas lights (common in European cities during the 19th century) did five or six times as well; and the carbon filaments of Edison's early bulbs hardly did better than that. Efficacies took a leap with metal filaments, first with osmium in 1898 to 5.5 lm/W, then after 1901 with tantalum to 7 lm/W, and a decade or so later tungsten radiating in a vacuum got up to 10 lm/W. Putting a tung­sten filament in a mixture of nitrogen and argon raised the efficacy of common household lamps to 12 lm/W, and coiling the filaments, beginning in 1934, helped to bring incandescent efficacies to more than 15 lm/W for 100-watt lamps, which were the standard source of bright light during the first two post-Second World War decades.

Original carbon-filament bulb from Thomas Edison's shop in Menlo Park

"Lights based on different principles -- low-pressure sodium lamps and low -- pressure mercury vapor lamps (fluorescent lights) -- were introduced during the 1930s, but they only came to widespread use in the 1950s. Today's best fluorescent lights with electronic ballasts can pro­duce about 100 lm/W; high-pressure sodium lamps put out up to 150 lm/W; and low-pressure sodium lamps can reach 200 lm/W. However, the low-pressure lamps pro­duce only monochromatic yellow light at 589 nanometers, which is why they aren't used in homes but rather for street illumination.

"Our best hope now rests on light-emitting diodes (LEDs). The first ones were invented in 1962 and pro­vided only red light; a decade later came green and then, during the 1990s, high-intensity blue. By coating such blue LEDs with fluorescent phosphors, engineers were able to convert some of the blue light into warmer colors and thus produce white light suitable for indoor illumi­nation. The theoretical limit for bright white LEDs is about 300 lm/W, but commercially available lamps are still a long way from achieving that rate. Philips sells LEDs in the United States -- which has a 120-volt standard -- that offer a luminous efficacy of 89 lm/W, for 18-watt soft white and dimmable bulbs (replacing 100-watt incandescent lights). In Europe, where the voltage ranges from 220 to 240 volts, the company sells a 172 lm/W LED tube (replacing the 1.5-meter-long European fluorescent tubes).

"High-efficacy LEDs are already delivering signifi­cant electricity savings worldwide -- it also helps that they can provide light for three hours a day for about 20 years, and if you forget to switch them off you will hardly notice on your next electricity bill. But, much like all other sources of artificial light, they still cannot match natural light's spectrum. Incandescent lights gave out too little blue light, and fluorescent lights had hardly any red; LEDs have too little intensity in the red part of the spectrum and too much in the blue part. They don't quite please the eye.

"Light efficacies of artificial sources have improved by two orders of magnitude since 1880 -- but replicating sunlight indoors still remains beyond our reach."



Vaclav Smil


Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World


Penguin Books


Copyright 2020 by Vaclav Smil


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