From Delancey Place:
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....
....MUCH MORE
