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The luminous efficacy of a typical CFL is 50–70 lumens per watt (lm/W) and that of a typical incandescent lamp is 10–17 lm/W. [38] Compared to a theoretical 100%-efficient lamp ( 680 lm/W ), CFL lamps have lighting efficiency ranges of 7–10%, [ 39 ] versus 1.5–2.5% [ 40 ] for incandescents.
For example, a 100-watt, 1000 hour, 120-volt lamp will produce about 17.1 lumens per watt. A similar lamp designed for 230 V would produce only around 12.8 lumens per watt, and one designed for 30 volts (train lighting) would produce as much as 19.8 lumens per watt. [76] Lower voltage lamps have a thicker filament, for the same power rating.
[32]: 20 The efficacy of fluorescent tubes ranges from about 16 lumens per watt for a 4 watt tube with an ordinary ballast to over 100 lumens per watt [51] with a modern electronic ballast, commonly averaging 50 to 67 lm/W overall. [52] Ballast loss can be about 25% of the lamp power with magnetic ballasts, and around 10% with electronic ballasts.
Luminous efficacy can be normalized by the maximum possible luminous efficacy to a dimensionless quantity called luminous efficiency.The distinction between efficacy and efficiency is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage.
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In the US lighting industry, foot-candles are a common unit of measurement used by architects to calculate adequate lighting levels. Foot-candles are also commonly used in the museum and gallery fields in the US, where lighting levels must be carefully controlled to conserve light-sensitive objects such as prints, photographs, and paintings, the colors of which fade when exposed to bright ...