From Neil Knowles
Shuki Nakamura repeats the canard that “only 10 per cent of the input power [of a lighting system] is converted into light, and the rest is lost as heat” (6 January, p 44). In colder climes this heat is very often useful, warming our houses for us and saving money on central heating.
His statement that LED lighting “can convert 50 per cent of input energy into light” seems optimistic. The standard measure of luminous efficacy is lumens per watt, and the theoretical maximum is 683. Typical tungsten sources are rated at around 20 lumens per watt, which is indeed inefficient. The best commercially available LED light sources are rated at around 45 lumens per watt – twice as good but nowhere near the 103 for linear fluorescent sources, or more than 200 for low-pressure sodium lamps, which is why these are used as street lights.
London, UK
