Artificial light has long been a significant contributor to the quality and productivity of human life. It expands the productive day into the non-sunlit hours of the evening and night, and even during the day it expands productive spaces into the non-sunlit (windowless) areas of enclosed dwellings, offices, and buildings.
Because we value artificial light so highly, we consume huge amounts of energy to produce it. The production of artificial light consumed an estimated 6.5% of total global primary energy in 2005. These percentages are large and, coupled with increasing concern over energy consumption, have inspired the development of new and more energy-efficient lighting technologies. In particular, we are currently witnessing a transition from incandescent technology to fluorescent and high-intensity-discharge (HID) technologies – a transition being accelerated in many nations through legislation.
In their current forms, however, all of these “traditional” technologies have limitations.
Filament-based incandescent technology, for example, emits light approximately as a blackbody. Because of the breadth of the blackbody spectrum, however, the majority of the emitted radiation lies outside the human visual response. Even at a hypothetical filament temperature of 6,620K, for which the blackbody spectrum optimally matches the human visual response, the luminous efficacy of radiation (lm/W, lumens per watt of radiant energy content of the light, a standard measure of the “visual efficacy” of radiation), is only around 95 lm/W, about 23% that of an optimal multi-component white light source. And, in practice, filaments are typically limited to much lower operating temperatures in the 2,700-3,200K range, and hence to a luminous efficacy of only 16 lm/W, about 4% that of an optimal multi-component white light source.
Both glow-discharge-based fluorescent and HID technology depend on the acceleration of free electrons in a gas discharge, the collisions of those energetic electrons with atoms in the discharge, the resulting excitation of those atoms into excited electronic states, and finally the generation of luminescence as those excited electronic states decay. A gas-discharge environment is a complex one, however, and there are many energy-loss channels aside from excitation of atoms into luminescent electronic states. As a consequence, less (and often much less) than 50% of the injected electrical energy typically ends up in luminescence at the desired wavelengths. Moreover, for mercury-based fluorescent technology, there is an additional energy loss associated with the phosphor conversion of ultraviolet luminescence at 254 nm to visible luminescence. The luminous efficacies of fluorescent and HID technologies (in an aggregate average over the various lamp types in use in the U.S. in 2001) are about 71 lm/W (lumens per watt of electrical power required to produce the light) and 96 lm/W, respectively, about 18% and 24% that of an optimal multi-component white light source.