The luminous efficacy of radiation (LER) in lumens per watt is defined as the ratio of the photometrical flux (luminous flux of visible light) and the radiometrical flux of the light source (radiant power of the total emitted spectrum).
The LER indicates how much of the light output we humans perceive (as white light or its visible components) versus the total energy put into emitting the light. Pier gave some examples of light sources, a tungsten light bulb typically delivers 15lm/W, while the sun spectrum including the non-visible bands is only 93lm/W, but the LER of the sun spectrum truncated to its 400-700nm visible bands is at 251lm/W for natural white light at a 5800K colour temperature.
Now, according to Pier, a combination of two or more individual spectral lines should yield optimum efficacy. Nowadays, most “white LEDs” are in fact Blue LEDs covered with a mixture of phosphors. The blue light (of higher energy) pumps or excites the phosphors that re-emit light at different set spectrum bands, depending on their chemistry.
“The LER depends very much of the spectrum bands being used and is relatively independent from the drive current or the temperature” explained Pier, taking as an example the Luxeon M white LED, a 5700K device capable of delivering 316lm/W. Another device put forward as being the state of the art was Philips Lumiled’s Luxeon Altilon core, combining four dies into a single 3.5x3.76 chip package and exhibiting a luminance of 75Mcd/m 2, a flux of 1000lm and a LER of over 200lm/W. This is to compare with halogen lamps with a 30Mcd/m 2 luminance, high intensity discharge (HID) lamps at 60Mcd/m 2 and the sun 1600Mcd/m 2. The colour rendering index (CRI) achieved by this product is considered very good at 93.
Today, about 95 percent of GaN LEDs are manufactured on sapphire wafers but the scope of using cheaper and more widely available