Several recently published road maps for solid-state light have called for systems with efficacies of 200 to 240 lm/W. Luminous efficiency, or efficacy, is a common performance metric for light sources. It is the ratio of the perceived brightness to the input power and, because the brightness of a light source is measured in lumens and electrical power is measured in watts, the units of efficacy are lumens per watt. Several assumptions were made to arrive at the 200 to 240 target, with only passing comments on color rendering properties.

Of the current lighting technologies, low-pressure sodium lamps have the highest efficacy and operate at up to 200 lm/W. But because their output is essentially monochromatic light at 589 nm, low-pressure sodium lamp sources have extremely poor color rendering properties. In fact, the color rendering index (CRI) for low-pressure sodium is negative (–47). The theoretical efficacy for a monochromatic light at 589 nm is 525 lm/W, so this translates to a wall plug efficiency of 38.1 percent. Although other light sources, most notably infrared laser diodes, have even higher efficiencies (electrical power to optical power), low-pressure sodium remains the benchmark technology in terms of efficacy...