Skartveit, "Observed and modelled hourly luminous efficacies
under arbitrary cloudiness," Solar Energy, vol.
The challenge in the design of white light LED clusters with CCT tunable consists of achieving excellent color rendering index (CRI) values  over a reasonable range of color temperatures while at the same time maximizing their luminous efficacies
Thus, luminous efficacies
(denoted by the symbol [[nu].sub.[psi]]) in units of lm/[W.sub.c] are equivalent to those in units of lm/We multiplied by one of these efficiency factors: the luminous efficacy of an incandescent lamp powered by grid electricity could either be written as 14 lm/[W.sub.e] or 4.4 lm/[W.sub.c], depending on whether one chooses units of wall-plug grid electricity or units of the primary chemical energy used to produce that wall-plug electricity.
As we learned with semiconductor LEDs, having light sources with high luminous efficacies
and long lifetimes is only the beginning of the story.
Current typical source luminous efficacies
are 14 lm/W for incandescent, 83 lm/W for fluorescent, and 72 lm/W for warm LEDs.