Lumen is a term used to describe the measurement of light conversion. One lumen equals one candela across one Ste-radian and is a measure of luminous flux, which is adjusted to the human eye’s perception of the light’s power. Lighting efficiency or efficacy is measured in lumens per watt. Incandescent typically have efficacy in the range 15–20 lm/watt. Ultra-high efficacy lighting can achieve over 120 lm/watt.