MrSeb writes: "One of the ways — if not the best way — to track the progress of LEDs over the past few years has been through the metric of lumens-per-watt. As you can gather from the name this is an efficiency rating that is based on the amount of visible light emitted relative to the amount of power consumed. A lumens-per-watt (lpw) rating is especially interesting because it works regardless of the light source — the lpw rating for an incandescent bulb is a lowly 15 (or so) while newer LED bulbs are in the range of 75. While 75 lpw is plenty efficient, it’s no where near what manufacturers like Cree are working on. In fact, the company has just put out a 200 lpw LED known as the XLamp MK-R."