The efficiency of a light source is measured in lumens per watt- simplified, the luminous power measured in lumens divided by the electrical power measured in watts. The more lumens per watt, the better the light source in terms of energy efficiency. A lot has changed in the last few years; what do you think is the most efficient light source? Answers after the fold, but no peeking! We can tell.
After spending hours making my own list, of course there was this neat table from Wikipedia which agrees with our other sources. I was surprised by the results.
1) Low pressure sodium lamps were far and away the most efficient, at 200 lumens/watt, but they give out a single orangy -yellow frequency of light and have been used mainly as roadway lighting. While I always found them easy on the eyes when driving, evidently the police didn't like them because it is impossible to tell the colour of a car, everything is grey or orange. They are being replaced by less efficient high pressure sodiums.
2) Next best are the high intensity discharge lamps like the metal halides (seen in big box stores and gymnasiums) and high pressure sodium (exterior and roadway lighting). at 150 they are every efficient but always big and bright, not suitable for home use.
3) Conventional fluorescent tubes are currently in third place, but that really depends on the colour temperature (cool white is more efficient than warm white)
4) Compact Fluorescents are slightly less efficient than long tubes at this time, but obviously far more efficient than the incandescents they replace.
Tied for 3 or 4) LED's, at 26 to 70, are not yet more efficient than flourescents, but are catching up; in the lab there are prototypes up to 131. They are also coming out in MR16 sizes to replace existing halogen fixtures, although they are still expensive.
5) Quartz Halogen fixtures are better than conventional incandescent at 24 but are evil little things. They are in fact, a very small incandescent that would melt if not made from quartz. 10 years ago they were all the rage, and I have a ceiling with 18 of them. They are extremely hot, they require transformers that burn out, and you can't just change the bulbs to CFL,s but have to wait until the LED replacements are bright, affordable and a decent colour temperature, which they are not yet. There have been many recalls of cheap quartz halogen lamps causing fires. They should be put out to pasture as quickly as incandescents. IKEA has lots of lovely quartz halogen designs that look so modern, but keep away.
6) Way down there at the bottom:
The incandescent lightbulb, all of which belong a museum with this one.
Be sure to read How to Green your Lighting again!