Research Topic: How much light for how much power?
By Ian Mander, 30 January 2006, updated 31 May 2007.
Question: How much light can I get from a halogen lightbulb compared with a high power LED for the same input power?
Answer: Probably about the same. At time of writing, most high power LEDs available (eg, 3W Luxeon) are only about the same efficiency as quartz halogen bulbs. However, the light quality will be different. 5mm LEDs normally have a beautifully diffuse light beam with a bright central area, but lightbulbs (with reflector) have a very bright central spot (good for distance illumination) and poor side lighting. So your choice won't be based on efficiency, but on what you want to use it for, price, complexity (high power LEDs need to be heatsinked), etc.
Update: 1 March 2007. Just over a year later, high power LEDs are slowly trickling through to the market which can manage 60 lumens/watt even at a relatively high 3 to 5 watt output. I say trickling through because their availability is seriously limited, especially in New Zealand. For higher wattage, well, the incandescent isn't dead yet.
Update: 23 April 2007. I've found a couple of sites to inexpensively buy high-power efficient LEDs: www.kaidomain.com and www.dealextreme.com (the latter seems to be higher recommended). I've bought several Cree XR-E (P4 bin) LEDs from the latter, and including shipping the latest price worked out to about NZ$7 per LED - not too bad. These LEDs are seriously bright even at low current, and work well in various situations.
The efficiency of various light sources
Originally from Wikipedia, updated by me.
Note: A candle typically produces about 12.6 lumens of visible light and 40 watts of heat, although this can vary depending primarily on the characteristics of the candle wick. For comparison, note that a 40 watt incandescent light bulb produces approximately 500 lumens for the same amount of power. The unit candela was originally defined to indicate the 'brightness' of a naked candle flame.