First, the figures:
The most efficient modern available white LEDs (as of March 2013) produce about 90-170 lumens of light per watt of electricity delivered to the LEDs when the LEDs are supplied "typical" current or that at which their characteristics are specified. Among high and mid power LEDs, this figure is 90-153 lumens/watt for very good ones. Many others that are in recent LED products achieve merely 35-50 lumens/watt. Most such white LEDs are and will be slightly more efficient when moderately underpowered and will usuallty be less efficient when overpowered. A few white LEDs achieve 150-180 lumens/watt at reduced power.
A laboratory prototype of a white LED achieving 276 lumens/watt is mentioned in this 2/13/2013 Cree press release. These figures are for cool white LEDs. Warm white ones typically achieve 75-135 lumens/watt for the best ones.
More in my efficient/bright LED page, updated 3/20/2013.
UPDATE 3/26/2012: There are some line voltage LED lighting fixtures achieving or slightly exceeding 100 lumens/watt. There are some with "warm color" and good color rendering index around or somewhat over 60 lumens/watt - including Cree LR-4 and LR-6 ones.
UPDATE 3/26/2012: Philips LED light bulbs are now available at Home Depot, and 3 with light output in the range of 470-1100 lumens and good color characteristics achieve 59-65 lumens/watt. A premium model achieves 90-93 lumens/watt.
Compare to 14-17.5 lumens/watt for standard "A19" 120 volt 60 to 100 watt incandescents, 16 to 21 for most halogen lamps, 50-70 (at optimum temperature) for most compact fluorescents rated 13 to 26 watts, and 85-100 for 32 watt T8 fluorescents operated with electronic ballasts.
Now, table of contents explaining how LEDs outperform incandescent lamps so well in more specialized applications:
Why batteries last so long in LED flashlights
Some LEDs work well for night vision
A special disadvantage of low power incandescents
Indicator lamps - LED advantage is greater here
Why LEDs have 10x the efficiency of incandescent in traffic lights
Where some of those really impressive lumen per watt figures for LEDs come from!
If the bulb is an incandescent, then when the batteries run down the filament runs cooler. For one thing, its resistance drops, which causes the current to not decrease as much as the voltage applied to the bulb does. Compare to this LEDs having an increase in resistance when fed less power, so when the batteries weaken the LED typically conserves the remaining energy in the batteries. Another significant difference is that incandescent lamps operate much less efficiently when the filament is not as hot, while most LEDs used in flashlights operate slightly more efficiently than normal at reduced power.
If an incandescent flashlight and an LED flashlight have the same number of batteries and the batteries are of the same type and condition, and the flashlight bulbs take the same amount of power from fresh batteries, the incandescent one will fade more rapidly when the batteries weaken. The LED one will continue working for many times the amount of time that the incandescent one does.
As it turns out, photometric units such as the lumen, lux, footcandle and the candela are defined in terms of photopic vision. Two light sources with different spectral content and having equal photometric measurement will appear equally bright to a "standard human eyeball" that is in photopic mode. But in scotopic mode, human vision has reduced sensitivity to red wavelengths and increased sensitivity to wavelengths from mid-blue to mid-green. Two light sources with equal photometric measurements can have very unequal performance to a dark-adapted eye if their spectral content is different.
Most high brightness green and blue LEDs and all of the usual high-brightness blue-green LEDs have a spectrum that is greatly more scotopic-vision-favorable than the spectrum of incandescent lamps. A nightlight made with non-yellowish-green, blue-green or turquoise blue LEDs will appear to illuminate a room more brightly than an incandescent or neon nightlight with equal lumen output.
In fact, most white LEDs have a spectrum somewhat more favorable to scotopic vision than the spectrum of typical incandescent lamps, especially 120 volt incandescent lamps 15 watts or less and super-long-life ones up to 150 watts.
A 4 or 7 watt 120 volt night light bulb usually has an efficiency (more accurately, overall luminous efficacy) of only 4 to 6 lumens per watt. Most 15 watt 120V incandescents get around 7.3-8 lumens per watt. This is less than the 14-17.5 lumens/watt of most "regular incandescents".
In addition, colored "lenses" were often used, and most of those removed a majority of the visible light that would otherwise get through. LEDs succesfully replaced most incandescent indicator lamps while using .02 amp, even when their efficiency was .5 lumen/watt (half a lumen per watt) or less.
The major reason is that traffic lights are colored. The red and green lenses block about 75 percent of the light! (Updated from 70% on July 13 2009.) In addition, LEDs normally specialize in producing a particular color of light. There is hardly such a thing as a practical white LED chip - the usual white LEDs have a fluorescent substance over a blue LED chip to convert some of the blue light to a broad spectral band from red to green. As of 2004, the red and green LEDs used in modern LED traffic signal units had about 2-2.5 times the energy efficiency of incandescent traffic signal lamps, before filtering by the colored lenses. (Yellow does not play a major role in energy savings since red and green are used so much more than yellow.)
Add to this the fact that LEDs can be made in custom-designed molded "bulbs" with tailored optical properties. This makes it a little easier to get light concentrated to where it needs to go and waste less power in producing light where one does not need as much.
Given all of this, one can see why a bunch of LEDs drawing 8-14 watts can do as good a job in a traffic light as the usual 67 to 116 watt incandescent traffic signal lamp!
Any wavelength of visible light has a characteristic lumen/watt luminous efficacy figure. This is approx. 683 lumens/watt times the Photopic Function of that wavelength. Light having more than one wavelength would have a luminous efficacy which is an average of those of all wavelengths present, with weighting for the amount of light present at each wavelength.
"White LED light" typically has 250-350 lumens per watt.
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