First, the figures:
The most efficient modern available white LEDs (as of June 2008) produce about 80-100 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. Many others that are in recent LED products achieve merely 20-45 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 laboratory prototype of a white LED achieving 161 lumens/watt has been announced on 11/19/2008.
More in my efficient/bright LED page, updated 11/26/2008.
For efficiency of LED line voltage "bulbs" and LED lighting fixtures, one should check out data available at the Department of Energy's CALiPER program. The most efficient LED item I could find there is a retrofit fixture that achieved 61 lumens per watt. I did not notice any mention of brands or models.
The most efficient line voltage lighting products that I am aware of by brand and model are the Cree LR-4 and LR-6 models. Efficiency according to what I have seen so far in Cree Lighting's website ranges from 47 to 54 lumens per watt, with 54 for the LR-6 models.
Compare to 14-17.5 lumens per watt for standard "A19" 120 volt 60 to 100 watt incandescents, and typically 16 to 21 for most halogen lamps rated to last 2,000 hours or more, typically 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.
Most line powered LED lighting products other than Cree LR-4 and LR-6 units as of June 2008 achieve anywhere from under 20 to as high as 52 lumens/watt.
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
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 ususally 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 flashlights will probably appear roughly equally bright. But 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, especially lower wattage / lower current, longer life 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, the usual 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. In dim conditions, white LEDs can outperform most incandescents watt-for-watt. But most fluorescent lamps are much more efficient than both incandescents and most white LEDs (though this is in early stages of changing), both for photopic and scotopic vision.
The next few paragraphs may bore some - click here to bypass.
The most obvious disadvantage of lower current incandescents is that the thinner filament must be operated at a slightly cooler temperature to reduce the evaporation rate accordingly. The lower temperature disfavors radiating visible light, while increasing the already-high percentage of radiation that is infrared. To some extent lower voltage incandescents can be more efficient than higher voltage ones of the same wattage since the filament is shorter and thicker and can be operated at a higher temperature for a given life expectancy. The most efficient incandescents of a couple hundred to a few hundred watts are designed to operate at 20-28 volts, the most efficient ones of around 10-100 watts have design operating voltages not far from 12 volts, and the most efficient ones (of a given life expectancy) of around a watt to a few watts have design operating voltages not far from 6 volts, and the most efficient ones of a fraction of a watt generally have design voltages close to 5 volts. But lower-than-optimum design voltages have the disadvantage of heat conduction from the ends of the short thick filament.
Another advantage of thicker filaments is the advantage of a fill gas.
The fill gas slows down filament evaporation because gas atoms "bounce"
evaporated tungsten atoms back to the filament. This permits a higher
filament temperature for a given life expectancy. A disadvantage of the
fill gas is that the gas conducts heat from the filament, and that is
energy that cannot be radiated and this is a loss.
A thinner filament has a thinner "boundary layer" of hot gas around it,
and a higher temperature gradient within this "boundary layer". For this
reason, the amount of heat conducted from a filament by the gas is nearly
proportional to the visible apparent length of the filament wire, and
hardly varies with filament diameter. A thin filament has almost as much
loss per centimeter as a thick one, despite the thin one having much less
input power per centimeter.
The cheapest fill gas, sometimes argon but more usually a traditional
mixture of argon and nitrogen (nitrogen impairs destructive arcs which
form excessively easily at over approx. 12 volts in pure argon), generally
is better than no gas at all (vacuum) if the wattage exceeds approx. 8 to
10 watts per centimeter of visibly apparent filament length. Premium gases
such as krypton and xenon are better, but are still worse than a vacuum if
input power is less than a few watts per centimeter of visibly apparent
filament length.
In addition, with ratio of power consumption cost to lamp cost lower with lower wattage ones, incandescents of lower wattages tend to be designed for longer life - which further impairs the efficiency of lower wattage incandescents.
If you only half-followed the previous few paragraphs, don't sweat it. But 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 8 lumens per watt. This is obviously much less than the 14-17.5 lumens/watt of most "regular incandescents".
LEDs have a special advantage as indicator lamps in electronic equipment. The incandescent lamps that used to do this job were low power, low current, vacuum-filled and often designed to last thousands or often tens of thousands of hours. A "standard" 5 volt .06 amp 20,000 hour incandescent only gets 2.1 lumens per watt!
The major reason is that traffic lights are colored. The red and green lenses block about 70 percent of the light! 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 are slightly more efficient than the most efficient white LEDs (although white started advancing faster than red, yellow and green in early 2005 or so). (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 somewhere around 12 watts can do as good a job in a traffic light as the usual 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" has typically roughly rough-ballpark 300 (often a little less) lumens per radiated watt, with one recently announced especially efficient prototype having 331 lumens per radiated watt (and "only" 91.7 lumens per input watt, meaning 27.7% efficiency of converting electricity to light). A more recent still laboratory prototype white LED (announced 12/22/2006) achieving 150 lumens out per watt in probably has slightly higher lumens out per watt in by having a more favorable more yellowish shade of white (but more "pure white" than yellowish, 4600 Kelvin color temperature) than most white LEDs including the 91.7 lm/W one, so I guesstimate 350 lumens out per watt out, so 150 lumens out per watt in is about 43% efficiency of converting electrical energy to light.
Bottom line: LEDs with datasheets claiming 150-600 lumens per watt are specifying lumens per watt of light output, not lumens per watt of electrical input.
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