The Most Efficient LEDs!

(And some really bright ones also)

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News and links! UPDATE 4/14/2012
General Notes and Disclaimers
Pure Red / Deep Red LEDs - 14 to 20-22 lumens/watt
Orangish Red LEDs - 46-96-100-105 lumens/watt
Red-Orange LEDs - 66-123 lumens/watt
Truly Orange LEDs - 18-22 lumens/watt
Yellow and amber LEDs - 34-108 lumens/watt, with a way for 50-110-plus for less-amberish yellow.
Yellow-Green LEDs - 3-8-140 lumens/watt.
Green LEDs - 70-120 lumens/watt.
Better InGaN green LEDs achieve 100-120-165 possibly 200 lumens/watt at reduced currents
Blue-Green LEDs - 57-93 lumens/watt
Blue LEDs - 25-39 lumens/watt
White LEDs - 81-192-210 lumens/watt
Warm White LEDs - up to 150 lumens/watt
Increased Efficiency of InGaN LEDs at lower currents
Important Notes on Gallium Nitride and InGaN LEDs!
LEDs with high candela "beam candlepower" figures
Notes on Where To Get Efficient LEDs
Converting / comparing lumens, candelas, millicandelas

News and links!

UPDATE 2/27/2016 this 3/26/2014 Cree press release:

Cree announced 303 lumens/watt at 350 mA. I did not see mention of die size in this press release, as in whether it was a roughly 1 mm square or larger. Correlated color temperature was mentioned - 5150 K.

UPDATE 3/18/2013 this 2/27/2013 Cree press release:

Cree announced 276 lumens/watt at 350 mA. I did not see mention in this press release of die size, as in whether it was a roughly 1 mm square or larger. Correlated color temperature was mentioned - 4401 K.

UPDATE 9/15/2010 - Nichia is mentioned by this LEDs Magazine 9/6/2010 news item to have achieved:

* 135 lumens/watt at 1 amp through an LED lamp having four 1x1 mm dice in series, 1,913 lumens from approx. 14.2 watts. The color temp. was mentioned as 4700 K.

* 183 lumens/watt at 350 mA in a lamp with a 1 x 1 mm chip. (added 8/11/2011)

* A 4600 K LED lamp with a .45 x .45 mm die (larger than the usual generally roughly .3 x .3 mm die in most low power LED lamps) achieving 250 lumens/watt at 20 mA.

The above article mentions that manufacturing techniques necessary to achieve such efficiency are costly, and that Nichia is not prioritizing getting such efficient LEDs onto the market if they are dispropotionately expensive for their light output.

Cree is mentioned in the above link in the "Comments" as providing production samples of XM LED lamps that achieve 135 lumens/watt at 1 amp, but I suspect the die size is larger than 1x1 mm in those.

UPDATE 5/14/2010 - Cree announced in this 5/10/2010 press release a 2 by 4 foot LED light fixture (LED luminaire) that achieves 100-plus lumen/watt overall luminous efficacy from AC line voltage electrical input to light exiting the luminaire. Cree's LR24HE achieves this even with color temperature of 3500 K and color rendering index of 90.
Light output is said to be 3200 lumens, which I figure to be around 56-76% of that of most 2-lamp 4-foot fluorescent fixtures.
Cree mentions this LED light fixture being shown at their booth at Lightfair 2010.

UPDATE 11/28/2009 - 3M achieved a green LED achieving 181 lm/W at 350 mA by combining a green-emitting phosphor with an available blue LED chip. This is mentioned in this 11/23/2009 item in LEDs Magazine.

UPDATE 11/2/2004 - WITH THANKS TO FANS OF THIS WEBSITE:

In August 2004, Osram announced a prototype of an orange-red LED that achieved 108 lumens/watt, producing over 2 lumens at 10 mA with a voltage drop presumably of about 2 volts.

IMPORTANT NOTE: Some laboratory prototypes mentioned above are not yet in production even if years old! Barriers apparently include but may not be limited to patents held by competitors, also cost of production with materials and processes used to achieve higher efficiency. Also, gallium nitride based LED chips just may not be cheap (substrate material is generally about 9 on the Moh's hardness scale where diamond is 10). Production of chips of size favoring a more optimum lower current density may price such things out of the market or increase the size of optics needed to control where the light goes.

General Notes and Disclaimers:

Luminous efficacies were largely determined by one or more of several methods, mostly almost laughably crude. I give NO warranty as to accuracy. I have at times slightly changed my estimates for some models. Your mileage may vary.
Unless otherwise noted, luminous efficacy is stated for a 25 degree Celsius (77 degree F.) ambient with a current of 20 milliamps (350 mA for heatsinkable models) in an environment that does not build up heat around the LED, unless otherwise is stated. Heatsinkable models are generally stated to achieve their claimed performance when cooling is sufficient to cool either the heatsinkable surface or the junction temperature to 25 degrees C.

Red, orange, yellow, and yellow-green models (other than high power / heatsinkable ones) mentioned here generally have maximum efficiency at currents around 20-25 milliamps. Efficiency of these is typically less at low currents of a few milliamps or less. The decrease is not as bad with GaAlAsP and "T.S." AlGaAs and the most efficient InGaAsP red ones as it is with most other InGaAsP (orangish red through yellow-green).
Most ultrabright blue, blue-green, non-yellowish green, and white LEDs have luminous efficiency that increases with decreasing current, although the increase will reverse as current decreases below the current at which efficiency is maximized. This current is usually 1-3.5 mA for most blue and green low power LEDs and 2.5-7 mA for most white low power LEDs. For high power LEDs with 1 mm square dice, the current that results in maximum efficiency is typically 25-80 mA for white, and 15-45 mA for blue and green ones with dominant wavelength of 470 to 530 nm.
Please beware that performance of many LED models at currents below a few tenths of a milliamp or a percent or so of maximum rated current may be unreliable.
There may be significant tolerance in light output. Your mileage may vary.

Some of these LEDs radiate light of slightly different colors in different directions. This is most apparent with many older white models and narrow-beam yellow models, as well as the most efficient yellow-green models.

PLEASE NOTE that the lumen per watt figures in this document are lumens of light produced per watt of electricity delivered to the LED. Some LED manufacturers stated much higher figures indicating the lumens per watt of radiated light. The latter, high figure is typically in the ballpark of 60-100 for red, 130-180 for orange-red, 220-265 for red-orange, 440-500 for yellow, 400-plus to near 660 for green (656 for Avago HLMP-C115), and around 50-90 for blue. The main purpose of this "high" figure is for conversion between radiometric and photometric units for the emitted light. If the conversion efficiency and the "high" lumen/watt figures are both known, multiply them to get the lumens out per watt in.

Beam brightness and width figures below are mostly ones claimed by manufacturers/distributors and I have NOT confirmed most of these.

Most Efficient Deep Red / Pure Red LEDs - 14-20-22 lumens/watt

UPDATE 5/11/2009: LEDEngin makes some deep red high power LEDs. These LEDs have rated peak wavelength of 655-670 nm, typically 660 nm. These appear to me to use GaAlAsP chemistry and to be unusually efficient for GaAlAsP.

The LEDEngin LZ4-00R208-0000 at 700 mA and 25 C is claimed to have typical voltage drop of 10.5 volts and minimum radiant flux of 2 watts. This is minimum efficiency of 27 percent. Minimum efficiency is 24 percent at 60 C.

One update of mine is that spectral analysis indicates dominant wavelength of 643-644 nm and luminous efficacy of the emitted light to be about 60 lumens/watt. If radiometric efficiency is indeed 27%, then overall luminous efficacy is 16 lumens per watt.

Efficiency increases slightly as current decreases to 400 mA.

Update 3/22/2011 Osram LH W5AM-1T3T-1-L-Z appears to me to be good for typically 17-18 lumens/watt at 350-400 mA with very good heatsinking, maybe 22-25 lumens/watt with good heatsinking and a lower current of 100 mA.

Update 7/23/2011 Philips-Lumileds LXM3-PD01 is now available. My guess is that it's LXML-PD03-0260, rated to produce typically 290 mW at 350 mA with a typical voltage drop of 2.2 volts, for typical radiometric efficiency of 37%, at a junction temperature of 25 C. This indicates overall luminous efficacy around 20-22 lumens/watt. The DS68 datasheet indicates achieving around or a little over 92% of this when the heatsinkable surface is at 50 degrees C. Peak wavelength is typically 660 nm, dominant wavelength is not specified. The active material is said to be AlInGaP.

Low power red LEDs with GaAlAsP and Avago's similar AlGaAs in my experience, when doing well, tend to achieve 7-8 lumens per watt. But unlike the more efficient orangish red ones below, they are truly fairly pure red in color, with dominant wavelength (color specification roughly meaning hue) of 637-645 nm, and peak wavelength of 654-665, usually 660 nm.

Some favorites of mine are Stanley H-3000L, Lumex SSL-LX5093SRC/E, Lumex SSL-LX5093XRC/4, Panasonic LN261CALUR, and Avago HLMP-D150. A favorite of mine with especially deep red color and good usefulness as an indicator lamp at low current is Kingbright WP7113LSRD. All of these bright truly red LEDs are available at Digi-Key.

Radio Shack red LEDs of this chemistry include 276-086, 276-087 and 276-307.

Most Efficient Orangish Red LEDs - 46-96-100-105 lumens/watt

UPDATE 9/5/2015 - Digi-Key is showing Cree's XPEBRD-L1-R250-00801. At 350 mA, it is rated to produce a minimum of 73.9 lumens and typically 77 lumens, with a voltage drop of 2.2 volts. This works out to 96-100 lumens/watt.

UPDATE 9/5/2015 - Digi-Key is showing Cree's XPERED-L1-R250-00801, but not yet as in stock. At 350 mA, it is rated to produce a minimum of 73.9 lumens and typically 77 lumens, with a voltage drop of 2.2 volts. This works out to 100-105 lumens/watt.

UPDATE 3/22/2011 - Lumileds LXM2-PD01-0050, according to its DS-68 datasheet, achieves minimum 50, typically 53 lumens at 350 mA when junction temperature is 25 degrees C, with typical voltage drop of 2.1 volts. This amounts to 68-73 lumens/watt.

Expext these LEDs to produce 10-11% less light than mentioned here with more realistic thermal conditions, namely heatsinking that cools their thermal pads to 35 degrees C.

UPDATE 1/1/2008: The most efficient low power red LEDs I tested so far are a batch of Nichia NSPR510CS ones. Most units in this batch achieve about 46 lumens/watt at 20 mA according to my tests. Forward voltage drop averages 1.95 volts and light output averages 1.8 lumens according to my testing.

UPDATE 10/30-2005 - I purchased some Avago HLMP-ED33-SVOOO LEDs from Digi-Key. These achieved 28 lumens per watt, possibly 30. With this efficiency advancing so slowly and tight competition between at least some brands in achieving this efficiency in recent years, I expect some LEDs of some other brands to achieve similar efficiency.

Avago HLMP-E*** types mostly come in 2-3 brightness-sorted subtypes.

Most Efficient Red-Orange LEDs - 66-123 lumens/watt

UPDATE 4/14/2012 - Philips-Lumileds LXM2-PH01-0070 produces minimum of 70 lumens, typically 72 lumens at 350 mA with voltage drop of typically 2.1 volts, maximum of 2.8 volts. This works out to typically 97.9 lumens/watt, minimum of 71.4 lumens/watt. This is with junction temperature of 25 C. Expect about 13% less typically when the thermal pad temperature is at 35 degrees C in continuous operation, which means about 62 lumens/watt minimum, 85 lumens/watt typical.

As for low power ones:

UPDATE 10/31/2009: Nichia NSPRR70AS, version with nominally 615 nm dominant wavelength (NSPRR70AS is nominally red): Dominant wavelength of 615 nm has a color that sometimes appears orangish red and usually appears reddish orange. At 50 mA, typical voltage drop is supposed to be 2.4 volts and typical light output is supposed to be 9 lumens. This works out to 75 lumens/watt.

UPDATE 10/12/2009: Osram LA E65F-CBEA-24-1-Z, in stock at Digi-Key, typically achieves 66.7 lumens/watt according to its datasheet. The datasheet states that typical performance at 50 mA is 7.17 lumens and a typical forward voltage drop of 2.15 volts.

Most Efficient Truly Orange LEDs - 18-22 lumens/watt by Toshiba, 18-21 lumens/watt by Avago.

Avago has the HLMP-DJ** LEDs, which are truly orange. The dominant wavelength is supposedly 605 nm, which is about that of an NE-2H neon lamp, and less red than the color of most other orange LEDs. The HLMP-DJ08 supposedly has a 9.5 candela 6 degree main beam.

UPDATE 5/27/2000 - HLMP-DJ24, 18-21 lumens/watt! The HLMP-DJ24 is a 23 degree model with a typical beam intensity of 2 candela.

Also note that these HLMP-D*** part numbers are obsoleted and the current ones have HLMP-E*** part numbers.

Most Efficient Yellow / Amber LEDs - 34-108 lumens/watt

UPDATE 3/12/2016 - Lumileds L1C1-PCA1000000000, a phosphor "amber" LED, typically produces 94 lumens at 350 mA with a voltage drop of 2.75 volts, which works out to 98 lumens/watt, at a junction temperature of 85 degrees C. At 25 C, the DS144 datasheet indicates light output is 1.15 times higher or typically 108 lumens, and the typical voltage drop is 2.85 volts, which works out to typically 108 lumens/watt. At 100 mA and 25 C, the datasheet indicates light output about 31% of that at 350 mA or 33.5 lumens and typical voltage drop of 2.78 volts, which works out to typically 120 lumens/watt.

UPDATE 5/28/2016 - Lumileds LXM2-PL01-0070, produces minimum of 80 lumens, typically 96 lumens at 350 mA with a typical voltage drop of 2.90, maximum 3.51 volts. This works out to minimum of 65, typically 94 lumens per watt. This is with junction temperature of 25 C. Expect about 2% less with thermal pad temperature of 35 C.

UPDATE 7/13/2009 - Nichia NS6A083B is an amber-yellow LED whose datasheet shows a spectrum indicating that it is a phosphored LED. It typically produces 55 lumens at 300 mA with a typical voltage drop of 3.3 volts, working out to typically 55 lumens/watt.

UPDATE 10/7/2015 - White LEDs can be filtered to yellow with yellow filters or "lenses". Typically 60-75% of the luminous/photometric output of a white LED gets through as reasonably yellow light. 50-125 lumens/watt (from 85-210 lumens/watt as of mid 2016) is possible by filtering the most efficient white LEDs. The downside is that white LEDs often have worse aging characteristics than yellow LEDs unless they are underpowered.

The loss is greater if filtering to an orangish or "amber" shade of yellow suitable for traffic signals. Phosphor yellow/amber LEDs may be better here.

As for low power yellow LEDs:

UPDATE 10/12/2009 - Osram LY E65F-CADA-46-1-Z, in stock at Digi-Key, typically achieves 44.6 lumens/watt according to its datasheet. Typical performance at 50 mA is output of 4.8 lumens and forward voltage drop of 2.15 volts.

As for low power yellow LEDs in my testing: UPDATE 4/3/2010 Cree C503B-ABN-CW0Z0251, one sample tested so far, produces 1.36 lumens at 20 mA with a voltage drop of 1.99 volts, meaning 34 lumens per watt. Digikey has these LEDs. Their website says 2.5 lumens, but I suspect they botched an oversimplified formula and meant to say .63 lumen based on 5 candela and 23 degree beam angle, by using 23 degrees as half-angle.

I expect that Osram, Toshiba, Avago and others have fair chance of achieving similar luminous efficacy with low power non-phosphor yellow / amber LEDs.

Most Efficient Yellow-Green LEDs - 3-8-140+ lumens/watt

Some Toshiba LEDs and AND ones that look like Toshibas have an overall luminous efficacy around 3-4 lumens per watt as of 2003. These are very yellowish green LEDs with a dominant wavelength around 570-573 nm.

Examples include Toshiba TLGA183P, TLGE185EP and AND AND183HGP.

LEDs of similar color but with overall luminous efficacy similar to, maybe slightly higher than that of white ones can be made by suitably adding a suitable fluorescent dye to suitable white or blue ones. That means about 70-140-plus lumens/watt! If you put a white or blue LED into a piece of green-fluorescing yellow acrylic this will probably work! Go Here for LED Hacking for Fun and Danger.

UPDATE 3/12/2016: Philips-Lumileds LXML-PX02, AKA LXML-PX02-0000, is a high power "lime" phosphored LED with dominant wavelength of 566-569 nm and its lowest brightness bin achieves luminous efficacy of 140 lumens/watt minimum at 350 mA when its junction temperature is 85 degrees C. Digi-Key says the LXML-PX02-0000 typically produces 165 lumens at 350 mA with a typical voltage drop of 2.75 volts, which works out to 171 lumens/watt.

The datasheet indicates that 25 degrees C, the light output is 1.1 times that at 85 C. The voltage drop is probably the same as with royal blue, or 2.9 volts at 25 C. This indicates typically 181 lumens and 178 lumens/watt at a junction temperature of 25 C.

At 200 mA and a junction temperature of 25 C, the DS68-1 datasheet indicates a typical typical light output at least 58% of that at 350 mA and a typical voltage drop of 2.74 volts, which works out to 105 lumens and 191 lumens/watt. At 100 mA and a junction temperature of 25 C, the datasheet indicates a typical voltage drop of 2.65 volts and a typical light output at least 28% of that at 350 mA, which works out to 50.7 lumens and 191 lumens/watt. The overall luminous efficacy is probably typically 193 lumens/watt at 140-150 mA.

Most Efficient Green LEDs - 70-120 lumens/watt

UPDATE 4/14/2012 - Lumileds Luxeon "Rebel" LXML-PM01-0100 is a top brightness rank green one with worst case performance of 100 lumens at 350 mA and worst case forward voltage drop of 3.51 volts. This works out to 81.4 lumens/watt worst case minimum. According to the DS68-1 datasheet, typical overall luminous efficacy at junction temperature of 25 degrees C is 100 lumens/watt at 350 mA (102 lumens, 2.9 volt voltage drop). At 100 mA, the datasheet indicates light output being 38% of that at 350 mA or 38.7 lumens typically and a forward voltage drop of typically 2.65 volts, which works out to typically 146 lumens/watt. I expect even higher overall luminous efficacy around 160 lumens/watt at 50 mA.

Now for low power green LEDs:

Update 7/15/2015 Cree CP41B-GFS-CN0P0674 is a 4-lead "high flux" / "spider" / "piranha" LED lamp available at Digi-Key. The middle of its flux range on log scale is 8.5 lumens and typical voltage drop is 3.6 volts, at 30 mA. This works out to 78.8 lumens/watt. The 67 in this part number means dominant wavelength at 30 mA is 515-525 nm.

The datasheet for these says that the minimum and maximum luminous flux is 6.6 and 11 lumens respectively. Assuming 3.6 volt voltage drop, this works out to 61.1 - 101.8 lumens/watt. Based on my experience with high efficiency LEDs generally, my expectations are 70-75 lumens/watt, which exceeds the 64.1 lumens/watt that I reported for an older version (CP41B-GFS-CM0N0674) on 4/14/2012 based only on square root of the multiplication product of minimum and maximum flux. However, I recently purchased some of these newer ones and will soon do actual testing.

Also, like unphosphored high efficiency blue and related non-yellowish green LEDs in general, these will have much higher efficiency when underpowered. I expect around/over 100 lumens/watt at 1-8 mA (updated 7/17/2015).

The -674 units have dominant wavelength of 515 to 525 nm at 30 mA, which sometimes appears as a shade of green that is slightly bluish and/or whitish. The -784 versions are more purely green at 30 mA with dominant wavelength of 520-530 nm, but I have only known them to be available in the -M0N0 brightness which is one step down. The -674 versions are more purely green at reduced current such as 1-10 mA.

UPDATE 8/26/2015 - Nichia NSPG510BS (which I have yet to test) has claimed beam candela 45-60% greater than that of the below NSPG510AS as I remember data mentioned in Nichia's website. This means overall luminous efficacy can be anywhere from 87 to 115 lumens/watt at 20 mA.

UPDATE 1/1/2008 - I tested several units of Nichia NSPG510AS and NSPG520AS. My tests indicate at 20 mA average forward voltage drop of 3.05 volts and light output ranging from 3.9 to 4.4 lumens. The datasheet says typical forward voltage drop is 3.2 volts. This indicates 61 to 72 lumens per watt overall luminous efficacy.

Better InGaN Green LEDs get 100-120-165 possibly 200 lumens/watt at reduced currents

UPDATE 8/26/2015 - Nichia NSPG510BS is (untested) 45%-plus more efficient than the NSPG510AS and 40%-plus more efficient than the NSPG520AS, as I remember and understand data from Nichia's website. This means 165 lumens/watt is somewhat likely and 200 lumens/watt is possible at favorable low currents.

UPDATE 12/25/2007 - I just got a batch of NSPG510AS units that average 72 lumens/watt at 20 mA. At 3 mA, they had a forward voltage drop averaging 2.6 volts and produced on average .94 lumen according to my testing, for an overall luminous efficacy of 120 lumens/watt.

UPDATE 2/16/2014: - On 7/28/2009, I tested a NSPG520AS at 1.69 mA, and found forward voltage drop of 2.45 volts and quantum efficiency of 29.5%. The color matched that of a Nichia green LED whose spectrum was analyzed with results indicating average photon energy of 2.339 eV and luminous efficacy of the emitted light being 512 lumens/watt. This indicates conversion efficiency of 28.15% and overall luminous efficacy being about 144 lumens/watt.

As for high power LEDs: As mentioned above, the Lumileds LXML-PM01-0100 and some Cree XPEBGR-L1-R250-* LEDs are likely to achieve 160 and 180 lumens/watt respectively at 50 mA according to their datasheets.

Most Efficient Blue-Green LEDs - 57-93 lumens/watt

UPDATE 5/29/2016 - Lumileds Luxeon "Rebel" LXML-PE01-0080 is a top brightness rank "cyan" sometimes-in-stock one with worst case performance of 80 lumens at 350 mA and worst case forward voltage drop of 3.51 volts. This works out to 65 lumens/watt worst case minimum. Typical is 81 lumens/watt. In "real world" heatsinking, these figures can be a couple percent less.

Lumileds LXML-PE01-0070 is more reliably available and has luminous efficiency of 57 lumens/watt worst-case and 74 lumens/watt typical when the junction temperature is 25 C.

Now for low power LEDs:

UPDATE 7/30/2000 - TESTED BG Micro LED-1052 narrowbeam bluish green Nichia NSPE590S - preliminary figure 24-26 lumens/watt on the single sample ordered. This model has an irregular beam with a small central bright pattern (roughly 7 by 4 degrees) with a claimed typical brightness of 20,800 mcd at 20 mA.

TESTED Hosfelt Electronics 25-376, apparently a Nichia model. Tentative figure for overall luminous efficacy 24-27 lumens/watt. Optical power output 7 mW at 20 ma with a 3.75 volt voltage drop - 9.3 percent conversion efficiency! Expect even better efficiency in the future since the voltage drop will usually be 3.3-3.6 volts.

Most Efficient Blue LEDs - 25-39 lumens/watt

UPDATE 2/16/2014 - Cree XPEBBL-L1-0000-00201 produces typically 43 lumens at 350 mA with a typical voltage drop of 3.1 volts. This works out to typically 39 lumens per watt at 25 C junction temperature.

Now for low power blue LEDs:

UPDATE 8/26/2015 - UNTESTED, SIGHT UNSEEN: Nichia has new blue LEDs recently becoming available. One is NSPB510BS, which achieves 25-34 lumens/watt according to my extrapolation from Nichia published data and results of my testing on older models. Expect optical output at 20 mA to be about 22-30 mW, which means conversion efficiency around 34-46 percent.

UPDATE 5/11/2009: Digi-Key has in stock Cree CP41B-BFS-CG0H0454, which is mentioned by Digi-Key to produce typically 3.13 lumens at 30 mA with a voltage drop of typically 3.6 volts. This works out to 29 lumens/watt.

Most Efficient White LEDs - 81-192-210 lumens/watt

UPDATE 5/21/2016 - The Philips-Lumileds L135-4080CA35000P1 typically produce 53 lumens from 100 mA of current with a typical voltage drop of 2.8 volts which works out to luminous efficacy of 189 lumens/watt, although its datasheet says 186 lumens/watt. At 65 mA it typically produces 34 lumens with a voltage drop 2.72 volts which works out to 192 lumens/watt, although its datasheet says 194 lumens/watt.

UPDATE 7/18/2016 - Samsung SPMWHT541ML5XAR0S5, expected to be in stock at Digi-Key on July 20 2016, is rated to typically produce 36 lumens at 65 mA with a typical voltage drop of 2.75 volts. This works out to 201 lumenms per watt.

UPDATE 5/21/2016 - Samsung SPMWHT541ML5XARKS0 at 65 mA typically produces 35 lumens with a voltage drop of 2.8 volts, for luminous efficacy of 192 lumens/watt.

UPDATE 5/25/2016 Samsung modules SI-B8R102250WW and SI-B8R104280WW achieve typically 1495 lumens and 173 minimum, 192 typical, 213 maximum lumens/watt with module temperature of 35 C and at their specified current of 700 mA with a voltage drop of 11.2 typical 12.2 maximum volts.

UPDATE 5/21/2016 - Cree XPLAWT-00-0000-0000V5051 cool white LED produces 513 lumens minimum, (extrapolated) 524 lumens typical, at 1050 mA with junction temoperature of 25 C. With junction of 85 C, minimum luminous flux output is 460 lumens, and typical is 470 lumens. Typical voltage drop at 1050 mA is 2.95 volts at 85 C, most likely 3.07 volts at 25 C. This works out to typically 162 lumens/watt at junction temperature of 25 C, 152 lumens/watt at junction temperature of 85 C.

This is a very high power LED, comfortably producing 776-794 or more lumens at 2 amps when its junction temperature is 85 C, and its absolute maximum current rating is 3 amps.

When significantly underpowered with 350 mA, this LED appears to typically produce (conservatively) 36% of 511 lumens with a forward voltage drop of typically 2.76 volts at 25 C, based on its datasheet. This works out to extrapolated as typically 190 lumens/watt.

A higher output version of this LED is mentioned in the above datasheet, and may become available. It is Cree XPLAWT-00-0000-0000V5051. The information that I have at hand supports 200 lumens/watt at 25 C at 350 mA. Cree claimed in this 5/22/2014 press release that this series of LEDs is to achieve up to 200 lumens/watt at 350 mA.

The datasheet for Cree XP-L LEDs is here.

UPDATE 5/30/2016 - Cree XPGDWT-01-0000-00ME3 and XPGDWT-01-0000-00ME2 typically produce 177 lumens at 350 mA with a typical voltage drop of 2.73 volts at 350 mA and junction temperature of 85 C. This works out to 185 lumens/watt. When the junction temperature is 25 C, the voltage drop increases by .1 volt and the light output increases by 11-12% according to the datasheet, so this indicates luminous efficiency of about 190 lumens/watt.

The datasheet indicates that when current is 125 mA, light output is typically about 36% of that at 350 mA, and voltage drop is about .08 volt less than at 350 mA. At junction temperature of 25 C, this appears to be about 70.5-71 lumens with a voltage drop of 2.75 volts when current is 125 mA, which works out to about 205 lumens/watt. According to how I see the graph of relative luminous flux vs. current in the datasheet, I expect about 205 lumens/watt at 25 C to be achieved with current from 125 to 150 maybe 160 mA.

UPDATE 5/28/2016 - Lumileds "Luxeon TX" L1T2-5070000000000 and L1T2-5770000000000 typically produce 275 lumens at 700 mA with a typical voltage drop of 2.8 volts when the junction temperature is 85 C. This works out to 140 lumens/watt. Worst case is 260 lumens with a voltage drop of 3.0 volts, which is 124 lumens/watt. At 350 mA, typical luminous efficiency is 159 lumens/watt when the junction temperature is 85 C. At junction temperature of 25 C, light output is 6% more and voltage drop is not stated, but other recent datasheets for similar LEDs indicate 4% more, meaning luminous efficiency is 2% more or 162 lumens/watt. Luminous efficiency increases from that as current is decreased to 100 milliamps, maybe even less.

UPDATE 2/27/2016 - As for a Nichia high power LED: The NVSW119B-V1 version with color temp. of 5000 K and the lowest color rendering index rating (none specified) typically achieves 305 lumens at 700 mA with a typical voltage drop of 2.98 volts, which works out to typically 146 lumens/watt. The version with second-lowest CRI (70 minimum) typically produces 302 lumens and achieves 144.5 lumens/watt.

At 350 mA, the typical forward voltage drop is 2.83 volts. Light output is typically 168 1nd 164 lumens for the lowest and second-lowest CRI versions respectively. This works out to 169 and 165 lumens/watt respectively.

NOTE - All high power white LED data above is from datasheets and datasheet interpretations and not personally experienced actual test results.

UPDATE 5/11/2009 Cecol / Citizen Electronics produces their CL-L233-C13N with nominal light output of 1335 lumens at 720 milliamps with a typical voltage drop of 18.6 volts, which works out to 100 lumens/watt.

Now for low power white LEDs:

UPDATE 6/30/2012 - Nichia announced on 5/10/2012 their NSPW510HS-K1, which has a claimed typical luminous efficacy of 170 lumens per watt at 20 mA. Typical light output is 10 lumens and typical voltage drop is 2.8 volts at 20 mA. Nominal beam width is 30 degrees. This LED may have a chip similar to that of NSPWR70CS-K1, which is larger than usual for low power LEDs. The beam may have different characteristics from those of other Nichia NSPW-510 series LEDs.

Update 5/11/2009: Nichia has an extremely efficient 4-lead LED, NSPWR70CS-K1, A.K.A. NSPWR70CSS-K1. At 20 mA, it is said to typically produce 8.7 lumens with a typical voltage drop of 2.9 volts, which works out to 150 lumens per watt. At 50 mA, typical luminous flux is stated to be 20 lumens and typical voltage drop is stated to be 3.1 volts, which works out to 129 lumens/watt.

I have tested this LED. My testing indicates 140-145 lumens/watt at 20 mA for a few that I have tested.

Update 8/3/2014: This LED may now be discontinued.

UPDATE 4/10/2010: As for a 2-lead through-hole 5mm non-Nichia DigiKey-available LED, there is the Cree C535A-WJN-CU0V0231. My composite result from 2 different testing methods is 81 lumens/watt at 20 mA, 97 lumens/watt at 8 mA, and peaking at 102 lumens/watt at 2.2-3 mA.

This LED appears to me to be one where the light-emitting surface is a convex surface that most rays of outgoing light are largely close to perpendicular to both before and after exiting the forward surface of the LED. It appears to me that this reduces internal reflection losses, but also results in a wider beam angle. This LED has a nominal beam width of 110 degrees.

The above white LED lamps have blue chips plus a phosphor to convert some of the blue light to yellowish light of broadband spectral content from mid-green to mid-red. The color is usually a cool white to "daylight" slightly bluish or sometimes violetish-bluish white, usually of about 5000-6500 K. Color rendering index of the most efficient ones as well as most other white LEDs is usually stated to be 65 or 70 when stated. There are white LEDs with improved color rendering index, but their overall luminous efficacy is compromised.

Most Efficient Warm White LEDs - up to 150 lumens/watt

Increased Efficiency of InGaN LEDs at lower currents

Overall luminous efficacy typically peaks at 1.5 to 3.5 mA and remains above that of 20mA usually at least down to .5-1 mA. Efficiency and light output become less predictable below .4-.5 mA.

White LEDs have most of their photometric output from a phosphor, and so the luminous efficacy of emitted light does not improve with decreasing current as much as with colored InGaN LEDs. In fact, spectral shifting of the LED chip's output away from wavelengths best-utilized by the phosphor can impair overall luminous efficacy at low currents. However, photometric output per milliamp seems to usually improve with decreasing current down to about 6 milliamps, sometimes as low as 2.5-3 milliamps. I have tested one low power white LED (Cree C535A-WJN-CU0V0231) at various currents, and found ratio of photometric output to current to be maximized at 4-5 mA, achieving about 1.135 times that achieved at 20 mA. This LED had efficiency maximized at 2.2-3 mA, with efficiency about 1.26 times that at 20 mA.

Important Note on Gallium-Nitride LEDs!
Gallium nitride and indium gallium nitride LEDs are fussy, and do not like their ratings to be exceeded. Exceeding 5 volts reverse voltage even with low current is supposedly damaging to them. I suspect that destructive electrolysis is what goes wrong. Peak forward currents in excess of 100 mA may also be bad for low power ones. Many of these LEDs are also supposed to be considered static sensitive.

High candela (beam candlepower) LEDs

UPDATE 5/14/2012 - Cree C503B-GAS-CB0F0791 and C503B-GAN-CB0F0791 typically produce 53.6 candela at 20 mA.

Brightest white 5 mm LED lamp at 20 mA that I am aware of as of 5/11/2009 - 3/20/2013: Nichia NSPW500DS (15 degree beamwidth, 27 candela) and Nichia NSPW500GS-K1 (same beam angle, 30-33 candela, but has compromised life expectancy).

Notes on Where To Get Efficient LEDs

Some Avago, Chicago Miniature, and most AND (apparently Toshiba) LEDs are available from Newark Electronics. Call 800-4NEWARK or visit http:/www.newark.com. They are in a position to collect sales tax if you live in or order from any U.S. state with a sales tax. Minimum order is US$ 25. Shipping is extra and duplicate shipping charges apply if you order more than one type of item not all in stock in the same warehouse. Higher minimums may apply to non-stock merchandise - often 500-1000 pieces of any non-stocked Avago LED lamp and maybe even for some stocked ones, and this applies with some other Avago distributors as well.

Future Electronics is the main distributor of Lumileds products and is a distributor of other LEDs.

Nichia LEDs are best obtained from their sales offices. In North America, check out Nichia America.

Toyoda Gosei makes bright green and blue LEDs.

Toyoda Gosei LED info is now on the web at http://www.toyoda-gosei.co.jp/led/e-index.html.

Converting / comparing lumens, candelas, millicandelas

A lumen is defined as the "luminous flux" of 1/683 of a watt of monochromatic light that has a frequency of 540 terahertz, or a wavelength of approx. 555.5 nm.
One thing worth noting is that a lumen is defined secondarily, in terms of the candela (which is 1 lumen per steradian), and the candela is defined primarily (it's the "beam candlepower" of 1/683 watt per steradian of 540 THz monochromatic light.)
Light of wavelengths other than 555.5 nm have a different amount of lumens per watt of radiation. The number of lumens in a watt of wavelength other than 555.5 nm is 683 times the photopic function of the wavelength in question, divided by the photopic function of 555.5 nm (which I believe is very close to but maybe not exactly 1).

A recently banned "USA-usual" 100 watt, 120 volt, 750 hour "regular" (A19) lightbulb usually produces 1710 lumens.

Lumens per watt is a measure of efficiency in converting electrical energy to light. Multiply this by the watts dissipated in the LED to get lumens. A typical red, orange, or yellow or yellow-green LED has a voltage drop around 2 volts and is getting around .04 watt at the typical "standard" current of 20 milliamps. A blue, white, or non-yellowish-green one typically has a voltage drop of 3.25 volts at 20 mA and gets .065 watt at 20 mA.

A candela is a lumen per steradian, or "beam candlepower". (Actualy, as mentioned above, the candela is a primarily defined metric unit, while the lumen is defined in terms of the candela.)
So lumens are candelas times the beam coverage in steradians. Candelas are lumens divided by the beam coverage in steradians. Ideally, that is - assuming that all light is within the beam and the "candlepower" is constant within this beam.

So you may now be wondering what a steradian is. It is 1 / (4 * pi) of a whole sphere or 1 / (2 * pi) of a hemisphere or about 3283 "square degrees", to the extent there is such a thing as a "square degree". To get steradians from the beam angle:

Steradians = 2 * pi * (1 - cos (.5 * (beam angle)))

(NOTE: There are a few other expressions equal to this. Proving that is homework for 12th graders taking trig / "elementary functions".)

So if you determine the steradian beam coverage and multiply that by the candela figure (or 1/1000 of the millicandela figure), you get the lumen light output - very roughly! The beam is not uniform and it does not contain all of the light. Obtaining lumens from beam angle and candela can easily be in the +100 / - 50 percent range. Actual lumens are generally higher than predicted by this formula with smaller beam angles of 8 degrees or less since the nominal beam does not include a secondary "ring-shaped" "beam" that usually surrounds the main one. Also note that some beam angle figures are optimistic and could lead one to expect a lot more lumen light output than actually occurs.

Please read my disclaimer.