Why LEDs can be 10-20 times as efficient as incandescents in some applications, but only 5-8 times as efficient in general home lighting!

UPDATED 7/18/2016 (excluding updates of the linked file led.html)

First, the figures:

The most efficient modern available white LEDs (as of July 2016) produce about 120-205 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, at temperatures at which they typically operate. Many others that are in recent LED products achieve merely 40-70 lumens/watt. Most such white LEDs are and will be slightly more efficient when moderately underpowered. A few white LEDs achieve 185-210 lumens/watt at reduced power and/or with very good heatsinking.

These figures are for cool white LEDs. Warm white ones typically achieve 80-190 lumens/watt for the best ones.

More in my efficient/bright LED page, updated 5/30/2016.

UPDATE 7/17/2015: LED light bulbs are now available at Home Depot and Lowes. Most "A19 style" ones achieve 50-75 lumens/watt, and extreme examples achieve about 94 lumens/watt, such as 800 lumens from 8.5 watts.

UPDATE 9/5/2015: 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 achieving around or somewhat over 75-95 lumens/watt - including some Cree ones.

A laboratory prototype of a white LED achieving 303 lumens/watt is mentioned in this 3/26/2014 Cree press release.250-plus lumens/watt white LEDs are still too expensive to be practical for any purpose having a scale justifying mass production, despite ability to reduce energy usage costs slightly from ones that are already greatly reduced.

Compare the above figures to 14-17.5 lumens/watt for "standard" "A19" 120 volt 60 to 100 wat 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 10-20 x the efficiency of incandescent in traffic lights
Where some of those really impressive lumen per watt figures for LEDs come from!

Why batteries last so long in LED flashlights

The main difference has mostly been how incandescent lamps and how LEDs behave when the batteries start running down. However, more recently (~2007) flashlights have started showing up on retail shelves with LEDs that are much more efficient than incandescent lamps.

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. When an incandescent is starved of power, it gets hungry. LEDs do the opposite, having an increase in resistance when fed less power, so when the batteries weaken the LED conserves the remaining energy in the batteries. Another significant difference is that incandescent lamps operate much less efficiently when underpowered, while LEDs used in flashlights mostly work 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.

Some LEDs work well for night vision

There is photopic vision and there is scotopic vision. Photopic vision is "day vision", which sees detail and color and works better in brighter light. Scotopic vision is "night vision" which is low resolution and black-and-white. In dimmer environments, there is "mesopic vision" where both scotopic and photopic vision are functioning.

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 150W or less.

A special disadvantage of low power incandescents

Incandescent lamps have a few economies of scale, resulting in lower wattage and lower current ones being less efficient than higher wattage and higher current ones of the same life expectancy. As a result, many low power incandescents are less efficient than high efficiency LEDs from as far back as 2001-2002.

A 4 or 7 watt 120 volt night light bulb usually has 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".

Why LEDs have very large advantage over incandescents for indicator lamps

LEDs have a special advantage as indicator lamps in electronic equipment. The incandescent lamps that used to do this job were mostly low power, low current, vacuum-filled and often designed to last thousands or tens of thousands of hours. A "standard" 5 volt .06 amp 20,000 hour incandescent lamp only achieves 2.1 lumens per watt, and most of its light was not directed towards where it would be seen from.

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 in the 1980s when their efficiency was .5 lumen per watt or less.

Why LEDs have 10-20 x the efficiency of incandescent in traffic lights

One minor reason is that incandescent traffic light bulbs are not as efficient as "standard" incandescents. Incandescent traffic light bulbs (traffic signal lamps) are designed to withstand the vibration of trucks passing over potholes, and are designed to last usually 8,000 hours. Being turned on and off a lot is not much of a factor (explanation somewhere in my Incandescent Lamp File), but it might be a little significant if it has to be turned on and off nearly a million times in its design life expectancy. With the design compromised to keep working after 2 years in a traffic light, its efficiency is only about 60-65 percent of that of a "standard" incandescent of similar wattage. So an LED 10 times as efficient as this is only 6-6.5 times as efficient as a regular incandescent. More below why LEDs can easily achieve this in a traffic light.

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 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 7-14 watts can do as good a job in a traffic light as the usual 67 to 116 watt incandescent traffic signal lamp!

Where some of those really impressive lumen per watt figures for LEDs come from!

I remember seeing "luminous efficacy" in LED datasheets, with very high numbers. And these numbers were always high, even back around 1980. Yellowish green LEDs always had numbers around 600 lumens per watt for this. The catch is that this is the luminous efficacy of the emitted light, a characteristic of the emitted light used for converting between radiometric units (watts) and photometric units (lumens). Divide the millicandela by this figure to get milliwatts per steradian. Divide mW/steradian by the square of the distance (from the LED) in meters to get milliwatts per square meter.

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, and typically 270-350 lumens/watt for LEDs manufactured around/after 2009.

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Written by Don Klipstein.

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