LED Efficiencies

John Sun
November 29, 2010

Submitted as coursework for Physics 240, Stanford University, Fall 2010

The electricity that goes into lighting up a room is often over-looked energy expenditure for many households. It may seem at first that an average 60W incandescent light bulb wouldn't waste so much energy, when compared to 1000W air-conditioner units and 5000w electric clothes dryers. However, due to people using light bulbs for extended periods of time, and multiple light bulbs to power certain light fixtures, lighting is actually the second largest user of energy in buildings. [1]

The problem is also that incandescent bulbs are incredibly inefficient. In terms of electrical efficiency, light bulbs are usually measured from how many lumens of light they can produce per watt of power they require. For example, the average 60W incandescent light bulb produces 720 lumens of light, which is an efficacy of 12 lumens per watt. Also, approximately 95% of the energy input into a bulb ends up being wasted as heat. [2] This is the reason that incandescent light bulbs are typically rarely used nowadays. Instead, the most dominant type of lighting is the compact fluorescent lamps. These lamps have an efficiency of around 35-60 lumens/watt (lm/W), and around 20% of their energy is turned into light. [3]

However, the new trend is leaning towards using light emitting diodes as a replacement for the compact fluorescent light bulbs, as LEDs can reach even higher efficiencies. In fact, research on LEDs in the past several years has been extraordinary; The company Cree showed a 65 lm/W bulb in 2003, and issued a press release in February of 2010 showing a prototype LED which achieved a remarkable 208 lm/W bulb. [4] This is of course an extreme, but the United States Department of Energy testing of commercial LED lamps in 2009 showed around a 46 lm/W average efficiency. [5]

What this means that LEDs have the potential to greatly surpass the efficiencies of CFLs. LEDs also have better durability, which means they don't have to be changed as often. An example of how a single-color LED can make a huge impact is when they are used in traffic lights. LED traffic lights can last up to ten years, compared to two years for their counterparts. They also use roughly 8-25 Watts, versus incandescent lamps which use around 67-150 Watts per lamp. [6] These advantages translate directly into using less electricity, as well as spending less money on replacements. The city of Santa Barbara, California converted all of its traffic lamps to LEDS by 2004, and has seen 70-80% reduction in their lighting electricity cost. [6]

Of course, traffic lights use single color LEDs, which means they can be very efficient. LEDs normally only produce a small range of wavelengths, depending on the material used to construct the diode. Since for everyday lighting, white light is normally desired, certain tricks had to be used to create the impression of white light. One way was to use 3 different colored LEDs (the primary colors red, blue and green) and mix their lights to create white light. This method is worthy of note, because it allows the possibility of mixing different proportions of light to obtain a large range of colors. However, this type of LED's emission power decreases at higher temperatures, which limits their feasibility for common usage. [7] The other method for producing white light from LEDs is to coat an LED with a phosphor which absorbs the specific light from the LED and emits a broad range of wavelengths. This method is a lot simpler, but also has its own problems. The phosphor method decreases the efficiency of the LED, due to the phosphor material absorbing some of the heat. However, this method is much cheaper than the multi-color LEDs, and the majority of white LEDs on the market use this phosphor light conversion technique.

LEDS are already used in a wide variety of technologies, ranging from the backlight in cell phones to flashlight bulbs. Some car companies even use white LEDs for their headlamps. Autommotive companies have been actively developing LED technology for headlamps since 2004, and the first series production LED headlamps were installed on the Lexus LS 600h, in 2008. [8] The major limiting factor for replacing all of the headlamps in cars with LED lights is that the LED technology is more expensive than what is used presently, but they are still increasingly used for a variety of functions.

In conclusion, there is a lot of room for growth for LED technology as compared to the more mature technologies of CFLs and incandescent light bulbs. It is very possible that the energy efficiencies of the LED lights will increase even more over time. They also circumvent the toxic mercury that the CFL lamps require. Once they become cheaper to make and research makes them even more efficient, we will most likely see LED lights dominate in almost every aspect of lighting.

© John Sun. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] "Buildings Energy Data book," U.S. Department of Energy.

[2] S. Sundhar, "Light Bulb Having Increased Efficiency," U.S. Patent 6285119, 4 Sep 01.

[3] C. J. Humphreys, "Solid-State Lighting," MRS Bulletin 33, 459 (2008).

[4] "Cree Breaks 200 Lumen Per Watt Efficacy Barrier," Cree, 3 Feb 10.

[5] "CALiPER summary Report," Pacific Northwest National Laboratory, October 2009.

[6] "California Says 'Go' to Energy-Saving Traffic Lights," U.S. Department of Energy DOE/GO-102004-1916, May 2004.

[7] E. F. Schubert and J. K. Kim, "Solid-State Light Sources Getting Smart," Science 308, 1274 (2005).

[8] S. Landau and J. Erion, "Automotive: Car Makers Embrace LED Signals," Nature Photonics 1, 31 (2006).