LEDs: notes on efficiency

Light emitting diodes (LEDs) are solid state lighting (SSL) devices and come in a variety of shapes, sizes and cost. The choice of what type to use for a particular application is dependent on many factors but the one that most people pay most attention to is the efficiency usually given in light output per unit energy supplied to the device. For LEDs the efficiency is usually rated in lumen per watt (lm/W) where the lumen (lm) is the unit of "luminous flux" or the energy within the range of frequencies we are able to see as light. A wax candle outputs about 15lm while a 100W 240Vac incandescent bulb outputs about 1,300. The lumen is defined in terms of candela steradians. What is a candela or a steradian? The candela is the intensity of light. For those who want to get deep the SI definition of the candela is "one candela (cd) is the monochromatic radiation of 540THz with a radiant intensity of 1/683 watt per steradian in the same direction". For Joe Bloggs, a wax candle generates about one candela. The steradian (sr) is another SI unit and is the unit of solid angular measurement. Since the surface area of a sphere is 4*Pi*r*r there are 4*pi or 12.566 steradians in it.

Most LEDs will normally be rated in millicandela (mcd) and some (usually high power ones) in lumens. If we are going to use LEDs for general lighting we should have the light output in lumens (all round) rather than candela (per narrow beam). To convert cd to lm use the following formula

lm=cd*2*pi*(1-cosX/2). where X is the beam angle

The power supplied to the LED can easily worked out by measuring the current, I (Amps)
and the voltage, V (volts) supplied to the LED. It is recommended that LEDs are driven with 20mA of current for the varying forward voltages (depends on colour emitted by the LED). Power, Watts = V*I. Luminous data is usually supplied by the manufacturers but there are independent bodies that carry out tests to verify manufacturers claims. One such independent testing agency is the US Department of Energy (DoE) in their Commercially Available LED Product Evaluation and Reporting (CALiPER) Program, formerly called the Commercial Product Testing Program, that supports testing of a wide, representative array of SSL products available for general illumination. The Department allows its test results to be distributed in the public interest for noncommercial, educational purposes only. Get info here

http://www.led-professional.com/content/view/819/61/

As I mentioned earlier, the lm/W data is what most people will look at when considering lighting solutions. I feel that this figure does not provide me with all the data I need when deciding what type of LED to use so I have begun to use lm/$ as another yardstick when considering LEDs. Additionally, I also apply a third ratio and that is the lm/sqcm especially when dealing with limited space. I use all three of these efficiency markers when comparing all types of LED against each other. I have not got much data (only 8 LEDs) yet to put up here but one can look at these sites that offer some efficiency data and lots more interesting LED reading:

http://www.otherpower.com/otherpower_lighting.html
http://members.misty.com/don/ledx.html