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

Poverty reduction by bloggng

If a child in a poor nation gets infected with some disease but the parents, friends and neighbours are all convinced that the child is sick because he/she climbed a mango tree belonging to the local traditional healer/spirit medium. If the disease is so serious that the poor child dies do we say that the child died because of the disease or because of lack of correct information?

If one of the ways of defining poverty is a lack of resources then the high infant mortality rates that reduce the number of human resources is an indicator of poverty. The same can be said of literacy/numeracy levels within a population. Uneducated populations are generally poor - to a greater extent than poor populations being uneducated.

With correct information at hand one can make choices that improves lives. Isn't it widely agreed that education lifts us out of poverty? Well, that's information alleviating poverty. Though I must add that some information indirectly leads to poverty - books with titles like "How to make a bomb with grocery store ingredients" are not what I am talking about.

So what am I saying here? INFORMATION, INFORMATION, INFORMATION freely flowing throughout the globe, universal access to this information etc. These are the things that fight poverty by working on the causes and effects. EDUCATION, EDUCATION, EDUCATION. Made freely available to all will fight poverty from the root causes.

Look at the image in this blogs header. See how dark Africa is? Well, there dosen't seem to be enough power to light it up for the kids to do homework at night, or for researches to power up IT. It was enough for me to quit massspectrometry to assemble and install low power computers.

EXCHANGE INFORMATION (blogging for instance) to REDUCE POVERTY. That simple!

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My Own Way: LED arrays to replace halogen floodlights

In this blog I will share with you the "why?" and "how?" that LED (forgive the pun) to the creation of what you see here on the left.

Stay connected.
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