Piezoelectric Effect and LED’s

White light emitting diodes in general are not bright enough to be used for general room lighting. There are two general approaches to making visibly white light from LED’s today.

The least expensive and more common method of making white light from an LED is to combine a gallium nitride UV LED with a fluorescent material that converts the UV into visible light. This approach basically replaces an electric arc through mercury vapor as the source of UV with an LED that produces UV light. The efficiency of this approach is very similar to fluorescent lighting. Fluorescent lamps achieve around 18% efficiency, while incandescents achieve around 3%, so fluorescent lamps offer about a five fold efficiency increase over incandescent.

Single color super efficient light emitting diodes can approach 100% efficiency, presently this is most true for red and blue LED’s. To produce what appears as white light to us, green light is also necessary.

The human eye contains receptors for red, green, and blue light, as well as receptors that are receptive to a broader spectrum that are largely used for dark vision. Color televisions produce the visible range of colors that we see with only red, green, and blue light. A combination of red, green, and blue can appear white, or any other color in the spectrum except violet.

You may think you see violet on television but actually what you see is purple, and there is a subtle difference that you will only see if the violet is extremely saturated. Purple consists of a mixture of red and blue light. It appears similar to violet because of the way our visual system processes color.

The red receptor in our retina has a secondary response peak in the violet range. The blue receptor is also still sensitive in this range. So when our brain sees a signal from red and blue together, it “sees” violet. Where red and blue light fail to reproduce this is that the green receptor is also somewhat sensitive to blue light, so red and blue light together activate the red and blue sensors strongly but also somewhat activate the green giving the reproduced color a less saturated or washed out appearance that we call purple.

An LED lighting system based upon red, green, and blue, will produce a light that appears white and natural except that it can’t illuminate violet in a way that will make it appear violet and not purpose. For the most part this isn’t something people would notice. However, violet gallium nitride light emitting diodes exist so that could be addressed with a 4-color LED if it was really important.

A much more efficient lighting system can be created using super efficient single color LED’s. They will actually be more efficient because super efficient single color LED’s exist. Because each LED is producing light at the peak of one of our eyes receptors sensitivity they will be effectively even more efficient.

At present no super-efficient high output green LED exists. Researchers are trying to find a way to make super-efficient high output green LEDs. If they can then a new lighting device that will be as much of an improvement over compact fluorescent as compact fluorescent is over incandescent, will become available. Also, because of the small form factor of LED’s it will be possible to design these basically into any form factor.

Given this one can understand why it is desirable to find a way to make such high output super efficient green LEDs. Thus this article on physorg.com caught my attention. It describes an approach one researcher is pursuing to utilize the piezoelectric affect but gives no details.

The piezoelectric effect is that if you stress certain crystalline materials, an electric charge is produced on the crystal. Conversely, if you apply an electric charge to the surface of the crystal, it will twist or deform. In short, it converts electricity into mechanical energy or vice versa. I fail to see how this has any application to LEDs. If someone out there is familiar with this research I would welcome enlightenment.

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