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Prudent RVer explains:

How Do LED Lights Work

This article is about how white LEDs, an abbreviation for Light Emitting Diodes, create light that we can use in our RV or home. Wikipedia says a LED is a semiconductor light source. White LEDs are commercially available to compete with incandescent and fluorescent lighting. The importance of LEDs is that they use only one-sixth of the electrical energy required by incandescent bulbs to produce the same level of light, and their lifetime is 1,000 times longer, measured in years.

SIDEBAR: For the geek, semiconductor light source means that when a light-emitting diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. When a blue LED is coated with a YAG phosphor, blue photons are absorbed in the phosphor and re-emitted over a broad range from red to green which, with suitable "tuning" can be made to look and act like a very pleasant white light.

Why is all this so important? It helps if you understand the basics of just what light is, how light works, and how our eyes see light.

Anyone reading this article is seeing light. The light you see is actually composed of trillions on trillions of little packets of light energy called photons. Some photons have blue energy, some green, some yellow, some orange, some red. These are the "visible" photons. There are other photons that humans cannot see, like the ultra-violet or infra-red ones, but other creatures can see them.

A person's eye is a marvelous instrument that detects those photons and sends a stream of nerve signals to the optic center in the brain. There the signals are interpreted as some kind of image. Our eyes and optic center are so precise that we can detect the shape of letters on the screen, and our minds have learned to translate those letters into words and ultimately into concepts. This is all an amazing transformation of data and transfer of information from a computer screen to thought.

Light entering the eye goes through a convex lens that focuses photons onto the the retina in the back part of the eye. The retinal surface is composed of "rods" that measure the intensity of the light and different color -sensitive "cones" spread like the pixels of a TV screen. Each cone absorbs photons of its color and send impulses to the optic center of the brain for analysis. The brain analyzes the pixel pattern of signals to determine both the shape of the image the eye is seeing and the color for each pixel position. When rods and cones in an area see no photons, the brain interprets that area as black; when they receive a mix of blue, green, and red, the brain interprets that area as some shade of white. The mix of photons gives us all the shades and hues of the colors we recognize, over 10 million colors.

Physicists describe photons as discrete packets of electromagnetic energy that interact with the atoms and molecules that make up matter, either through creation, absorption, or reflection.

Atoms and molecules create photons to dispose of excess energy. For example, burning hydrogen gas in an oxygen atmosphere happens when two atoms of hydrogen get together with an atoms of oxygen and combine to form a molecule of water. The acts of combination leaves some excess energy, and the process creates a photon of a specific energy and sends it on its way (at the speed of light). Burning wood does the same, but the mix of photon energies is more diverse. Heating a metal filament in a light bulb with an electric current gets the metallic atoms so hot, they spontaneously emit photons of a variety of energies. Our sun is a giant thermonuclear furnace that emits a huge supply of photons of all energies in all directions into space, and the small portion (0.00000005%) that strikes our earth provides for our existence.

The energy of a photon can be absorbed by some materials to transform an exact equivalence of the photonic energy to electrical, mechanical, or heat energy. The operation of the eye is one example where the selective absorption of photons allows us to see. Chlorophyl in plants absorbs photons from the sun to create the sugars that become cellulose, etc making up the plant. When you spread your hands above a campfire, your skin absorbs the infra-red photons to warm your body. Solar photovoltaic cells absorb certain energies of photons and transform their energy into electricity.

The reflective properties of light allow us to see the objects around us. Light created by a source shines upon a surface that does not absorb all the photon energy. Instead, some portion of the photons bounce back. The color of the object we see depends on which photons are absorbed and which are reflected. If we see black, all the photons were absorbed. If we see white, most all of the photons were reflected. If we see green it is because the red and blue photons were absorbed. The patterns of white and black that show the characters making up the words in this article are formed in one way or another by this reflective property of light.

LEDs are a new source of light. They illuminate the surfaces around you so you can see your surroundings. LEDs are the best method for creating light for use by humanity in our future world. But in this early stage of LED development, their advantages are fighting against shortcomings and misconceptions.

Relatively speaking, it is expensive to install LEDs at this, but they are the least expensive operating source of light available; they create photons at 20% to 50% of the cost per photon as our other common light sources, incandescent bulbs and fluorescent tubes. Despite their initial high costs, the lower operating costs and extended lifetimes of LEDs tip the cost/benefit figure into the LED camp. In time, the cost of LEDs will drop, but the cost of the traditional light sources will stay the same. And, some of those traditional sources are being banned from the shelves of our markets.

Tradition is a second shortcoming. We already have incandescent bulbs and fluorescent tubes that use electricity to produce light; why should we re-invent the wheel? The easy answer is that creating light with LEDs costs only one-half to one-sixth the cost using current lighting fixtures. This may seem small at $0.18/Kw-hr today, but when electricity costs $0.50/Kw-hr, it will become much more important. For RVers who boondock off the grid and depend upon solar power to keep going, the cost of a Kw-hr may become priceless.

A third misconception, that LEDs cannot produce adequate lighting for our style of living, simply needs to be set aside. LEDs are adequate. We just need to understand how LED lighting works and how to make it meet our needs.

White LEDs came to the market less than ten years ago. In the beginning they had a decidedly blue cast, what we now call "cool, cool white." The first commercial use for white LEDs was in flashlights, and the more powerful variety of LED had a single emitter encased in a plastic hat that focused the light forward as was needed for that application. About four years ago the first of the multi-emitter LED ceramic chips came to the market. These had a larger phosphor cap over the blue emitters and spread the light beam into a broader 120 degree cone of light. They were still in the cool range of white. In time, the technology of tuning the phosphor mix became more exact, and reliable "warmer" LED chips became available and at a  cheaper cost. More emitters were added to the chips, and higher intensities became available. Today, a number of designers are working to produce a variety of mixes of intensity, color rendering, and color temperature for new LED offerings, and the costs are dropping.

The complaint that you cannot get enough light from an LED fixture is simply met by adding more LED emitters to the light source. The complaint that the color is not right can be met with proper attention to the mix in the phosphor cap, and more choices are becoming available. The complaint that LEDs make little bright spots of light can be addressed with the use of proper diffuser materials to give a spread of light. In the future, Organic LEDs will come out which offer large areas of light.

Do not expect the LED lighting to look just like the old standard incandescent bulb or fluorescent tubes. Think about what you are trying to accomplish with the light in your life, and then choose LED light fixtures for your home and RV that offer a good solution to what you need.


copyright Sam Penny, September 6, 2010