So I mentioned in my last post that there are two main reasons LED are such a big change from way we have previously generated light for ourselves. I discussed the electric versus electronic issue. Now I want to introduce the second issue: directionality.
All of our previous methods of generating light have been omnidirectional. That means that the light is radiated nearly all directions more or less equally. If you picture a candle, the light shines above and below and all around it fairly evenly. Some light is blocked below the candle, but that’s actually a shadow. The flame is still shining light down on the top of the candle.
That means that we can take as single candle, stick it in a room, and illuminate the entire space. It may not be very bright, but the light goes everywhere. The same thing can is true for light bulbs. The design of a traditional incandescent light bulb is pretty much the same as candle. There is a filament providing the light, just like a flame, and there is a base for holding the filament in place, just like the candle. We have developed various shapes for light bulb since then, but the only real difference is if the bulb has sockets on two ends so the shadow is on two ends instead of just one.
All the different sources still have the same distribution property. Incandescent, halogen, fluorescent, compact fluorescent, mercury vapor, sodium discharge, metal halide… They all send the light in all directions. Often, we don’t want the light to go in all directions, we want to be able to control it. So we have developed reflectors and refractors to help send the light the ways we want it to go and stop if from going where we don’t want it go. Just for clarification: reflectors are things like mirrors that bounce the light back either toward where it came from or in another direction we intend; refractors are things like lenses that let the light pass through them to the other side, but while passing through the light changes direction.
Going back to our candle in the room example, if we stick the candle on the wall so it stays out of the way, there is a bunch of light shining on the wall where it isn’t very useful to us. So, we stick a mirror on the wall behind the candle and (most) of the light that would have landed on the wall is now bounced back into the room where we want it. If we want to get even more useful light out of it, we can use a curved mirror instead of a flat one so that we can start to bend the edges around the flame to catch more of the light that would have gone onto the wall further from the candle and use that too. Finally, we can stick that entire thing into a lantern and use a lens on the side toward the middle of the room to catch stray light and refocus it across the room. We have created a lighting fixture, and we have been using the basics of reflectors and refractors to control omnidirectional sources for thousands of years. We’re pretty good at it.
Now, we can take those omnidirectional sources and make them directional. PAR lamps and MR-16 lamps that you buy at the hardware store are examples of lamps that have the reflectors built onto the source. The filament inside is still omnidirectional, but it is built into a solid unit that does some of the work of the fixture. That’s why you often see light coming out the back of such sources either through gaps or through what appears to be opaque surfaces when the lamp is off.
The big change with LED sources is that they are truely directional. The diode is made up of layers and the light is only emitted out of one side. If you stick an LED in the middle of the room you don’t get light everywhere. It only lights the part of the room that the LED is “facing.” The other side of the room remains dark.
It may seem like a minor point, but it really is important. Think of lighting up a space a building an object. The old way was to start with everything, both good and bad, then take away the bad parts. The LED way is to start with nothing and just starting adding things. The question becomes, how much do you add? With the old way, once you take away the bad parts which is pretty much the light going where we don’t want it, whatever is left over is useful. With the LED way, someone has to decide whether or not to keep adding.
So far, much of the LED lighting development has been driven by the need to reduce energy consumption. That means that LED sources have been designed by people who want to stop adding as quickly as possible. The problem is that people have lighting needs that are established without regard to the source or energy consumption, but by whether or not they can see well enough for whatever is it that they are doing.
Return to the example of a light bulb in a room. If we want to illuminate the room with LED sources, we have to add LEDs pointing in all directions. To illustrate, let’s imagine we take the guts out of a light bulb and glue LEDs onto the outside. How many do we have to add? One every square inch? One every square half-inch? Do we have the cover the surface completely? Then, we have to address the point of the previous discussion of the LEDs being electronic but the power is electric. Can we fit all the electronic components and drivers inside that light bulb?
In summary, the directionality of LED modules means we have to rethink and re-imagine how to develop a light source. It doesn’t mean that these problems can’t be solved, or aren’t being solved, it just means it is more difficult.
Tags: LED lighting
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