LED: Lightsource of the future

What are LEDs really?

 

LED stands for "light emitting diode." A diode is an electrical component with two terminals which conduct the electricity only in one direction. With an electrical current, the diode emits a bright light around the small bulb.

The first LED was developped in 1969 and emitted a red light. Everyone has seen these LEDs, in television sets, etc. A couple of years later a green LED was developped and in 1993 the blue LED. Only by 1999 was a LED developped that produces white light.

The possibility to produce white LEDs brought with it a revolution in the lighting world. The small, economically and maintance friendly LEDs have a lot of benefits. There do remain a couple drawbacks though.

 

The operation of LEDs, and the different kinds of lightsources

 

Lightsources can be divided into 3 main categories:

1. Incandescent lightbulbs

They produce light when an electric current passes through a filament and causes it to heat up and glow. Because they are less energy efficient than other light sources, they are best used for lighting tasks that demands high levels of brightness.

Only 5% of the current gets turned into light, all of the other energy is lost as heat.

Halogen bulbs are a variation of incandescent bulb technology and produce 10% more light. Since recently halogen bulbs exist in energy efficient versions (aka Haloco lamps). These are 40% more efficient than the previous generation of halogen lamps.

2. Fluorescent bulbs

These bulbs work by passing a current through a tube filled with argon gas and mercury. This produces ultraviolet radiation that bombards the phosphorous coating causing it to emit light. There are many different types of fluorescent bulbs. They are all very efficient and produce very little heat, they are still more efficient than most LEDs!!! But they have one big disadvantage: the gas contains mercury, making these bulbs harder to recycle and bad for the environment!

3. Solid state lighting

These are LEDs. Light Emitting Diodes are bulbs without a filament, that are low in power consumption and have a long life span. Photons are created, and thus light, within a solid object (the semi-conductor crystal) when electrons pass through it from a high energy state to a low energy state. A low electrical current is necesarry to produce the wanted effect.

 

Light created by a LED

 

Everyone knows LEDs from seeing them in lounge bars, advertisement panels, .. We can see a lot of colors and dynamics in these lighting solutions. How is this possible?

As mentioned earlier LEDs produce a red light (these were the the first LEDs). By changing the composition of the semi-conductor, it's possible to create other colors, like green and blue. But to be truly useful as general lighting we need white light. To create this with LEDs isn't that easy. There are three techniques to achieve the desired effect:

1. Mixing a red, green and blue LED (RGB)

Using this technique it's possible to create a color dynamic, by controlling the three color sources seperately. The disadvantage is the possible color differences. The closer the three color sources are to each other the smaller this effect is.

2. A second way is to use a UV-LED, and use the built in red green and blue fluorescent materials to decide on a color.

3. A third way is to use a blue LED, and surround it with fluorescent material that absorbs parts of the blue light and in turn emit yellow and red light. These colors in combination with the original blue light will emit a white light. An important drawback is however that through this process the LED becomes less efficient and after a while there could be color changes.

 

The future of the LED

 

When light is generated with the aforementioned techniques then heat is also generated. Controlling this heat is important for the future of the LED and its many possible applications.

Most people think a LED will last them a lifetime. This is correct for certain and extremely low current LEDs (5mm LED, like the ones in televisions). The biggest disadvantage these LEDs have, is that they produce very little light (a couple of lumen), and for this reason their use is limited to orientation lighting. A life span of 50.000 hours is possible for these kinds of LEDs.

When the power of a LED is increased (the so called POWERLEDs) then the temperature increases considerably as well. Powerleds as well still only produce a couple of dozen lumen. Philips, one of the largest producers of LED lights, notes that these lights have a lifespan of 6000 - 30.000 hours. This is a far cry from the 100.000 hours you often notice in magazines and slogans! There are however constant improvements to how powerleds are cooled. This improved cooling results in more lumen and a longer lifespan. We have arrived at a point where almost every possible lighting problem has a LED solution.

Another important fact is that LEDs are very shockresistant. Which in turn makes them highly useful for vehicles.

 

LED and colors

 

By constantly improving LEDs we can now use them in our livingrooms, museums, stores,.. An important factor is the LEDs color. Noted in Cri or Ra. A traditional incandescent lamp was used as a standard and got the following index value: Ra100. This means that all of the colors of an incandescent lamp are considered perfect.

The method mentioned above to create light with LEDs, makes it hard to get good color with LED lights. The techniques to improve colors in a LED, have a side effect that they reduce the LEDs efficiency. It is possible though to create LED lights with a color value of Ra80 and upwards. To give you an example of what this means: TL-tubes and gasdischarge lamps until recently only had values of just above Ra80.

It's also important to note that the Ra values (the ones you find on the lamps packaging) don't mean that the lamp you just bought will be perfect for its intended purpose! The Ra value is just an average of the 15 (CIE norm) test colors. If the light peaks in the blue color spectrum, then the Ra values could be very high, however you would end up with a very cold and unpleasant light. The most important colors that produce a desirable light are R9 (red) and R15 (skintone). These colors also happen to be the hardest colors for a LED...

 

Dimming a LED

 

Dimming a LED is completely possible, but depends on a lot of factors. The driver (the electronic component that allows the LED to work) needs to be dimmable. The driver and dimmer need to be tuned to each other and the dimfunction is influenced by the amount and type of LEDs!

Remember that dimming a LED using an old dimmer will almost never work. To not damage the old dimmer, assume that you can't connect LEDs of more then 10% of the power output of the dimmer. Usually you'll find more info on the website of the producer of the dimmer to find the ideal combination of dimmer and LED lights.  Your best option still remains to just buy a specific LED dimmer. There are many affordable LED dimmers available now.

 

Conclusion

 

The ideal LED does not exist!

What we mean is, that there isn't a LED that would provide a solution for every possible problem, and there will probably never be one! The immense variation in LED lights, colors, lumen, lifespan, ... make it very important to consult your lightingspecialist!

 

Sources: 
Linda Claeys, “De ideale led bestaat niet” Spotlight, maart 2013
“Compatibiliteit tussen bestaande dimmers en retrofit ledlampen”, Elektrovisie juni 2013
S. Schoordijk, “RARA hoe kan dat” Allicht, juni 2013
P. Hanselaer, C. Lootens “LED’s toegelicht”, Groen Licht Vlaanderen, Laboratorium voor lichttechnologie ; Ingelicht, magazine voor de vakhandel van Philips Lighting van mei 2007

 

design & implementation by e2e NV