White LED light - coating a
big plus or a major pain?

Bright white LED lights are behind the push of lighting to new phase. Coating was a major plus, now a problem. Find out more and how light engines are rethought to move on.

With the emergence of the white light LED package all lighting products have been pushed to an exciting new phase. So it might be good to find out what they actually are…


But what are WHITE LEDs then?


White color is the combination of all visible spectrums. And a single LED unit can only emit in one spectrum - one pure color. So how can this light be white?


Actually, you get white LED light from LED units that emit in the blue or ultraviolet (UV) spectre. In simple words - they are made from blue LEDs. But there is a little trick involved - mostly with phosphor.



Where does the coating goes?


In most currently available white LEDs, the phosphor is either placed directly on the surface of the light chip (called conformal coating or doping), or is scattered in the solid material placed over the LED chip, such as silicone or resin.

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Then the white light is produced...


white light single LED unit

A single unit LED is coated with a material that has the ability to shift some of the blue light into green and red. This then - together with the blue light that remains - makes a decent quality white light.


white light with rgb led

We can also use a combination of different colored LEDs. Typically a package of three LEDs called rgb (for Red, Green and Blue) can achieve this nicely.



White degradation and color shift


Did you notice that your lower grade LED light became bluer over time? What really happened is that the phosphor has started to fade under the bright light and your LED unit has started to turn back into a decent blue LED.

In time the blue light output gets worse not better as the LED ages it keeps going to the blue side. Because blue is naturally less visible your LED slowly loses its output.

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Lifetime rating for White LED lights


Lifetime rating for good quality white LEDs should be between 50,000 - 100,000 hours (give or take.)
Still, this is not the full story. It is very likely that the phosphor component start fading after a few thousand hours of use. The bight light for which you buy the stuff will over time degrade the whitening effect of the phosphor.


Heat economy of white LEDs


It may be clever how white LEDs are constructed, but a significant portion of the light emitted by your sturdy blue LED diode is going to be absorbed. As a result it is not going into light. It will be lost as heat waste. So the heat economy of current white LEDs is not so good after all.

Is it going to be changed any time soon?

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Good White LEDs are on the way...


SPE from Rensselaer


A technology holding great promise is called Scattered Photon Extraction (SPE).


Developed at Rensselaer Polytechnic Institute SPE technology is used to create white LEDs using a more advanced phosphor-conversion. In this format, phosphor is placed separately from LEDs so as to avoid the intense heat loss and fading. Also, they play around with shape of the reinforcing optic surrounding the LED chip to extract more white light.

They also trial other white-conversion material placed at a remote to the LEDs, such as a material affectionately called quantum dots.



Ultra bright white light from Laminaceramics

Folks at Laminaceramics play around with another concept. Similar to what I described for LED bulb heat management the Enlux type product, they tinker very successfully with the light engine.

Guess they might want to talk to those tinkerers at Rensselaer to avoid the rapid fading that the too bright light may mean for the phosphor deposits. Lamina also uses a thin phoshor layer on the LED cavity.


Would these improved White LED lights hit market anytime soon?


Typically, both Rensselaer and LaminaCeramics are renowned for their quick patent to market cycle. So wait and see is a pretty good option.




Related Articles

Superbright white LEDs have awesome brightness for their tiny size

Organic LEDs may be thinner, but will be just as bright if not brighter

Read more about the quantum dots on the articles page

More in-depth
http://ledsmagazine.com




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