different coloured LED's

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different coloured LED's
« on: 25/10/2007 22:48:22 »
tech or science section...not sure.

anyway, why are LED's not multi-coloured? I have never seen a pink, blue, or black one. Why?


Offline techmind

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different coloured LED's
« Reply #1 on: 25/10/2007 23:59:26 »
anyway, why are LED's not multi-coloured? I have never seen a pink, blue, or black one. Why?

We didn't get any blue LEDs until about 1990. You get different colours from LEDs depending on the bandgap of the semiconductor material.
LEDs emit a fairly narrowband spectrum of light, so they will normally be fairly saturated spectral (loosely "rainbow") colours. This precludes pink, since pink is a desaturated red, or equivalently red+white. Similarly, you don't "naturally" get white LED's (see below though).

The first LEDs discovered (in the 1960's) were infra-red.
Then they discovered materials which had bandgaps suitable for making deep visible red LEDs (typically gallium arsenide), followed brighter more orangey-reds and then yellow and green LEDs (widely available in the 1980's), and that was how it stayed until the 1990's, when a persistant chap at Nichia discovered a compound from which you could make blue (and even near-UV) LEDs. There's all sorts of stories about this with varying degrees of truth; variously I've heard that the Nichia chap only studied the material he did not because it looked especially promising, but simply because no-one else had looked at it, so there was little risk of his work not being original. At one time he was supposed to have been banned from doing the research, but continued it in secret ... and then discovered how to make revolutionary blue LEDs! Google searches will find lots, but for starters try:
 - http://www.sciencewatch.com/jan-feb2000/sw_jan-feb2000_page3.htm
 - http://www.eetimes.com/story/OEG20010824S0051

Modern high-brightness Red/orange/yellow LEDs are often made from Aluminum Indium Gallium Phosphide (AlInGaP). Modern green and blue LEDs are made from Indium Gallium Nitride (InGaN).

To make "white" LEDs you actually take a physically blue LED, but coat or cover it with a phosphor which glows yellow under the blue light - so blue+yellow = white. There's all sorts of issues with the uniformity of thickness of the phosphor coating, which leads to uneven bluey or yellowy tinged shades of white in different directions. Philips/LumiLEDs has an interesting patented ceramic phosphor tile which overcomes some of these problems.

Not sure about your suggestion of a "black" LED. Unless you mean "blacklight" as in UV. You can get some near UV LEDs now - I guess just pushing the blue technology a bit further.

It you've never seen a blue LED then you need to get out more! They've been all the rage for the past 5+ years!  Shop-shelf lighting, blue-LED keyfobs/cigarette-lighters, computer front-panels, USB memory devices... Blue LEDs are the facilitating technology in "VidiWall" or other billboard-sized outdoor TV screens. You obviously need Red + Green +Blue primaries to make a full-colour display.

In the 1980's and 1990's you only ever put a few 10's mA through an LED. Now with advances in the thermal design (heat dissipation) we have huge LEDs from LumiLEDs/Philips, Cree, Osram which operate at 700mA or more than 1A of current.
For some information/examples, see http://www.lumileds.com/products/line.cfm?lineId=2

For lots more information on LEDs see the Wikipedia entry: http://en.wikipedia.org/wiki/LED
« Last Edit: 26/10/2007 00:27:04 by techmind »
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Offline RD

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different coloured LED's
« Reply #2 on: 27/10/2007 09:59:17 »
Not sure about your suggestion of a "black" LED. Unless you mean "blacklight" as in UV. You can get some near UV LEDs now.

Ultra Violet LED