From n1joy at arrl.net  Sun Oct 26 12:51:12 2008
From: n1joy at arrl.net (Roland Daignault Jr.)
Date: Sun Oct 26 12:51:25 2008
Subject: [76] How Bright White LED's Are Manufactured
Message-ID: <4904A000.6040506@arrl.net>


 Hey Gang,

  Here is an interesting message post regarding how the new super bright 
white LED's are manufactured.  It was posted on one of my aircraft 
message boards regarding one member who is using these as an airplane 
landing light!  It's a little technical, but not too difficult to follow.

  SEE YA!!

  Roland - N1JOY

  *****************************************

At the risk of boring everyone to death, I'm going to talk about white light LEDs, since I spent many years developing these animals and if I don't write it down I'll forget the details.

How do they work?
The light produced is actually a mixture of blue light from an LED blue die and a yellow light from a "phosphor".  The phosphor is not from the element phosphorus, it is something quite different.  The phosphor is placed in the light path of the blue LED and a part of the blue light is absorbed by the phosphor and then re-emitted as yellow light.  The mixture of the blue which passes through and the yellow from the phosphor is perceived as white by our human eyes.  There is very little green and red in the light mixture produced, only monochromatic blue and a peaky yellow.  Red and green phosphors are becoming available, but they are very expensive or have other problems.
The LED is a gallium nitride type of die, usually doped with indium so these are called InGaN die.
The most common phosphor used is a YAG, doped with cerium.  Yttrium Aluminum Garnet is the full name.  The garnet refers to the crystal structure.  The cerium dopant creates a special energy level from which yellow photons can be produced.  Sometimes gadolinium is used as a dopant.  There are other types of phosphors, some are silicates, some are nitrides, which can work in this application.

How are they made?
A disc or dollop of phosphor powder dispersed in clear silicone polymer is dropped over the InGaN die and then cured into position.  The disposition of the phosphor is a very complicated process.  The whole affair is then encapsulated in clear optical silicone.  The die is wirebonded or flip chip bump mounted on a leadframe or silicon submount.
Because of limitations in chip fabrication and heat removal, the die is about 1 mm square, not larger.
The forward voltage of each chip is about 4 volts dc, so a row of three can be powered by a 12v system.  Arrays of three then can be assembled to make whatever amount of light that is required.

How much light can they make?
Light quantity is measured in lumens.  A big 8' ceiling fluorescent puts out about 2000 lumens.  A 100 watt simple lightbulb about the same.
So, for a useful landing light we'd need about 2000 lumens.
Since the best white light LEDs put out about 80 lumens per watt, we'd need an array of about 25 LED's.  The low efficiency simple white light LEDs that you see in flashlights only put out about 15 lumens per watt, so we'd need 133 of those.

How much do they cost?
The highly efficient ones cost about $2 each to make, but the selling price is much higher than that, say $5 per LED in volume.  That makes an array of 25 cost $125 just in LEDs alone.

What are their advantages?
Solid state lighting devices are incredibly rugged and ideal for aircraft use.  The useful lifetime of a lighting device is usually measured in the time it takes half the population to die catastrophically.  But LEDs don't fail like that, they slowly get dimmer.  Even so, they last perhaps 50,000 hours before they get to 50% of their light output.  Incandescents fail at about 1,000 hours.

What's the future of white light LEDs?
100 lumen per watt devices are being produced now, 200 lumen/watt devices are on the horizon.  Full spectrum devices can now be produced (that is defined by the Color Rendering Index, where 100 is excellent, they are around 82) which makes their use for domestic and industrial lighting very attractive.

So there you have it, a data dump on white light LED's.

Chris Lowery