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CromagNet said:
LOL govt... So what about the coveted XO bin. Why are they so hard to get. Or sell for more. I was 'assuming' that it was because of the imprecision of the process of coating the blue LEDs with yellow phospor. I've always wondered this but only recently figured it was the phospur. /ubbthreads/images/graemlins/icon3.gif
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Any manufacturing process has acceptable tolerances. If the # of parts out of tolerance are low enough and the parts cheap enough, you can just scrap, repurpose, recycle etc the rejects.
In the case of LEDs, tint/wavelength are a tolerance. All semiconductor manufacturing operations have yield issues - a signifigant percentage of parts are out of tolerance that they have to be rejected - that require broader tolerances than many other manufactured parts. Cutting-edge processors reputedly have a yield rate of approximately 20%. In order to increase yields without, bins have been devised. This lets the customer know what they're getting without making a product with huge margins on specifications. Imagine if white LEDs weren't binned and the Vf could be anywhere from 2.8V to 4.2V, or if brightness varied by up to 100%...
In the case of flux and tint for white LEDs you have two factors with their own major tolerance issues - the diode itself (wavelength, flux), and the phosphor (spectrum, efficiency, quality of application).
The fact that it's rare to see a "poor-performing" Luxeon III indicates that Lumileds has improved their processes.
Specific bins are difficult to get for a variety of reasons...
• It's cheaper to order unspecified bins and hope for the best
• They're made in limited quantities
• Volume buyers can afford to pay a premium for specific bins
• Lumileds doesn't assign part numbers by bin, just by overall product, so you can't just spec a part number and get exactly what you want