Question about LED maufacturing.

I keep reading (here and other places) about LEDs being sorted by tint before being assembled into flashlights.

What is the deal with the manufacturing of the LEDs? Is it that large of an inexact science?

LEDs can vary in efficiency, forward voltage, etc, but white LEDs add another variable into the mix, and that variable is tint.

white LEDs are in fact blue LEDs coated with a yellow phospher that absorbs and re-emits the blue light as yellow light. ideally the ratio of blue light to yellow light should be 1:2 (i can't remember where i got this from), since that would result in pure white light. however, the process of coating the blue LED die with the phospher is far from an exact science, as a slight variance in the thickness of the phospher coating would substantially affect the tint of the light emitted. if the coat is too thick, you'll probably get a yellow or green tint and if the coat is too thin, you get a blue or violet tint (also can't remember where i got this from, so someone correct me if i'm wrong).

the white LEDs are usually sorted at the factory by their temperature colour, but that doesn't always accurately reflect the tint of the light produced by the particular LED. hence, flashlight manufacturers like arc have had to sort the LEDs individually by hand, selecting only the whitest LEDs.

in addition, the tint will vary with the brightness of the LED, where underdriving the LED will make it take on a green/yellow tint and overdriving it will produce a blue or violet tint. i gather this is due to the phospher having a non-linear rate of phospherence relative to the amount of blue light it is exposed to. i.e. at dimmer light levels, it emits relatively more yellow light and at brighter levels, it emits less yellow light.

disclaimer: a fair bit of the information i supplied above is speculation and hearsay, thus completely inaccurate. anyone feel free to set me right /ubbthreads/images/graemlins/tongue.gif