Xref: utzoo sci.electronics:3849 rec.audio:8351 Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!nrl-cmf!cmcl2!rna!dan From: dan@rna.UUCP (Dan Ts'o) Newsgroups: sci.electronics,rec.audio Subject: Re: Looking for Blue LEDs Keywords: blue blue blue, not yellow or red or green Message-ID: <255@rna.UUCP> Date: 21 Sep 88 03:14:54 GMT References: <1138@nmtsun.nmt.edu> <862@ritcv.UUCP> Reply-To: dan@rna.UUCP (Dan Ts'o) Organization: Rockefeller University Neurobiology Lines: 24 In article <862@ritcv.UUCP> cep4478@ritcv.UUCP (Christopher E. Piggott) writes: >I have several, but have no idea where they came from. (They look more >purplish than blue to me, as if they were red L.E.D.'s with blue paint >mixed into the glass. I have technical reasons to suspect this, also: they >are not as grossly enefficient as blue L.E.D.'s should theoretically be). I'm not sure (therefore I know I shouldn't post this...), but I doubt that your blue LED's could possibly be red LED's with a blue filter. This seems impossible since LED's (which, with red are GaAs? gallium arsenide) are second cousin's to semiconductor lasers, and are very monochromatic. Just try to view a red LED through a green or blue interference filter -- you'll see nothing. Therefore output in the blue region is nil as well. In any case, I also believe that blue LED's are fabricated from silicon nitride. BTW, why should a blue LED necessarily grossly inefficient ? I do believe that blue LED's made from silicon nitride are, but why must a blue LED (regardless of implementation) be inefficient. As far as why your blue LED appears purplish. It probably is psychophysical/physiological, rather than any red output. Just as the extreme shortwavelength end of the rainbow appears violet/purple, the blue LED probably is emitting light at, say 430nm. Your red cones in the retina actually have significant sensitivity in the deep blue region -- more so than your green cones. Thus deep blue appears purplish or actually bluish/red, an anomolous sensation.