Why does the diffraction reflection of my green LED have red in it?

[Bluemoon]

I was looking at the diffraction reflection of an illuminated early 1980s green LED using a CD in a dark room

and the spectrum was red-yellow-green; why?

I thought that LEDs were pure monochrome light sources.

 

Also, a modern blue LED had green in its spectrum.

Again, does anyone know why?

 

[Robittybob1]

I was looking at the diffraction reflection of an illuminated early 1980s green LED using a CD in a dark room

and the spectrum was red-yellow-green; why?

I thought that LEDs were pure monochrome light sources.

 

Also, a modern blue LED had green in its spectrum.

Again, does anyone know why?

 

So what colour was it to your naked eye? I assume it was green.

What light mixes to form green, is it red and yellow?

 

With monitors (RGB) "Red and green mixes to form yellow". I'm interested but I'm not an expert in this topic.

[Enthalpy]

Most often, Led manufacturers try to produce light over a narrow spectrum. This was certainly the case for red, yellow and green in the 1980s.

 

Though, the process isn't perfect. If the material's gap (or doping) gives naturally green light, the components may contain some impurity which can radiate some red through a different transition. Often also, the dopant with the desired deep level brings more deep levels, of which some may radiate. Or if the component uses a heterojunction, some carrier pairs may recombine through a secondary process.

 

Then, there are some Led which are designed to emit in several bands. The usual case is the white Led, in which GaN produces hard blue (405nm, almost UV) which is converted in part to red and green by phosphors to produce an approximately white light.

 

Detail: Led emit normally over a narrow band whch isn't monochromatic, because the carriers have a thermal energy (mean 25meV if in the conduction or valence band) that adds to the gap, and because heavy doping also makes the band limits a bit fuzzy. Heavy doping could also make the deep levels broader, but I doubt it's the case in a Led.

 

Not related directly with Led, but a band diagram with a deep level is there

http://www.scienceforums.net/topic/68556-preparing-gold-colloid-nanospheres/#entry703828

pretty standard diagram, so you may find a better one elsewhere.

[John Cuthber]

...

I thought that LEDs were pure monochrome light sources.

...

 

Why?

[Bluemoon]

Robittybob1> So what colour was it to your naked eye? I assume it was green. What light mixes to form green, is it red and yellow?

It looked ...{"early 1980s green LED"} ... green, (I don't think that any mix of non-green emitted light produces green). I suspect that red & yellow mix to give orange. (red & blue give magenta).

 

Enthalpy>

Thanks for that, (I gave you +1)

 

John Cuthber> why

Well, because whenever I've read about LEDs the explanation talks of a definite band gap (ie: one wavelength).

[Enthalpy]

Why supposedly monochromatic or single-band: because datasheets tell it, as well as books, courses... Secondary radiations aren't mentioned usually, it's nice to read that the naked eye observes them.

 

Colours in the 1980s: there was one material for red resulting directly from the bandgap; yellow and green were obtained from GaP, through a deep dopant level for yellow at least. Blue is a more recent colour, I don't quite remember if it resulted from GaN right at the beginning. A true nice blue demands a deep dopant level in GaN also.

 

Meanwhile, GaN is used for more colours, either with a dopant level, or by conversion phosphors.