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What is the halo of minuscule dots seen around some LEDs?

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RoGeorge:

--- Quote from: Someone on January 30, 2023, 09:11:14 pm ---https://www.nature.com/articles/s41598-017-06215-x

--- End quote ---

That link revealed the proper name, thanks!  :D
It's 'speckle', Wikipedia has more details, 'subjective speckle pattern' and better pics:


Photo source:  https://en.wikipedia.org/wiki/Speckle_(interference) 

Benta:
Yes, well, that's about lasers. The speckle effect is well known there.
Your OP was about "some models of LEDs".
What exactly do you mean? Is your question about laser LEDs or plain LEDs?

tom66:

--- Quote from: Someone on January 30, 2023, 09:11:14 pm ---
--- Quote from: tom66 on January 30, 2023, 06:50:00 pm ---No.  Ordinary LEDs emit incoherent, diffuse light.  If you check the datasheet for an LED it will show the typical emission pattern.
--- End quote ---
That would be incorrect, LEDs are producing light in roughly the same mechanism and as the emitters get smaller interference effects get stronger:
https://www.nature.com/articles/s41598-017-06215-x

Like polarization it is not a binary yes/no but a question of how much. Zero or 100% are not the answers in real life engineering.

--- End quote ---

I am not sure if we are reading the same thing.  That paper states "For a light source with a broad spectral bandwidth such as LED, it should be regarded as an incoherent source."  They talk about sLEDs being coherent; these are *not* LEDs, though they are an interesting device that I only found out about from that paper, so thanks for that.

LEDs are not coherent light sources.  To be coherent the emission of light from the device must be parallel, or as close to that as practically measurable, and there is effectively no coherence in the output of a typical LED.  LED emission is almost exclusively from stimulated emission, the electron directly dropping through the bandgap and emitting light, whereas laser diodes experience gain through stimulated emission.  The gain process means the emission of light from a laser diode is coherent like other laser sources.   

RJSV:
Noticed similar effect, with old school incandescent colored lights:
   Had noticed, with a little blue incandesent bulb, it had a RED halo!  Wondering how that could be, the halo consisted of a sparse pattern of thin (red) lines, emanating, seemingly from that blue lamp.
But, a bit of head bobbling and I realized that there WAS a red bulb, but you just couldn't see any of it, being barely blocked,.by the blue bulb.
   Was easy to duplicate, when I got home later.

Someone:

--- Quote from: tom66 on January 30, 2023, 10:27:32 pm ---
--- Quote from: Someone on January 30, 2023, 09:11:14 pm ---
--- Quote from: tom66 on January 30, 2023, 06:50:00 pm ---No.  Ordinary LEDs emit incoherent, diffuse light.  If you check the datasheet for an LED it will show the typical emission pattern.
--- End quote ---
That would be incorrect, LEDs are producing light in roughly the same mechanism and as the emitters get smaller interference effects get stronger:
https://www.nature.com/articles/s41598-017-06215-x

Like polarization it is not a binary yes/no but a question of how much. Zero or 100% are not the answers in real life engineering.

--- End quote ---
I am not sure if we are reading the same thing.  That paper states "For a light source with a broad spectral bandwidth such as LED, it should be regarded as an incoherent source."  They talk about sLEDs being coherent; these are *not* LEDs, though they are an interesting device that I only found out about from that paper, so thanks for that.

LEDs are not coherent light sources.  To be coherent the emission of light from the device must be parallel, or as close to that as practically measurable, and there is effectively no coherence in the output of a typical LED.  LED emission is almost exclusively from stimulated emission, the electron directly dropping through the bandgap and emitting light, whereas laser diodes experience gain through stimulated emission.  The gain process means the emission of light from a laser diode is coherent like other laser sources.
--- End quote ---
You again jump to the binary yes/no quantification, which is plainly incorrect. Observing coherence in non-laser sources is easy:
https://iopscience.iop.org/article/10.1088/1361-6404/ab3035
Showing coherence from LED pumped phosphor, or for a more in depth look:
https://opg.optica.org/prj/fulltext.cfm?uri=prj-10-11-2460&id=509788
"The radiation from white LEDs is generally understood to have low spatial coherence across the spectrum and be more or less unpolarized"
see how the language is softened to acknowledge there is some coherence and polarisation. Keep saying its none/zero and I'll keep replying with the same point, this is not a binary all/none one/zero yes/no characteristic.

Coherent light does not need to be parallel, it can be point/spherical source. They are independent characteristics that you lump together (which could be simultaneously necessary for some applications) as some generalisation which is incorrect.

People really need to avoid making claims about fields which they don't understand.

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