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

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RoGeorge:
Some models of LEDs light up with a visible halo.  :-//

That is visible with the naked eye, especially against a black background.  The halo can be captured with a camera, too, though to the eye the halo is not that thick.

To the eye it looks like the halo is made out of minuscules dots of light sprinkled around the LED.  Those sprinkled dots seem "fixed" in the air when changing the viewing angle.  More visible from red SMD LEDs, or from clear 5mm red LEDs, especially for punctiform light sources.  LASER pointers always have that halo when pointed to a wall, much stronger than the red LEDs, and visible no matter the pointer's color.

What causes that visible halo?

AndyC_772:
I think with a laser pointer, it's an interference pattern. The light from the laser is coherent, and where there are multiple paths to reach the same point on the wall (eg. because of dirt, imperfect lens coatings etc), those different paths represent a different number of wavelengths. Hence in some cases they add constructively, and others, destructively - hence the pattern.

I'm not really sure what I'm looking at in that photo - it just looks overexposed, I don't see any structure or pattern.

AndyBeez:
Agree with @AndyC_772, it's the interference pattern created by the light waves inside the laser beam.

Point the laser beam into a glass of water and you might see the pattern split apart. Do not point laser's into your eyes to see the effect.  8)

RoGeorge:
If so, do ordinary LEDs emit coherent light, too, just like a LASER diode would emit?

tom66:

--- Quote from: RoGeorge on January 30, 2023, 06:35:46 pm ---If so, do ordinary LEDs emit coherent light, too, just like a LASER diode would emit?

--- End quote ---

No.  Ordinary LEDs emit incoherent, diffuse light.  If you check the datasheet for an LED it will show the typical emission pattern.

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