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| What is the halo of minuscule dots seen around some LEDs? |
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| tom66:
--- Quote from: Someone on January 30, 2023, 11:24:26 pm ---"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. --- End quote --- I didn't say the light had no coherence, I said it was incoherent. Here is a definition of incoherent light: --- Quote ---Incoherent light is light that contains waves whose wavelengths are not in phase with each other and that do not oscillate at the same frequency. Not only do the photons in incoherent light oscillate at different frequencies, but they also oscillate in different directions. --- End quote --- That matches what an LED emits. Yes, there may happen to be an coherent point of emission on the LED by chance arrangement or, possibly on some device that could be constructed in a lab environment. The point is, the emission of light from an LED in a normal application is not, on the whole, coherent. It therefore is not a coherent source of light. As far as any normal use of an LED is concerned, the light exhibits little to no coherence (it is "incoherent", like ABS is, "inflexible" -- you can flex it, but not really that much.) --- Quote from: Someone on January 30, 2023, 11:24:26 pm ---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. --- End quote --- Ok, fair enough, a simplification on my part applicable to the most notable difference between LEDs and laser diodes. But still, LED light is not coherent. Not for any normal LED in any normal application. I await the datasheet for a coherent LED that I can buy. |
| David Hess:
Coherent sources, and LEDs are somewhat coherent, have speckle: https://en.wikipedia.org/wiki/Speckle_(interference) |
| Someone:
--- Quote from: tom66 on January 30, 2023, 11:37:05 pm --- --- Quote from: Someone on January 30, 2023, 11:24:26 pm ---"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. --- End quote --- I didn't say the light had no coherence, I said it was incoherent. Here is a definition of incoherent light: --- Quote ---Incoherent light is light that contains waves whose wavelengths are not in phase with each other and that do not oscillate at the same frequency. Not only do the photons in incoherent light oscillate at different frequencies, but they also oscillate in different directions. --- End quote --- --- End quote --- Except the LED does have coherence in its emissions in prefix = negation, in-coherent is without coherence. Except this entire thread is someone asking why they observe coherence effects from an LED.... perhaps because the LED emits (some/significant/enough) coherent light? Light as a practical use is not incoherent or coherent as binary choices, please just stop trying to label things with primary school level science. Light from most practical sources is somewhere between and quantifiable by various metrics, there most certainly is coherent light coming from an LED, significant amounts of it which cause all sorts of interesting optical effects. It seems everyone else here gets it as a parameter of qualities of light that has some continuum of more/less rather than yes/no. If you want to get silly, please present the incoherent light source (an imaginary construct which does not exist) and we can illuminate all the spherical cows with it. Even thermal sources of light still contain coherence! |
| T3sl4co1l:
Indeed it is a speckle pattern, and despite LEDs not being coherent sources, the spacial coherence can still be enough to observe such effects -- thanks to their intensity and small size, they make good point sources. I've observed this with my strings of, uh, whatever they were, 7x7mm (4-pin THT) red superbright LEDs. Presumably the speckle is characteristic of the component lens, the air between (mostly irrelevant at indoor conditions), and your eyes. So it varies as you move around, and seems to track your eyes say as you rotate your head. You can see similar patterns in the bokeh when defocused, or viewing through a magnifier or such. (I feel a loupe/microscope shows these fairly often, as the depth-of-field is quite poor, and we often work with strongly specular materials e.g. "bright" solder joints and flux, glossy plastic parts, solder mask, etc.). Also contributes (or contributed by) normal eye wear/aging; I've noticed a little bit of, astigmatism I suppose, in the last I don't know five years or so; bright sources (particularly at night, both because of contrast and wide iris) having more of a halo, plus some streaky shapes which can be shadowed in certain regions (e.g. hold my finger near my eye, partially shadowing the light source from one side or another). Well, I was told a long time ago that I probably wouldn't need glasses until I was 40, and there's a few years left before that, so that seems on track. :) Tim |
| SL4P:
BTW, consider that many lasers, even visible beams, can emit IR in those ‘side lobes’, so even not looking into the beam, or observing lasers kicking off glass - might expose your eyeballs to harmful doses of invisible IR laser emissions. |
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