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Old 07-01-2011, 09:09 PM #1
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Default Diffuse laser reflection; why "shimmer"?

First of all, I DID try to google this, and I DID try a search on the forum.

Okay... I've wondered about this off and on for a long time. I first noticed it with my ~ 5mw red HeNe back in the late '70s.

Put a focused laser spot on say, a white wall (glossy paint) - it doesn't seem to matter what TYPE of laser or COLOR, or for that matter, output. I can see this with a 5mw green laserpointer.

... and observe the diffuse "backwash" or "splash" of light on, say, the top of a nearby bookshelf or most other objects.

Objects illuminated by the "splash" seem to "shimmer". As if they are coated with a thin layer of vaseline mixed with "sparkly stuff".

Countless tiny "sparkles"... if you remain perfectly still, these countless tiny "spots" seem more-or-less stable... but move your head, and the sparkles seem to "move"... they kind of follow your head movement / eye movement.

Countless tiny interference patterns? The light in the splash can't be spatially coherent anymore, GENERALLY, though it is still chromatically coherent. Zillions of little coherent bits, from micro-reflections, interfering w/ each other, going in and out of phase? Or what?



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Old 07-01-2011, 09:16 PM #2
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Default Re: Diffuse laser reflection; why "shimmer"?

walls are far from perfect smotheness. and you can NEVER make your hand perfectly still. so those 2 things alone cause what you are experiencing. and with so many small speckles that would account for the illusion of it seeming to move.

and the reflection is still coherent from what i have read.

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Old 07-01-2011, 09:44 PM #3
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Default Re: Diffuse laser reflection; why "shimmer"?

I can get it with the laser mounted on a tripod, though; it doesn't have to be hand-held.

"Far from perfect smoothness"; yes that is kind of what I mean. That maybe there are countless mini-reflections splashing off of a glossy white wall (works best if glossy, anyway).

It can't be SPACIALLY coherent anymore, because all of the zillions of tiny surface reflections from microscopic bumps, cracks, lumps, etc., are each providing light going off at all different angles.

The splash of light, then, spilling across another object (say, a tabletop near the wall) can't be spacially coherent - the tabletop is at an ANGLE (say 90 degrees) to the wall - we are no longer observing one clean beam of laser light.

BUT the light should still be CHROMATICALLY coherent - all one color.

So I am thinking that the little "sparklees" are an interferance pattern created by light that is all matching in wavelength, BUT is no longer "in-step", spacially; all of the tiny reflections from the diffuse splash are out of phase with each other. So we see this as "zillions of tiny bright spots".

Move your head, and the phase relationships change, and the sparkly little lights seem to move or "flow".

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Old 07-01-2011, 09:48 PM #4
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Default Re: Diffuse laser reflection; why "shimmer"?

it is still coheren. as my under standing, each relected angle it is coherent. it jus looks non coherent because of so many relections.
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Old 07-02-2011, 01:48 AM #5
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Default Re: Diffuse laser reflection; why "shimmer"?

It is caused when the light interferes with itself. See laser speckle
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Old 07-02-2011, 02:10 AM #6
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Default Re: Diffuse laser reflection; why "shimmer"?

Even with it faults you gotta love wikipedia.
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Old 07-02-2011, 02:32 AM #7
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Default Re: Diffuse laser reflection; why "shimmer"?

OKAY! Thanks... if I would have used the word "speckle" in my searches instead of "shimmer", I would have found it.

Oh well. One man's shimmer is another man's speckle.

The wiki page makes sense, and I THOUGHT that it was something like that (which I described above in my own clumsy way).
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Old 07-05-2011, 01:08 AM #8
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Default Re: Diffuse laser reflection; why "shimmer"?

I've heard that depending on how much the surface "moved" with respect to your head/eyes/object, you can determine the person's vision rating.
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Old 07-05-2011, 10:13 PM #9
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Default Re: Diffuse laser reflection; why "shimmer"?

Quote:
Originally Posted by JBTexas View Post
OKAY! Thanks... if I would have used the word "speckle" in my searches instead of "shimmer", I would have found it.

Oh well. One man's shimmer is another man's speckle.

The wiki page makes sense, and I THOUGHT that it was something like that (which I described above in my own clumsy way).
I remember trying to search for this. What a pain! Glad you found your answer!
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Old 07-27-2011, 08:26 PM #10
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Default Re: Diffuse laser reflection; why "shimmer"?

"Vision rating"

Turn your head, and the speckle moves differently if your nearsighted or farsighted. One will go left to right, and the other right to left. Some one had a patent on that way before diode lasers were common. As a quick vision test for children and seniors.

If you have 20-20 the magnitude of the movement is small.

Steve

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Old 07-27-2011, 09:06 PM #11
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Default Re: Diffuse laser reflection; why "shimmer"?

LSRFAQ / Bionic-Badger; Thanks for the update! Found this:
Eye testing using speckle - Wikipedia, the free encyclopedia
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