What causes laser source speckle patterns, and why do they remain constant even if the observer moves?
November 14, 2010 4:43 PM   Subscribe

Why doesn't the speckle pattern that you see around the source of a laser change when you move your head, and what causes the pattern?

The pattern I'm talking about is like this one: http://i120.photobucket.com/albums/o173/bugbread/LaserSpeckles.jpg. I could understand if it were something along the lines of reflections within the laser lens or mechanism, but it would seem that the pattern would change when you moved your head, but it doesn't -- it remains the same. Why is that (and, probably the key to that question, what are those speckles?)
posted by Bugbread to Science & Nature (3 answers total) 1 user marked this as a favorite
 
They're called "speckle" (I know, I know, incomprehensible jargon). How they move when you move your head actually depends on whether you're nearsighted or farsighted.
posted by hattifattener at 4:48 PM on November 14, 2010 [2 favorites]


The pattern is actually formed inside your eye, so moving across the beam doesn't produce the same sort of movement that an image formed by a real object would.

The pattern is the result of in-phase (coherent) photons adding up (bright spots) or cancelling out (dark spots), based on the relative path length from the light source to the retina of your eyes.

With normal (incoherent) light, such as that from the sun, a light bulb, or whatever, the billions of photons hitting your retina don't have an ordered, predictable relationship in phase, so they "add up" randomly, creating an average light intensity which is based on the amount of source light - this is what happens when you look at any ordinary object, throughout your day.

With laser light, the special, ordered relationship of all the photons allows coherent interference to create these patterns.

The links in hattifatteners post above do a decent job of explaining the process. I want to emphasize that speckle is subjective (that is, its actual appearance depends on things like the optical power of your eyes, so it looks different with and without reading glasses), but very real. You see it around laser light in the lab, and on objects illuminated by laser light. It looks like a real "texture" on the object, but it only exists inside your eye.
posted by IAmBroom at 11:12 PM on November 14, 2010


Thank you both. Succinct and totally clear. Also, the near-sighted versus far-sighted thing is tremendously interesting, as is the fact that the speckle is always in focus, no matter how your vision is. Next time, I have to try taking my glasses off to see the difference in perceived speckle movement.
posted by Bugbread at 4:29 PM on November 15, 2010


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