Speckle noise reduction

Holography related topics.
Dinesh

Speckle noise reduction

Post by Dinesh »

Joe Farina wrote:Too much speculation, I know.
Well, yes...and no. You can think a problem through in the greatest depth, throw the most abstruse and complex mathematics at the problem and you might get a good handle on the problem. Or, you can simply try it out and See What Happens. My method (for what it's worth) is to think through these theoretical ideas/problems/issues. Then you've got to ask yourself, "Are these serious impediments to a solution?" At the end of the day, I usually end up saying, "Heck, there's only one way to find out. Let's try it and see." Theory prepares you for possible problems/explanations of what you observe afterwards, but a certain amount of care is necessary that the theory does not dissuade you from trying it. You can get lost in the maze of theory.
Jeffrey Weil

Speckle noise reduction

Post by Jeffrey Weil »

You could consider a diffuser to be like an array of lenses. A random array, but an array none the less. If you moved a single lens, like a spatial filter, in one of your beam paths you would get movement and no holo. I think if you moved a diffuser during an exposure the same thing would happen. Unless it's a single beam setup.
holomaker
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Joined: Wed Jan 07, 2015 8:01 am

Speckle noise reduction

Post by holomaker »

Could you use a blocking card with a large hole in it ,in the reference beam (collimated then expanded)and move the card (rotate it) causing only partial speckle to form in one area, and then another to form in an overlapping type of way ? Yes more light would be lost but could be adjusted to work ?

I very much like this statment Dinesh, it will be a classic of yours for sure !
Dinesh wrote:You can get lost in the maze of theory.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Speckle noise reduction

Post by Joe Farina »

Dinesh wrote:You can think a problem through in the greatest depth, throw the most abstruse and complex mathematics at the problem and you might get a good handle on the problem. Or, you can simply try it out and See What Happens. My method (for what it's worth) is to think through these theoretical ideas/problems/issues. Then you've got to ask yourself, "Are these serious impediments to a solution?" At the end of the day, I usually end up saying, "Heck, there's only one way to find out. Let's try it and see." Theory prepares you for possible problems/explanations of what you observe afterwards, but a certain amount of care is necessary that the theory does not dissuade you from trying it. You can get lost in the maze of theory.
I have to agree. My engineer friend seems to think that I have a big mouth, he often chastises me with "just do it," "prove you have a problem and fix the problem," "don't think it to death" and on one occasion he got really pissed off and said "VENUS HERE WE COME."

Dave, that's an interesting idea.
BobH
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Location: Mesa, AZ

Speckle noise reduction

Post by BobH »

A moving aperture with a diffuser is another way to go, and published in the '60s. At Simian, the diffuser was moved but held stable for the exposure.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Speckle noise reduction

Post by Joe Farina »

BobH wrote:A moving aperture with a diffuser is another way to go, and published in the '60s. At Simian, the diffuser was moved but held stable for the exposure.
Thanks a lot. Could you please point me in the direction of the paper, if possible?

The advantage I see to a moving aperture w/diffuser during exposure is that the speckle pattern can be re-arranged many times.

Thanks again Dave.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Speckle noise reduction

Post by Joe Farina »

I haven't read this yet, but it looks interesting:

http://www.jpier.org/PIERB/pierb34/01.11070601.pdf
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Speckle noise reduction

Post by Joe Farina »

I've been thinking about Dave's idea of using an aperture which changes size during exposure, and I would like to clarify how I imagine it works.

Say we have a 5mm circular aperture in front of a diffusion screen for the object beam (for a split-beam reflection hologram with the reference beam and one object beam). The object beam comes from the beamsplitter, then goes to any re-directing mirrors, then goes to a beam-expanding lens (say a negative lens), then to the aperture. Say the spot of light on the back of the aperture is 10mm, but light is only getting through the 5mm hole. Then the light goes to the diffuser, and then to the object. There will be a certain speckle pattern on the object.

Now, say the aperture “magically” changes to 7mm. The original speckle pattern from the 5mm hole will still be there, exactly as before. However, since an additional 2mm “ring” has been added to the diameter of the aperture, new areas of the diffuser will now be exposed to light, and those “new” areas will generate another speckle pattern, which will cover the object completely. In other words, the 2mm “ring” will cause an “overlapping” speckle pattern (as Dave said). If the aperture slowly “grows” from say 2mm to 10mm during the course of the exposure, maybe substantial “blurring” of the speckle will occur?

The aperture could be isolated from the rest of the table (say for example by using a rigid "arm" support which extends over the table), and designed so that no vibrations are transferred to the floor.
Jeffrey Weil

Speckle noise reduction

Post by Jeffrey Weil »

The speckle pattern from the 5mm aperture is a part of the 7mm's pattern. It's more course but it's still part of it.

I think the only difference between an exposure being 100% from the 7mm aperture and one where it's only part is that some of the courser parts of it's speckle pattern have been exaggerated by the 5mm exposure.
holomaker
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Joined: Wed Jan 07, 2015 8:01 am

Speckle noise reduction

Post by holomaker »

my thought was not too change the size of the aperture, but to move it to a new area within the collimated light area, creating a new speckle zone ...after the collimated light area the beam is then focused again and becomes diverged and hits the film plate ..
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