Speckle noise reduction

Holography related topics.
Dinesh

Speckle noise reduction

Post by Dinesh »

Joe Farina wrote:By the way, I wonder if it's possible to use a color reflection (not transmission) master, then make color copies. Maybe it would avoid crosstalk problems in the master? (But then again, my knowledge of copy configurations is extremely limited.)
Well, you're right, it would avoid cross talk. But, colour holograms are designed to be reconstructed with white light. Hey, Phil, got a white laser?!

Yes, you can reconstruct with three (or two) lasers, but there must be almost zero swelling. White light reconstruction allows a little "slop room", so that if your "633 fringes" have swollen to 650 ( a 2.6% swell), then your eye will see little difference since the cones overlap quite a bit. There'll be a slight shift of the image vis a vis the other colours, but at 2.6% shift, the eye/brain easily compensates. It's actually quite amazing how distorted an image has to become before the brain gives up, even random images become meaningful (which is why people see Jesus on a tortilla, but that's another story!). But, if you reconstruct the shifted red fringes with a laser, then the laser bandwidth is outside the 17nm "window" and it'd be dim at best. Of course, the same holds for all your other colours. In a single beam Denisyuk, there is always an angle at which the image comes out bright, since the replay colour shifts with angle. However, the three colours will shift by different amounts, so this technique may not work for colour.
Joe Farina
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Speckle noise reduction

Post by Joe Farina »

Dinesh wrote:But, if you reconstruct the shifted red fringes with a laser, then the laser bandwidth is outside the 17nm "window" and it'd be dim at best. Of course, the same holds for all your other colours.
I forgot about that, yes, the master would (of course) need to be reconstructed with laser light onto the copy, and the efficiency would depend on lack of shrinkage or expansion in the master.
142laser
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Speckle noise reduction

Post by 142laser »

It is too bad there seem to be no reliable inexpensive RGB lasers with good color balance and narrow linewidth for both making and reconstructing holograms. :) We do have HeNe for red, doubled YAG for green and 480 nm or 488 nm from Coherent OPSL for blue available...:)
142laser
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Speckle noise reduction

Post by 142laser »

I was wondering if "shaking" the plate holder for viewing (at a frequency higher than 60 Hz) would average out the speckle like it does for viewing a laser spot on a screen; shaking the viewer's head might not be so good. :) A new product...Vibra-View plate shaker! OK guys and girls, keep it clean here...:)

Who has tried this? :) It works for images on a screen.
142laser
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Speckle noise reduction

Post by 142laser »

Maybe use one or more PZT's on the display frame (plate holder) with a small driver box? Who can try this...Ed? :)

This may not work at all as you may still get speckle or the holographic image will blur; interesting physics questions. I just set up Physics Demonstrations for a living so stupid s*** comes to mind on a regular basis...:)
Dinesh

Speckle noise reduction

Post by Dinesh »

142laser wrote:I was wondering if "shaking" the plate holder for viewing (at a frequency higher than 60 Hz) would average out the speckle like it does for viewing a laser spot on a screen; shaking the viewer's head might not be so good. :) A new product...Vibra-View plate shaker! OK guys and girls, keep it clean here...:)

Who has tried this? :) It works for images on a screen.
Well, firstly, it's interesting you say this. There seems to be an opinion that reconstructing a hologram with incoherent ("white") light will result in speckle. Not so. The reason that incoherent light will not create speckle is that the phase changes are so rapid, that the the speckle "blurs out". Light from an incandescent source has a coherence time of about 10^(-8), so in essence, when you look at something with white light, be it a hologram or a white rabbit, you're eyes are shaking at a frequency of 10^8 Hz, or about 100,000,000 times a second. There are several SF movies and TV episodes where "time slows down" ( a Star Trek episode comes to mind - "Wink of an Eye"). But, as Kirk wanders around the Enterprise in his accelerated state, the walls seem to be "unaccelerated". If time did slow down, as per the episode, Kirk would have seen slowly forming coloured interference fringes on the walls, slowly shifting.I thought that this would have enhanced the effect, but clearly the writers of the episode were unaware of coherence theory!

Would this work on a laser illuminated hologram? Yes, it probably would, though I'd suspect that 60Hz may be a bit fast. The eye retains an image for roughly 1/15 seconds, and the saccadian response is roughly 20ms, or at a frequency of 50Hz. But, the image would probably blur. However, if you passed the image through an FT filter and transformed the image, then inverse transformed it, then it may not blur. This is because the Fourier Transform of a shifted function merely shifts the phase, not the amplitude.

So, Phil, there's your physics demonstration. Illuminate a hologram with a laser, oscillate it at 50Hz or so and show the speckle disappears. Then do a "double pass" FT and show that the blurring also disappears!
142laser
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Speckle noise reduction

Post by 142laser »

Someone needs to try this who has a table set up with laser, spatial filter and plate holder. If the amplitude is small there may be almost no blurring. Oh, Ed.....Ed...:) Maybe he has done this before. :)

What about modulating the laser at say 100 Hz with an AOM? Or would you simply get "pulsed speckle"?

I use white round and square thin metal plates for resonance demonstrations by placing them vertical and illuminating with a spatially filtered HeNe. When mechanically driven (center mounting post) at a resonance (by a special speaker like apparatus and a matched driver) you can see the nodal lines as they have clear sharp laser speckle but the antinodes have the speckle time averaged out. It is cool. The issue is it is also LOUD at resonance...that is Physics. :) Laser speckle interferometry. 8-)
Dinesh

Speckle noise reduction

Post by Dinesh »

142laser wrote:What about modulating the laser at say 100 Hz with an AOM? Or would you simply get "pulsed speckle"?
No, speckle is a function of the surface being illuminated, not of the source or the path, unless the path has a phase altering component.
Ed Wesly
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Speckle noise reduction

Post by Ed Wesly »

Sorry, don't have any PZT shakers and drivers in house, so I can't try shaking the hologram to eliminate speckle at the moment. It sounds like you have the primo Pasco shaker device, maybe send that to me? Just kidding, I will try to send you a laser reconstructed hologram for you to try, but probably not until next week as I am cleaning house in preparation for the onslaught of out of town holographers converging here soon.

One speckle reducing and spatial filter eliminating set up that I have seen work is one of TJ's, a laser beam incident on a metal ball on a spinning motor shaft, looks pretty speckle-free, the big trick is in making sure the ball is centered on the shaft, otherwise the image wobbles. He said he suggested that to Paula Dawson, maybe she used that trick on some of her meter square pieces.

When I worked at FermiLab, we reduced the speckle when photographing bubble chamber holograms by using a 500 mm Reflex Nikkor lens with an extension bellows, as we had to find small stuff fairly far away in the fiducial volume. I rigged up a revolving disc with an off-center hole in front of the lens so the aperture rotated during exposure, averaging out speckle.
ReflexNikkorFront.jpg
For those not familiar with the lens, the reflex style camera lenses bounce light back and forth inside of them to make a very compact design; however the convex mirror glued on the inside of the front corrector plate gives a donut-shaped entrance pupil, and donut-shaped out f focus highlights!
ReflexNikkorGuts.jpg
Don't have any final images to show, sorry, but that was over 30 years ago! And they weren't very interesting anyhow!

BTW, there is speckle in white light reconstruction of holograms, viz. a rainbow hologram with its slits too small. That is the classic trade off when engineering those kinds of set ups, color saturation versus speckle.
"We're the flowers in the dustbin" Sex Pistols
Dinesh

Speckle noise reduction

Post by Dinesh »

Ed Wesly wrote:BTW, there is speckle in white light reconstruction of holograms, viz. a rainbow hologram with its slits too small. That is the classic trade off when engineering those kinds of set ups, color saturation versus speckle.
That may well be diffraction as opposed to speckle. Speckle is a function of spatial coherence. The phase relationships in white light are too random to show an effect due to a coherence. Speckle is largely a reflective effect; speckle in transmissive media are caused by non-uniform index variations of the order of lambda. However, (Raman Nath) diffraction is a transmissive effect where the spatial frequency is given by the grating equation:

lambda*nu = sin(theta),

differentiating,

d(nu)/d(theta) = cos(theta)/lambda.

Thus, if theta is small, there may be a large variation of nu against theta, which may resemble speckle.
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