is this speckle?

[Joe Farina]

I tried to take some photos of a hologram to show the "grainy" effect which may (?) be laser speckle. The issue of speckle was discussed at length in the "speckle noise reduction" thread currently on page 3 of this section.

As can be seen in the close-ups, there is a grainy appearance to the image. It's like a multitude of tiny dark spots, which seem to be present in the emulsion itself (not on the objects). So the graininess appears to be imbedded in the emulsion, i.e., it's like looking through a dirty window, the specks move in the direction opposite to the direction of the eye movement. This was a single beam Denisyuk with spatially filtered light.

I think all the holograms I've ever seen or done show this effect to an extent, some more than others. Sometimes it's not very noticeable, sometimes it's obvious.

[Steven]

I would think so Joe.
I get the same "grain" effect in my holograms, but we both know that there's no grain in DCG.
I believe the amount of speckle, or how visible it is, will be proportional to the coherence length of the laser. You get less speckle with a laser that has a shorter coherent length. It's a problem manufactures have producing laser televisions, they have to "spoil" the coherent lengths of the three lasers to reduce the speckle.

Steven.

[bobh1357]

Coherence is needed to record a hologram, and speckle results when coherent light illuminates a surface. The size of the specks depends on how rough the surface is, and the size of the source illuminating it, the spectral bandwidth of the source, and the diameter of the pupil in the eye of the observer. Polarization of the source also counts, but to a lesser extent (random polarization produces less speckle than linear polarization). If you want less speckle in your pics, use the largest aperture on the lens, keep the camera as close as possible to the image, or vibrate the hologram or source at a high frequency (so it's moving during the exposure) but not so much that it blurs the picture.

[Joe Farina]

Thank you Steven

[Colin Kaminski]

You could re wire your fringe locker to provide some speckle reduction.

[Joe Farina]

You could re wire your fringe locker to provide some speckle reduction.

I can? Please tell how By the way, this is how I store DCG plates worth keeping. They will probably keep indefinitely, as long as the silica gel is monitored.

[Colin Kaminski]

I have not read about this for years but if my aging memory serves the idea is to put a moving diffuser in the object beam. The diffuser spreads out the speckle. I usually use broken light bulbs as diffusers. Perhaps, someone here remembers more. I'll post when I run into it again as I go through my books.

[Joe Farina]

Thank you Bob, that helps quite a bit. (I saw your response just now). I didn't know about the polarization factor, but that definitely makes sense.

Thanks Colin, this was also discussed in the speckle noise reduction thread.

[psmaelme]

Wow !! amazing