Diffraction gratings

BobH · Post by **BobH** » Tue Dec 01, 2009 3:09 pm

A spreading lens before the diffuser will do the same thing. The graininess comes from the size of the beam being small when it illuminates the diffuser. Anything that makes the beam bigger will make the graininess smaller, and reduce the speckle too.

The_Stranger · Post by **The_Stranger** » Tue Dec 01, 2009 3:11 pm

dave battin wrote:yes, you have it correct above, this method works very well for doing rear-lit screen situations, and i like the double sided frosting idea, when i use the single sided frosted glass i make the laser hit the frosted side first, as i think it may be more efficient this way :think: :

ok, thank you, but what is the difference above usig a "normal" lens to light the object? Does ik effect quality or does it effect the total picture? that rudy paper gives al lot of info, but without any practical experience it is almost totally incomprehensible.

(I could try and see for myself, if the laser was up and running, but now I have to ask here. The replacement brick is in and I expect to start shooting in two weeks, after my holiday)

Joe Farina · Post by **Joe Farina** » Tue Dec 01, 2009 3:50 pm

dave battin wrote:when i use the single sided frosted glass i make the laser hit the frosted side first, as i think it may be more efficient this way :think: :

Now I really feel dumb that I didn't realize that, thanks Dave

When a simple lens is used, the beam will often have swirls, etc., due to imperfections in the lens, or dust. Also, the lens alone might provide a limited amount of beam expansion. Back in the old days, they often used spatial filters in the object beams (for display holograms) to solve the first problem, but I guess nowadays people just use diffusers.

The_Stranger · Post by **The_Stranger** » Tue Dec 01, 2009 3:55 pm

Joe Farina wrote:
dave battin wrote:when i use the single sided frosted glass i make the laser hit the frosted side first, as i think it may be more efficient this way :think: :
Now I really feel dumb that I didn't realize that, thanks Dave

When a simple lens is used, the beam will often have swirls, etc., due to imperfections in the lens, or dust. Also, the lens alone might provide a limited amount of beam expansion. Back in the old days, they often used spatial filters in the object beams (for display holograms) to solve the first problem, but I guess nowadays people just use diffusers.

So what you are saying is that instead of cleaning the object beam with a spatial filter, a lens or grating along with a diffuser can also be used to produce a clean hologram? Can that also be used for the reference beam, since Dave did send me two gratings and two pieces of frosted glas a a lot cheaper and easier to use than two pinholes/spatial filters (I expect not, that would be too simple

)..

Joe Farina · Post by **Joe Farina** » Tue Dec 01, 2009 4:14 pm

The_Stranger wrote:(I expect not, that would be too simple )..

Right

The reference beam needs to be spatially coherent.

BobH · Post by **BobH** » Tue Dec 01, 2009 4:21 pm

Diffusers in the object beam path will soften the shadow edges and reduce laser speckle. Both are very nice things for hologram images. The price you pay is wasted light, and illumination of the entire area around the object which often needs to be baffled away some way. That's not a problem if the scene works well being lit that way.

You also loose a bit of polarization control, which can be brought back by putting a polarizer after the diffuser, but that wastes more light.

The_Stranger · Post by **The_Stranger** » Tue Dec 01, 2009 5:55 pm

Joe Farina wrote:
The_Stranger wrote:(I expect not, that would be too simple )..
Right The reference beam needs to be spatially coherent.

Just what I thought, it didn't make sense...

Danny Bee · Post by **Danny Bee** » Tue Dec 01, 2009 6:35 pm

The_Stranger wrote:
Joe Farina wrote:
dave battin wrote:when i use the single sided frosted glass i make the laser hit the frosted side first, as i think it may be more efficient this way :think: :
Now I really feel dumb that I didn't realize that, thanks Dave

When a simple lens is used, the beam will often have swirls, etc., due to imperfections in the lens, or dust. Also, the lens alone might provide a limited amount of beam expansion. Back in the old days, they often used spatial filters in the object beams (for display holograms) to solve the first problem, but I guess nowadays people just use diffusers.
So what you are saying is that instead of cleaning the object beam with a spatial filter, a lens or grating along with a diffuser can also be used to produce a clean hologram? Can that also be used for the reference beam, since Dave did send me two gratings and two pieces of frosted glas a a lot cheaper and easier to use than two pinholes/spatial filters (I expect not, that would be too simple )..

there are so many ways to control object light depending on how much power you have and how stable your table is. I use some times 3 to 4 lights, taking this from photography studio lighting, main, fill, and hight-lite...and some times background lighting. main and fill can be anywhere from 1 to 2 (means if main light is X then fill light is 1/2 of that) or 1/3 (means if main light is X then fill light is 1/3 of that) or 1 to 4 (means if main light is X then fill light is 1/4 of that)then i have the high-lite 1.25 to 1.5 brighter that the main. depending what i want my results to be I set my refrence beam 1/4 of the main or fill light.