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Bright H2 copies - how?
March 12 2004 at 5:58 AM
Tony  (no login)

Lately I’ve been concentrating on making reflection copies of transmission masters. I’m using 8E75 and a 10 mw hene laser. My results have been pretty good, but I’m really having trouble understanding all the factors that go into making a really bright H2. It seems like there are some factors that are not under control, as my results are mixed – some setups yield much brighter results than others, and I’m not sure why. So far I have covered a lot of ground in terms of beam ratios and overall exposure, so I’m pretty sure there’s more to getting a bright H2 than that.

My question is if the beam angles could have something to do with it. In one case, the brightest viewing angle for a good H2 seems centered between the viewing angles where the image ‘winks out’. In another not-so-bright case, the brightest image occurs right at the ‘wink-out’ angle – it seems as if the brightest image could be (should be ?) centered between the ‘wink-out’ angles.

I’m thinking that the angle between object light and reference beam onto the H2 might have something to do with this. I’m keeping the reference beam at 56 degrees to the film to eliminate woodgrain. The object light is approximately normal to the film – so the total angle is around 146 degrees. Would a lower angle give brighter results?

Looking for some advice from the experts here… (or anyone who knows how to get a bright H2 everytime…)

 
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AuthorReply
Martin
(no login)
Re: Bright H2 copies - how?March 12 2004, 6:26 AM 

Beyond the choice of recording material and its processing, you could limit the size of the area of the transmission hologram (master) to be illuminated. E.g. if you are willing to sacrifice a part of the vertical parallax, you are likely to get a significant brightness increase.
 
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Tony
(no login)
aperture restriction on H1March 12 2004, 6:54 AM 

Thanks - yes, Martin, I have tried that too. It does help some, but I still don't get the brightness of other similar setups with full aperture, and the brightest viewing angle is still not centered in the range...
 
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(no login)
Re: aperture restriction on H1March 12 2004, 7:07 AM 

I am sure I am stating the obvious but beam ratio is also very important.
 
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(Login MichaelHarrison2)
Re: Bright H2 copies - how?March 12 2004, 7:32 AM 

For reflection copies you want to get as close to a 1:1 ratio as you can. This will depend a great amount on the quality of your master.

You should also restrict the aperture as much as you can vertically (the actual amount depends on your master scene) since that will reduce the amount of "noise" and increase brightness.


---------------------------
If someone says it can't be done but they haven't tried it, don't believe them.
http://www.dragonseye.com/Holography
 
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Tony
(no login)
beam ratiosMarch 12 2004, 7:55 AM 

I've been all over the map on beam ratios, so I'm pretty sure that's not the whole explanation. One issue I run into on measuring beam ratios is as follows:

I'm measuring using a radio shack solar cell, which is about 1.25" x 0.75" in size. So I'm measuring the average intensity over that area. This is fine for the reference beam, which is nice and large and clean. But for the light from the H1, there are hot spots which are significantly smaller than this area. So to get a numerical ratio close to 1:1, the actual peak brightness in the H1 light can be brighter than the reference. When this happens, I get 'burnout' areas in the H2 which are visible as white smudges before painting black, or dark smudges which are more color shifted on viewing. It's like the old SLR's, where you want a 'spot' metering capability...
 
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(Premier Login colinholo)
Forum Owner
Re: beam ratiosMarch 12 2004, 9:30 AM 

Dinesh and Joy taught me how to set beam ratios by eye in order to avoid this. For transmision masters it was an easy skill to learn by watching. For reflections it seemed much harder. I have not tried it yet but the nextime I am making a reflection H2 I am going to replace the H2 plate with a piece of ground glass so I can observe both beams from the same side while setting the beam ratio.
 
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Tony
(no login)
right brain / left brain ratioMarch 12 2004, 9:50 AM 

After getting frustrated with electronics, I started trying to do it by eye. I put a card at the film plne; position my head over it so each eye can see one side of the card - then try to figure out which eye sees a brighter intensity. Kinda weird in the brain!
 
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Bob
(no login)
Re: beam ratiosMarch 14 2004, 10:42 AM 

Make a mask for the detector with a small hole in it to decrease its active area. When necessary, I've made a fiber optic probe for my detector so that I could make the "hot spot" beam ratio between 2 and 5, depending on the composition of the scene. You need to put a piece of flashed opal diffuser in front of the probe end so that it isn't sensitive to input angle.
 
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Dinesh
(no login)
Re: Bright H2 copies - how?March 12 2004, 12:37 PM 

"My question is if the beam angles could have something to do with it."
Certainly. In a volume hologram, such as most of us make, the hologram is pretty angle sensitive and will "wink out" as you go past the "right" angle. The factors that affect this are the Bragg plane angle and the spatial frequency, which is the sum of sins and not the sin of sums. I also suspect that although the H1 and H2 are parallel, I suspect you're object beam is not normal to the H2. There is more light hitting the H2 at an angle other than at 0 degrees maybe because your object was not at the center of your original H1. In other words, a line drawn from the center of the object scene through the H1 did not hit the H2 at zero degrees.

The angle of the Bragg planes is the bisector of the reference angle and the object angle. If the object beam is normal to the plate, ie 0 degrees, and the ref is at 56 then the Bragg planes are orientated at 28 degrees. If you view the hologram perpendicularly with a reconstruction beam coming in at 56 you should see tyhe brightest image. However, if the plate was twisted so that the reference and object beams were not "in line", you've created a shear twist on the Bragg planes. This would alter the brightest point of the H2. Also, the spatial frequency for non-zero object beams is:
lambda = d*(sin(ref) + sin(obj))
You see, it's the sum of the sins of the beams, nott the sin of the sum. The total angle has little to do with it.

Another factor may be the beam divergence. If you're not collimating the H2 ref, the expanded beam may not be symmetric. You're not hitting the H2 with a cone of light but a "twisted" cone with its axis off-center. This would also create a twist in the Bragg planes.
 
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Jonathan Head
(no login)
Re: Bright H2 copies - how?March 13 2004, 10:56 AM 

Some added thoughts to what has already been suggested. The quality of the H1 is crucial, and it's not always obvious it'll make a bright H2. I've used what looks like a good master and it simply won't produce better than a mediocre transfer, no matter what. If you have a master that gives good results check your lab notes. Maybe you used a higher than normal beam ratio when shooting the master (that can help with contrast).

Also I would say that lower beam angles may just help. I know wood grain can be a problem, but aiming exactly for Brewsters angle isn't strictly necessary (being close can be enough). The higher your angles the less efficient is the interference process, which is why closer to 45 degrees can be better than 56 or 57 degrees. (In a similar fashion although not under direct control, beam polarization affects interference efficiency too). The point is to improve the quality and contrast of the fringes, so another area to look at is vibration - basic but so very important.
 
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Tony
(no login)
H1March 13 2004, 11:06 AM 

Jonathan, thanks for the input. I think I may have one of those 'stubborn' H1's. It looks really good, but the results are not up to par. After 12 test shots, I'm going to go with the best setup and make a glass H2 and then move on. One strives for perfection, and sometimes gets lucky, sometimes not.

I'm pretty sure my setup is stable. Results are consistent, even out to 2 minute exposures. Most exposures run around 20 seconds.

Maybe on my next subject I'll investigate how far I can deviate from Brewster on the reference angle, to see if that helps much. This has been fixed in this series of experiments.
 
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Tony
(no login)
brighter - but why?March 21 2004, 8:08 AM 

Well, after a week of business in Chicago, I'm back at it, and this morning finally got a bright one on this subject.

I did try changing the reference angle to the film - Brewster, Brewster+10, Brewster-10, with no noticeable improvement. I was just about to give up and move on, when I had the idea to recreate exactly a successful setup I had used in the past. That turned out to give a much brighter result - but I'm not sure why!

The setup that didn't work as well has, in order from laser to film, a spatial filter, a variable beam splitter (40x lens), and mirrors to orient object and reference beams to H1 and H2.

The setup that did work well has the variable beam splitter in front of the spatial filter. The spatial filter is in the reference beam path. For the object beam path, having only one spatial filter, I used a DCV 6 x -6 lens to spread the object beam. The object beam wasn't perfectly clean, but was pretty good. This resulted in a somewhat smaller illumination of H1 in both dimensions, but not really much different than I had restricted the aperture on the previous setup.

I'm thinking that the explanation is that the illumination of H1, while not absolutely smaller, did concentrate the illumination more tightly in the center of H1. Sort of like an apodization function in sonar/radar.

Sad thing is I was so busy in Chicago I never got to the museum. Oh well, maybe next time!
 
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dcgman
(no login)
Re: brighter - but why?March 21 2004, 10:04 AM 

"The setup that didn't work as well has, in order from laser to film, a spatial filter, a variable beam splitter (40x lens), and mirrors to orient object and reference beams to H1 and H2.
The setup that did work well has the variable beam splitter in front of the spatial filter."

Maybe worth checking out how the variable beam splitter (BS) performs before and after the spatial filter (SF). I'm wondering if the split beams were polarised at rightangles to each other when the BS was placed after the SF- due to the range of angles incident on the BS.
 
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Bob
(no login)
Re: brighter - but why?March 21 2004, 10:43 AM 

If the variable beam splitter is one of those disks with a variable reflective coating on it, your beam ratio may be significantly different than what you think. It will vary across the plate when used in an expanded beam, making accurate power readings difficult (especially with a large area detector).
 
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Tony
(no login)
variable splitterMarch 21 2004, 12:08 PM 

No, the variable splitter is a straight one. The cool thing about a Gaussian beam profile, is that if you apply a linear function to it, it's still a Gaussian!

I've been all around the park on ratios. I think the polarization is more likely to have something to do with it.
 
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Bob
(no login)
Re: variable splitterMarch 21 2004, 1:48 PM 

How does your linear variable beam splitter work? Disk shaped variable beam splitters have a linear variation of reflectivity. Gaussian shaped beams are no longer gaussian when used with this kind of beam splitter. Rectangular shaped variable beamsplitters also have a linear reflectivity variation (usually), and so act the same way.
 
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Dinesh
(no login)
Re: brighter - but why?March 21 2004, 2:25 PM 

As I understand it, the setup that didn't work had an expanded beam hitting the beamsplitter while the one that did had a raw beam on the beamsplitter. If this is so, have you considered that an expanded, diverging beam reflecting off an optic has different angles of reflectivity across the beam? The reference beam may not have been uniform.
 
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Colin Kaminski
(no login)
Re: Re: brighter - but why?March 21 2004, 8:06 PM 

"The reference beam may not have been uniform."

I noticed when I visited you, you paid very close attention to getting a very uniform reference beam intensity. While I do this myself I did not go to the extremes you did before. Why does this make such a difference?
 
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Dinesh
(no login)
Re: Re: Re: brighter - but why?March 22 2004, 4:41 PM 

"Why does this make such a difference?"
You get non-linearities. Especially directional non-linearities. I'm in the process of writing up my theory talk at the PCG meeting and I'll explain more fully there.
 
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(Login MichaelHarrison2)
simple interference?March 22 2004, 3:07 AM 

I've read the other responses fairly quickly last night (before having to go get some stitches put in, a story for another time) so if someone's already suggested this, please pardon the dupe...

The setup that didn't work as well has, in order from laser to film, a spatial filter, a variable beam splitter (40x lens), and mirrors to orient object and reference beams to H1 and H2.
The setup that did work well has the variable beam splitter in front of the spatial filter.


I think you've mentioned that your beamsplitter is a plate splitter. Is it similar to the beamsplitter sold by Edmunds as pictured here?

(photo from Colin's site, I don't have any good close-ups of mine)

If the answer is yes, do you have additional glass epoxied to the back as shown in the picture?
If not, you're very likely going to get intereference from the from the front and back reflections.

Also, why the beamsplitter after the spatial filter? You're adding an element that will dirty the beam just after you've cleaned it up.

---------------------------
If someone says it can't be done but they haven't tried it, don't believe them.
http://www.dragonseye.com/Holography
 
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Tony
(no login)
variable splitterMarch 22 2004, 8:53 AM 

Yes, this is the type of splitter I have. However, it has a very small angle (15 arc minutes) in the short axis between front and back surfaces that prevents interference fringes.

I have mine mounted horizontally instead of vertically.

I placed it after the spatial filter as it considerably simplifies the setup, and I only have a single filter. This way, both reference and object beams get the benefit of a clean beam. Of course, if the results are dim, then who cares?

 
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(Login MichaelHarrison2)
Re: variable splitterMarch 22 2004, 9:08 AM 

Yes, this is the type of splitter I have. However, it has a very small angle (15 arc minutes) in the short axis between front and back surfaces that prevents interference fringes.

Are you sure it completely seperates the beams? Do you see two distinct dots in the reflected side when firing an unspread beam through it? Do you card off the secondary beam?

In any case you won't be able to card off the secondary beam if you're firing a sreading beam through the splitter. You're going to get interference between the two beams.

If you're keeping the beams in the same plane before and after the splitter, I seriously doubt that you've got polarization problems. I think you've just got self-interference caused by the beam splitter.

I have mine mounted horizontally instead of vertically.

Is that the way the splitter is intended to be used? Is the axis of your wedge aligned with the plane of reflection?

I placed it after the spatial filter as it considerably simplifies the setup, and I only have a single filter. This way, both reference and object beams get the benefit of a clean beam. Of course, if the results are dim, then who cares?

Exactly.

---------------------------
If someone says it can't be done but they haven't tried it, don't believe them.
http://www.dragonseye.com/Holography
 
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(no login)
Re:brighter - but why?March 22 2004, 5:07 AM 

Sound like polarization. Can you take a polarizer, even cheap polarizing sun glasses and put them in the expanded beams just at the plate holder. Place a white card in the plate holder. Block the object beam, put the glasses in the reference beam just before the white card (about 2 or 3 inches away) and rotate the glasses until the most light shines through the glasses and hits the white card and record the orientation of the glasses. Now open the object beam and block the reference. Rotate the glasses until the most light shines through to the white card. Do this for both set-ups and record the differences. Let us know what you find.

Are all you optics equal distance from the table and are you using side lighting? Would/could you draw your two configurations?

 
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Tony
(no login)
polarizationMarch 22 2004, 8:56 AM 

This sounds like a good test to look at. I'll try this tonight. Not sure how accurately I'll be able to measure the polarization, but I'll give it a try. I might have a camera-style polarizing filter around I can use.
 
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(no login)
Re: polarization (Tony)March 26 2004, 4:29 AM 

Tony, have you tried to view your polarization? What did you find?

Can you post your two configurations? A simple drawling in paint will do. It does not have to be to scale. I would like to see the actual geometry of the two set-ups.

Peace!
John
 
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Tony
(Login barockteer)
Setups - but can't upload the files!!!March 26 2004, 5:58 AM 

I haven't had a chance to do the polarization experiment, mostly because I doubted that was the problem. Everything is in the vertical plane, so I don't see how anything could rotate the polarization of one beam relative to the other. That would be a good explanation for the brightness difference, but I don't see how it could be occuring in these setups.

I can't seem to upload my paint drawings. They are .jpg files, only 23K each. I am logged in. I hit the Insert Object button and get the applet. Then I browse to the file and click upload, but I see nothing appear in the temporary files. help...
 
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(Login MichaelHarrison2)
Re: Setups - but can't upload the files!!!March 26 2004, 6:04 AM 

If you'll send the files to me I'll put them on dragonseye and send you the links to post.
That'll have the added benefit that they won't disappear in two weeks.


 
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(Login MichaelHarrison2)
interference?March 26 2004, 6:07 AM 

what about...
http://www.network54.com/Forum/message?forumid=205174&messageid=1079975292
 
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(no login)
Rre:Setups - but can't upload the files!!!March 26 2004, 6:25 AM 

Jpeg is the format you want to use and the file size is ok.
Click on Insert Object.
Click on Temporary files in the left hand pane.
Click on Upload at the beneath the left hand pane.
Click on Browse in the left hand pane.
Drill to the file you want and double click it.
Then click on the Upload button IN the left hand pane NOT the Upload button at the bottom of the left hand pane.
Then click on insert link below the right hand pane.
 
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Bob
(no login)
Re: Bright H2 copies - how?March 21 2004, 1:59 PM 

Well, I just re-read your first post and the problem sounds more like plateholder motion. Sounds like you have a big fat fringe pattern; sometimes centered, sometimes not. I'd recommend rebuilding the plateholder, or looking for a source of stress in it.
 
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Tony
(no login)
film holderMarch 21 2004, 6:51 PM 

Nope, I doubt it's the film holder. Too many shots come out the same for it to be moving, then not moving when I change the setup and get a bright one. Much more likely the shape of the H1 beam or the position of the beam splitter.

The beam splitter is like these:

http://www.newport.com/store/xq/ASP/lone.Optics/ltwo.Filters+%2526+Attenuators/lthree.Linear+Variable+Metallic+Neutral+Density+Filters/lfour./id.3569/lang./qx/product.htm


 
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