Advice please on shooting a trans holo at tip angle

Justin W · Post by **Justin W** » Sat Jul 04, 2009 8:38 pm

Hello all!

Is there just something I don't know about making a hologram with the object off the normal to the film?

My goal is a transmission hologram shot with the object 35degrees off the normal of the film. This, according to the Good Book, is the proper angle for making a master that can be transferred to a rainbow H2 in multiple channels where all the colors will be in correct registration (for an H2 that will be burned and reconstructed at 45 deg).

I can't get my dumb @ss past the first step of this procedure - making the master. Nothing but the center of the object shows (and dim at that). The center part that does appear in the holo is of course the hottest portion of my object beam, so initially I assumed that my beam ratios were just too high for this shot, but then lowering them dramatically did not change anything...

It stands to reason that my reflected object light will be thinned out and weakened some as it covers a larger area of the film than it would on the normal - in fact, if my math is correct, my object light at the film is reduced by a factor of 1.75. I check my light levels with a LaserCheck, which needs to be pointed straight at the source of the light, so I've been figuring my beam ratios from checking my object light at the film, then dividing that number by 1.75 to compensate for the angle. I'm shooting on VRP-M, and have kept my ratios during my attempts in the area of between 4:1 and 6:1, which usually works swell for me.

Am I just attempting something that's never going to work? With my object beam and reference beam arriving at the film 90 degrees to each other, I should be creating 45 degree fringes, yes? Please glance at my geometry and tell me if you see an obvious reason that this has not been working. I feel in my bones that there is just a fundamental flaw in my setup that I am overlooking.

Tom B. · Post by **Tom B.** » Sat Jul 04, 2009 9:45 pm

Since the fringes are slanted, emulsion shrinkage during processing could cause problems. The Good Book (3rd ed., p 201) suggests checking that the brightest replay angle is as expected the same as the reference angle and to try swelling the film with 5-10% sorbitol if not.

Justin W · Post by **Justin W** » Sat Jul 04, 2009 10:24 pm

The replay angle seems OK - I've turned the film this way and that and the meager images I'm getting are brightest at the angle it was all burned at...

BobH · Post by **BobH** » Sun Jul 05, 2009 12:29 am

I should point out that in your drawing, you're showing 35 degrees measured to the film plate, not to the normal of the film plate.

And with the geometry shown, the fringes will essentially be perpendicular to the film plate so I wouldn't worry too much about film shrinkage.

I'd be using an s-polarized reference beam with a black glass backer plate index-matched to the film plate in the plateholder. That gives the best fringe visibility. But even with a p-polarized reference beam, you should be OK from a geometry perspective. I'd be looking at how the image looks to the plate through a polarizer, oriented to match your reference beam. Look at it, through the polarizer, from all four corners of the film plateholder. Maybe the image is really dim in the polarization that counts, despite it looking bright to your eyes. And lighting a generally round object with diverging beams is going to give you hot spots in the center of the object, one for each beam. Especially rqapidly diverging beams like in the drawing! :doh: :naughty:
Then I'd focus on air currents and bracing or stiffening the mounts. That's where the problem probably is.

Justin W · Post by **Justin W** » Sun Jul 05, 2009 2:12 am

Hi

OK, yes, I misspoke; the plate angle is 35 deg off the intended H2 plane I guess.

The drawing was a quickie to show the basics of what I'm doing... The actual object is much larger and more oddly-shaped than what I showed there. The object beams are also probably (definitely) not so quickly diverging - I just wanted to show the basic orientation of the beams involved.

I reckon I should go back to my DIY antihalation that I've been using before, which consists of index matching my film to a piece of glass with the back painted black. And yes, sounds like good advice, taking measures to increase rigidity of my various components!

JohnFP · Post by **JohnFP** » Tue Jul 07, 2009 5:34 pm

You're not running out of coherence length are you?

Jeffrey · Post by **Jeffrey** » Tue Jul 07, 2009 6:05 pm

This geometry should not be your problem, it IS preferable for a rainbow master, and the fringes are perp to the plate, so shrinkage means little.
Either vibration, coherence, or polarization. A large object can move, too, maybe you are detecting some ambient vibrations you haven't encountered before. Ratio of even 10:1 should be fine. IMHO

Ed Wesly · Post by **Ed Wesly** » Tue Jul 07, 2009 7:04 pm

Here is my experience with an extremely deluxe and perfected three-color slit fully registered system. I cannot take credit for this set up; it is due to Professor Craig Newswanger, one of holography’s greatest inventor/engineers. This system was the fruit of his years and years of making big and small stereograms for meter square displays (viz. Mark Diamond’s Dizzy Gillespie and I think he did the big Michael Jackson amongst others) and embossed masters.

The “object” in the incarnation of the set up I had the good fortune to work with was a TI DMD device projecting an image onto a back projection screen instead of motion picture film, so this was a stereogram system that was capable of doing true color via separations with a 458 nm beam. Either totally synthetic imagery alone or in combination with live action captured with a single camera on a tracking rail could be done, with decent flesh tones!

Like the diagram above, the H1 plate was not parallel to the projection screen but tipped at the alpha angle or Benton angle as it is sometimes referred to. What its actual measure was, I don’t recall, but it could be calculated, and it was certainly in the ballpark of 35 degrees above, if not the same.

But unlike the diagram above, the reference impinged upon the master along its normal! Which goes counter to the old philosophy of splitting the angle between the reference and object to get the fringes to stand up straight to not be deformed by processing shrinkage.

Processing of Agfa Millimask plates to make these masters was of the develop-rehalogenation bleach regimes to combat shrinkage. I guess that Craig’s prior experience taught him to use the normal incidence for some reason, my conjecture is that you can always find the normal by looking at back reflections. In electronics you can always find ground, and that is why some circuits use a trigger to ground to turn something on.

Also this relaxed the resolution and stability requirements a bit, as the intra-beam angle is not the extreme 90 degrees in your set up (where the fringes are on wavelength apart) but about half that much of an angle.

The replay of flushing out the real image of this H1 to make the copy was extremely critical. The holder for the master had swivel and tilt adjustments so that we could examine the real image on a screen at the exact same plane of the holographic plate with a magnifying loupe to see if all three colors were in register. We could reliably get white parts of images with no color fringes around the edges. It wasn’t always a struggle, either! Sometimes you could just plop the plate into position, and voila, no problems!
The real image of the H1 was focused onto the H2 plate, and its reference beam was around 45 degrees but it had to be calculated to be compatible with the tip angle of the master, as they work as a team. The first order approximation of the tip angle says that the plate “points” at the replay reference angle, see the Benton and McGrew articles in the 1982 ISDH for details for example. (I will e-mail anyone a pdf of same if you PM me here.)

458 nm was used throughout as this transfer was in photo-resist. This created an interesting alignment of the master plate with respect to the center of the object/image from the project, with the blue slit being on the top third of the plate and looking straight on to the object. The green slit was lower, and looking ever so slightly upward to the screen, and the red at the bottom of the plate even more so. What a weird concept, but it works!

There were 3 slits arranged about one inch wide with their centers about 2 inches apart. The reference beam covered the whole plate, and a mask allowed only one color’s slit to be exposed at a time. On replay the whole master was illuminated with a mask of three slits so that all colors were exposed simultaneously. This sounds like a waste of light since only half of the plate’s surface area was utilized, but this prevented a whole lot of monkey motion trying to move a beam through three slits sequentially. Surprisingly this arrangement hardly ever required any tweaking for color correction! And what we would do to fix it would be to only move the Gaussian bright spot to the weak slit.

I forget the exact distance of the H1 from the object, (I never wrote this stuff down, much to my chagrin, as I don’t work there any more, but I am confident that I could recompute and reconstruct the set up for my own profit and gain, if I feel the urge or the incentive of a consulting check in my hand!) but it was around an arm’s length, as the final holograms were meant to be viewed at an arm’s length away, and you want the position of the real images of the slits of the master to focus on the viewer’s eyes. A bit bigger distance than what you might be used to!

Collimation in both steps was critical. Big telescope mirrors were used with the spatially filtered beam coming in at the shallowest angle possible to minimize off-axis aberrations.
You may have seen the fruits of this system if you have Microsoft Software CD’s that have the holographic pattern on the surface, with the dropped in Windows flag, or some Intel chip packages with security holograms, or the Beanie Baby labels with the holograms on it. Pretty amazing state of the art expertise! Hats off to Craig!
Whew! So don’t let this intimidate you, Craig had lots of dough to get this thing working, just play with this stuff, get the knack of it, this is how Sam and Dan and Rudie did all their amazing pieces in the Golden Age!

Coming down to earth, I think your dimness problem is due to the old killers of holograms, vibrations and maybe even coherence as noted above, and not the geometry.

Justin W · Post by **Justin W** » Wed Jul 08, 2009 5:28 am

Thanks guys for getting back to me on this one, and apologies for not keeping up with it.

I'm sitting here right now letting a piece of test film settle for the transfer. Yep, I finally got the master and indeed my issue was vibration; it's a larg-ish heavy resin statue that I hadn't mounted with anything approaching a lick of common sense. Some seat-of-the-pants interferometer testing showed me my instability and some additional bracing got me a pair of images I am about to attempt to blend together with good registration. I'm not holding my breath. The masters are a bit dim (underexposed because I was overestimating speed increase from a prewash), and moving my replay stripe from one to another sure does give the appearance of jogging the real image from one spot to another right along with it...

Ed - it's surely gonna take a while to process your post. I'll keep crunchin' it down until I understand. Thank you for stopping by with a method that's proven to work! I hope to "get it" soon.

Justin W · Post by **Justin W** » Wed Jul 08, 2009 10:42 pm

Well I'm back with an update per my test shot.
The generic tip angle seemed to have worked for the most part, but not perfectly; Both channels will replay simultaneously but not in perfect registration.
I've had so damn many complications with this project - and ironically enough, it turns out much of my trouble with mastering has been due to using developer that is just done. Straight done. Cooked. Finished. Slow and reluctant.
Which accounts for dimness even after stabilizing this blasted rig.
So with a cornucopia of lessons under my belt I will re-master the image...

After digesting Ed's post for an evening, I think I grok that some mobility with my master(s) would be a good thing for registration. I'll have to address this later. For now I'm seeing what I can do with a straight "Benton Angle" master. Maybe this time I'll shoot for 41degrees so I can burn my h2 with a 60degree reference angle.

It all almost worked. Close enough to be tantalizing, especially for a greenhorn like myself.