BEAM RATIOS

MichaelH · Post by **MichaelH** » Wed Jun 16, 2004 10:42 pm

I have to say I rarely measured the beams but did it by eye.

Doing it that way, all you can say is that the ratio looked "about right".

Colin Kaminski · Post by **Colin Kaminski** » Thu Jun 17, 2004 1:20 am

Here is some more stuff from Dinesh from the old forum...

Q Factor August 13 2003, 8:43 AM

The problem is that this Q factor was designed in late sixties and early seventies when there was no Kogelnik and the major theories of hologram efficencies were on the basis of MTF. Also the major theories of hologram formation was still either a 'pure' Bragg grating in the manner of X-Ray diffraction or of diffraction screens. Even back then observational evidence seemed to show some anamolies with this Q in Lithium Niobate and DCG, where the Q should have shown volume diffraction but observation showed Raman-Nath diffraction:
Alferness R. "Analysis of Propagation of Second Order Bragg Angle of a Thick Holographic Grating." J. Opt Soc, vol 66, pp353-62, 1976
Magnusson R. & Gaylord T.K. "Analysis of Multiwave Diffraction of Thick Gratings"" J. Opt Soc, vol 67, pp 1165-70, 1977
The problem is that Kogelnik assumed a pure sinusoidal grating with slow energy transfer, infinite plane wavefronts and only the propagation of Bragg diffracted waves inside the grating. The existance of higher order waves found by Alferness (1976) and Magnusson and Gaylord (1977) shows Raman Nath diffraction in thick holograms for index modulation, dn >=0.005. (I've heard several people - some quite distinguished - claim they've 'disproved' Kogelnik like these 'Relativity is wrong and I can prove it!' types. They never look at the assumptions of the Kogelnik theory.)
Detailed criteria for 'thin' and 'thick' by:
Moharam M. G. and Young L. ""Criterion for Bragg and Raman Nath Diffraction Regimes" Applied optics vol 17, pp1757-9, 1978
Benlarbi B. Cooke D. J. and Solymar L. "Higher Order Modes in Thick Phase Gratings" Optica Acta, vol 27, pp885-95, 1980
show that for low index modulation, the assumptions of Kogelnik are valid and Q is valid, however as the index modulation rises and and/or non-linear effects come in and the gratings need to be described as a sum of higher order Fourier terms, index modulation becomes important. Nath introduced another parameter for these situations:
P = (L^2)/(d^2*n*dn)
L = shooting wavelength
d = fringe distance
n = bulk index
dn = index modulation
Formulae for thick holograms can be used for P>>1 and relative power diffracted into higher order modes are proportional to P^(-2).

Kaveh · Post by **Kaveh** » Thu Jun 17, 2004 3:28 am

Dinesh wrote:
Hmm. Maybe my memory is finally fading.
You were the one who first told me this. Remeber Adrian Lines coming to shoot "Mirror Man" at Icon? he insisted on 4:1 and you got mad in that little basement office and kept saying how Adrian didn't understand that once he bleached it didn't <i> have</i> to be 4:1 anymore. It's a phase hologram now. Neither I nor John Drinkwater understood this and you kept repeating, "It's all in Kogelnik! Read Kogelnik!" I didn't know what in h*ll a "Kogelnik" was, but it sounded important!

This is frightening. A mixture of pain and pleasure is coming back into my head! What days we had...

javamiggs · Post by **javamiggs** » Thu Jun 17, 2004 8:26 am

Hi. This new format is great.

I have done a few reflection holos with fair results (thank you guys for answering my queries on the previous forum) and im now working to make my first transmission hologram.

When someone says 4:1 (obj:ref) beam ratio, is it the ratio before any of the split beam reached the object or plate? What is the ideal ratio between the reference beam and the beam reflected from the object?

Martin · Post by **Martin** » Thu Jun 17, 2004 11:43 am

Kaveh wrote:
Bruce wrote:Can some kindly soul tell me again what the beam ratios are for Slavicz VRP used as a trans master? Seeme to me it was 4:1 ref.- obj.
First of all, I don't see why beam ratio should be dependent on the particular plates you use.

4:1 seems quite low to me, as you will get intermodulation noise (due to different parts of the object interfering with each other), leading to a halo around the image. I seem to remember using around 10:1. But it all depends on the object.

Have a look at http://perso.wanadoo.fr/holographie/GB/ ... nglais.htm
Gentet points out:

"As these emulsions have been specially developed for Denisyuk Holography, this means that even low contrast fringes during your recording will result in a very high efficiency hologram.

This is important to note when making reflection hologram. Due to the great dynamic range of the material, a low beam ratio, often needed in some concurent low contrast materials, say 2:1, will produce very bright but over-modulated images. We recommend a beam ratio of 10:1 for reflection holography and as high as 30:1 for transmission holography for the best bightness and image contrast."

So in short, the beam ratio might relate to the index modulation of the recording material.

Dinesh · Post by **Dinesh** » Thu Jun 17, 2004 2:01 pm

[quote:8f9a9a7f3b]When someone says 4:1 (obj:ref) beam ratio, is it the ratio before any of the split beam reached the object or plate? What is the ideal ratio between the reference beam and the beam reflected from the object?[/quote:8f9a9a7f3b]
4:1 refers to the ratio between reference beam and object beam at the plate. ie the reference needs to be 4 times as bright as the reflected object beam at the plate. Since the object light will vary at different parts of the plate because it reflects differently, it'll have bright points and dim points. The difference between these is called the dynamic range of the object. You want to get a ratio of 4:1 using a midrange of brightness from the object at about the center of the plate.
This 4:1 is a sort of mantra that holographers use for H1s. It's a reasonable working starting point, but it depends on the film and the developer you use, as Martin pointed out. If you go too high, say 2:1, you'll get noise and a halo will appear around the object, as Kaveh pointed out. If you go low, you'll get a dim image, as Michael pointed out.
Like everything else in holography, you've got to play with it!

MichaelH · Post by **MichaelH** » Thu Jun 17, 2004 3:17 pm

At every point in holography where someone says "do this", keep in mind that there's a huge range within which you'll get a hologram.

Start off with the recommendations from the people who've done it and then play. You may find that you get better or more desirable results by changing a few variables. It's all highly subjective anyway unless you're making a HOE.

Guest · Post by **Guest** » Thu Jun 17, 2004 4:15 pm

MichaelH wrote:At every point in holography where someone says "do this", keep in mind that there's a huge range within which you'll get a hologram.

Start off with the recommendations from the people who've done it and then play. You may find that you get better or more desirable results by changing a few variables. It's all highly subjective anyway unless you're making a HOE.

Thanks, Michael. This is an EXTREMELY valuable statment, especially for people who have made zero to a couple of holograms with anything at all on them!

Colin, maybe you can think about using a piece of that statment at the top of the General Holography section?

Kaveh · Post by **Kaveh** » Sat Jun 19, 2004 6:56 am

Martin wrote:So in short, the beam ratio might relate to the index modulation of the recording material.

This makes sense. The discussion is broadened a bit because the original posting related specifically to transmission masters. In reflection you don't have intermodulation noise around the image.

Going back to transmission, there is always a compromise when choosing beam ratio. There are different types of 'noise' in your hologram. Grain noise, i.e. scattering, and intermodulation are the two that come to mind. I think there are other sources but I can't think of them off hand.

Grain noise is generally dependent on the optical density of the hologram before bleaching. Simplistically, the more grains have been converted to silver, the more they will scatter light during reconstruction. To counteract this noise, you want your image to be as bright as possible in comparison the background. This means, in general, a lower beam ratio, which gives you a higher index modulation and therefore more diffraction. But too low a beam ratio and you start hitting intermodulation noise.

What Gentet is saying is that the index modulation is so high in his plates, you can use very high beam ratios and still get a bright image, therefore not getting any intermodulation.

Martin · Post by **Martin** » Sat Jun 19, 2004 10:50 am

Kaveh wrote: What Gentet is saying is that the index modulation is so high in his plates, you can use very high beam ratios and still get a bright image, therefore not getting any intermodulation.

I am wondering about the effect of grain size: assuming you have a very fine grain emulsion, say "Ultimate Ultra Fine" (which is said to have 8nm grain size). What happened if you were able to process that emulsion in a way you would get even smaller grains?
Based on what I saw on PFG-03, certain physical/colloidal developers do actually yield smaller grains upon processing. The trend then may go in a direction where the emulsion resembles more and more a pure (transparent) "phase" layer - since absorption is decreasing.