Beam Ratios

Jem · Post by **Jem** » Fri Oct 09, 2009 4:16 pm

Hi folks

So, now I have my small optical table set up i'm becoming more adventurous and moving onto simple split beam work. So far i've made a couple of transmission holo's that appeared to turn out fine (ish) and an assisted Denisyuk. The Assisted Denisyuk was a nice clean image but wasn't particularly bright and I would like to get it better, same with the transmission holo's.

I think i'm a little confused over beam ratios. Am I right in thinking that for a split beam white light reflection hologram where the object is behind the plate it is advantagous to have the object beam brighter than the reference beam (i've used kicker mirrors, but these don't seem to make that much difference, so i'm going to try a split beam assisted Denisyuk where the object is illuminated directly with two expanded beams, whilst the reference beam hits the plate at approx 50 degrees from my collimating mirror). Whilst with a transmission hologram it is better to have the reference beam brighter than the object beam?

Maybe i'm a little confused, but that's how it appears to be in the reference books and i'm not convinced i've got the grasp of it correctly. Could anyone please give me a simple to understand guide as to what ratios I should be looking at for a reflection hologram (assisted Denisyuk type) and for a transmission hologram.

I'm using Colour holographic BB520 plates.

Many thanks

Jem

Joe Farina · Post by **Joe Farina** » Fri Oct 09, 2009 5:14 pm

It seems that a 1:1 ratio is usually recommended for split beam reflection holograms, and that's what I would suggest.

In the Holography Handbook (1982) by Unterseher et al. (which might be described as the "practical holographer's bible" in the US in the 1980's) they state that: "Split beam holography is considered real holography." I definitely agree. Objects just don't look very good when illuminated with a cone of undiffused light coming from the front (i.e., unassisted Denisyuk).

BobH · Post by **BobH** » Sat Oct 10, 2009 12:45 am

I can't think of any time I'd go with more object light hitting the plate than reference light. HOEs are typically made with a BR of 1. As a diffuse object gets bigger than a point source, though, the beam ratio for the least amount of intermodulation noise starts going up (more reference light).

Why not set up a nicely diffusing object and do a beam ratio bracket, keeping the total power at the plate constant? Do BR=1, 2, 4, 6, 10, and 20. Do that for an assisted Denisyuk, a transmission hologram, an image-planed reflection transfer, and a Rainbow hologram and you'll know everything there is to know about beam ratios in 24 shots!

Actually, I think beam ratio brackets are just as important for a new image as exposure brackets. Exposure brackets make the material work best, BR brackets optimize the image.

Ed Wesly · Post by **Ed Wesly** » Mon Oct 12, 2009 2:44 pm

Couldn't agree more with Bob on that exercise above! I did that myself a million years ago, comparing the then recently introduced Pyrochrome developer versus the old develop-fix-rehalogenate bleach method of processing transmission holograms with the same BR's and exposure series, and found you could go with lower (more object light ratios) and get brighter holos with the Pyrochrome process!

For those who are seat of the pants-ers, what you are trying to avoid in intermodulation noise is like a halo around the object. If you get that, then raise the ratio (more reference beam).

The beam balance ratio may need to be tweaked depending on the object, but once you see the effect you will know how to proceed!

I am curious to try the effect of the ratio on reflection holograms, as the intermodulation noise caused by the self-interference of all the object points with each other creates a transmission grating, which might rob efficiency from the reflection fringes, but whether or not it makes the halo around the object remains to be seen.

Jem · Post by **Jem** » Mon Oct 12, 2009 8:41 pm

Thanks Joe, Bob & Ed

Great explanations and suggestions as always

. I don't know what I would do without the expert guidance from yourselves (& others).

I love this forum and I love making holograms, just wish I had more time (and money

)

Cheers

Jem

BobH · Post by **BobH** » Wed Oct 14, 2009 2:20 pm

Ed Wesly wrote:proceed!

I am curious to try the effect of the ratio on reflection holograms, as the intermodulation noise caused by the self-interference of all the object points with each other creates a transmission grating, which might rob efficiency from the reflection fringes, but whether or not it makes the halo around the object remains to be seen.

It certainly does make a halo. More of a glow around the image, in the space of the image. The low frequency intermodulation noise can be greatly reduced in a transmission hologram by index-matching it, either with adhesive or oil, to a cover plate. Thw fringes that create it show up as a surface modulation, and that component goes away with index-matching. That also allows a lower beam ratio, and therefore a bit more diffraction efficiency.

Another related effect is the dim ghost image that can show up between the image and the bottom of the hologram (for a top lit hologram), caused by over-modulation. It looks like a chunk of glowing mess if the imgae is large and bright, but if the image is a small thing floating in black space it can take the shape of the object. That also indicates a higher beam ratio, or moving the object a bit farther away from the master hologram.

Colin Kaminski · Post by **Colin Kaminski** » Wed Oct 21, 2009 10:49 pm

It takes some time to get the skill but when I set beam ratios I take my fingers and wiggle them in front of the object and look at the shadows on a white card placed in the plate holder. I make sure that the shadows are visible but dim. It has worked better for me than a meter at finding hot spots. I saw Dinesh doing this in his lab once and copied him. It took 20 or so exposures to figure it out but now it feels more natural than making a measurement.

John Klayer · Post by **John Klayer** » Wed Oct 21, 2009 11:02 pm

Visible but dim? Your fingers?

Colin Kaminski · Post by **Colin Kaminski** » Thu Oct 22, 2009 11:11 pm

It is a vague description but it does work. It takes a few tries...

Tom B. · Post by **Tom B.** » Sat Oct 24, 2009 6:54 am

I find that sometimes overexposure on hot spots can create quite magical effects, such as ghostly trails, colors in an otherwise monochromatic image etc. Also - the best exposure for the image is not the average best, or the worst-case (hot-spot) best, but the one the best shows what you want the viewer to see. If the essential texture is kind of dim, emphasize that and let the hot spots take care of themselves - unless you get complete burnout in which case you must of course back off - unless you consider burnout to be a magical effect, too, which it might be in some situations.