BEAM RATIOS

Bruce · Post by **Bruce** » Tue Jun 15, 2004 12:59 am

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.
I think I have C.R.S. disease!
Much Thanx in advance
Bruce

MichaelH · Post by **MichaelH** » Tue Jun 15, 2004 7:25 pm

I believe it all depends on your application but 4:1 should get you in the ballpark for display work.

Dinesh · Post by **Dinesh** » Tue Jun 15, 2004 9:33 pm

Ideally it should be 1:1 if you're bleaching. The problem is which part of the object do you choose for the "object 1"? If you choose the brightest part, the dimmer parts are less and vice versa. Practically, 4:1 works fine based on roughly the middle of your dynamic range if your dynamic range is not too great.

Bruce · Post by **Bruce** » Tue Jun 15, 2004 10:18 pm

"Ideally it should be 1:1 if you're bleaching"

If I remember correctly, bleaching tends to bring out noise, so ideally, a reasonably bright, un-bleached image is prob. best? Expose and develop VRP to about ND 2 or so, or would you go further to say , about 0.6?

Also Dinesh.... Are carrots fattening? Even if you eat bushels of them daily? LOL

Bruce

Dinesh · Post by **Dinesh** » Wed Jun 16, 2004 2:17 pm

[quote:8a1259ef5c]
If I remember correctly, bleaching tends to bring out noise, so ideally, a reasonably bright, un-bleached image is prob. best? Expose and develop VRP to about ND 2 or so, or would you go further to say , about 0.6?[/quote:8a1259ef5c]
In the early days they didn't bleach because of the noise issue. If you don't bleach, you can get very clean H1s but they'd be bloody dim! Usually about 5% or so. These days bleach formula are not as noisy. I've got about 70% with VRP using CWC2/PBQ with very little noise by taking it to about 2. I don't actually measure the density, I just look at the safelight until it looks grey-ish red rather than red in the dark room (rods are color-insensitive. If the light is low enough in a dark room, it'll eventually look grey)

[quote:8a1259ef5c]Also Dinesh.... Are carrots fattening? Even if you eat bushels of them daily? LOL [/quote:8a1259ef5c]
When I tried this, I never got fat but I never quit either. In them thar days, it was possible to smoke in your office and I got my best inspiration with feet on desk and smoke curling up to the ceiling while staring at a whole mess of spectrophotometer plots I'd hung up on the wall. You do get a tendency to go ,"heh! heh! What's up Doc!"

Kaveh · Post by **Kaveh** » Wed Jun 16, 2004 8:06 pm

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.

MichaelH · Post by **MichaelH** » Wed Jun 16, 2004 8:21 pm

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.

I've quite regularly used ratios of 2:1 to 4:1 and never seen any kind of halo. The only time I've gotten what you describe was one time when I didn't bother to measure the ratios and I'm pretty sure they were more like 1:2.

Also, for most display work you want to keep the contrast low. If you've got a subject with some really strong specular reflection and the rest being matte, I could see going as high as 10:1 but you'd end up with a image that's pretty dim overall.

Dinesh · Post by **Dinesh** » Wed Jun 16, 2004 8:40 pm

The reason for the "4:1" originally was due to the H&D curve (Actually, strictly speaking, holographers used a D vs E as opposed to D vs Log E). The justification was to use the straight line portion of the curve so that no non-linearities and distortions occured in the image. You'd set the exposure at the center of the H&D curve and then set k so that the extremes of delta E would not take you into the toe or neck of the curve. The curve decides the gamma of the particular film (it's the slope of the curve), which in turn depends on the grain size and grain distribution. The 4:1 figure was based specifically on the 10E75 and 8E75, since early work centered around HeNe. Today, Slavich and BB plates have a much smaller grain size and very different grain distributions so the gamma is higher and the curve steeper. As the steepness of the curve (the gamma) increases, the allowable variation delta E decreases. This is true so long as you don't bleach. Once you bleach, you can throw the H & D curve out the window 'cos there is no density and so you can't plot d vs anything. Bleaching converts the amplitude variation into a phase variation for which the ideal ratio is 1:1. Effectively, Kogelnik now kicks in. So if you were making just a grating with two planes waves they'd be at 1:1. However, a real extended object has a dynamic range and in order to take this dynamic range into account, you'd the range to some lower ratio so as to compensate for the variations in intensity. This compensation would have to take account of intermod noise as well as other factors (polarisation etc). Intermodulation noise becomes important as the ratio of the object to plate increases and is also dependant on the variations of reflectivity of the object. For instance, a clay jug full of metal coins about 7 in in diameter shot about 2 in from an 8x10 plate would probably badly suffer from intermod. Take the same object and move it to 10 in from the plate and intermod decreases. As a rough starting point, I'd aim for 4:1, but I'd alter it depending on factors like reflectivity of object, ratio of object extension to plate size, mixture of materials in the object scene etc.
The 10:1 is a favourite for Resist holographers because masters for mass production need it to be super clean. However we were shooting 4:1 at ABNH with no problem (look at your Visa or Mastercard card!). Mainly, I suspect, because the object was so small relative to the plate.

Kaveh · Post by **Kaveh** » Wed Jun 16, 2004 9:20 pm

Hmm. Maybe my memory is finally fading.

I have to say I rarely measured the beams but did it by eye. Perhaps I just assumed it was 10:1 and it was less. I would say if there is not intermodulation, then there is not harm in a low ratio.

Dinesh · Post by **Dinesh** » Wed Jun 16, 2004 9:50 pm

[quote:05a696fdac]Hmm. Maybe my memory is finally fading.[/quote:05a696fdac]
Yeah, old man!
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 have 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! The next day both Drinwater and I were buried deep in Kogelnik!