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Thank you John. That's quite true about the distance from the object to plate having a large effect with SBR's in DCG. It may be even more true with MBDCG, since the dye is absorbing a lot of light. By the way, thanks for your research with sodium metabisulfite and aluminum sulfate. This combination works really well as a post-exposure reducing/hardening bath for MBDCG.

One thing I found out about DCG and I believe it may pertain to MBdcg is the distance the object is from the plate in a SBR. Here is the "object" I made for using with test stripping. As you can see, there are differences in the same strip (exposure time) relative to distances from plate.

Basically think of it as, the further away from the plate the less object beam which yields a greater difference in light intensities (obj to ref). I had 3 set of Cover Masks. Used 1 2 3 then reversed 2 for 4 and 1 for 5 to have 5 different exposures.
Oh, the blocks were found in a dollar store being sold as a girls necklace. Nice and white cubes. $1 total cost of cubes. The dividers where black foam board. Plated lated right on it.

Thanks for putting that into layman's terms, Dinesh.

I can't really answer the question, because I've done only a few tests with varied exposures. On one plate, an extra two minutes exposure caused a noticeable decrease in efficiency, with that particular kind of development. (It wasn't a large decrease, but noticeable.) I realize that exposure and development are intertwined, so to speak. That is, more development can "correct" an underexposed plate, kind of like silver halide, I suppose.

With my lab conditions at least, it seems that exposure time is the easiest parameter to adjust. The development could also be varied, of course, but it doesn't seem so convenient.

[quote="Joe Farina"]his works very well. I've found that exposure time is quite important for MBDCG. I used to think that because it was so slow (a typical exposure with HeNe for a 4 X 5 holo would be ~20 minutes), a couple minutes here or there wouldn't matter. But I used the shutter described above, to do a series of exposures varied by two minutes. And it turns out that two minutes over-exposure decreases the diffraction efficiency noticeably.[/quote]

Sorry, Joe, I thought you had some mathematics.

Let me explain, you state that a small change in exposure causes a large and negative change in efficiency. However, think of amplifiers; if you run them in the linear region, then a small change in volume does not cause a large change in output, ie does not cause distortion. This is similar to a hologram, since developing a hologram is simply amplifying it's characteristics; essentially, the hologram acts like a tuning and amplifying circuit for light. So, to cause a small change in efficiency for a small change in exposure, you need to find the point on the curve where the rate of change of efficiency as a result of rate of change of exposure is low, or zero. In other words, where on the curve is the point where a small shift in the "x" axis (exposure) does not cause a large change in the "y" axis (the efficiency) over a limited range over which you're varying the exposure.. Of course, the amplifier analogy is limited and cannot be adopted willy nilly in all circumstances.

Ah, I see. Your not stopping the exposure during the move of the curtain. As long as there's no diffraction problems coming off your mask your method is fine.

Sorry I didn't get it the first time.

Jeff, let me phrase this another way. I have two shutters, one for the main beam, and the other is the "curtain." I open the main shutter (the plate is still in darkness at that time). Then I gradually raise the curtain (in 2 minute increments for example), so that the bottom edge of the curtain cuts across the plate at various points for each of the 5 sections (I have been working with 5 increments lately). There are no multiple exposures, each area of the plate sees a single continuous exposure with no interruptions. It works extremely well, no banding on the image whatsoever.

Joe, maybe I'm confused but it seems like you are doing multi exposures. Your exposing each area more then once.

If I understand this correctly your exposing a bit on one side of the plate, top or bottom, then moving the curtain so the next area is exposed, but the first area is now getting more exposure, so on and so on.

When you finally expose the whole plate during the last exposure the original area has now been hit 4-5 times.

Try cutting a single hole in your curtain so each area is only getting hit once. Two 30 second exposures is not the same as a single 60 second one. Even if it's the same master and ref angle.

Sorry if I'm misunderstanding this.

Dinesh, i am sorry, but i can't even come close to answering that. Please remember that my sicence and & math education ended in the 8th grade ;)

[quote="Joe Farina"]This works very well. I've found that exposure time is quite important for MBDCG. I used to think that because it was so slow (a typical exposure with HeNe for a 4 X 5 holo would be ~20 minutes), a couple minutes here or there wouldn't matter. But I used the shutter described above, to do a series of exposures varied by two minutes. And it turns out that two minutes over-exposure decreases the diffraction efficiency noticeably.
[/quote]
Joe, you've posted eta - time curves from a number of sources. Now consider d(eta)/dt. Where is d(eta)/dt almost flat?

Considering the figures for the density chart, consider that the amount of illumination through an uniformly illuminated aperture is proportional to the aperture area, which in turn is proportional to the square radius of the aperture
L = k*A = k*pi*(r^2) = k'*r^2

Consider now that in order to double the amount of light through the aperture, the aperture area must now double. How much greater must the radius be in order to double the area of the aperture and so double the amount of light through the aperture?

Tony, I haven't done much of anything yet with mbdcg. The one i did last weekend is posted in the gallery section. Jeff, no, there are no multiple exposures with that shutter. I just move it up (alternately it could go down) like a stage curtain. For example, it moves up 1 inch for the bottom 1/5 of the plate for two minutes, then up another 1/5 (the bottom 2/5ths are now being exposed), for another 2 minutes, and so on. The final "exposure" for the whole plate (including the top 1/5) is longer, say 18 minutes in my case. These would result in "single" exposures (going from top to bottom) of 18, 20, 22, 24, and 26 minutes. Hope i managed to be clear.