chirped fringes and whitening in DCG

[Joe Farina]

I've noticed in some recent MBDCG holograms, that when the initial water bath temperature gets too high, the whitening in the film does not occur throughout the depth of the emulsion. Instead, it occurs mainly at the gelatin-glass interface. This is obvious, because the virtual image is not visible from the glass side of the plate (it just looks opaque white), but the real image from the emulsion side is very strong, with high efficency. I took a razor blade, and scraped off a bit of the emulsion, and compared the glass-side of the emulsion to the front side. The gelatin that was scraped away from the glass looked like opaque white paint. A small piece of the front surface of the layer was cut away separately, and it was quite transparent (it would have to be, to get such strong diffraction visible in the real image).

I am wondering if this kind of whitening is just the same thing as "chirped" fringes, but going too far with respect to "broadband-type" processing. This whitening effect is very common when using Jeff's MBDCG, and the water bath temperature has to be at a precise level, to get maximum diffraction without excessive whitening.

While I've also done standard blue-exposed DCG, I always used room-temperature water and alcohol baths, and never saw any whitening. But looking at the old literature, whitening seems to have been a pretty big issue, especially in the papers from the late 60's.

I am wondering if all of the "whitening" problems in DCG are one and the same thing -- excessively "chirped" fringes.

Surprisingly, the papers seem to be lacking in a precise reason why chirped fringes occur in DCG. Usually it's said that immediate immersion in 100% IPA (for example) causes the outer surfaces of the gelatin to dehydrate rapidly, while the inner areas dehydrate more slowly. So far, the clearest explanation was given by McGrew, but I don't know if this is an accurate explanation of what happens:

"If the processing is altered so that the transition from 100% water to 100% alcohol occurs with no intermediate stages, and if the alcohol is heated so that dehydration occurs very rapidly, the hologram differs dramatically from the ideal thick hologram. Its reconstruction bandwidth becomes extremely large. In this case of fast processing, the gelatin dehydrates at different rates throughout its depth. At the gelatin/alcohol interface, dehydration is accomplished immediately. The alcohol heats the water trapped deeper in the emulsion and drives some of it back towards the glass substrate, further swelling the deeper regions before drying them.

Fast processing produces a differential swelling through the gelatin layer so that the regions closest to the substrate reconstruct at long wavelengths while the regions near the outside surface reconstruct at short wavelengths."

From McGrew's statements, it might seem that "whitened" DCG is due to the inner areas (near the glass) getting too swollen, due to heat and trapped water. But I don't understand this heating effect he is referring to.

[Tony DCG]

I have a bit of a cold so my head is a bit foggy. I can only comment on DCG not MSDCG
If some other idea occurs to me I will comment further.

I am wondering if all of the "whitening" problems in DCG are one and the same thing -- excessively "chirped" fringes.

Let me think out loud. It seems to me chriping is when a sinusoidal signal get chopped off at its peaks which hence in our case allows more exceptance of light angles and hence more broadband.
Whitening to me has more to do with the springyness of the gelatin and when we exceed that spring breaking ponit.
Now you do bring up a good point in that we are always playing the two, meaning processing as hot and as fast as possible without whitening. I think we do want a level of mirco cracking to occur that is when chriping occurs.

I think you nail it however regarding with the pseudoscopic side in that if it is brighter and slightly clearer vs. the virtual side, It then it means that it was not fixed deep enough in the layers. This of coarse gets a bit more crazy when we are talking about film age and thickness and such. And just to confuse things more, the top will get fixed more than the bottom. Also look at areas in which the film was either thicker or thinner, that can also indicate fixability (i made that word up:) It may have been Dave who told me to look that a reason he likes light hardening is that it tends to penitrate the layers bether than fixer. I know you like light hardening. I think when doing light hardening it is worth exposing both sides of the plate since in (at least DCG) the concentrations tend to be higher, less light is getting to the back side of the film. Maybe you can try on one plate fix on the film side then on another plate fix the glass side.

When my cold meds kick in I might need to add or change stuff but here's my first chemically induces thoughts for now.
Good luck Joe

[Joe Farina]

Thanks for the comments, Tony, they are helpful. Regarding the definition of "chirped" DCG, I've always thought this simply refers to a varied spacing of Bragg planes in a reflection hologram, with the variety of spacings resulting in a broadband replay.

It seems to me (from what I've read about DCG and seen with MBDCG), that when broadband DCG technique (warm or hot liquids for processing, and straight 100% IPA, etc.) gets too extreme, the areas at the interface get too swollen (past the "spring breaking" point) and become white. I'm guessing that the whiteness begins at the gelatin/glass interface (like it seems to with MBDCG), and when the processing temperatures get increasingly higher, the whiteness begins to extend more throughout the entire depth of the emulsion.

But with MBDCG at least, there is a rather surprising "middle zone" where the glass-gelatin interface is opaque white, and the gelatin-air interface is totally transparent. The depth of the opaque or transparent zones is hard to tell, a microscope might help.

[Johnfp]

You mean like this one? I thought mastering the technique but reversing the process such that the image you want to see is the back thus white. I love the brightness of the white. So white it washed out during photographing.

LightHouse.JPG (12.59 KiB) Viewed 3235 times

LightHousePseudo.JPG (17.05 KiB) Viewed 3235 times

[Joe Farina]

Hi John, no, I was referring non-image white, i.e., the milkyness of the film. But since I started using sulfite/alum after exposure, this doesn't seem to be a problem anymore.