Thanks for pointing to that paper by the Bulgarian group (Mazakova et al).
It seems to deal mainly with gelatin hardening prior to exposure:
"The results from the experiments show that if for bias hardening of dichromated gelatin layers a hardening agent is used which binds not the carboxylic but, say, the amino groups of the gelatin molecules, the light energy is used more thoroughly, i.e. the sensitivity improves." (p. 319)
DCG Hardening (Dinesh)
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Martin
DCG Hardening (Dinesh)
"In DCG, it seems to me that to increase the overall hardening by post processing methods would decrease the delta n. If the density of the carboxyl (cross linked) gel remains unaffected but the hardness of the dissolved gelatin were to increase in hardness through hydroxyl hardeining, wouldn't the n of the surrounding gel approach the cross linked gel and decrease delta n?"
This seems to be linked to the problem of a consistent general theory on the actual mechanism behind DCG recording. Since the early days of DCG there has been relatively little effort on doing research about that problem.
From a more practical side and resuming Usanov's approach to SHSG,
formaldehyde (vapor) hardening is said to provide a means to "seal" the layer surface. Upon dehydration in the alcohol bath this "sealing" leads to a high vapor pressure within the gel. Under such condition the extremely small voids/vacuoles are said to expand and thus to enhance DE.
I am not sure how DCG fits into that scheme though. I am aware of attempts to incorporate formaldehyde into the final alcohol baths. And of course there has been Graube's vapor hardening to improve DCG resistance to humidity.
This seems to be linked to the problem of a consistent general theory on the actual mechanism behind DCG recording. Since the early days of DCG there has been relatively little effort on doing research about that problem.
From a more practical side and resuming Usanov's approach to SHSG,
formaldehyde (vapor) hardening is said to provide a means to "seal" the layer surface. Upon dehydration in the alcohol bath this "sealing" leads to a high vapor pressure within the gel. Under such condition the extremely small voids/vacuoles are said to expand and thus to enhance DE.
I am not sure how DCG fits into that scheme though. I am aware of attempts to incorporate formaldehyde into the final alcohol baths. And of course there has been Graube's vapor hardening to improve DCG resistance to humidity.
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Martin
DCG Hardening (Dinesh)
"Obviously, the greater the concentration of gelatin, the greater the number of Chromic-COO- bonds, but too high a concentration will cause the gelatin to "precipitate" and cause oh-so annoying "whities"."
Interesting, I didn't know that.
By the way in the paper by Phillips, Rallison, Barnett, Schicker, Coleman,Dichromated gelatin - some heretical comments, SPIE 1914, there are some quite interesting statements about gelatin hydrolysis:
"What we propose in this paper is that the large values of index modulation observed in D.C.G. are plausibly caused by gelatin hydrolysis in the nodal parts of the image structure. If the collapse of the recording layer can then be reversed or inhibited, void modulation occurs, ranging between the refractive indices of gelatin (augmented by added Chromium) and air." (p.101)
And: "(...) When D.C.G. layers are processed, then a gradual build-up of deposited gelatin is observed in the processing dishes or vessels. This observation has been confirmed by one of us (RR) over many years of work in the field of D.C.G. processing. One has to ask (...) is it relevant to the underlying mechanism in the formation of the D.C.G image? (p. 105)
And: "It seems extremely unlikely that the mean index of a D.C.G. layer can be in excess of its original unprocessed values." (p.105)
Interesting, I didn't know that.
By the way in the paper by Phillips, Rallison, Barnett, Schicker, Coleman,Dichromated gelatin - some heretical comments, SPIE 1914, there are some quite interesting statements about gelatin hydrolysis:
"What we propose in this paper is that the large values of index modulation observed in D.C.G. are plausibly caused by gelatin hydrolysis in the nodal parts of the image structure. If the collapse of the recording layer can then be reversed or inhibited, void modulation occurs, ranging between the refractive indices of gelatin (augmented by added Chromium) and air." (p.101)
And: "(...) When D.C.G. layers are processed, then a gradual build-up of deposited gelatin is observed in the processing dishes or vessels. This observation has been confirmed by one of us (RR) over many years of work in the field of D.C.G. processing. One has to ask (...) is it relevant to the underlying mechanism in the formation of the D.C.G image? (p. 105)
And: "It seems extremely unlikely that the mean index of a D.C.G. layer can be in excess of its original unprocessed values." (p.105)
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Martin
DCG Hardening (Dinesh)
"A new efficient injection, upon actinic radiation could be used to do this job, I guess some solar cell type dye, using with a semiconductor (AgBr)(TiO2) to transport the electron could eventually reduce the dichromate."
Using TiO2 might be an interesting approach - particularly since TiO2 can be spectrally sensitized similar to silver halides.
Using TiO2 might be an interesting approach - particularly since TiO2 can be spectrally sensitized similar to silver halides.
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Martin
DCG Hardening (Dinesh)
"From what I understand is that cross linking is broken down with heat. Then the gelatin becomes soluble in water."
I would suggest to put it that way: crosslinked gelatin can be broken down by hot water (depending on the degree of crosslinking/hardening). The more it had been previously crosslinked, the more it withstands hot water. E.g. a
non-hardened gelatin gel may dissolve at 35°C in water, whereas hardened gelatin may not dissolve in hot water of say 60°C (actually it can be crosslinked to a degree where it does not dissolve at all, even in boiling water).
"(...) the water absorption goes down with additional cross linking? Right?"
Yes.
I would suggest to put it that way: crosslinked gelatin can be broken down by hot water (depending on the degree of crosslinking/hardening). The more it had been previously crosslinked, the more it withstands hot water. E.g. a
non-hardened gelatin gel may dissolve at 35°C in water, whereas hardened gelatin may not dissolve in hot water of say 60°C (actually it can be crosslinked to a degree where it does not dissolve at all, even in boiling water).
"(...) the water absorption goes down with additional cross linking? Right?"
Yes.
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JohnFP
DCG Hardening (Dinesh)
"This Chromic ion forms a complicated bond with the carboxyl group, COO- across two of the gelatin structures".
Do you mean between the helical and single strands or any combination of? Or just between helical or just between strands or both but not mixed?
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Hardening
So we post harden in fixer with hardener or in the oven. Why? If this increases the hardness evenly throughout the emulsion how is it helping to achieve a greater index modulation? Is it just to build up a base?
That is, let’s say our emulsion is 25 microns thick and our exposure caused cross linking 15 microns deep at the constructive areas and only 5 microns at the destructive areas. Does the hardening take both of these deeper such that we have 25 microns at the constructive and 15 microns at the destructive areas?
Or is it not a deepness thing but a density of cross linking thingy. So in our 25 microns thick emulsion we have cross linking in the constructive and destructive areas all the way through the 25 micron layer but where there is constructive interference the density of cross linking is higher, let’s say 50% and in the destructive areas the cross linking is less concentrated, let's say 10%. Does the hardening process increase both linearly so that the constructive area is 75% and the destructive area is 35%?
So ideally if the first case were true we would want a hardening process that penetrates deeper at the already hardened areas.
If the second case were true we would want a hardening process that attracts the cross linked molecules from the destructive sites to the constructive sites.
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Drying
So in drying we need to have more gelatin concentration (or thicker) at the constructive areas then at the destructive areas. Now lets work backwards from an already dry hologram that we put back in water to get a uniformly thick coating (we have not exceeded the crack point). So this is where I get confused. It would seem to me that we need a sample at the destructive areas that holds more water but has less polypeptide molecules, whereas at the constructive areas has to be a higher concentration of polypeptide molecules with less water.
What may be happening is that the higher concentration of cross linking falls off to the lower concentration of cross linking gradually. So when the emulsion is dried the higher concentration pulls some of the helixes and strands into the high concentration areas as they curl up and constrict. Probably mostly due to strands that are not helical and cross linked. This pulling is like putting a sponge through an old time wringer and the water come out of the destructive areas. Then when water is added again, the strands relax and acting like springs want to return to their original shape and thus will migrate back to the destructive areas and acting as a decompressing sponge absorb water.
Do you mean between the helical and single strands or any combination of? Or just between helical or just between strands or both but not mixed?
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Hardening
So we post harden in fixer with hardener or in the oven. Why? If this increases the hardness evenly throughout the emulsion how is it helping to achieve a greater index modulation? Is it just to build up a base?
That is, let’s say our emulsion is 25 microns thick and our exposure caused cross linking 15 microns deep at the constructive areas and only 5 microns at the destructive areas. Does the hardening take both of these deeper such that we have 25 microns at the constructive and 15 microns at the destructive areas?
Or is it not a deepness thing but a density of cross linking thingy. So in our 25 microns thick emulsion we have cross linking in the constructive and destructive areas all the way through the 25 micron layer but where there is constructive interference the density of cross linking is higher, let’s say 50% and in the destructive areas the cross linking is less concentrated, let's say 10%. Does the hardening process increase both linearly so that the constructive area is 75% and the destructive area is 35%?
So ideally if the first case were true we would want a hardening process that penetrates deeper at the already hardened areas.
If the second case were true we would want a hardening process that attracts the cross linked molecules from the destructive sites to the constructive sites.
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Drying
So in drying we need to have more gelatin concentration (or thicker) at the constructive areas then at the destructive areas. Now lets work backwards from an already dry hologram that we put back in water to get a uniformly thick coating (we have not exceeded the crack point). So this is where I get confused. It would seem to me that we need a sample at the destructive areas that holds more water but has less polypeptide molecules, whereas at the constructive areas has to be a higher concentration of polypeptide molecules with less water.
What may be happening is that the higher concentration of cross linking falls off to the lower concentration of cross linking gradually. So when the emulsion is dried the higher concentration pulls some of the helixes and strands into the high concentration areas as they curl up and constrict. Probably mostly due to strands that are not helical and cross linked. This pulling is like putting a sponge through an old time wringer and the water come out of the destructive areas. Then when water is added again, the strands relax and acting like springs want to return to their original shape and thus will migrate back to the destructive areas and acting as a decompressing sponge absorb water.
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Dinesh
DCG Hardening (Dinesh)
"This is not the MB sensitized mechanism?"
Yes, true. I was thinking of simultaneous actinic reduction of the dichromate and electron injection so that both effects would be occuring at the same time and the electron injection would increase the local refractive index higher than just the dichromate alone. I think John's question was: can you increase the local hardness during exposure, presumably standard exposure with blue/green. It brings up an interesting (though maybe stupid) question. What happens if you put MB in DCG and shot with 488 and 633 simultaneously? Thinking further, you'd need to alter the geometry of the 633 so that the average spatial frequencies at both wavelengths were similar. OK, it's stupid!
Yes, true. I was thinking of simultaneous actinic reduction of the dichromate and electron injection so that both effects would be occuring at the same time and the electron injection would increase the local refractive index higher than just the dichromate alone. I think John's question was: can you increase the local hardness during exposure, presumably standard exposure with blue/green. It brings up an interesting (though maybe stupid) question. What happens if you put MB in DCG and shot with 488 and 633 simultaneously? Thinking further, you'd need to alter the geometry of the 633 so that the average spatial frequencies at both wavelengths were similar. OK, it's stupid!
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Dinesh
DCG Hardening (Dinesh)
"What we propose in this paper is that the large values of index modulation observed in D.C.G. are plausibly caused by gelatin hydrolysis in the nodal parts of the image structure. If the collapse of the recording layer can then be reversed or inhibited, void modulation occurs, ranging between the refractive indices of gelatin (augmented by added Chromium) and air."
Surely one way to test this would be to fill the voids with some other material with an index between air and gelatin. It seems that the DE of the DCG hologram could be used to find the exact modulation, a la Kogelnik, and this would settle the question. I've not seen any other papers address this, since it's usually assumed that the modulation is caused by hardening due to cross linking. It's pretty heretical, I think we need a Holographic Inquisition!
Surely one way to test this would be to fill the voids with some other material with an index between air and gelatin. It seems that the DE of the DCG hologram could be used to find the exact modulation, a la Kogelnik, and this would settle the question. I've not seen any other papers address this, since it's usually assumed that the modulation is caused by hardening due to cross linking. It's pretty heretical, I think we need a Holographic Inquisition!
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JohnFP
DCG Hardening (Dinesh)
What about if you did what you stated but set the red up in such a way soas not to get an interference of any sort just an even illumination. For a Denisyuk, bring the red beam in from the object side.
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Dinesh
DCG Hardening (Dinesh)
"Do you mean between the helical and single strands or any combination of? Or just between between strands or both but not mixed?"
I think between just the strands. As I understand it, the helices are attracted by polar bonds due to the polarity of the molecules, while the strands are isolated and "capped" with the COOH group. The reduced, Chromic ion reacts with a pair of G-COOH to attach them (simplified): G-COOH + Cr3+ + G-COOH --> G-COO-...Cr3+....-OOC-G with the H's going off as water.
"So we post harden in fixer with hardener or in the oven. Why?"
Mainly because the gelatin in DCG is considerably softer than the commercial gelatin in Silver plates. If you didn't harden, too much water would get into the gelatin and the stuff would break apart internally. Too much washing and the gelatin would come off. Of course, too much hardening and no water would get in at all. The bias hardening hardens it just enough to absorb just enough water to get just the right amount of modulation. The modulation is then incremented by just enough dehydration at just the right rate. All these "justs" are the real trick of DCG holography.
Hardening.
Ideally, the DCG has a uniform density. When you zap it, where there is light you have cross linking, where there is no light, there is no cross linking. Where there is cross linking, there is an increase in density, while where you have no cross linking the density remains unchaned and is the density of the original gelatin (but note Martin's post of the "heretical comments". The last point about the average density of gelatin). So, in your 25 micron emulsion, if the spatial frequency of the hologram is 3000 l/mm then the fringe seperation is 1/3000 mm or 0.3 microns. This means that you have a cross linked plane every 0.3 microns. You have to take this with a pinch of salt because in actual fact the fringes are sinusoidal so that the cross linking varies as a sin wave. However, assuming a square wave (which all the books do!) you have therefor a cyclic variation of crosslinked and uncrosslinked every 0.3 microns and therefore a cyclic variation of hardness. There is an effect such that the refractive index of a material is proportional to the hardness, so that you have a cyclic variation of refractive index. This is basically like a anti-reflection coating, the cyclic variation of refractive index forms an anti-reflection layer that reflects light of just the right wavelength and allows all the other through. The amount of light reflected depends on the difference in refractive index and so the difference, or modulation, in hardness. If the overall, uncrosslinked stuff is too hard, the difference is that much smaller and so the brightness is that much less. If the overall stuff is too soft, it'll break up during the processing stage. Again - aim for "just"!
There's no "pulling" of uncrosslinked molecules to the cross linked areas. I suggested you might "pull" electrons that you've somehow injected into the material into the cross linked areas and Sergio has said this is how MB works (although I had something different in mind - see my reply to his post). The local cross linked areas just get harder. It's like putting bread in an oven. If you could carefully apply heat in the oven in layers, maybe by putting in shelves in the oven and placing dry ice on these shelves, the dough would become bread along the heat layers and so get harder, leaving the colder areas as just dough. Actually frying eggs in layers is more appropriate, since this is also a polymerisation process. Eggography?
I think between just the strands. As I understand it, the helices are attracted by polar bonds due to the polarity of the molecules, while the strands are isolated and "capped" with the COOH group. The reduced, Chromic ion reacts with a pair of G-COOH to attach them (simplified): G-COOH + Cr3+ + G-COOH --> G-COO-...Cr3+....-OOC-G with the H's going off as water.
"So we post harden in fixer with hardener or in the oven. Why?"
Mainly because the gelatin in DCG is considerably softer than the commercial gelatin in Silver plates. If you didn't harden, too much water would get into the gelatin and the stuff would break apart internally. Too much washing and the gelatin would come off. Of course, too much hardening and no water would get in at all. The bias hardening hardens it just enough to absorb just enough water to get just the right amount of modulation. The modulation is then incremented by just enough dehydration at just the right rate. All these "justs" are the real trick of DCG holography.
Hardening.
Ideally, the DCG has a uniform density. When you zap it, where there is light you have cross linking, where there is no light, there is no cross linking. Where there is cross linking, there is an increase in density, while where you have no cross linking the density remains unchaned and is the density of the original gelatin (but note Martin's post of the "heretical comments". The last point about the average density of gelatin). So, in your 25 micron emulsion, if the spatial frequency of the hologram is 3000 l/mm then the fringe seperation is 1/3000 mm or 0.3 microns. This means that you have a cross linked plane every 0.3 microns. You have to take this with a pinch of salt because in actual fact the fringes are sinusoidal so that the cross linking varies as a sin wave. However, assuming a square wave (which all the books do!) you have therefor a cyclic variation of crosslinked and uncrosslinked every 0.3 microns and therefore a cyclic variation of hardness. There is an effect such that the refractive index of a material is proportional to the hardness, so that you have a cyclic variation of refractive index. This is basically like a anti-reflection coating, the cyclic variation of refractive index forms an anti-reflection layer that reflects light of just the right wavelength and allows all the other through. The amount of light reflected depends on the difference in refractive index and so the difference, or modulation, in hardness. If the overall, uncrosslinked stuff is too hard, the difference is that much smaller and so the brightness is that much less. If the overall stuff is too soft, it'll break up during the processing stage. Again - aim for "just"!
There's no "pulling" of uncrosslinked molecules to the cross linked areas. I suggested you might "pull" electrons that you've somehow injected into the material into the cross linked areas and Sergio has said this is how MB works (although I had something different in mind - see my reply to his post). The local cross linked areas just get harder. It's like putting bread in an oven. If you could carefully apply heat in the oven in layers, maybe by putting in shelves in the oven and placing dry ice on these shelves, the dough would become bread along the heat layers and so get harder, leaving the colder areas as just dough. Actually frying eggs in layers is more appropriate, since this is also a polymerisation process. Eggography?