by Din » Fri May 05, 2023 12:19 pm
Joe,
The first thing that occurred to me was the size of the reproduction source. But, you say you can get sharp holograms with silver, so this does not appear to be the problem. Joy asked if you used the same source, at the same distance, for the sharp hologram as the blurred ones.
Assuming it to be a problem unique to DCG, other things that occur to me:
1. Scattering of light from nearby objects such as mounts etc, including the table itself. We used to lay down black velvet cloth along the table from the objective to the plate. This matters less to silver halide because DCG is some 100's of times softer than silver, so low frequency gratings from spurious sources are more of a problem. I cannot overstate the importance of baffling, especially on DCG because it's so much softer. DCG, as opposed to silver, will produce modulation from any light, specifically low levels of spurious reflections, since the actinic reaction of DCG is a hardening of the exposed parts. In silver, the actinic reaction is the reduction of the silver ion to elemental silver, and requires a minimum of energy. Not so for DCG, there is no threshold. Another point is to put iris's both behind the objective and in front of it. Light from the mirror behind the objective spreads around the objective and creates noise.
2. The emulsion is too soft. Either there's too much gelatin for the dichromate, or the pre-hardening stage, before the water wash, did not preharden it enough. The result of this is that the emulsion starts to expand as water penetrates it, starting from the top and working it's way downward. This means there's differential swelling, since the upper layers continue to absorb water and expand as the water penetrates deeper to levels that have not yet started to swell. The upper emulsion layers will twist the Bragg planes by a differential amount relative to the planes deeper in the emulsion. This means that the recon angles are variable, and correct only for specific depths of the image. Also, a softer emulsion will give rise to a surface grating which will act as a transmission grating and create noise, which will manifest as blurring.
For what it's worth, we used Knox gelatin (250 bloom for display); our formula was 45 gms of gelatin in 300 mls of water. We used a magnetic hot plate, poured the mix into a beaker, and put in a magnetic stirrer. We adjusted the stirrer control on the hot plate so as not to stir too fast, and set the plate heater control so that the temperature of the 'goop' (Joy called it, the 'goop') was about 50 deg C (important to avoid 'greenies'). We also covered the beaker so as not to lose water through evaporation, but allowed a thermometer into the 'goop' . I checked temperature and stirring every 20 minutes or so. After about 2 hours, when the gelatin was thoroughly dissolved and mixed, we dissolved 8 gms of dichromate in 30 mls of water, added it to the goop, and let it dissolve for a further half hour, still stirring. Afterwards, we filtered the now-sensitive, goop into a second beaker, with coffee filters, adding a few drop of photoflo in the goop in the filter to assure uniformity of the emulsion - again covering the filter to minimise loss due to evaporation. The filtering stage was also on the hot plate to ensure the temperature was kept at about 50 C We used Rapid Fixer, with hardener, for pre-hardening. If the plates were fresh (just a few days old) I fixed for about 2.5 to 3 minutes, if the plates were older (over about 10 days old), I cut down the fix time to about a minute, sometimes even less when I wanted broadband.
3. I passed this by Joy and she pointed out what had not occurred to me: scattering within the emulsion by particulate matter while recording. This would occur either because of pockets of density in the emulsion or to particulates of gelatin within the emulsion that have not completely dissolved, or to particulates of dichromate crystals, also because they have not completely dissolved. Due to the softness of the gelatin, and because there's no threshold of exposure, these 'internal scatterings' will produce spurious gratings. See above my method for making sure everything is completely dissolved and the density is uniform, including dissolving the dichromate in water before adding to the goop.
This is all I can think of now. If anything else occurs to me, I'll post again.
Joe,
The first thing that occurred to me was the size of the reproduction source. But, you say you can get sharp holograms with silver, so this does not appear to be the problem. Joy asked if you used the same source, at the same distance, for the sharp hologram as the blurred ones.
Assuming it to be a problem unique to DCG, other things that occur to me:
1. Scattering of light from nearby objects such as mounts etc, including the table itself. We used to lay down black velvet cloth along the table from the objective to the plate. This matters less to silver halide because DCG is some 100's of times softer than silver, so low frequency gratings from spurious sources are more of a problem. I cannot overstate the importance of baffling, especially on DCG because it's so much softer. DCG, as opposed to silver, will produce modulation from any light, specifically low levels of spurious reflections, since the actinic reaction of DCG is a hardening of the exposed parts. In silver, the actinic reaction is the reduction of the silver ion to elemental silver, and requires a minimum of energy. Not so for DCG, there is no threshold. Another point is to put iris's both behind the objective and in front of it. Light from the mirror behind the objective spreads around the objective and creates noise.
2. The emulsion is too soft. Either there's too much gelatin for the dichromate, or the pre-hardening stage, before the water wash, did not preharden it enough. The result of this is that the emulsion starts to expand as water penetrates it, starting from the top and working it's way downward. This means there's differential swelling, since the upper layers continue to absorb water and expand as the water penetrates deeper to levels that have not yet started to swell. The upper emulsion layers will twist the Bragg planes by a differential amount relative to the planes deeper in the emulsion. This means that the recon angles are variable, and correct only for specific depths of the image. Also, a softer emulsion will give rise to a surface grating which will act as a transmission grating and create noise, which will manifest as blurring.
For what it's worth, we used Knox gelatin (250 bloom for display); our formula was 45 gms of gelatin in 300 mls of water. We used a magnetic hot plate, poured the mix into a beaker, and put in a magnetic stirrer. We adjusted the stirrer control on the hot plate so as not to stir too fast, and set the plate heater control so that the temperature of the 'goop' (Joy called it, the 'goop') was about 50 deg C (important to avoid 'greenies'). We also covered the beaker so as not to lose water through evaporation, but allowed a thermometer into the 'goop' . I checked temperature and stirring every 20 minutes or so. After about 2 hours, when the gelatin was thoroughly dissolved and mixed, we dissolved 8 gms of dichromate in 30 mls of water, added it to the goop, and let it dissolve for a further half hour, still stirring. Afterwards, we filtered the now-sensitive, goop into a second beaker, with coffee filters, adding a few drop of photoflo in the goop in the filter to assure uniformity of the emulsion - again covering the filter to minimise loss due to evaporation. The filtering stage was also on the hot plate to ensure the temperature was kept at about 50 C We used Rapid Fixer, with hardener, for pre-hardening. If the plates were fresh (just a few days old) I fixed for about 2.5 to 3 minutes, if the plates were older (over about 10 days old), I cut down the fix time to about a minute, sometimes even less when I wanted broadband.
3. I passed this by Joy and she pointed out what had not occurred to me: scattering within the emulsion by particulate matter while recording. This would occur either because of pockets of density in the emulsion or to particulates of gelatin within the emulsion that have not completely dissolved, or to particulates of dichromate crystals, also because they have not completely dissolved. Due to the softness of the gelatin, and because there's no threshold of exposure, these 'internal scatterings' will produce spurious gratings. See above my method for making sure everything is completely dissolved and the density is uniform, including dissolving the dichromate in water before adding to the goop.
This is all I can think of now. If anything else occurs to me, I'll post again.