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25 holography lessons

Glossary FAQ


11. Hologram drying

Hologram drying operation is considered in a separate lesson, since special attention should be paid to non-obvious processes taking place by drying and to influence of the drying operation on the hologram quality. It can be very pity when after so labour-intensive work relating to recording and chemical treatment of the hologram and after completion of the last operation you'll see on the hologram not good-looking colored stains and spots which sharply worsen image quality (see photo). And the most unpleasant point consists in the fact that these defects are unrepairable even if you repeat the hologram drying operation.
You'll probably be astonished if we tell you that as a matter of fact this operation can't be called drying operation in the strict sense of this term. In reality the purpose of this operation is removal of water only from the hologram surface. And namely the remains of water on the emulsion layer surface cause the above-mentioned defects. Water inside the emulsion layer evaporates gradually and doesn't cause any harm to the hologram.

It may seem strange but the emulsion gelatinous layer of the photographic plate partly has hydrophobic properties - it repels water. You can easily make sure of this while looking how water flows down from the hologram by chemical treatment. Water is gathered into separate streams and drops which remain on the photographic plate surface even if you keep it vertically over the tray. Mechanisms of water (and water solutions) penetration into hydrophobic emulsion layer and deformations of the emulsion layer by surface water drying aren't yet found out in details but it's obvious that just the remains of these streams and drops are the main reasons of colored stains on the hologram.
For uniform removal of surface water it's possible to add the so-called surface-active substances (SAS) into water. SAS always contain in shampoos and detergents. They increase hydrophilic properties of the emulsion layer and facilitate water flowing down from the photographic plate. But this process proceeds slow (it takes several hours) and drying quality is heavily deteriorated because of dust particles, small glass splinters and suspension from running water which stick to emulsion layer surface. Colored stains inevitably appear around every such small particle.
More rapid and reliable method of the hologram drying is drying using ethyl alcohol. A research worker S. Mc Grew was one of the first who paid attention to peculiarities of alcohol drying of holograms on dichromated gelatin (DCG). He discovered that the alcohol penetrating into emulsion layer displaces water from it (this process is called "dehydratation") only in the direct vicinity from the emulsion layer surface. As though a thin "crust" of dry gelatin is formed which prevents going out and evaporation of water located inside the emulsion layer. This circumstance promotes carrying-out high-quality hologram drying.

For alcohol saving three alcohol solutions with 50%, 80% and 100% alcohol concentration are usually used. Accurately take a photographic plate from the tray with running water and after water flowing down place it into the tray with 50% alcohol solution. For removal of surface water raise one edge of the hologram from the tray under angle of 40-50 degrees and let alcohol flow down during 10-15 seconds. Alcohol drags water behind it from the photographic plate surface. If you use trays of transparent plastic you can see faint transparent inhomogeneities - swells (see photo) near the lower edge of the hologram. This is water flowed down from the photographic plate and not yet dissolved in alcohol. Having raised the photographic plate out of the tray 4-5 times in such a way you can make certain that the number of strias becomes progressively less - that is water leaves the photographic plate surface. This procedure is repeated in every tray successively raising alcohol concentration.
Immediately after termination of drying process in a 100% alcohol it's possible to remove easily the adhered small particles from the photographic plate surface using a small soft brush wetted in alcohol. This would be totally impossible before alcohol drying. The rigid "crust" prevents damage of gelatin layer. Begin to remove particles moving from the upper edge of the hologram to the lower edge following alcohol flowing down. Remove only the biggest particles since very small particle aren't visible on the hologram after alcohol drying up. Nevertheless be very careful by removal of glass splinters. The fact is that a touch of the brush can lead to a deeper penetration of the sharp glass splinter into gelatin layer. Therefore if the splinter can't be removed after several attempts leave it at its place. After alcohol drying up it possibly will create a hardly noticeable point on the hologram.
After removal of small particles place the photographic plate on the filter paper or ordinary tissue paper under an angle. So the second step of drying process begins - evaporation of water from the emulsion layer. This process is slowed down through existence of dry "crust" but since there is already no surface water and since the volume of this water considerably exceeded water volume inside the emulsion layer duration of the drying process doesn't exceed half an hour.
Image gradually appears on the hologram. More exactly it is transformed from the infrared spectral region into visible spectral region as the thickness of the emulsion layer loosing water returns to the original state. But the final image color won't be red as laser radiation color any longer but will be within the limits from orange to green color depending on the humidity level in the room and on the degree of the emulsion layer tanning. This effect is connected with the fact that after fixing and removal of not developed silver bromide the emulsion layer thickness decreased slightly. Now the hologram is ready for final decoration (see photo).

1. S.P. Mc Grew "Color control in dichromated gelatin reflection holograms", SPIE, v. 215, 1980, p.24-31.
2. S. Vorobyov "The sensitization of VR-P Russian photoplates for recording pulsed holograms", SPIE, v. 3358, 1997, p.67.