Methods of hologram chemical treatment

We start with the most prevalent method of hologram recording on PFG-03m photoplates. Index "m" means a modernized photoplate of PFG-03 type, which sensitivity is increased 1.5-2 times. Technical characteristics of the "Slavich" photoplates are given in the "Files" section.

Development

Holograms on PFG-03m photoplates are better developed in GP-3 developer, though quite good results could be achieved with GP-2 developer. According to our investigations, the brightness of holograms in the latter developer is less than in GP-3.

Here are the compositions of both developers:

GP-2

GP-3

Methylphenidone may be dissolved in a small quantity of hot water or alkaline solution, and then poured into the general solution.

Our research has proved that GP-3 developer works better when the quantity of thiocianate is reduced to 30 g.

The working solution for both developers is formed by dissolving 15 ml of the concentrated solution in 400 ml of water. It is advisable to allow the working solution for 10-15 minutes prior to the development, having thoroughly stirred it with a glass stick.

Development time for GP-2 is 15 minutes, for GP-3– 8 minutes. Development temperature is 17 C⁰.

Fixation of PFG–03 runs very quickly, since the size of the bromide silver crystals is very small (tens of nano-meters). Some specialists do not advise to fix holograms at all. It is useful when a minimum emulsion layer shrinkage is required, for example, for manufacturing of reflecting master-holograms.

We, however, fix holograms in an ordinary neutral fixer with the following composition:

Optimisation of the drying of the holograms

Careful drying after photochemical processing plays an important role in the final visual quality of the hologram. For Russian photoplates we have used drying by ethanol. The plate is consecutively dipped in baths containing 50%, 80% and 100% ethanol for 1-2 mins and constantly agitated. However this process by itself does not give stable results.

The analysis of the drying process shows that only complete and homogeneous removal of the water from the surface of the emulsion layer guarantees a high quality of the final hologram. The presence of water inside of the emulsion layer does not influence the uniformity of the hologram. However a thin layer of surface water is separated with large effort from the surface and frequently remains even after processing in 100% ethanol. Consequently coloured stripes and spots appear on the hologram. To remove the surface water it is necessary to change the method of drying as follows [1]:

In all the ethanol baths the holograms are lifted from time to time as shown in figure and are kept in this position for 10-20 sec. Between raisings of the hologram it is necessary to rock one time the bath to mix the solution of ethanol. When the water flows down from the surface of the hologram near to the part immersed in the solution of ethanol some knots becomes visible. When all the water has flowed down this knots disappears. This serves as a sign of the end of dehydration in the given ethanol bath and the necessity to proceed to the next ethanol bath, where the procedure repeats. In last 100% ethanol bath the surface of water vanishes completely. This drying method can be used for any holographic photoplate or film.

The processing of holograms in spirit baths occurs in a thin surface layer of the dry gelatin. It is as though a "crust" prevents the exit of water. The water begins to leave the emulsion layer only after the hologram is taken out of the last ethanol bath and the spirit has evaporated from its surface. If isopropyl alcohol is used for drying silver-halide holograms it is only necessary to use two baths containing 50% and 80% spirit solutions. In 100% isopropyl part of the water is linked into the emulsion layer and does not leave even on complete drying. As a result, a strong (more 100 mcm) shift of colour in the image to longwave occurs. This is exhibited clearly for DCG holograms (photoplate type PFG-04) where use of a bath of 100% isopropyl is obligatory for good final brightness of the image. The mechanism of this linkage of water into the gelatin is not clear.

The formation in the 100% ethanol bath of a dry and rather firm surface emulsion layer allows one to easily remove (with the help of a brush) any impurities which have adhered to the plate in the previous solutions. The most convenient time for such cleaning is as the hologram is raised above the ethanol for the last time.-->

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