General Description. Homolka and Rudolf Fischer The former discovered that certain intermediates, such as those that gave indigoid dyes, can develop exposed silver bromide producing simultaneously an insoluble dye. Such agents are indoxyl, 3-hydroxy-thio-naphthene, etc. There is another way, namely that of using the development process indirectly to form dyes. Fischer discovered that dyes, such as those belonging to the quininimine and azomethine class, can be formed when exposed silver bromide is developed with p -phenylene diamine together with coupling substances.
During development p -phenylene, or its derivatives, reacts with phenols or naphthols or substances containing a reactive methylene group through the medium of oxidized silver bromide, to form strongly coloured insoluble dyestuffs, the developed silver image being at the same time produced. Fischer thus conceived the idea of mixing a developing agent, such as an aromatic diamine, or amino phenol and a coupler which could be a phenol or a body containing a negative methylene group.
The next step was to incorporate the coupler in the silver halide layer or layers instead of adding it to the developer, but owing to the apparent impossibility at that time of making the dye-forming substances non-diffusing, Fischer, in , had to abandon his project for making a multilayer colour film by applying these discoveries.
The physical structure of a gel is such that the conditions for molecular diffusion are ideal. Hence our ability, amongst other things, to develop a gelatine emulsion. As gelatine can only trap coarse-grained particles insoluble in water, the dyes produced by colour development must be insoluble in water and large in structure. Next, the substances to be incorporated must not adversely affect the sensitivity of the silver halide emulsion. The next important name is that of Dr. William Schneider of the Agfa organization I. He tells us that in instructions were issued that a colour process must forthwith be perfected.
In Schneider and G. Wilmanns reported Agfa-Veroefentlichungen , Vol. Colour couplers similar to those discovered by Fischer were used but these were made fast to diffusion by attaching a long-chain alkyl group, since diffusion depends upon the physical dimensions of the particles determined by molecular weight and constitution.
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In general, a chain-like structure containing twelve carbon atoms is usually adequate. By virtue of these couplers containing a group soluble in alkali, it was feasible to incorporate them in the gelatine emulsion in an alkaline solution. In Agfacolor Reversal film was made available in 8 mm. These films were of both daylight and tungsten types, the speed being roughly the same as that of Kodachrome. Immediately following the occupation of Germany, investigation by the Anglo-American Service experts of the Agfa film factory at Wolfen, Germany, revealed that the Germans had developed to a commercial stage the long-promised mm.
None of the colours were particularly saturated, the reds being quite dull. It was not as good as Technicolor. It does not appear that any new developing machines or printers were constructed for the processing of the film, but modifications were made in existing equipment.
Between and thirteen feature pictures were made see list below and some fifty shorts consisting of one or two reels.
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The material is particularly suitable for newsreel production, and a picture was made of actual combat in Russia. Ranger U. The latter was. The method is capable of good results if proper control and equipment were used, but under the wartime conditions in Germany there was an apparent lack of good equipment and a generaly laxity in quality control, resulting in many inferior prints being released.
Darrel Catling, A. Moreover, a complete change of locale— coinciding with the change of the reels—often helped to cover up the lack of balance. Incidentally, some of the trick work was of a very high order. Therefore, for the moment at least, it seems apparent that the Agfacolor system is unable to yield cheap prints for commercial release purposes; the cost cannot fail to be very much higher than that of a print consisting of one ordinary emulsion and three different dyes.
Post-war efforts have been confined to the Russians, who have made a number of news films of events such as Dr. Both these films were publicly exhibited in the British theatres, and the writer heard innumerable people express very strongly their view that the colour was to be preferred to that of Technicolor.
The expert saw that there was anomalous colour rendering due to the very defective magenta coupler, but here and there shots turned up of quite exquisitely satisfying colour, and it is perfectly certain that the Germans were at the beginning of a most important development.
The men who were responsible for the achievement have since been interrogated, the opinion unanimously expressed being that they were forced into production before they had completed their task, and that great improvements would shortly have been made in the saturation of the couplers and particularly in the design of the coating machinery. In the following table, cost figures per foot of rnm. Agfacolor negative and positive film are compared with those for black-and-white film. Late in and early in , monthly production of Agfacolor film at the Agfa film factory at Wolfen amounted to an average of:.
It is claimed that under the Russians this has been boosted to more than 15,, ft. Release prints were always made from the original negative and no duplicate negatives were made. Special effects were made by indirect and trick methods which were not very satisfactory. No masking was used. All Agfacolor films have three emulsion layers and one filter layer Fig. The film base is first coated with a red sensitive emulsion. Directly on top of this is coated a green sensitive emulsion.
Next comes a thin layer of colloidal silver which functions as a yellow filter. The final coating is an emulsion sensitive to blue only.
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The yellow filter, needless to say, has the duty of absorbing blue light transmitted by the top layer, which would otherwise record in the green and red recording layers which are inherently blue sensitive Fig. No non-stress surface gelatine coating is used and no gelatine coating divides the green and red recording emulsion. The colour couplers dispersed in these emulsions are chosen such that:.
The anti-halation coating is on the back of the film base in the negative-positive films and beneath the bottom layer of emulsion in the reversal film see Table The total thickness of the multiple layers does not exceed that of normal film, therefore the separate coatings are required to be extremely thin. Nitrate base is used for negative.
Acetate base is used for positive.
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Thickness microns. This type of sensitizing is typical of most monopacks, such as Kodachrome, Kodacolor, Ektachrome, and Ansco Color film. The ideal sensitivities can be mathematically calculated relative to either practical or theoretical synthesis, by additive or subtractive methods but the best compromise has to be effected with the materials available Fig.
Since it is desirable that the positive material should record in each of its respective layers light modulated by only one layer of the negative, the sensitizing should be designed to correspond to the maximum absorption and minimum overlap of the spectral transmission of the negative dyes Fig. The relation between negative colours and positive sensitizings can in fact be such as to overlook any relation whatever to what we might expect to be the normal appearance of colours in the negative. By this is meant that the negative colours need not be complementary to the colour of the light recorded.
See below. The negative cyan coupler in the present product has remarkable absorption in the deep red and by placing the red sensitivity of the positive far into the red we are assured of the cyan layer of the negative being efficient to modulate this layer only Fig. Thus the relation between negative and positive can be expressed as follows:. Schneider and others have suggested that there is no reason theoretically why a positive material should not be sensitized, say, in the infra-red, green, and ultra-violet regions, respectively, in order to obtain a more complete colour separation Fig.
In this case the absorption maxima of the three dyes present in the layers of the negative would also have to be in the infra-red, green and ultra-violet Fig. A dye that absorbs only in the infra-red or in the ultraviolet would be colourless to the dye. Such negative material would be magenta only. The development of Agfacolor negative or positive film requires only minor modifications of the standard machines.
We have to provide for a bleaching bath and for washing for a more prolonged period than usual. It is recommended that washing should be entirely carried out by fine jets, or better still by the use of a flat jet issuing from an inclined flattened nozzle. Afterwards the solution is again brought to the prescribed working temperature,. The simplest layout consists in inserting this apparatus in the developer circuit, otherwise the developer must be strongly agitated to ensure uniform development.
Plastic materials have been found satisfactory for pumps, and in general, plastics have stood up well for piping and tanks. Precautions must be taken to prevent the formation of foam in circulated solutions to avoid rapid ageing of baths by oxidation. Subsequent to development the washing must be very thorough otherwise a general blue fog will be formed in the bleaching bath if the washing is inadequate. It is absolutely essential that hypo should be completely removed in the final wash. Tanks and tubes have been entirely abandoned for washing at any stage. Figs, and show the type of spray wash recommended.
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Subsequent to the first spray wash, or in any case after the bleaching bath the remainder of the processing can be carried out in ordinary illumination. The explanation of the inclusion of a stop bath in the positive sequences is that there is some advantage to be gained from the fact that the developer remaining in the layers continues to be active during the washing, but whereas the developer is soon exhausted in the fully exposed areas the shadows still retain a sufficient supply of fresh developer which continues to produce silver and dye images.
The resulting slight fog is of no consequence in the case of the negative film. In the positive film the highlights must be free from fog, and so the action of the colour developer must be interrupted immediately after development. For this purpose acid salt solutions are suitable, for in spite of the permanent addition of alkaline colour developing solution they retain their hydrogen ion concentration at a p H value of 4 to 5, for some time.
At the same time, by virtue of the formation of easily dissolved salts the developing agent is washed off more quickly. It should be noted that when a silver sound track is desired it is necessary to use an additional and preliminary weak bleach to eliminate the colloidal silver yellow filter layer before applying the final viscous bleach on the picture area see below.
This can be done with a weak bleach which is not strong enough to affect any of the silver in the sound track image. Furthermore it is desirable to include a brief immersion in a five percent, solution of sodium sulphite immediately after the final bleach, to inhibit its action before it has a chance to flow over on to the sound track during the spray wash which succeeds the bleaching bath. The bleach bath is a preparatory stage in the removal of the developed metallic silver entirely from the dye image.
Having formed a silver salt, such as silver ferrocyanide, it is easily eliminated by the solvent action of hyposulphite. Simultaneously the unexposed silver halide is eliminated. The conventional photocells in use in most projector sound reproducing systems are of the type using a sensitive coating of caesium—silver—oxygen, which is mainly sensitive to red and infra-red radiation. The dyes used in colour development processes transmit radiation in this region, with the result that a dye track is unsatisfactory when used in conjunction with a large proportion of the phototubes used today.
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While new photocells have been recently developed having high sensitivity in the green-blue and blue regions of the spectrum, and with these dye tracks are capable of reproduction equalling in quality that of silver tracks, it is nevertheless desirable for the time being to continue to employ the silver image sound track. The German solution of this problem was ingenious. Obviously the silver track must not be bleached if it is to be retained.