In the main, the procedure is to add colorless dyestuff-formers to the emulsion, and, after the exposure, treat them to form the dye. Often such after-treatment may interfere with the latent image. In that case the dyestuff-formers can be stabilized against developing and fixing baths, and dye formation made to take place after the silver image has been formed. When the film is to be used as a copy material, an entirely different story is true.
First of all, color correctness is not essential in the master from which the copies are to be made. It is important only that the differential absorptions and transmissions of the three part images be such that they print in different layers in the copy material. Since the coloring and sensitization can be controlled at will, it can be arranged that the top yellow-dyed layer be made red- or green-sensitive.
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These colors will not be absorbed by the yellow, hence will produce a latent image throughout the entire depth of the layer. Thus there is no great loss of speed or of contrast. In order for the exposing light not to affect the layer underneath, that layer must be dyed with a color which will absorb the red or green used to expose the top layer.
By careful choice of dyes and sensitizers, it can be arranged that each layer in the pack will be exposed by a light that will be transmitted by that layer and absorbed by the layer beneath. The method for doing this is disclosed in United States patent , and its extension to printing from lenticular and screen plate originals is disclosed in United States patents and The basic principles are discussed in the first mentioned patent cf. The arrangements of the layers are such that consideration is paid not only to the absorption and transmission of each of the layers, but also to the relationship which must exist between all the layers.
Thus in one embodiment of the invention, three silver halide layers are poured one on top of the other, the bottom layer being colored cyan, the middle magenta, and the uppermost yellow. This last is made green-sensitive by the addition of pinaflavol or erythrosin. The magenta layer is made red-sensitive, while the bottom cyan layer is sensitized to the infrared.
Each of the colored layers acts also as a protective filter for itself and for the layers underneath. Thus the yellow dye in the top layer acts as a minus-blue filter for the entire emulsion pack. In the same manner the cyan dye in the bottom layer will act as a barrier to whatever red light may spill over the exposure of the central layer. All three of the dyes freely transmit the infrared so that there will be no barrier to its passage. Only the bottom layer has infrared sensitivity, so only this layer will register an image. A pack of this type is ideal when printing is done in stages from separation positives.
It can also be used for printing from a colored master positive. This can be done if the blue-filter separation is converted into a color which will transmit red and infrared, but which will be modulated by green. A magenta or red is such a color. The green-filter image must be printed by red light, but must be transparent to green and infrared. A cyan color would serve here, if it is transparent to the infrared.
But then most dyes are. The red-filter image must be printed with infrared light, and be transparent to red and green. This is a rather broad statement, and somewhat questionable, since the green character of the dye is indicative of red absorption. However, there are no doubt many substances which will absorb only in the infrared, and which would therefore serve. When the material is to be used for printing from lenticular positives, the banded series of filters must not be the familiar red, green, and blue set.
The blue must be replaced by a green filter which is opaque to the infrared, the green sector must be replaced by a red which is opaque to the infrared, and the red by an infrared which is opaque to the visible U.
In making duplicates from screen plates, where the screen is separated from the emulsion, as in the Finlay rather than the Dufaycolor techniques, a separate copying screen is used, with a pattern that is identical to that of the original, but having filter elements transmitting green, red, and infrared, rather than blue, green, and red U.
No mention is made as to how such a screen could be prepared. Another attack on the problem of correlating the spectral sensitivity of the layer with the color of the dye dispersed within that layer is made in United States patent Eng. Instead of shifting the sensitivity of each layer toward the red, the layers are dyed weakly, and instead of having the emulsions on one side of the base, they are coated on both sides, each side containing the three layers.
There is insufficient yellow dye to prevent the blue light from reaching through the entire depth of the yellow layer, and a similar condition holds with the magenta and cyan layers and green and red light. But instead of printing one image, two identical images are printed on each side of the base. When the two weakly colored images are combined, the effect will be that of a strongly colored image.
It may be easier to utilize a duplitized monopack, in which two layers are coated on one side of the base, and a third layer on the other side U. The two outer layers are colored magenta and cyan, while the central layer is colored yellow. This means that two of the layers could be printed by means of blue light, and since both the magenta and cyan transmit blue freely, no loss of contrast would be had.
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Of course, each printing must be done separately, but this is a preferred method anyway. The central yellow layer forms a barrier for the further passage of any blue rays that may spill through the magenta or cyan layers. The yellow layer could be sensitized to the infrared, or to the red. Many other arrangements of sensitizations could be had, which would allow the differential copying of the three partial images.
The preparation of a monopack suitable for two-color work, has also not been overlooked U. But instead of coloring each layer with a single dye, two are used. Thus the orangered layer will contain pyramine orange and azo fuchsin, or azo fuchsin and mordant yellow, while the green layer will contain benzo pure blue and pyramine orange or benzo pure blue and metanil yellow.
The rate at which the two dye components are acted on by the bleach solutions varies with the dyes. Thus it becomes possible to obtain dichroic effects in each layer, orangy highlights with deep red shadows in the orange-red layer, and sky-blue highlights with deep green shadows in the other. It becomes possible in this manner to obtain pleasant flesh and sky renditions, and at the same time, maintain true blacks in the middle tones and shadows.
Other types of monopacks are outlined in United States patent and in English patents and The last is especially interesting in that it takes advantage of the difference in the spectral sensitivities of silver bromide and silver chloride emulsions. The range from to will serve to print on to an unsensitized or color-blind silver bromide emulsion, while the range beyond will act only upon the sensitized silver chloride. A monopack that is suitable either for use in the camera, or for the making of a master positive for subsequent printing, is disclosed in United States patent Eng.
The layers are coated in the following manner: A blue-sensitive emulsion is coated on to a celluloid base. The emulsion is not dyed, but contains dye intermediates which can easily be converted into a yellow dye at a later stage. Upon this is coated a layer containing a yellow filter dye, then a green-sensitive emulsion containing colorless intermediates for the formation of a magenta dye, and finally a red-sensitive emulsion. After the exposure, the pack is treated to form the dyes in the blue and green-sensitive layers.
The top layer, which is undyed, contains a precipitant for a cyan dye, so that this layer can be dyed without affecting the other two. Thus a colorless pack receives the exposures so there is no loss of speed nor of contrast.
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The formation of the dyes and the preferential dying of the uppermost layer is accomplished by the methods outlined earlier in this chapter. It is to be noted that the blue-sensitive emulsion is on the bottom. This means that the pack is to be exposed through the carrier side.
Since the processing yields a direct positive, there is obtained an inversion of right and left. This is rectified by exposing through the base, rather than in the normal manner. Gaspar also suggested that the dye-bleach monopack could be used as one element of a Troland bipack.
Here, it may be recalled, a bipack is formed with a two-layered monopack as one of the elements. Several different possibilities suggest themselves as to the arrangement of layers. In five of the six arrangements, two of the layers are separated by the thickness of one of the supports. In these, one of the elements is a single-coated film, the other contains coatings on both sides of the carrier.
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Obviously this leads to poor definition and a possible loss of registry for the layer that is so separated. The sixth arrangement is the conventional one where the two supports form the outside surfaces, and the three emulsion layers are one directly behind the other. The front element contains a single emulsion layer, and is colorless.
The rear element is a two-layered monopack, dyed and processed in accordance with the Gasparcolor technique U.