International photographer (Jan-Dec 1941)

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l6MM. dEpARTMENT Some Notes on Color There are three nerve fibrils in the human eye: one of them is sensitive to red, the second to yellow, and the third to blue. All of the colors as we see them are made up of various combinations of these three basic colors: magenta, for example, exciting the red and the blue fibrils simultaneously and giving rise to the sensation of the purplish hue as we know it: any variation in the shade of magenta will cause a variation in the ratio of the excitation of these two fibrils and will cause a corresponding response. All color photography consists basically of breaking down the original scene into its components so that these components may be recorded photographically, and then combining them again for the final print. This process is known as color separation. All of the many methods of color photography have this process of color separation in common, though their methods may — and do — vary considerably. The earliest and simplest form of color photography was employed in still photography. Here a still life object was photographed on three different negatives — one negative through a red filter, one through vellow, and one through blue. All three of the negatives so obtained were black and white. The final color print was obtained by making prints from these negatives on thin tissues which were pigmented with a color determined bv the color of the filter used to expose that particular negative, and by combining these three tissues. The exposure of three separate negatives at three different times precludes the possibility of taking moving objects, and for a time it was possible to photograph only still life objects. With the poising of the problem of obtaining color separation negatives simultaneously, several different methods presented themselves. The simplest of these is known as the bi-pack method. In this system two films are used — one of them ( the one towards the lens ) is a color blind material not sensitive to red and having a red backing, against which is placed a standard panchromatic emulsion which is highly sensitive to red. Color separation is obtained by recording the blue and yellow (or green) light on the firsl film, its blindness to red light creating the same effect as placing a green filter in front of this emulsion. The green light is then filtered out by the red backin" on this film acting as a filter for the panchromatic emulsion behind it. This gives us what is known as a two-color process, because only two films are used in the color separation. A second process developed is the Technicolor process. In this system three color separation negative are obtained. Two prisms are used, and their hypotenti cemented together so that their appearance resembles a cube. By means of a coating on the surfaces in contact we have a unit which acts as a partial transmitter and a partial refractor. In other words part of the light entering the prisms from the lens will go right on through while part of it will be reflected to one side. Having divided the light in this manner, it is possible to place filters in any desired combination, utilizing the bi-pack principle of two films on one side and a single film on the other. Here, too, the printing process is one of pigmentation of the prints from black and white negatives. Unfortunately, these two basic principles are not adapted to amateur work. They are expensive, and their complicated nature would not make for the convenience and light weight that is of such consideration in amateur work. It was apparent, then, that an entirely different approach would have to be made to the problem, in order to secure direct color on a reversible film with no appurtenances. The old Kodacolor process, which did make it possible to get direct color on reversible film, had the disadvantages of being very grainy, not being sharp, being very slow, and requiring special filters not only on the camera but on the projector as well — and the ultimate result was only mediocre color. Kodachrome, seems to have achieved the ideal. It is needle-sharp, capable of perfect color rendition, needs no corrective filters if the properly balanced emulsion (either interior or exterior) is used for the condition at hand, needs nothing in the way of projector gadgets, is reasonably fast, and has no grain. While the exact prcesses involved in Kodachrome are a manufacturer's secret, the known facts should make it possible to present a working knowledge of the system. The sensitive emulsion on Kodachrome consists of three separate emulsions, cemented together by a very thin layer. Each one is sensitive to one of the primary colors, allowing for some overlap. Underneath the coating adjacent to the lens is a thin coating constituting a yellow filter. It is in this manner that the actual separation is achieved. When Kodachrome is developed, it is first treated in the usual manner of a reversible film — that is, it is first developed negative, then the reduced silver bleached off, the film exposed to light, and developed again to obtain the positive. Up to this point there is no color on the film. The black and white in the three-colorseparation emulsions at this state corresponds to the negatives made by exposing three separate films through three filters. except, of course, that the image has already been reversed in the Kodachrome and we have a positive. The color is obtained by the reaction of a "color developer" upon a "color coupler." We are all familiar with the sight of red rust on steel rails after a rain. When a copper penny is found in the mud it will have a greenish-blue color. Notice, too, the yellow flame after salted water has boiled over on the stove. All metals have a characteristic color, and when they are combined in a salt of the metal it becomes visible. The rain acting on the rail forms iron oxide; the water in the mud acting on the penny, copper oxide. The "table salt" in the boiling water is the salt of sodium known as sodium chloride; color is not visible until placed in a flame, when its characteristic color of yellow becomes visible. Now, in the examples of the steel rails and the copper penny we can call the rails and the penny "color couplers" and the rain and the mud "color developers." In the case of the boiling water, the salt water will be the "color coupler" and the flame the "color developer." In Kodachrome three different color couplers are used to treat the film after reversal, and these are acted upon by the color developers to obtain the desired color. Actually, the process is considerably more intricate. First, all three layers of the film are treated with one coupler and developer. A bleach is then used to remove the color from the two upper emulsions, but not on the one closest to the base. Another coupler is then used for these two layers, but not the third already colored, and this is acted upon by the color developer. The bleach is then used on the top layer, but is not permitted to act on the bottom two. A third color coupler is then used on the top layer, but not permitted to act on the two layers already colored. These color couplers are chemical compounds which, when acted upon by the color developer, will yield the color desired for that particular color-separation positive. The black and white positive permits more or less density of the color to be evident, resulting in the gradations in the original scene. It is to be emphasized that the exact process is a secret of the manufacturer. By means of varying the correction of the yellow filter underneath the first layer, we can "correct" the film for the blue light of the outdoors or the yellow light of mazdas. Because of patent complications, Kodachrome has thus far been balanced only for photoflood light in cinema film. It has, however, been balanced for standard studio mazdas in the still films. New B&H 2000-foot Film Reel Completing the Bell & Howell line of 16mm. projection reels is the new, 2000-foot spring-steel reel recently introduced. The newest addition will permit an hour's continuous projection of sound film, an hour and a half of silent film. Trice $4.00. 22