THE PROCESSES OF COLOR PHOTOGRAPHY.
PART IV. THE
KODACOLOR PROCESS C. E. K. MEES,EASTMAN KODAK COMPANY, ROCHESTER, NEWYORK
In 1923 a new process of amateur cinematography was placed on the market by the Eastman Kodak Company. The process depended upon the use of a film 16 mm. wide instead of 35 mm., the width of standard motion picture film. This film, after exposure, is not developed to a negative and printed, as is customary with 35-mm. film, but is developed by what is known as the reuersal process. In this reversal process, the exposed image is first developed, and then the developed silver is dissolved in a bleaching bath, as i t is called, which oxidizes the silver. This leaves behind the silver bromide which was not affected by the-developer because it was not exposed to light. After a fresh exposure to light, this remaining silver bromide is developed in its turn and gives a positive. This process is illustrated in Figure 23 which is a drawing made from pictures ' taken through the microscope. In section 1, we see the grains of the light-sensitive silver bromide in the emulsion, and in section 2 we have marked by crosshatching those which have been affected by light during exposure in the camera. They would not show any change to the :* -' eye, of course, because the change by light is not visible. The grains form what is called the latent image. Now, after development, these exposed grains FIGURE 23 turn into black metallic silver, and this is shown in section 3 of the diagram. Then, the bleaching bath removes all the silver, and it leaves behind the silver bromide grains which were not exposed in section 2. These are re-exposed and developed and make the final positive, as we see in section 5. Since 1923, amateur cinematography has developed until now it is a considerable industry. The early apparatus has been supplemented by other forms of cameras and projectors, so that a t the present time a wide range of extremely convenient apparatus made by several different manu-
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facturers is available for the public. As a general rule, a t the present time, the cameras are driven by spring motors which are wound between exposures, and the trend of design is in the direction of cameras as small and compact as possible, so that an amateur motion picture camera is little more bulky than the very small and compact cameras used for still photography. In view of the success of this process of amateur cinematography, it is natural that the Kodak Company studied the various processes of color photography in order to find one which would be suitable for application to amateur cinematography. It was not easy to find such a process owing to the requirements which were considered necessary, one of which' was that it should he possible for anybody without any special knowledge of the subject to take color as easily as he could take monochrome pictures or still pictures in a kodak. The most satisfactory process of amateur cinematography in colors would clearly be one depending upon the use of a screen film in which the film would carry the filter units preferably dispersed regularly in dots or,.? lines. There are, however, great practical difficulties in the preparation of such a screen and in securing the adherence of the emulsion to it, which have hitherto prevented such a process from attaining pra'ctical realization. In 1908 R. Berthon patented a procFIGURE 24 ess which realized most of the advantages of a screen film process without involving too great difficulty in making the film. In this process the color filters are placed in the lens, while the film is embossed on the support side with a number of small lenses. In Figure 24 the corrugated edge represents the surface of t6e film base, in which the small lenses shown have been embossed. We see below this the thickness of the base itself and then a thick black line which represents the thickness of the emulsion. When this film is put into the camera, the embossed lenses will form miniature images of the diaphragm of the camera lens on the film emulsion. Suppose, for instance, that the lens contains a stop with three holes in i t - m e covered by a red filter; a second, by a green; and a third, by a blue filter. Then, behind each film lens there would be formed on the emulsion a group of three dots-one dot corresponding to the red filter; one, to the green; and the thud, to the blue filter. The effect produced by the embossed film and the filters in the lens is exactly similar to that which would be obtained by the use of a screen film in which the filter units were in contact with the emulsion, but the process has the advantage that the film is much easier to make and to use. The
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lenses can be either spherical or cylindrical. If they are cylindrical, the filters used in the camera lens can be strips parallel to the embossing on the film, and we shall then get line images of them on the film. Some years after this invention, Berthon became associated with Authon Keller-Dorian, the owner of an engraving plant in Alsace, and the SocieM Keller Dorian-Berthon was formed to develop the process for commercial purposes. In 1925 this process was offered to the Eastman Kodak Company and was thought to offer possibilities of success, especially as it was closely akin in its requirements to the method of amateur cinematography which the company had already developed. Rights in the process were therefore purchased and development was started with a view to perfecting the process for use with the amateur 16-mm. film. A great deal of study was involved. It was necessary to standardize the methods of making the lenses on the film; to design and make a suitable emulsion strongly sensitive to green and red light and yet with sufficiently fine grain for the minute structure of the separate color elements to be resolved; and, especially, to work out suitable methods. by which the fihn could be developed and reversed while the rendering of color was retained. When all this work had been done, there was available a process which is simplicity itself from the user's point of view. In order to make the pictures, a photographer using a standard Cine-Kodak fitted with an fll.9 lens has only to insert a color filter into the lens and thread the Kbdacolor iilm in the camera. After the film has been expo>ed, it is sent away for processing and comes back to the photographer as a roll of black and white film which can be projected in an ordinary projector and will give a black and white picture on the screen. But if the projector is fitted with a special color filter like that which is used in the camera, then a colored picture will be obtained on the screen. The operation of the Kodacolor process is as follows. The color filter is composed of three separate areas-red, green, and blue-and slips into the front of the lens. The film is threaded into the camera with its back, that is, the side of the film opposite from the sensitive emulsion, facing the lens, so that the emulsion surface itself is away from the lens. The back of the film is embossed, by running through steel rollers, with tiny cylindrical lenses composed of the film base material and extending lengthwise of the film. The lenses on the film are about four times narrower than the dots making up the illustrations in a magazine, and they are therefore invisible except under a microscope. They cover completely the surface of the side of the film opposite from the sensitive emulsion. When the trigger of the camera is pressed, light reflected from the subject passes selectively through the three-color filter, on through the camera lens, and thence through the tiny embossed lenses on the film to the sensitive emulsion coating on the opposite side, where i t is recorded (see Figure
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25) The function of the lenses embossed on the film is to guide the rays of light falling upon each tiny area and lay them on the sensitive emulsion as three distinct impressions corresponding to the three filter areas, so that the three colors covering the lens are imaged behind each tiny cylindrical lens as three parallel, vertical strips, because the tiny cylindrical lenses are parallel to the stripes of color on the filter. Thus, the width of each of the minute areas of emulsion is subdivided into three parts related to the three filter areas and affected by light that is able to pass through the F ~ o m e25 different colors. The sum of these small affected areas of film constitutes the whole photographic image. A red ray from an object in front of the camera, for instance, reaches the sensitive film a t a spot related to the red area of the filter. The reversal arocess turns this affected s ~ o t into a transparent area, leaving opaque the adjoining, unaffected areas related to the green and blue segments of the filter; so, also, with green and blue colors and with combinations of colors. The sum of the points on the scene containing red makes a photograph from red light on the emulsion areas.related to the red filter area; the sum of the blue also makes a separate photograph; and similarly, the green. In Figure 26 is shown a photomicrograph of a Kodacolor picture of a number of strips of colored paper, the strips being very narrow and photographed FIGURE 26.-Photomicrograph of Kodacolor a t some distance, so that a considPicture of Colored Bands. erable number of different colors are included in the small section shown. It will be seen that the pattern is divided by dark, vertical lines, between each one of which the same pattern repeats. These vertical lines are the edges of the small film
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lenses, and the pattern shown between each pair of lines shows the distribution between the red, green, and blue emulsion areas corresponding to each of the colored strips. Thus, in one strip it will be seen that twothirds of the strip is clear and only one-third is darkened. This corresponds to a color such as yellow, for instance, in which only the hlue area is darkened, the green and red being transparent. In projection, the optical system of the camera is reversed. Behind the film are placed a condenser and a source of light. On the projection lens is a color filter of the same kind as that used on the camera lens and of the same three primary colors-red, green, and blue. When the picture is projected, the opaque areas of the film prevent the light from coming through the corresponding filter on the lens and thus falling upon the screen. For each color, theiefore, the emulsion areas, whose density is determined by the exposure given in the camera and the subsequent reversal processing, regulate the amount of light transmitted through the red, green, and hlue segments of the filter on the projecting lens and thus determine the color which is projected upon the screen; so that on the screen we obtain a reproduction of the original colors of the scene Owing to the absorption of the light by the colors of the scene photographed in projection, it is necessary to be content with a small picture on the screen; so that with the highest power of tungsten light available, only small screens can be used, such as those which are suitable for use in the home. In the camera, jt is necessary to have some method of controlling the exposure for the three different colors. The film may vary somewhat in its sensitiveness to red, green, and blue. Even if a film be made which has even sensitiveness to the three different filters, it does not follow that this can he duplicated exactly, and on the exactness of adjustment depends the color rendering. The adjustment for the varying sensitiveness -of the emulsions is accomplished by cutting down the length of the color filters used in the camera. This is done by what is termed a ratio diaphragm cap. After the emulsion has been tested, these caps are chosen to correspond to the color sensitiveness of that particular batch of emulsion, and the cap is then placed on the end of the film in such a position that it cannot be missed by the user. All that is necessary when placing a new film in the camera is for him to take his color filter out of the camera, and replace the ratio diaphragm cap by a new one from the end of the fresh film. In this way, he can always he sure of getting the correct ratio of exposures. As will be realized, the projector system is optically an exact inversion of the camera system, but there is in general one difference between the camera and the projector: the focal length of the projection lens is usually made greater than that of the camera lens. The relative apertures must be the same; that is, the cone of light coming through the projector lens
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must have exactly the same angle as the cone of light coming through the camera lens, since otherwise the optical conditions will not be reproduced correctly, and the colors will not be correct. A difference in focal length can, however, be compensated by the use of a supplementary lens, which places images of the color filters a t the right point in the optical system; and this lens, which is termed a cmpmsator, is fitted on to the lens of the projector. It makes no difference when black and white pictures are being projected except to change slightly the focal length of the projection lens, but it is necessary to obtain an even screen when Kodacolor pictures are being projected. The results obtained with this Kodacolor process are of very good quality indeed, as would he expected from a three-color additive process. The process has, it will be observed, certain fimitations. As with a screen plate process, the small size of the separate units makes a considerable demand upon the resolving power of the emulsion,and a finegrained emulsion is necessary, which is consequently limited in speed. The pictures can be taken only a t a very large aperture, approximately f/2, in full sunlight; that is, about three times as much light is required as is necessary for other processes of color cinematography employing filters but in which coarser grained and therefore faster emulsions can be utilized. The preparation of duplicates or prints offers very considerable difficulties, and while some progress has been made in the preparation of these duplicates, especially fot' standard 35-mm. sized films, the process is not simple, and the ordinary mamfacturing processes used in the making of standard motion pictures are not applicable. In projection, there is a very serious loss of light caused by absorption of the filters, which always occurs in additive processes of color cinematography but is somewhat greater than that shown by other processes. The subtractive processes are free from this defect. There are therefore a considerable number of difficulties to be overcome in the adaptation of Kodacolor to the production of color pictures for the motion picture theater, but its instantaneous success as an amateur process seems to show quite clearly that it has solved the problem of color motion pictures in the borne.
