44
JOURNAL on CEEMICAL EDUCATION
JANUARY,
THE PROCESSES OF COLOR PHOTOGRAPHY. COLOR CINEMATOGRAPHY
1929
111
C. E. K. MEES,EASTMAN KOnAK COMPAN~',R O C A B SNEW ~ R ,YORK
The various processes of color photography which have been described are applicable to motion picture work, though, owing to the long exposures required and the loss of light by absorption, no practical process utilizing the principle of the screen plate has as yet been developed for cinematography. A process of very similar type has, however, recently been introduced for amateur cinematography and will be described in the fourth of these articles. The motion picture processes, like other
FIGURE 17.-Color Filter Rotating Disk for Kinemacolor Camera. processes of color photography, are divided clearly into additive and subtractive processes, and, as in the case of the photography of stationary objects, the additive processes developed first. Two-Color Processes One possible simplification in color photography is the substitution of two colors for three. While with the three-color process we cannot expect to get results from the subtractive method equal to those given by the additive, in a two-color process the possibility of getting a satisfactory result with the subtractive method is somewhat greater than with an additive method. In order to get the whites and blacks with the
additive two-color process, it is essential that our two colors should be exactly balanced or complementary, as it is called, because otherwise they would not give white on the screen; and this condition that the two colors should be complementary to each other greatly limits the range of colors which can be obtained. When working the subtractive process, on the other hand, whites are obtained by the absence of any color a t all, and blacks by both colors in full strength, so that colors can he used which are not exactly complementary to each other, and the range of uossible colors wbicican he obtained is greatly extended. Additive Processes In the case of motion pictures, there are two methods of additive synthesis. Either the three pictures may be projected successively, persistence of vision being relied upon for the blending of colors, or the color units may be projected simultaneously, as is done in the triple projection lantern. Successive Projection.-The earliest practical cinematographic color process was a two-color process utilizing persistence of vision for the addition of the pictures. By means of a rotatJ 17) of color filins disk (Fimre . ters placed in front of the camera, F~CURB 18.-Kinemaco or Projecting Machine. pictures are taken on panchromatic film alternately through a red and a green filter, the pictures being taken a t twice the normal speed, so that for each complete picture two negatives are made, one through each filter. The positives from these negatives are so projected through a machime similarly equipped with a rotating shutter, which is made to operate synchronously with the picture, that the pictures taken through the red filter are again projected through a red filter, and the green pictures similarly through a green filter (Figure 18). The succession of red and green pictures upon the screen produces complete synthesis by persistence of vision and gives the effect of a twocolor additive picture. This process was known as the Kinemacolor process and enjoyed a considerable success. It has often been revived with various modifica-
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JOURNAL OP
CHEMICAL EDUCATION
JANUARY,
1929
tions, such as coloring the successive frames on the film instead of using rotating filters or modifying the colors of the filters in the hope of improving the color rendering.
Simultaneous Projection.-The most complete and satisfactory process of color cinematography is undoubtedly the simultaneous three-color additive process worked out in its complete form by Gaumont. Three lenses are used in the camera, and three pictures are taken through the primaly color filters upon the same strip of film. A normal motion picture is 1 inch wide by 3/4 inch high and corresponds to four sprocket holes in the film. The Gaum o n t pictures a r e three sprocket holes in height instead of four for each color, so that the whole set of color pictures corresponds to nine sprocket FIGURE20.-Diagram of Three Lenses in holes, and the film is two and Gaumant Camera.
a quarter times the length of the standard film (Figure 19). After each exposure the film is pulled down, therefore, the length of nine sprocket holes a t one time, and a new set of pictures is taken, sixteen pictures a second being exposed just as normal pictures. The positives are projected in a machine fitted with a correspondingpull-down and gate and with three condensers and three sets of objectives fitted with special registering devices (Figure 20). The results given by this process are admirable, all colors beingperfectly rendered and the quality being in every way first class. Subtractive Processes An advantage of the subtractive color process for motion pictures is that the film so produced can be shown in any ordinary machine, and this advantage is even greater in the case of motion pictures than in the case of ordinary lantern slides. The obvious method of making subtractive
TWO-COLOR TAKING CAMERA mOJCCIION O R N l l "
motion pictures would be to make three negatives and to superpose the negatives in printing, recoating the film each time, and using a dyed bichromate process or a process in which the silver image is made to mordant the dye and so is transformed into a color image. The recoatiug and re-registering, however, present very great difficulties in practice and make the process costly; and it appears likely that the subtractive processes of color cinematography will use only two prints, that is to say, they will be chiefly two-color processes and these two prints may be either in the one emulsion layer or in two emulsion layers on opposite sides of double-coated film, a method which appears a t the present time to offer considerable advantages in the direction of simplicity. It is clear that, since there are two sides to film, i t is possible to coat an emulsion on each side of the film and to print the red image on one side and the green on the other. This process has been worked out in several
ways to a practical end, some of the color images being produced by the mordanting of a dye upon an image obtained by the conversion of the silver into some suitable substance, others by the imbibition of a dye into the gelatin, and still others by toning to colored images by chemical means. A typical two-color subtractive process is that worked out by J. G. Capstaff in the Research Laboratories of the Eastman Kodak Company. The silver is treated in such a way that the reaction products harden the gelatin selectively and thus a silver negative is transformed directly into a dye positive. In order to apply this to motion picture work, the negatives are taken in a camera in such a way that red and green pictures are taken successively, one below the other (Figure 21). From this strip of negative film a master positive is made, and this is then printed by means of a special projection printer upon opposite sides of doublecoated film (Figure 22). In this projection printer, the red positive is projected on to one side of the film and simultaneously the green picture on to the other, the images being slightly displaced vertically, so that they exactly register one on top of the other on opposite sides of the film. The emulsions being exactly the same and the light intensities the same, there is no difficulty in obtaining equal results in the two pictures. The strip is then developed and fixed for the two pictures and without further delay is passed into the bleach bath, which bleaches the silver and locally hardens the gelatin where the silver was present. The silver is then fixed out, leaving a clear-coated gelatin strip of film bearing, on both sides, the images in the form of hardened gelatin. The two sides are then dyed by passing through a dyeing machine, the side containing the pictures taken through the red filter being dyed blue-green, and the side containing the pictures taken through the green filter, red. On viewing the film so prepared, a two-color subtractive picture is seen, which, being on standard film, can be run in any machine in the same way as black and white. A number of other workers in color photography have used similar processes; notably, Hernandez Mejia and Thornton, whose patents for film having images on opposite sides are of considerable value in this connection. The Technicolor process resulted in a film having images on both sides, but the two images were printed separately and then cemented together back to back to make a compound film. A very promising method of making subtractive pictures is the imbibition process. In this, dyed images are prepared upon master films, and the dyes are transferred in succession in register on to a single film coated with gelatin. Provided that the mechanical difficulties can be overcome and sharp pictures obtained, results should be excellent, and the process
VOL.6, NO.1
THEPnocEssEs on COLOR PHOTOGRAPHY. I11
49
has the advantage of giving a film which has exactly the same properties in regard to curl, etc., as the ordinary black and white film, so that it can be inserted with black and white film and does not involve any changes in projection. Theoretically, the imbibition process could be used for three-color as well as two-color pictures, although the difficulties increase rapidly as the number of transfers become greater. Combinations of imbibition with other methods of making dyed images are very common in motion picture color processes. Working Details for the Processes of Color Cinematography Taking the Negative.-There are three general methods of arranging a camera for color photography: In the first, which was that originally used in Kinemacolor, the pictures were taken serially, a rotating filter being used in front of the lens, so that the pictures were taken alternately through a red and a green filter (see Figure 17). The objection to this is that a quickly moving object is in a different position when the red negative is taken from that which it occupied when the green negative was taken; and there will therefore be shown on the screen alternate bands of red and green, which produce a colored. striping a t the edges of quickly moving objects. There is also the disadvantage that the exposure must necessarily be shorter because the two pictures must be taken in the same period as that in which one picture is normally taken. Sometimes an attempt is made to lessen this disadvantage by using very light filters. Such filters cannot be recommended, as they are invariably detrimental to the color rendering. Another method of working is to take the two pictures, or in the Gaumont process, the three pictures, simultaneously, using two or three lenses (see Figure 21). In this case, the red picture being taken from a slightly dierent standpoint from that of the green picture, it will be impossible to register both the distance and the near foreground simultaneously. This defect may be called stereoparallax. I t is partly in order to diminish this that especially narrow pictures are used in the Gaumont process, so that the taking lenses can be placed nearer together. Stereoparallax is a disadvantage only when dealing with really near objects. For any objects beyond twenty-five feet, it is negligible. The use of two lenses is advantageous as regards the exposure necessary, because the pictures are then taken at normal speed. The filters can be as light as is compatible with color rendering, and the whole aperture of the lens can be utilized. Both the striping difficulty and stereoparallax can be avoided by using one lens for taking the two pictures, the light from the lens being split into several parts by means of prism systems. This is entirely satisfactory except for the loss of light involved. In practice, it is convenient for
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J O ~ N on AL CHEMICAL EDUCATION
JANUARY, 1929
distant scenes to have cameras using two lenses fitted with so-called beam s&Jitters for near objects, where the stereoparallax would be fatal, while the method of successive pictures is now used chiefly in title cameras photographing still objects. Production of the Positive.-The practical making of the positives for projection in the additive processes presents little more difficulty than for black and white work. More attention to detail is required, and a rather higher standard of regularity of quality must be maintained, but the process does not involve any special difficulty. When we turn to the subtractive processes, the positive making is, of course, the key to the whole matter. From every other point of view, these processes are the most satisfactory, and the only thing that has kept them back is the intrinsic difficulty of mak'mg the positives. A detailed discussion of the difficultiesof the subtractive processes and of the methods by which they are overcome would occupy too much space here. Projection.-The additive processes require, of course, their own special projector, the Kinemacolor machine having rotary filters which can be thrown out of the way, thus converting the machine to a black and white projector; while the Gaumont process requires a triple projection machine with registering arrangement which cannot easily be converted into a black and white projection machine. These special projecting machines are the great disadvantage of the additive processes and will probably always restrict their use. Both the Gaumont and the Kinemacolor processes involve a considerable loss of light, so that more current has to be used in projection. The absorption of the Kinemacolor filters, which are as light as possible, is such that only two-thuds of the incident light reaches the screen, and for equal brightnesses two and a half times as much current must be used. With the Gaumont process, the matter is somewhat worse; the filters are not only slightly darker than the Kinemacolor filters, but the blue filter represents a loss of one-third of the light; since, while the blue contributes to the color of the scene, it adds very little indeed to the total brightness. The subtractive processes, having their high lights represented by clear film, do not require any increase of current whatever, the brightness being the same as for black and white. This is one of the great advantages of the subtractive processes. Among all processes of color photography, the rendering given by the three-color additive process is supreme. Indeed, under favorable circumstances, the color rendering by this process is almost perfect. The subtractive processes, even the best three-color processes, do not give such good results in regard to rendering as the additive processes, because no dyes can fulfil the theoretical conditions so correctly as can the projection filters.
As has been explained previously, when we turn to the two-color processes the position of the additive and subtractive processes in regard to color rendering is reversed, and the color rendering obtainable by means of the two-color subtractive process is intermediate between that of a three-color process and that of a two-color additive process. From this general discussiou of the processes of color cinematography, it will be seen that the additive processes, and especially the three-color additive process, while able to give photographic quality and color rendering superior to the subtractive processes, seem to have a somewhat limited field, owing to the necessity for the use of a special projecting machine. These are likely to survive for the very best three-color work. For the wide field of general motion picture work, it is probable that a two-color subtractive process having the color in the film, so that i t can be used in any projector without increase of light, must prove the most suitable, and that this process will be one having the two colors on opposite sides of double-coated film.
