Photographs in Natural Colors

Popular Mechanics, huhtikuu 1915

New Process Makes Reproduction of Unlimited Number of Prints Practicable for the First Time

The reproduction on paper of an unlimited number of photographic prints in the true colors of the original subject, which has been the dream of every inventor in the photographic field, is now possible by a process which is stated to be so simple that any intelligent amateur can master it. Frederick Eugene Ives, the American invertor of the half-tine process of printing and the three-color half-tone process, as well as of many devices in the field of applied optics, has taken out patents on this new photographic method.

Color photography on transparent slides has been in use for several years, but only one transparency could be made from a single negative. No reproduction in colors on paper has heretofore been possible by any simple and dependable rocess. The new "Tripack" process has been reduced to so uniform a basis that color prints can be accurately duplicated. Tests already made indicate that the prints will retain the brilliance of their original colors in spite of constant exposure to light.

The new process is based on the same principle as the three-color halftone. Three negatives are made simultaneously, by a single exposure, and from these, positives of different colors are printed. Two of the positives are on celluloid; the third, or bottom layer, is on paper. Each of the three negative plates separates out from the various colors of the object photographed one of the three colors of red, green, and blue. The negatives themselves are colorless, but one records in black and white the red rays of light reflected from the object photographed, one the blue rays, and the other the green rays. In the finished prints, one of the celluloid films is dyed red, and the other yellow, both being superposed upon a paper print of peacock blue. The print from the negative that recorded the green rays is on the red film, and the resultant image of the green portions of the subject is transparent and colorless. When this film is seen through the layer of yellow celluloid and the blue paper is also visible through it, the greens of the picture are the same color as in the original subject, since blue and yellow make green. By applying the same principle to the other positives, every object photographed will, when the prints are cemented together by a transparent varnish, appear in its natural colors. Intermediate shades, which are made up of various combinations of the three basic colors, are registered on more than one plate, so that in the finished print the correct blend is obtained.

The camera used in this new color photography resembles in outward appearance the well-known type of folding camera, and may be used for ordinary photography, with either plates or film pack. When a color photograph is desired, a set of three plates, arranged in a thin metal carrier in a special plate holder, is inserted. When the opaque slide in the from of the plate holder is lifted, a spring pushes the blue-sensitive plate forward on a hinge to the bottom, or "floor," of the camera. The plates for recording the red and green rays are held firmly upright, film surface against film surface. After the blue plate has fallen into position, a sheet of yellow glass is dropped to form an angle of 45° with the plate on the bottom.

When the shutter is opened, the light, after passing through the lens, is filtered through a small compensating glass. It next strikes the yellow glass, which reflects a part of it down to the plate in the bottom of the camera which is sensitive to blue rays only. The remainder of the light passes on through the reflector glass and first impinges upon the emulsion of the green-sensitive plate, then passing through a red, transparent coating on the film of the same plate, and registering on the red-sensitive plate. The length of exposure required is ordinaryli a trifle less than one second, in sunlight, with a rapid rectilinear lens. After the exposure, the pressure of a lever restores the yellow screen to its former position and the blue-sensitive plate into the carrier. The plates are developed in the usual photographic solutions with the aid of a holder which makes it unnecessary to remove them from the carrier.

The prints from the three negatives are made at one time in a single printing frame. That made from the red-sensitive plate is on a paper which gives a print in peacock blue. After printing, the celluloid film bearing the image from the green negative is dyed magenta pink, and that from the blue negative, yellow. The resulting yellow and red films are placed ypon the blue print so that the images exactrly co-incide, and the picture which results has all the colors of the original.

Transparencies may be made in the same way except that the blue print is also on a celluloid film. Experiments in the application of the process to moving-picture films are said to be very satisfactory.

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