Colour Photography

A process which is in some respects analogous to the screen plate process is one invented by R. Berthon in which film is em bossed on the back with minute lenses. A tricolour diaphragm is then placed at the lens of the camera and the film is exposed through the back so that the lenses form images of the coloured filters on the emulsion.

After exposure and processing by reversal, the film is projected with a multi-colour diaphragm attached to the lens of the pro jector, and the colours of the original object are thus reconsti tuted on the screen. This process is used entirely in motion pic ture work.

Bleach-out Processes.

Since many dyes are destroyed by the action of light, and since Grotthus and Draper showed that dyes are bleached by light of the colour which they absorb, it is clear that a printing process can be produced by mixing three colour dyes which are sensitive to light. Thus, if we make a mixture of auramine—which is bleached by blue light ; erythrosine—which is bleached by green light ; and methylene blue—which is bleached by red light; and coat this mixture in a layer of paper, we shall get a black surface. Now, if this be printed under a coloured original, the dyes which absorb the colours will bleach, and a coloured reproduction will be obtained, the accuracy of which will depend upon the correct adjustment of the conditions. The most important of these conditions is the relative sensitiveness of the different dyes and the choice of dyes which have the correct ab sorption. Since dyes such as those mentioned have little sensi tiveness to light, it is necessary to increase it by sensitisers, and a great deal of work has been done in this field in the development of suitable sensitisers, of which the best known are Anethol and compounds of a similar nature.

A commercial bleach-out paper was placed on the market under the name of "Utocolour" paper and was employed particularly for the printing of screen plate originals. The results obtained were

not very satisfactory, however, and great difficulties have been experienced in achieving any improvement in the process. It is difficult to adjust the three dyes to equality of bleaching. It is very difficult to remove the sensitiser and the bleached dye, and it is particularly difficult to stabilize the unbleached dye so as to pro duce a permanent picture. These various difficulties have pre vented the process from coming into any general use. (For colour photography as applied to motion picture work. see MOTION closer study of these the reader is referred to the works partic ularized in the short bibliography appended to this Section.

Incidentally, the consideration of photographic apparatus is complicated by continuous, practical developments in all types of gear. Camera construction, and to some extent the manufac ture of lenses and other gear, has hitherto been influenced mainly by three factors, the introduction of dry plates, enabling develop ment of the latent image to be postponed indefinitely, the substi tution of gelatine film for glass, and the perfection of methods of enlargement. Two factors are being added—the improvement of existing processes of colour photography and the swift expan sion of cinematography—the effect of which is already pro nounced. Today the production of amateur cine-cameras is enor mous and, although "still" photography can never be completely ousted by motion picture production, the design and supply of apparatus for the former is being as strongly influenced by the latter as the popularity of the old "stand" camera was by the ap pearance of the revolutionary Kodak.