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Three-Colour Photography

blue, yellow, red, hauron, colours and colour

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THREE-COLOUR PHOTOGRAPHY This particular branch of photography is, with the exception of the diffraction and Lipp mann's processes, practically the basis of all colour photography. It is based on the theory that by the use of three colours only all the colours of nature can be simulated, either by the use of three lights—red, green, and blue violet—when optical synthesis is used, or by the aid of three pigments—red, yellow and blue —when the subtractive process is employed on paper or glass.

The first idea of this process was enunciated by Clerk-Maxwell in a lecture before the Royal Institution, in i861.

Henry Collen, the miniature painter to Queen Victoria, writing to the British Journal of Photography (1865, P. 547), threw out a sug gestion, which, although theoretically incorrect, shows that he certainly conceived the idea, and apparently in ignorance of Clerk-Maxwell's sug gestions.

Ducos du Hauron sent, in 1862, a letter to M. Lelut, member of the Academie de Medecine et Sciences, in which he describes the principles of three-colour work, and though, like Collen's, they were erroneous, still they are of sufficient interest to warrant inclusion :— "Physical Solution of the Problem of Repro ducing Colours by Photography. method which I propose is based on the principle that the simple colours are reduced to three—red, yellow, and blue—the combinations of which in different proportions give us the infinite variety of shades which we see in nature. One may now say that analysis of the solar spectrum by means of a glass which only passes one colour has proved that red exists in all parts of the spectrum, and the like for yellow and blue, and that one is forced to admit that the solar spectrum is formed of three superposed spectra having their maxima of intensity at different points. Thus one might consider a picture which represents nature as composed of three pictures superimposed, the one red, the second yellow and the third blue. The result of this would be that if one could obtain separately these three images by photography and then reunite them in one, one would obtain an image of nature with all the tints that it contains."

Here also the theory of colour selection is erroneous, and founded on Brewster's theory, but the above note proves that Du Hauron had really conceived the idea of three-colour work independently of Clerk-Maxwell.

Du Hauron goes on to describe the use of three filters of deep red, deep yellow, and deep blue, and gives a sketch of a chromoscope by means of which the three positives could be seen visually, and even suggests a stereoscopic chromoscope. In November, 1868, Du Hauron took out a French patent for three-colour work, and here he used red, green and violet filters thus falling into line with modern practice. In r865, Rausonnet, of Vienna, attempted to produce three-colour photo-lithographs, but failed to obtain any result, and gave up the idea. In 1867, Charles Cros, a Frenchman, had quite independently been working on the same problem, and in 1869 published his ideas. Cros utilised the principle of monochromatic illumination of his subject, but he did not follow up the subject quite so energetically as Du Hauron, who was the first actually to produce a three-colour print. It was only after the discovery of the principle of orthochromatising that three-colour work made any advances. It is impossible to give a complete historical sketch of the subject, but the above includes reference to the first workers on the subject.

The commonly accepted theory of three colour work is that the filters and plates must be so adjusted as to give a reproduction in black—that is, in metallic silver—of the three sensation curves according to Clerk-Maxwell, but the requirements of the printing inks, or the projection colours, necessitate modifications ; and the generally accepted practice now is that the three filters should have a slight overlap, as follows : The red filter should transmit from A 7,000 to A 5,800 ; the green from A 6,000 to 4,600 ; and the blue from A 5,00o to 4,000 ; and equal density should be obtained under each filter.

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