Colour Photography

COLOUR PHOTOGRAPHY In 1810 J. T. Seebeck obtained a coloured effect by exposing to the spectrum silver chloride which had been darkened by ex posure to light. This experiment has often been repeated, but as the colours obtained cannot be fixed, it has no practical value. In 1891 G. Lippmann of Paris produced the first picture by interference heliochromy, although the possibility of doing so had been pointed out by W. Zenker in 1868, Lord Rayleigh in 1887, and 0. Wiener in 189o. In this method a perfectly transparent, grainless emulsion is coated on plate glass and exposed through the back with the sensitive surface in contact with a reflecting surface, such as mercury. The incident light is reflected back on itself, giving rise to interference, thus setting up stationary or standing waves in the emulsion with their crests or loops exactly half a wave-length apart. These standing waves produce ex posure effects so that on development, the silver is deposited in laminae half a wave-length apart and reflects light of double the separation, giving bright colours. Unfortunately the plates are very insensitive, and as the results can be viewed only in the hand or by projection with a special outfit, the method is not in general use.

Tricolour Photography.

The methods which are used at the present time for reproducing objects in colours by photography are dependent upon the suggestion made by Clerk Maxwell in 1861. Any colour may be reproduced by a mixture of the three primary colours, red, green, and violet. In order to reproduce any colour, therefore, we may analyze the different proportions of these three primary colours in it and then synthesize the colour by superimposing the three primaries upon a screen in projection. To illustrate this, Maxwell took three photographs of a coloured ribbon : one through a red solution, one through a green solu tion, and a third through a blue solution. From these three nega tives, three transparencies were made, each of which was pro jected by means of the coloured light by which it was taken, Max well adding a fourth picture taken and projected by yellow light.

In

1869 Ducos du Hauron published a small book on colour photography, in which he laid down the principles of three-colour photography, on which all later work has been based, and at ap proximately the same time Charles Cros published an article in which he had independently come to much the same results. Du Hauron's book is astonishingly complete and contains a very clear account of the two fundamental processes of colour photography including their application by different methods. These processes are known as the "additive" (that employed by Maxwell) and the "subtractive" processes.

The practical development of colour photography was, how ever, delayed for many years by the difficulty that the photo graphic materials available were sensitive only to the blue and violet of the spectrum ; it was with the greatest difficulty that materials sensitive to the whole visible spectrum could be ob tained, and with such materials very long exposures were required.

The introduction of colour sensitizing by Vogel in 1873 first made colour photography possible, and the introduction of the isocyanine and carbocyanine sensitizers at the beginning of the loth century made it practical.

The Additive Process.

Clerk Maxwell's picture was pro jected on to a screen by means of optical lanterns and was thus the so-called "additive" process ; that is to say, the colours were formed by adding light to light. This method was also used by du Hauron and Cros in a special instrument called a chromoscope, in which the three pictures were viewed in superposition, a plan later used and developed by F. E. Ives also.

F. E. Ives developed the additive process in practice. He took photographs through three dyed gelatine filters upon materials sensitised with the best dyes available at that time and projected the transparencies by means of triple lanterns devised for the purpose, these synthetic colour pictures giving extremely good colour reproduction upon the screen.