The Omnicolore Plate

The Omnicolore plate, though not yet actively marketed in this country, possesses a special interest as the actual invention of the veteran pioneer of colour photography, Louis Ducos du Hauron, in conjunction with his nephew, R. de Bercegol. Its introduction immediately followed that of the Autochrome plate, in 1907. In differs from the latter in the formation of the screen, which consists of a film of gelatine, dyed in a regular pattern of red and green rectangles crossed by blue-violet lines. The red rectangles have an area of about 1/420 x 1/600 in.; the green ones are somewhat larger, and the width of the blue lines is about 1/500 in. The dyed gelatine being more translucent than starch grains, the time of exposure is proportionately shorter than in the case of the autochrome plate.

The Thames Plate

The Thames plate is a British production, the inventor being Mr. C. L. Finlay, of London. In this plate also the screen consists of dyed gelatine, the colour units being a regular series of dots of red and green, each about 1/220 in. in diameter, the interstices being coloured blue-violet. This screen is particularly translucent, and as the emulsion is also a rapid one and its colour sensitiveness so adjusted that the compensating screen need only be of moderate depth of colour, comparatively short exposures may be given with successful results. A feature of the Thames plate is that the screen may be had either with or without the sensitive coating. In the latter case it is used in conjunction with a pan-chromatic plate, the two being placed together in the dark slide, film to film. They are separated again before development, and the plate may be fixed as a negative and any number of positives printed from it, each of which, when bound up in register with any Thames screen, becomes a complete colour photograph. This method of reproduction is, of course, only possible with a perfectly regular screen, such as the Thames.

The Dioptichrome Plate

The Dioptichrome plate, another French product, is the invention of Monsieur Louis Dufay, and was first issued in 1909. It is now readily procurable in this country, and the results shown are so excellent that a wide popularity may be predicted for it. In structure the screen is somewhat similar to the Omnicolore (described above), the pattern consisting of continuous green lines, interspersed with rows of red and blue rectangles. The colour-rendering of this plate appears to be exceptionally good; flesh tints and white (the latter a very severe test for a screen-plate) being reproduced with remarkable fidelity.

It will have been noticed, from our general description of the screen-plate method, that there is no superimposition of colours. The colour sensation is not produced by the subtraction, from the light falling upon the photograph', of one set of colour rays after another. The rays which pass through the photograph to the eye are of the exact colours of the light-filter elements - red, green, and blue-violet - and it is by the addition of colour ray to colour ray in the eye itself that the correct colour sensations are produced. This is known as the additive principle, in contradistinction to the subtractive principle of the superimposition processes before described.

No satisfactory way of obtaining colour prints on paper from screen-plate photographs has yet been invented. For this purpose, the triple-negative method is still the only practicable one.