Excellent as are the results obtainable by the indirect or triple-negative processes already described, no method which involves so many operations can be regarded as ideal in its simplicity; and although the invention of these processes made three-colour photography and three-colour printing a commercial possibility, they were too lengthy and troublesome to be adopted by more than a very small minority of amateurs. The long-felt want was a photographic plate by means of which a direct colour photograph could be obtained, with approximately the same ease and certainty as an ordinary photograph in monochrome. This desideratum has been realised in the screen-plate.
The fundamental principle of the screen-plate processes is the same as that of the triple-negative method already described - the separation of the colour rays by means of light-filters of red, green and blue-violet. It is in the application of the principle that they differ from the earlier method. The screen-plate avoids the necessity of making three separate negatives, by combining the three light-filters in the form of an infinite number of microscopic lines, dots, patches, or grains of the essential colours, evenly distributed over the entire area of the plate. Superimposed upon this composite colour-screen is a film of photographic emulsion, made sensitive to all colours. The plate is exposed with the glass side towards the lens, so that all the rays of light which go to form the picture must first pass through the microscopic light-filters. Naturally, when the plate is developed, silver is only deposited under the red particles of the screen in those places where red rays fell upon them, and those portions of the emulsion covered by the green and blue-violet particles of the screen are only affected in the areas where green and blue rays respectively fell. The result is a composite negative image - the three colour-records split up into countless thousands of separate but closely adjacent particles or lines. The colour-screen, being protected by a special varnish, remains unaffected by the development of the sensitive film above it, and if the plate is fixed after development and examined by transmitted light, a negative image is seen in complementary colours, the positive colours being blocked out by the silver deposit. Instead of fixing at this stage, however, the negative image is usually converted into a positive, by dissolving out the reduced silver bromide and redeveloping the plate after exposing it to daylight - two very simple operations, occupying only a few minutes. The effect of the redevelopment is to deposit silver over all those portions of the colour-screen through which the light rays did not pass during the original exposure : thus all particles of colour not required for the finished picture are blocked out; and as the original silver deposit has been dissolved away, the colours blocked out by the first development are disclosed, the result being a transparency in the natural colours of the objects photographed.
Micrographs Of The Colour Screens (X 100).
Autochrome. Omnicolore. Dufay. Thames.
The screen-plate idea was originated by Louis Ducos du Hauron as long ago as 1868, but no practicable screen was made until 1895, when Prof. John Joly, of Dublin, produced screens formed of fine lines of colour in juxtaposition. These screens were, however, so costly to manufacture that the Joly process was commercially a failure; and it was not until 1907 that the first really successful screen-plate was placed on sale, by Messrs. A. & L. Lumiere, under the name of the Autochrome Plate. The introduction of this plate marks a new era in colour photography. From being a costly and lengthy process, involving many delicate operations with proportionately numerous risks of failure, photography in natural colours became at once so simple and inexpensive as to be within the reach of almost every amateur.
The Autochrome screen is composed of millions of grains of potato-starch, of an average diameter of about 1/1800 in. The method of manufacture is, briefly, as follows. The starch grains, separated into three lots, and dyed respectively red, green, and blue-violet, are intimately commingled until the mixture appears of a uniform grey tint. The glass plate having been coated with a transparent tacky varnish, the coloured grains are "dusted on," until the whole surface is uniformly covered.
A Portrait Study.
Reproduced by Tri-Colour process, and engraved and printed by The Half Tone Engraving Company, Ltd., from a colour transparency by the Thames Colour Plate, Ltd.
The grains arc then rolled into close contact, no interstices being left, and a protective varnish is applied. The screen is then coated with a panchromatic emulsion, and the plate is complete.
The mode of using the plate is as outlined above, but it should be noted that, in this and all other screen-plate processes, a yellow "compensating screen" is used before or behind the lens, to cut off the excess of blue and violet rays.
Since the arrival of the autochrome plate, a bewildering number of methods of screen-plate manufacture have been patented by different inventors, and many varieties of screen-plates are now on the market. Space will only allow of our referring in detail to those which are most generally in use.