SCREEN - PLATE COLOUR PHOTO GRAPHY The process of producing photographs in the colours of Nature by means of a screen plate is based on the three-colour process enunciated by Clerk-Maxwell, but instead of using three sepa rate colour filters and three separate plates and prints, the colour filters are distributed on one surface in small areas, coated with a panchro matic emulsion, and the picture obtained on this support either by chemical reversal of the negative into a positive or by printing on to another plate of similar character.
The first idea of such a plate was conceived by Ducos du Hauron, in 1862, in a letter which, however, was not published till 1897. Ixt his French patent (No. 8,361, of Nov. 23, 1868) du Hauron says : " There is another method by means of which the triple operation can be done on one surface. The separation of the three elementary colours may be effected, no longer by three coloured glasses, but by means of one translucid sheet covered mechanically by a grain of the three colours." In a little work by him, Les Couleurs en Photographie, Solution du Prob lame (1869), he also deals with this subject, but a more elaborate description of this idea is given by Alcide du Hauron in Triplice Photographique des Couleurs, in which will be found the germ of all screen-plate processes, and any introduced of late years have been but variations of du Hauron's ideas.
In dealing with this subject the writer cheer fully acknowledges that he is following to a great extent and sometimes borrowing freely from a valuable paper on " Some Experimental Methods Employed in the Examination of Screen Plates," by Dr. Kenneth Mees and J. H. Pledge (Phot. Journal, May, 191o, p. 197). Screen plates may be classified under two headings : regular and irregular, or line and mosaic ; and can be further subdivided into the following methods of manufacture : (I) ruled lines, (2) dusting-on methods, (3) bichromated colloid methods, (4) section cutting, (5) mechanical printing or mechanical methods and dyeing, (6) other processes.
The first line screen by a ruling method was patented by Joly in 1895, patent No. 14,161, '95. In 1896, Jas. W. McDonough took out patent No. 12,645, '96. The Joly screen was com posed of lines having a width of in. (.12 ram.), and a separate viewing screen, with slightly dif ferent colours, was issued. The taking screen was placed in contact with a panchromatic plate, and from the line negative thus obtained a posi tive was made and bound up with a viewing screen so adjusted that the lines of the latter fell into contact with the correct lines of silver deposit on the positive. The McDonough lines were in. (0.8 mm.) wide, and some even as fine as in. were made. The dusting-on method was first patented by McDonough (No. 5,597, '92), and he claims the use of par ticles of glass, transparent pigments, gelatine, resin, or shellac suitably stained and subsequently coated with a panchromatic emulsion.
The autochrome plate, patented (22,988 and 25,718 of 1904, and 9,1oo of i906) by Messrs. Lumiere, is prepared by sifting suitably stained starch grains over a tacky surface, rolling them, and then filling the interspaces with a black filling. The average size of the starch grains is (.015 mm.). The plate is issued coated with an emulsion, and the image needs to be reversed after exposure, so that the original plate serves as the positive.
According to Mees and Pledge, Fawcett sug gested in the British Journal of Photography (February 22, isoor) the use of a screen plate with the emulsion coated thereon.
Palmer's patent (22,228, 1907) claims the use of ceramic colours or fluxes dusted on a tacky glass plate, and then fired in a kiln ; also in another plate he uses gum elemi. The Aurora screen plate (introduced by the inventor, E. Penske, in February, 1909) uses . mixture of three dyed materials dusted on a tacky plate, and fills the interspaces with a black filling. The fragments vary in size from vie in. (.03 mm.) to in. (.15 mm.), the average being about -3-hr in. (.07 mm.).