Halftone Colour Process.—When reproducing colour sub jects by the halftone process, the plates are made in the same way as halftones, but the photographic plates must be colour-sensi tive (panchromatic) ; when making the exposure the light not only passes through the prism, lens and screen but also is trans mitted through what are known as "colour filters." Generally considered, these are either dyed (coloured) films of gelatin cemented between two sheets of optical glass (and attach able to the lens by means of special holders), or they may be plain sheets of dyed gelatin, inserted in the barrel of the lens in a slot provided for Waterhouse stops.
The action of the filters is identical, in that the filter allows only rays of light of a certain colour to reach the photographic plate.
A special colour filter is used for each colour negative. Thus, a negative only records the colour and its component tints of one of the fundamental colours of the spectrum.
To make a negative for the yellow printing plate, a blue filter is used; for the red printing plate, a green filter, and for the blue printing plate, a red filter.
The light, passing through the respective filters, prevents the colours and tints not complementary to the inks in which the printing plates are printed from being recorded on the special photographic plate.
As in the production of the halftone plate, the ruled screen is placed in front of the photographic plate, but for colour work, a circular screen, which can be turned at different angles, is used for each of the colour negatives.
The angles of the screen must be different for each colour to prevent a "pattern" or "moire" effect when the three colours are superimposed in printing. For one colour, the screen angle is set at 45° from the vertical, and for the others at 75° and 15° respectively.
The negatives are developed, printed on metal and etched in the same way as when making a monochrome halftone plate, but much fine etching, demanding great skill, has to be done in the reproduction of colour subjects.
The "fine etcher" must possess a good knowledge of colour and its analysis so that the final results will approximate the colour original. He must correct the deficiencies of the colour separa tions. This he does by a series of fine etchings, painting out certain parts and continuing the etching by local application of acid.
This operation may have to be repeated a number of times, and many proofs are pulled from the plates before final satis factory results are obtained.
It is now usual to include a fourth colour-plate when repro ducing coloured subjects. This is usually printed in grey or black; the "colour," however, is not dissected in the ordinary sense of the term, although a yellow filter is used. The fourth plate represents the whole of the tones in the original in their proper relation to each other, and becomes the key plate. The fourth printing allows for cleaner and more rapid working on the printing press, and at the same time provides a depth of colour and "quality" which, owing to the scientific shortcomings of the process, are not always possible to obtain.
The plates are finished in the usual way, mounted usually on metal or sent unmounted to the printer along with progressive proofs which show the shades of colour used in the proofing and their sequence in printing.
Halftone colour negatives are made either by the direct or the indirect method. In the first instance, each colour negative is made in the same way as an ordinary halftone negative, but the exposure is made through a colour filter in addition to the screen, thus recording on the panchromatic plate one of the primary colours and its shades and constituting, when developed, one of the halftone negatives from which a plate will be made to print in a certain coloured ink.
The indirect method entails the use of three or four photo graphic plates for each colour. The first exposure provides a con tinuous tone colour-separation negative. It is obtained by making the exposure through a colour filter.
The full-tone negative can be retouched to assist in correcting colour values. From the full-tone negative a continuous tone positive is made and from it the screen negative.
This concludes an outline of the principles and processes of photo-engraving. The casual reader is warned against any attempt to use the chemicals and machinery mentioned. The necessary skill for their employment can only be attained after years of training and practical application of the process.