Screen plates are conveniently studied under the following heads:— I. Method of production of the screen.
II. Examination of the screen as a whole.
III. Examination of the filter units.
IV. Examination of the emulsion.
V. Adjustment of the compensator and emulsion to the filters.
I. The screen may be either regular or irregular. The methods of production may be classified as follows: (I). Ruled lines. Method used by Joly and MacDonough.
(2). Dusting-on methods. Employed to produce irregular screens in the Autochrome, Agfa, and Lignose processes.
(3). Printing by means of bichromated colloids. These are used in the Warner-Powrie, Finlay and Duplex processes.
(4). Various methods of section cutting. The best known ex ample is the Krayn screen introduced in Germany in 1907 and taken off the market shortly afterwards.
(5). The use of mechanical printing or a combination of me chanical printing and dyeing. The Omnicolore and the Lumiere regular screens, neither of which was marketed commercially, were made in this way, as is the Dufaycolor screen now in use.
(6). Other processes. A variety of patents have been taken out for experimental screens which must be classed under this heading.
II. The screen as a whole when examined should appear to be of a neutral shade. This is known as the "first black condition." The adjustment is obtained by alteration of the area of the units as far as possible rather than by adjustment of the depth of colour. The total visual absorption should be as low as possible.
III. The separate filter units should have the purest possible colours, preferably without overlap in the spectrum. The best possible compromise is for a red filter transmitting from X 590 up wards; a green filter from X 590-490; and a blue filter from X500 down to the violet. The size of the screen units is determined by their invisibility. In order to get bright colours, however, it is necessary that they should not be too small, since otherwise the emulsion will not have sufficient resolving power, and the images behind the units will overlap. For regular screens the units should not be larger nor smaller than of an inch. For irregular screens it is necessary that the units should not be larger than one-third the dimension of regular screens, since the average unit will tend to be a clump of about Io grains; that is, having approximately three times the diameter of the single unit.
IV. The emulsion should be sensitive to the whole visible spec trum and of a resolving power depending on the size of the units.
In the case of the small units of irregular grains, its resolving power must necessarily be high, so that the emulsion must be slow and fine grained. It must also be coated in very close con tact with the screen, since otherwise the scattering of the light in the intervening space will affect the colour.
V. The emulsion should be so sensitized that the effects pro duced under the red and green filters are equal if possible. The compensating filter used on the lens can then be adjusted to the emulsion and to the filter units, so that the "second black condi tion" is fulfilled; that is, a scale of greys is rendered as grey after exposure, development, and reversal.
The effective speed of a screen plate is governed by three factors: (I) The sensitiveness of the emulsion; (2) the multiply ing factor of the screen; (3) the multiplying factor of the com pensator. The effective speed of the screen plates on the market is lower than that of normal negative materials, although at the present time they are fast enough to enable snapshots to be made.
Owing to the very thin emulsion necessary to give sufficient resolving power, the latitude of the process is small, and in sub jects with wide contrasts the shadows may be underexposed, while the highlights lose colour owing to overexposure and the consequent irradiation. Except with very flat subjects, there is practically no latitude in the exposure given in the camera, the results depending entirely upon the accuracy with which the exposure is determined.
One drawback to screen plates lies in the fact that they must be viewed as transparencies, either by projection or in a simple viewing device. The magnification under which they can be viewed is limited by the fact that the image is broken up into small units.