ISOCHROMATIC SCREENS Only those rays of light that are visible to the eye should be allowed to affect the photographic plate ; besides the visible rays of the spectrum, there are two sorts of invisible ones—the ultra violet and infra-red—situated respectively before the violet and after the red rays. The ultra violet rays are particularly active photo chemically, and are present in daylight and all the kinds of artificial light usually employed for photographic work. It is the primary function of the isochromatic (or orthochromatic) screen to absorb these rays, and the ordinary screen is therefore a yellow glass of the requisite shade, which is front of or behind the lens, or immediately in front of the plate, or it is sometimes fixed in the lens diaphragm between the two components of .a doublet or other com pound lens. The next function of the screen is to cause rays of each colour to act upon the plate to an extent dependent on their visual bright ness. Thus a yellow daffodil photographed against a dark blue background should appear as a light grey flower against dark grey ; with an ordinary plate the daffodil would appear darker than the background. The isochromatic screen is yellow, and therefore " damps down " the action of the blue rays by partially absorbing them, so that in the negative the blue background would come out faint and would therefore appear dark, as it should be, in the print.
No plate, however carefully colour-sensitised, is sensitive to all colours in degrees proportionate to their visual luminosity. The erythrosine type of plate, for example, is always deficient in the bluish-green region. The screen must compen sate for such deficiencies by absorbing those colours to which the plate is most sensitive, to such an extent that the other colours are adequately recorded. The average " isochro matic " plate is not red-sensitive, and only an approximately correct colour rendering can therefore be obtained, unless a panchromatic plate and a properly adapted light filter are employed.
For general working, a very fair " ortho chromatic " effect is obtained by using a yellow green sensitive plate, and a yellow screen which cuts out all ultra-violet rays and depresses the blue-violet, though it is, of course, better to use one special type of colour-sensitive plate and a screen adapted accurately to it. The yellow screen is more usually made by dyeing the film of a piece of optically flat glass which has been coated with pure neutral gelatine. The dye should give as transparent a filter as possible ; that is, the screen should be pale, and exposure only increased by a minimum amount. Thus, while naphthol yellow and tartrazine are good dyes for the purpose, a screen can be made with filter yellow K, or rapid filter yellow, which gives equally good results, while exposure is only increased perhaps half as much as it is in the former case.
Gelatine-coated glass may be stained with a 1 : 50o solution of the yellow dye selected, and filters of various intensities should be made ; the best one to use may be found by experimental exposures on an actual coloured subject.
To adapt a light filter or screen correctly to a given plate, two methods are available. One is to test the experimental screens by photographing the spectrum through them, and varying the colour until each portion of the spectrum is recorded in the negative with a density pro portionate to its visual intensity ; the other is to photograph a test chart, which is most con veniently made by drawing concentric circles on a card and dividing each of these circles into two parts, one painted black, the other coloured ; blue, green, and orange are the best colours to use if a panchromatic plate is being tested, or blue-violet, blue-green, and gamboge if a green yellow sensitive plate. The circular card is attached to a small motor and revolved rapidly, and the amount of black in each ring is increased until all the rings appear of equal luminosity. If a correctly compensating screen be used, then each ring will appear, in a photograph taken of the disc, of equal density. The screen must be experimented with until this result is obtained.
A liquid filter is very convenient for experi mental work, as its colour may be varied at will by adding concentrated dye solutions. Thus by adding, say, m ccs. of a i : zoo solution of filter yellow and n ccs. of a z : 100 solution of crocein scarlet to water contained in a flat glass cell of 16 sq. cm. area, a liquid screen is obtained which satisfies the tests for a good colour rendering. Flat glass can then be coated with a 5 per cent. solution of gelatine, allowing, for example, 1 cc. to coat each io sq. cm. of surface ; each cc. of the gelatine solution must therefore contain if X or m g' of the yellow dye, of the orange dye, and so on. Stained gelatine filters are usually bound up with a cover glass, the two glasses being cemented together with Canada balsam.
Pot glass filters (made from glass coloured in the mass in course of manufacture) are usually inefficient, requiring prolonged exposures owing to a considerable percentage of grey in the glass.
The multiplying factor is important in iso chromatic photography, as by under- or over exposure and attempted correction in develop ment, the colour contrasts in monochrome may be somewhat falsified. All yellow screens necessitate an increase in exposure of from two to ten or fifteen times the normal. A screen with which five times the normal exposure is required is often designated a X 5 screen, and so on. The only satisfactory way to find the multiplying factor is to photograph a black-and-white object, giving varying times of exposure, and judging from the results which is the correct one. This should be verified then on a landscape.