AS the plates used for color photography are sensitive to all colors, it is obvious that we cannot use the normal red light for illumination of the dark room, and if this were reduced, as it can be, so as to be safe for the panchromatic plate, the light would be so weak that it would be practically impossible to see anything. It is an established fact, however, that the eye is seven hundred times more sensitive to green light than to red, therefore, a deep green light is used for color work. But even with this it must not be forgotten that the plate is sensitive to green, hence undue exposure of the plate should be avoided, and it is advisable either to work in the shadow of the lamp, or to provide a cover for the dish; the former is preferable, as it enables one to rock the dish during development.
Green safelights can be obtained commercially and also green-stained papers, which can be placed between two glasses and used instead of the ordinary red screen. They can be also home-made, and as they do not require such a careful adjustment of the quantity of the dyes as taking filters, they are fairly easy to make. They require no other dyes than those used for the filters, that is, one can use naphthol green, patent blue and tartrazin, or instead of the patent blue, brilliant acid green may be used. A suitable formula is:
Patent blue 1.75 g.
Naphthol green 1.75 g.
Gelatine, 8 per cent solution 700 ccm.
This is sufficient for 1 square meter. This screen should be bound up with another one prepared from:
Tartrazin 5.5 g.
Gelatine, 8 per cent solution 700 ccm.
When dry the two filters should be bound up together with a piece of tissue paper in between to diffuse the light. Only the two sides of this safelight should be bound; the top and bottom should be left free, so that any moisture, which might be driven out by the heat of the lamp, may escape. It is preferable to place the yellow-coated glass next the light source. The patent blue in the above formula may be replaced by brilliant acid green with equally satisfactory results.
It is possible, however, to make these safelights by merely soaking old plates in dye, or even old negatives may be used, provided the silver images be dissolved by the familiar hypo and ferricyanide reducer or potassium cyanide solution and the plate then well washed. The gelatinized glasses thus obtained should be soaked in either the patent blue and naphthol green solution or preferably in the following:
Brilliant acid green 5 g.
Naphthol green 5 g.
Water 1000 ccm.
Soak for thirty minutes and then rinse and dry. A glass should also be soaked in 2 per cent solution of tartrazin for the same time, and when dry the two screens can be bound up together with paper in between as already advised. To make these lights safer still, the tissue paper may be soaked in the acid green solution, or one or two thicknesses of paper used, or even thin blotting paper employed.
To those unaccustomed to work by green light the room will at first appear very dark, almost black in fact, but as the eyes become accustomed to the light it will be found that the illumination is quite sufficient and actually it soon appears so bright that one may get the impression that it is unsafe. The time needed for the eye to become accustomed to the light depends a great deal on the outside illumination; if one enters the room from a room which is brightly lit, particularly by daylight, it may be twenty minutes or more before full vision is obtained. With use one becomes so accustomed to the green light that it will be found preferable even for ordinary negative work.
But now we are in a position to state that even this dim green light is unnecessary, as it has been discovered that certain of the aniline dyes possess the peculiar property of desensitizing the silver salts, so that one has only to soak the plate in a weak solution thereof or add some dye solution to the developer, to be able in about a minute to use a bright yellow light for development, and this process in no wise affects the image. The best dyes for this purpose are phenosafranin or the ammonium salt of aurantia; stock solutions of these dyes should be made up, of the phenosafranin 1: 2000 and of the aurantia 1:1000. The former is rather more efficient, but it has the disadvantage of staining the gelatine deeply. It is also somewhat difficult to remove, but this will be dealt with presently. For the preliminary bath one part of the phenosafranin solution should be diluted with nine parts of water, while the aurantia solution should be used full strength. In either of the solutions the plate should be immersed for one minute and can then, without washing, be developed in the usual way. This preliminary soaking may be obviated by the addition of the dye solution to the developer, in the case of the phenosafranin at least. Ten per cent of its volume added to the developer is as efficient as the prior bath. As the dye has to penetrate the gelatine and its action is not instantaneous, it is obviously necessary to soak the plate in the dye solution, or if the dye-developer be used, allow it to act in a safelight for a minute or so. The soaking or initial development must be done first by the ordinary green light or in total darkness, and only after this can the bright light be used. Naturally also, as the silver salts are not completely deprived of all sensitiveness, this process must not be abused and the plate manipulated too near a very strong light.
Suitable safelights for this method may be made by soaking two gelatinized glasses in a three per cent solution of tartrazin for about fifteen minutes, rinsing and drying, and then binding up with tissue paper. It might be preferable to soak the paper in rose Bengal solution, as this makes a safer light, of a bright pleasant orange color.