ISOCHROMATIC PLATES Plates specially sensitised for colour, and used with compensating light filters or isochromatic screens. They may be roughly grouped into three classes : (i ) those which are sensitised for green and yellow rays of light, and give an approximately true colour rendering ; (2) those which are sensitised for the entire spectrum, often called " panchromatic " (which see); and (3) plates rendered sensitive to orange and red, but which lack green sensitiveness.
There are a great many varieties of the first type, but all possess what is termed a maximum of colour sensitiveness in the yellow-green region ; staining an ordinary plate with certain of the aniline dyes induces this sensitiveness, and ery throsine is characteristic of type i in its action. Panchromatic plates being discussed under their own heading, this article will deal only with the first and third types. The ordinary isochromatic or orthochromatic plate has a distinct maximum in every instance in the green-yellow region of the spectrum, and its sensitiveness generally ends off abruptly at about the D line (= A 5893). Such a plate can be prepared by bathing, this process giving excellent results provided that the plates bathed are used a day or two after being treated. Two solutions should be made up as follow : A. Erythrosine . . io grs. I g.
Alcohol . . . 4 oz. 166 ccs.
B. Liquor ammonim (.880) i drm. 52 Water . . . 24 oz. i,000 For use, I drm. of A is mixed with 6 oz. of B a few minutes before use. Great care must be taken only to bathe clean working dry plates of medium rapidity, about 150 or 18o H. and D. (pyro-soda or pyro-metol speed). A " safe " ruby light must be employed for the illumination of the dark-room. (See " Colour Screen or Filter.") Scrupulous cleanliness must be ob served, dishes etc., being thoroughly clean in the strict chemical sense. The quicker the bathed plates are dried, also, the better, so that a warm room in which they will become dry in two to three hours should be chosen if possible. The plates are placed in the dish, and flooded over with the solution, 2 oz. being allowed to each half-plate ; the dish is gently rocked for three minutes, and the plates are then placed in another dish and given six rinses in plain water or left under a running tap for two minutes. They should be placed in a new wooden rack to dry, and this rack should not be used for any other purpose. Dry in complete darkness.
A sensitiser for the green and blue-green rays, which tends to slow the plates, may be prepared thus : Auracin . . . . I gr. 23 g.
Boiling water . . . io oz. 1,000 ccs.
Allow to cool, then filter the solution, and add drm. of strong ammonia.
Plates of type 3 are difficult to prepare because, owing to their great red sensitiveness, the work must be done almost in darkness. A suitable solution for bathing may be made up as follows : Cyanine blue (ethyl cyanine or pinacyanol) . . I gr. 28 g.
Alcohol . . . . 8 oz. 1000 ccs.
Add oz. of this to 6 oz. of distilled water, add f drm. of strong ammonia, allow to stand for twenty minutes, and filter. Two ounces of solution will suffice as before for one half plate.
There has been a tendency of late to produce plates so highly sensitive to greenish-yellow that when used without a screen quite satisfactory colour renderings can be obtained. Such plates are by no means perfect, but they show a marked improvement over ordinary plates. A " no filter " plate may be prepared by means of the following solution : A. Erythrosine . . I gr. 23 g.
Distilled water . i oz. ioo ccs.
B. Liquor ammonia (88o) I 55 g.
Distilled water to . io I,000 ccs.
C. Silver nitrate . . 2 gr. 46 g.
Distilled water . . 1 oz. ioo ccs.
Mix equal parts of A, B, and C, and dilute the mixture with an equal volume of distilled water. Select slow ordinary plates, bathe them for two minutes each, rinse them well under the tap, and put them in the rack to dry. Very intense green-yellow sensitiveness is thereby produced, but no such plate will give an accurate colour rendering, because it is still sensitive to ultra violet rays, to extinguish which a yellow filter is necessary.
Isochromatic plates of commerce are made by adding carefully selected and purified dyes, dissolved in alcohol or water, to the liquid emulsion with which plates are coated. The sensitive particles of emulsion are thereby coloured, though often almost imperceptibly, and their colour absorption altered in conse quence, so that rays of light for which the plates are " sensitised " are absorbed. One of the earliest descriptions of an isochromatic emulsion for plates was given in Tailfer and Clayton's patent specifications of 1882-3.