PANCHROMATIC PLATES Plates made sensitive to the entire spectrum, and used either for isochromatic photography with a suitable screen where a full colour render ing is required, or for colour photography by the three-colour process. They can be made by bathing slow ordinary plates with certain aniline dyes, particularly some of the isocyanine deriva tives. Either of the two following baths is suitable for panchromatising :— A. 1 : 1000 alcoholic solution of pina chrome . . 8 mins. 2 ccs.
Ammonia (.880) • 4 ,, 1 ), Distilled water . 400 ,, 100 „ B. i : moo alcoholic solution of homo col (Bayer) . 12 mins. 3 ccs.
Ammonia (•88o) . 8 „ 2 ,, Distilled water . 480 ,, 120 „ Instructions for bathing will be found under the heading " Isochromatic Plates." Particular care must he exercised as to cleanliness, pure air for drying, and rapidity of drying in the case of red-sensitive plates, as they are liable to fog on the least provocation. A truly panchromatic plate should be evenly sensitive to the whole visible spectrum, but with practically all plates there are " gaps " in the sensitiveness—that is, comparatively less sensitiveness towards certain regions, particularly the bluish-green—due to minima of absorption in the dyes used. Both
pinachrome and homocol plates exhibit two minima in their spectrum sensitiveness, which can be filled up to some extent by the use of supplementary dyes.
Colour filters for either three-colour work or isochromatic photography must possess minima in their absorption spectra to compensate for any minima in the spectrum sensitiveness of the plates to which they are adapted.
Development should always be carried out with a cool solution, and in darkness or in a green " safe" light.
In process work, panchromatic dry plates are largely used for the three-colour process, the exposures being made directly through the colour filter and the half-tone ruled screen.