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BLACK AND WHITE REVERSAL 43
As the blacks (shadows) and the whites (highlights) have been changed from their inverse relationship such as is found in a still camera negative, the film is known as reversal film and the development method as reversal processing.
Reversal films are manufactured in two general types, universal reversal, which may be developed as either a negative or as a reversal at will, and regular reversal, which may be developed only as a reversal Generally speaking, universal reversal films, being intended primarily
Gelatin abrasionresistant coating (D)
Fast emulsion (B)~ Siow emulsion (A)
Film base (E)
Anti-curl gelatin backing (non-halation dye incorporated) (C) — — i
Fig. 1A. Photomicrograph of the cross section of a modern black-and-white film. This shows (A) the slow emulsion layer adjacent to the base, (B) the fast emulsion layer on top of the slow layer, (C) the anti-curl gelatin backing that contains a dye or a silver deposit to minimize halation, (D) the gelatin protective overcoating that prevents abrasion marks in the emulsion proper and resists abrasion because of its physical hardness, (E) the film base. Negative films and duplicating negative often have the anti-halation dye in the film base rather than in the gelatin backing. The film base is clear if the film is to be used for projection. Duplicating positive films often have a water-soluble yellow dye in the emulsion that is dissolved out of the emulsion when the film is developed.
for duplication and not for direct projection, utilize an anti-halation dyed base; regular reversal films, being intended primarily for direct projection and not for duplication, utitize a silver anti-halation undercoating that is removed during development. Reversal films are manufactured in different photographic speeds (exposure indices) ; common values are group 100 and group 50. These are ASA speed ratings given in accord