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FILM MANUFACTURE 25
are sensitizers. Some sensitizers are extremely unstable to light, others quite stable. The exact nature of the sensitizing action is still obscure. although numerous theories have been evolved to explain many of its aspects.
Positive-type emulsions intended for duplicating purposes usually have a relatively large quantity of yellow dye mixed into the emulsion ; the effect of this dye is more physical than chemical since it tends to prevent or reduce intercrystal reflection and diffusion and the reflection of light to the emulsion from the base after the light has passed through the emulsion during exposure. The use of the yellow dye materially increases the resolving power of an emulsion at the expense of sensitivity ; more incident light is needed to produce the same density. Yellow-dyed and similar emulsions have been used satisfactorily for sound negatives for 16-mm where high resolving power is needed for good sound records.
A finished liquid negative emulsion contains about 6% gelatin (dry weight) and 4% silver halides. This mixture is coated upon one side of the base after an extremely thin under coat or "subbing" coat has been applied to the base to cause the emulsion to adhere to the base. The emulsion is warm when applied and is chilled as it passes over a chilled roll after coating on the base. The emulsion is then dried. A typical coating in liquid form might be 1/100 in. in depth ; this dries to something like one-tenth its application depth. Modern emulsion coating techniques usually include multiple coatings for even black-and-white films; multiple coatings are absolutely essential in integral tripack color films such as Kodachrome and AnscoColor. The total emulsion thickness for all layers of color film is but little more than that of the simplest black-and-white films.
Slitting and Perforating
Film is usually slit and perforated just prior to shipment. It is slit into appropriate widths (in this case 16-mm in width) and perforated. Both the slitting and perforating operations must be performed very carefully ; film manufacturers are meticulous about maintaining the mechanical tolerances of the finished film. The film is cut to typical lengths of 50, 100, 200, 400, 800, 1000. 1200, 1600, and 2000 ft. Upon cutting, the film is mounted upon cores or spools (as required) and packed and prepared for shipment.
50, 100, 200, and 400-ft. lengths are in common use in 16-mm cameras. 400-ft lengths are usually used in sound recording machines, although some amateur-type and special equipment may use 100 and 200-ft. lengths.