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TYPES OF AVAILABLE FILM 41
be wider than the slit. An emulsion is said to have Low turbidity when the image size approaches that of tin test object. A Lippman emulsion
has zero turbidity for all exposure levels. Other emulsions show an increase in turbidity with an increase in exposure above a specific exposure value for a specific material. Turbidity is usually defined as the amount of spreading per unit increase in logic exposure.
Generally speaking, the numerical evaluation of turbidity is considered by film manufacturers to be in the province of the research laboratory of the film manufacturer and of no concern to the film user. This is presumed to be the case because of the difficulties encountered in evaluating the test results, which are usually in the form of a microdensitometer trace. Comparative tests are more feasible for film users; they are made by photographing a slit with an optical wedge placed against it to attenuate the light logarithmically from one end to the other. Photomicrographs are then made and compared visually.
If a number of small circular areas are exposed with a certain exposure and a number of large areas are likewise exposed with the same exposure, it will be found that in customary developers the density of the large areas is smaller than the density of the small areas. This effect, called the Eberhard effect, is attributed to the retarding action upon development caused by the products of development.
Another manifestation of the same effect is a somewhat higher density at the outer edge of a uniform exposure area; the density at the center will be a minimum. Developing machines are usually designed to sweep away the products of development rapidly, keeping the Eberhard effect to a minimum.
Types of Available Film
In 1924, when reversal film was first marketed, the number of available types of 16-mm films were few ; they consisted merely of one reversal type, one negative type, and one positive type. The volume of 16-mm film manufactured has since grown to the point where it is significantly large compared with 35-mm raw stock production. The increase in quantity has been accompanied by an increase in the number of types available. These are :*
* Most terms used in this chapter are in accordance with "American Standard Nomenclature for Motion Picture Film Used in Studios and Processing Laboratories' ' — ASA Z22. 56-1947. On the whole, the nomenclature used in films is likely to be confusing because there are many ambiguities. Alteration of standard definitions where made is for the purpose of reducing ambiguity.