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30 III. 16-MM FILM AND ITS CHARACTERISTICS
or it may be a parallel beam incident upon the film at an angle. Some of the incident flux is absorbed, some reflected — the remainder transmitted. In the case of a film, the transmitted flux must be "collected" in some manner as it is scattered by the film emulsion which is microcrystalline in structure. If numerical measurements are to have meaning, measurement variables must be reduced to a minimum; this is accomplished by the "American Standard Method of Determining Transmission Density of Motion Picture Films" ASA Z22.27-1947. American Standard diffuse density designates densities determined under the practical geometric conditions provided by any one of three standard means and methods. These conditions approach the ideal conditions for totally diffuse density given in Section 3.1.1 of the Standard as closely as practical equipment and methods permit. As the subject is extensive and involved, reference should be made directly to the American Standard for details.
Practical densitometers other than the specified standard types must be calibrated. As most commercial densitometers are not standard types, calibration consists in the measurement of density samples with a standard densitometer and comparison of the results with the practical densitometer under test. If the conditions under which the practical densitometer are used are consistent, measurements are ordinarily reliable. Practical densitometers are designed to measure density values directly on a density scale of the instrument ; they are also designed for convenience in operation. The type of practical densitometer used depends upon the accuracy with which the results are to be determined. For ordinary measurements in the range of 0.1-3.0, the Eastman 2A densitometer, a direct-reading optical wedge type instrument manufactured by Eastman Kodak, is satisfactory and low in price (approximately $100.00). For the measurement of fog (the density of unexposed but developed and fixed-out film) where the values are less than 0.1, a more accurate densitometer is required, particularly in measurements of finegrain positive film developed in good developers. Common fog values for fine-grain positive are in the order of 0.01. For measurements in this range, the Martens polarizing-head densitometer (obtainable through Ansco) and the Western Electric RA-1100 photoelectric densitometer (obtainable from the ERP Division of Western Electric) are suitable and reliable instruments. The Eastman 2A densitometer is quite rugged; the Martens and the Western Electric instruments require care in handling similar to that required by a good compound microscope.