The Cine Technician (1943 - 1945)

122 THE C I N E T E C H N I C I A N November— December, 19 A Precision Integrating Sphere Densitomete] J. G. Frayne & G. R. Crane (Electrical Research Products Inc., Hollywood, California) Reprinted in part from The Review of Scientific Instruments, Vol. II. No. 11. 330-355, November, 11>4« » Printed in U.S.A. Editors' Note. — As yet there are only two models of the E. R.P.I, densitometer in use in this country A member of the Editorial Committee has been using one of these for some months past and elaiiv it is the finest instrument yet designed, and for case of use, lack of eyestrain and absolute accuracy i has no compare. An integrating sphere type of physical densitometer has been devloped with a view to standardizing density measurements in the motion picture industry. The instrument accommodates films or plates up to 5" wide. Scanning beam dimensions of 0.025" X 0.100" are used but may readily be changed. Density values corresponding to (1) the equivalent of standard visual diffuse or (2) photographic printing density may be obtained by insertion of suitable optical filters. Densities up to 3.0 are read directly on a special three-scale logarithmic meter which provides a uniform density scale. A chopper in the light path makes possible a.c. amplification using a highly stabilised amplifier with a highly selective gain-frequency characteristic. This latter feature permits operation in daylight or in a room illuminated with ordinary a.c. lamps. Calibration is based on the inverse square law. The optica] schematic diagram of the integrating sphere densitometer devised by the authors i1shown in Fig. 1. Two important considerations have been kept in mind to ensure measurement of true diffuse density irrespective of the opacity of the deposit. First, the cone of the angle of incidence has been kept to a value slightly under 25°, which reduces to a negligible amount the error introduced by having the average light path somewhat longer than the thickness of the emulsion. Secondly, by proper placing of the cell and light baffle, no direct radiation through the entrance hole or direct reflection from the " hot spoi " at the bottom of the sphere is permitted to fall on the active surface of the photo-cell mounted within the sphere. This ensures that the cell response is, for all values of density, proportional to brightness of the snhere wall which is, in turn, proportional to total flux transmitted through the sphere opening. Density Range. Tn order to measure effectively the range of densities in photographic processes, it appears necessary to be able to read as high as 3.0. This means a range of 1000 to 1 or a 60-db input voltage ratio to the amplifier of the densitometer. Since it is extremely difficult to cover this range in one scale, it is desirable to use 0 multiple scale with three ranges of density ; namely, ranges of 0 to L.0, 1.0 to 2.0 and 2.0 I 3.0. This may he accomplished by inserting i. the amplifier circuit a 40-db loss for the first range 20-db loss for the second range and no loss for tl highest density range. Design. As shown in Fig. 2, this instrumei has been designed to be mounted in a table wit its panel flush with the table top. The head assen bly ami meter case are above the panel, located for maximum convenience in operating and r< ing. A steel case houses the sphere, amplifier and associated equipment. Optical System. The optical system is sho schematcially in Fig. 1. The light course is a stai c0*mns[» lehs diffusing bloc* objective, it us dard lamp with a prefocus base. The filament is operated at a relatively low temperature to ensure long lite and the current is supplied bj saturation type voltage regulator which maintains constanl current over a wide range of line voltages. Ballasl lamps are also used and in 1 tions where the line voltage variation is less thai 1 volt, the regulator is not required. The condenser lens assembly consists of a pa of plano-convex lenses whose mounting also tains the coloured glass filters. The image of th>