Journal of the Society of Motion Picture Engineers (1930-1949)

to about 120 rpm, an adequate speed for the shortest time during which impact occurs. The actual speed of the drum is measured just before and after each test by means of a Jagabi Speed Indicator accurate to within i of 1%. The optical system from light source to film on the camera drum is shown in Fig. 5. There is every possibility that other equivalent systems could be designed; however, the one shown in the figure was developed from the equipment and materials most readily at hand after a trial-and-error period in which the design was changed several times. The light source (not shown in Fig. 1) is mounted in the air stream of a centrifugal air blower immediately behind the cylindrical tube which contains the condensing lens, ground-glass plate, slit, and projection lens, in that order. The impact phenomenon occurs between a pair of field lenses, the first of which serves to collimate the light rays and the second, to focus the collimated rays on the surface of the objective lens. The light path between the field lenses is partially blocked by the knife edges on the bar ends, the distance between the edges at each instant of time indicating the deformation in the specimen. The camera lens, a Color Skopar I, //3.5, 105-mm coated lens, focuses the slit image on the film by means of a first-surface mirror set at 45° to the axis of lens arrangement. The image is brought into focus by viewing it on a piece of exposed film set over the holes in the drum and the camera case (see Figs. 1 and 2). There are two electronic circuits which are important to the proper operation of the camera: (1) the timing circuit of the bar release mechanism; and (2) the shutter delay circuit. The former operates through a cam on the shaft of the camera and times the release of the impinging bar so that the exposed strip of film will not include the region of the splice. In order that the bar will be released two contacts must be closed, a pushbutton controlled by the laboratory technician and the cam timing contact which is preset. It is expected that the bars will have essentially the same periods from release to initial impact for all anticipated values of drop height. Thus, the cam setting should not have to be changed, except for minor adjustments, until very large drops are attempted. The shutter delay circuit, as its name implies, causes the shutter mechanism to open the shutter just prior to the instant of initial impact. The actual exposure time is governed only by the speed of the drum and is preset on this basis. Thus far, Kodak Linagraph Panchromatic LP421 film and Dektol (D-76) developer have been used with good results. Summary The rotating-drum camera described here has been built to record an impact phenomena, which occurs in from 5 to 20 msec, in order to determine directly some of the dynamic properties of rubberlike materials at high rates of compressive straining. The optical advantages of a one-to-one slit-image relay system are utilized to produce, on a film strip, direct displacement-time data which could not be obtained by any other means. Electronic circuits are provided to time the release of the ballistic pendulums with the proper position of the film in the camera and to delay the opening of the shutter until just before the instant of initial impact. Acknowledgment The authors wish to thank R. E. Lewis, Physicist, Armour Research Foundation, and R. A. Einweck and C. R. Olson, Illinois Institute of Technology, for their help on this and other phases of the project. 48 July 1952 Journal of the SMPTE Vol. 59