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

dons the film and image had to be moved together. 4. In order to obtain the minimum amount of blurring due to the rapid movement of the combustion process, it was necessary to utilize the shortest possible exposure time. Since light intensity could not be varied at will, the other alternative was to maintain a maximum lens speed during the entire exposure. This type of operation is approximated with the use of a focal plane shutter. 5. The picture could not be so small as to lose the details of the combustion process. A standard 16mm frame was chosen as the smallest suitable picture size. 16mm pictures taken at 5000 frame/sec require a minimum film speed of 38 meters per second. 6. In order to make full use of the flame pictures the angular position of the crankshaft during the exposure had to be known. Similarly, the gas pressure in the combustion chamber had to be known at the same angle and in the same explosion for each picture. 7. Simplicity of construction was an important consideration that need not be expanded upon here. Of the optical systems available at the time, the system suggested by Wedmore2 seemed more nearly to fulfill the qualifications than any other. A camera incorporating these principles was built into the flywheel of the engine and is shown schematically in Fig. 5. Light from the combustion chamber, D, passed through the quartz window, F, and was reflected by the stellite mirror, H, into a stationary field lens, I, (a Zeiss Opal Tessar). The principal plane of the field lens was in the combustion chamber. The beam of parallel rays formed by the field lens passed through each of a series of small lenses, M, as they were moved through this beam of light by a large circular disc attached to the crankshaft and rotated in a plane perpendicular to the plane of the paper. Light from the series of lenses, M, was reflected by a corresponding number of right-angle prisms, N, also mounted on the disc and located one behind each small lens as shown in Fig. 5. Images of the flames inside the combustion chamber were formed upon the film, P, which was held against the inside surface of the rim of the disc by centrifugal force. With such a system, the image of a stationary object in the combustion chamber remains at rest with respect to the moving film (except for a slight twisting motion) despite the motion of the 30 small lenses. The duration of exposure of each picture was controlled by varying the width of the stationary aperture, O, which was close to the film and acted like a focalplane shutter. Another shutter, K, was provided with the necessary actuating mechanism so that exposure would take place through only one revolution of the disc. The moving lenses were f/2 motion picture camera objectives purchased from the Eastman Kodak Co. The focal lengths were closely matched, but slight differences could be compensated for by individual adjustment of the position of each lens in the large disc. The lenses were spaced 2.4 crankshaft degrees apart, therefore 5000 frame/sec could be obtained at an engine speed of 2000 rpm and adequate exposure obtained by adjusting the focal plane shutter to give an exposure of 2.2 crankshaft degrees for each picture or 91 -|% of the time between frames. It is interesting in this connection to calculate the amount of light lost in this optical system. The stellite mirror, H, Fig. 5, scattered about one-half the light incident upon it. The amount of light lost by Fresnel reflections at the glass-air surfaces of the rest of the optical system may be approximated from the equation where t is the total transmittance if F. W. Bowditch: Research Photography 477