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SECOND REFOCUSING LENS FIRST REFOCUSING LENS
A
PRIMARY IMAGE PLANE
Fig. 4. Manner of modification of image motion by refocusing lenses.
mirror pair, the order of reflection would be reversed, the incident ray first striking the upper mirror, then the lower. However, no change occurs in the manner of movement of the emerging ray at the time of reversal of the order in which the mirrors function.
As similar translational movement is imparted to each ray of light proceeding from the objective lens to the primary image, upon reflection from the pair of moving mirrors, the primary image as represented by the emerging rays must appear to move in the same direction and at twice the speed of the moving mirrors.
Figure 4 illustrates the manner in which the refocusing lenses modify the motion of the final image on the photosensitive film relative to the motion imparted to the primary image by the moving pair of mirrors. This figure represents a condition, approximated in the actual camera, in which the speed of the photosensitive film is the same as the speed of the mirror pairs. For this condition, the focal lengths of the refocusing lenses and their positions relative to the primary and secondary images must be such that the secondary image will be exactly half as large in any linear dimension as the primary image. Both primary and secondary image, as shown, include an arrow with a cross alongside at each end. As may be readily seen, remembering that the arrow in the primary image is twice as long as in the secondary image, the two crosses are twice as far apart in the
SECOND REFOCUSING LENS
Fig. 5. Schematic diagram of optical system as used in the Isotran camera at Battelle.
primary image as in the secondary. Hence, if one of the crosses in the primary image is regarded as a displaced position of the other, the displacement is half as great in the secondary image. Hence, the rate of displacement of the secondary image is half that of the primary image.
Besides the function of imparting apparent motion to the primary image, and consequent real motion to the final image on the moving film, each pair of mutually perpendicular mirrors serves as a focal-plane shutter governing the exposure on the moving film of the particular frame with which that pair of mirrors is concerned. As each pair of mirrors sweeps across the primary image, exposure of the different parts of the final image on the film proceeds in exactly the same manner as if the image of the reflectors themselves formed the moving slit of a focal-plane shutter on the film. All pairs of mirrors start and end the exposures of frames at the same absolute locations. The continuous motion of the film, therefore, effects the necessary displacement of the successive frames one from another throughout the length of the strip.
Figure 5 illustrates schematically the
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February 1953 Journal of the SMPTE VoL 60