Rectification of panoramic photographs
Ross, Leroy Ellsworth
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Panoramic photography is quite old, but is becoming increasingly important in the field of aerial photography. Due to increased efficiency of aircraft warning facilities and accuracy of anti-aircraft missles the problem of obtaining photographs for aerial reconnaissance and mapping and charting becomes extremely hazardous. The obvious solution is to fly higher and further away and to take oblique photographs. This doesn't seem to be difficult, at first glance, but the object to camera distance becomes extremely long, thereby reducing the scale of the photograph. Decreasing the scale of a photograph decreases its usability. The only way to keep a proper scale then, is to increase the focal length. Increasing the focal length increases the format size and the overall camera dimensions, and more important weight. Weight and size are at a premium in an aircraft and to combat this a folded optical system is coupled to a slit scan device which utilizes moving film. This reduces weight and size, but introduces other technical problems, which are presently being solved by the U. S. Air Force. Assuming that the panoramic camera is built and is operational suitable. A method of rectification must be devised in order to effectively utilize the photography. Three methods of rectification are discussed. First, a pancratic lens coupled to a slit scanning device. Second, a mechanical rectifier, utilizing moving film and paper. Third, a mathematical Solution. The pancratic lens-slit scan system tends to recreate the conditions in existence at the time of exposure. The Scheimpflug condition is neglected by selecting a slit width so small that the image remains within the focal range. During the exposure the images were in perfect focus from nadir to horizon, as the distances involved are considered to be infinity. But in rectifier the object to image distance varies from 4 feet at the nadir to 6 feet at 0 = 60°. Therefore a lens system had to be designed to keep proper focus for all object to image distances and the proper magnification, (to insure the correct scale). These two variables are a function of the angle of scan (0). The basic design for this pancratic lens starts with selecting 2 lenses of 22 inch focal length and computing the movements necessary to accomplish the required magnifications and focal distances. It was found that this could be easily accomplished, but the final lens system would have to be designed by a competent lens designer and would be much more complicated, due to a aberration corrections. For proper illumination the filament of the light source has to move as a function of 0 in order to keep imaged onto the aperture. The effective f/no very nicely stayed approximately the same, 14.75 to 17.25, throughout all the lens movements. For overall illumination the speed of the scan has to vary to compensate for loss due to the inverse square law, due to tilting of the image plane, and due to increased density of negative toward the horizon. The second system, the mechanical system, consists of either a pancratic or a fixed lens system, with the negative moving past a slit and the image being projected down on a table onto moving paper. The moving parts that have to be calibrated are: the negative, printing paper, light source filament, 1st projection lens, 2nd projection lens, rotation of lens system, image to object distance, and tilt of easel. Most of these relationships are easy to compute and control, but the paper speed when determined will be difficult to control. As has been observed in the field , continuous printing devices do not operate efficiently in operational organizations. The third system, the mathematical solution, uses surfaces which were derived by trial and error after extensive mathematical research. This system can be proved, mathematically, not to be perfect. In fact the distortion has not been reduced less than 14 per cent. This is too much for reconnaissance and charting purposes. In view of the limitations of the various rectifiers it is felt that the pancratic lens-slit scan system would be the most practical rectifier for field use. If accepted for field use this pancratic lens system could eventually replace the current tri-metrago method of charting. The panoramic rectification system would eliminate many operations and specialized techniques and save money by reducing man-power requirements per chart.
Thesis (M.A.)--Boston University