British Kinematography (1953)

September, 1953 HIRSCH, SYMINGTON AND PHELP I UNDERWATER TELEVISION 63 difference in frame blanking time in the television system and pull-through in the film camera. The pull-through and exposure times in the Cine Kodak Special are approximately equal so that at 25 frames per second only half the television information per frame could be recorded. By using a 2 : 1 interlaced raster one field could be recorded whilst the other was ignored as it occurred during the shuttered time. With this arrangement some 300 television lines were recorded in the camera frame height of 7.5mm. resulting in an image on the film of about 40 lines/mm. Due to the form of the light distribution in the C.R.T. spot no lininess was observed in the recorded picture. Experience has shown that very acceptable pictures could be obtained at this standard of definition. The synchronous camera motor was driven from the generator which supplied the television equipment. The synchronizing pulses were sufficiently rigidly locked to the supply frequency for there to be negligible float between camera and television. A simple stroboscopic viewing device was fitted in the camera driving shaft to show when the phase of the motor was such that the camera shutter was open at the correct time. Failure in this respect results in a " shutter bar" appearing in the picture due to the finite time taken for the shutter to uncover and cover the lens aperture. Underwater Difficulties The difficulties encountered in handling the equipment are numerous. The greatest problem is getting the camera to point at the required target. Obviously when the unit is on the end of several hundred feet of non-spin cable it is extremely difficult to manoeuvre the camera at all — particularly in a tideway. It is possible to control the camera to some extent by the use of a shot wire . attached to a second derrick and with a heavy shot weight on the end. The shot wire is attached to the camera, but allowed to run free. The camera can then be controlled by raising or lowering the shot and by moving the two derricks relative to each other. The ship can be moved by mooring fore and aft to buoys (in the case of working over a wreck) and paying off or taking up on the appropriate winches. Applications The number of possible applications for underwater television which spring to mind is apt to be bounded only by the imagination but some of the more obvious applications of general interest are : (a) Sea-bed survey, (b) Salvage and snip's hull inspection, (c) Dock and harbour work survey, (d) Marine biological and oceanograph ical research, (e) Fisheries. Sea-bed survey for determining the nature of the sea-bed surface is of considerable interest to oceanographers and forms perhaps the simplest application. Some incidents from trials undertaken by the Admiralty illustrated the possibilities although the existing simple equipment was not specifically adapted to this task, nor was this application directly under investigation. On the occasion of a demonstration, a diver was sent down to operate underneath the camera at about 120-ft. depth. The nature of the bottom was determined from the screen as being of coarse gravel ranging from ^-inch to 3-inch with a slight admixture of coarse sand. The diver was instructed to gather a sample, which he was observed to do. Examination of this completely confirmed the detailed information obtained from the screen. This result was obtained with the camera some 10 to 15 ft. above the bottom, a closer approach being impossible due to lack of remote optical focusing. One of the objects investigated in the search for the " Affray " was a formation on the bottom revealed by echo-sounder to be semicircular in plan, some ^-mile across and rising to a height of 50-60 ft. from the sea-bed, rather like a circular disc half buried at an angle in the bottom. As a possibility existed of the " Affray " being close to the steep edge of this formation where it might not be distinguishable acoustically, it was investigated by camera, the depth being rather more than