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62
BRITISH KINEMATOGRAPHY
Vol. 23, No. 3
required, eight 150-watt spotlight reflector lamps (each giving a beam angle of 30° in water) were used in pressure tight housings. The lamps were mounted on a tubular gantr) so that they would illuminate the target area evenly. In turbid waters illumination is exceedingly difficult and the viewing range is limited. To assess the percentage of light transmission in water a hydrophotometer is used. This consists essentially of a lamp and lens assembly at the one end and a photo electric cell at the other with a ^-metre path between. The photo cell is fully modulated in daylight and lowered into the water, the reading obtained indicating the percentage light transmission in water. The accuracy of the instrument in turbid water is adequate, but when used in clearer waters the accuracy is inadequate, a difference of a few per cent. giving a great change in visibility.
Method and Equipment used for Recording
A test was made in the Channel last year but as specially developed film recording equipment was not available at the time, the basic components were assembled to form a rudimentary recording system as shown in Fig. 5.
The picture from the television camera was displayed on a standard demonstration monitor using a 12-inch diameter cathode ray tube with a so-called white screen. A metalbacked phosphor on the tube helped to maintain the required brightness and rast. The
■/. 5
tube was supplied with E.H.T. at 8.5 kilo volts and run at a peak current of 100 microamps. The television standards were 625-line 50 fields interlaced using a 4 to 3 aspect ratio. The normal picture size was reduced to 6 inches by 8 inches in order to minimize the effect of tube face curvature, and under these conditions a highlight brightness of 50-ft. lamberts was obtained. Past experience had shown that at this brightness Super X reversal film and a lens aperture off/2.8 gave sufficient exposure.
A Cine Kodak Special 16mm. camera was mounted on a framework rigidly fixed to the monitor. The whole assembly was mounted on castors for easy manipulation, but was securely lashed in position on board.
It was originally intended to use the Kodak 63mm. telephoto lens at f/2.7, as this was known to give a good image at the required conjugates, but owing to space restrictions on board it was necessary to reduce the focal length to 50mm. As this lens has a maximum aperture of f/1.9 an increase of two to one in exposure was obtained. A difficulty frequently encountered in recording television pictures on film arises from the