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September, 1953
HIRSCH, SYMINGTON AND PHELP : UNDERWATER TELEVISION
61
Fig. 4. The first experimental assembly.
almost nil due to the enormous quantities of solid matter held in suspension. But when very clear water is found, off Malta and Cornwall for example, it is possible to see considerable distances by daylight only.
To obtain reasonable results in most British waters it is necessary to use artificial light, but this presents the difficulty of placing the lights. The best position is as far away as possible and behind the lens so that the minimum amount of spill occurs.
There appears to be no particular advantage in choosing lights with a specific colour temperature in using colour filters. The data obtained by Hulbert (giving absorption and scattering against wavelength) off the coast of America shows this quite well. The American bay water is similar to very good British coastal waters and is what we would interpret as oceanic.
These remarks apply only to television, as the spectral response of the Image Orthicon is similar to that of the human eye slightly extended at both ends of the curve.
For photography it would be advantageous to use filters with certain types of emulsion.
Lighting
Various light sources are available for underwater use and as the conditions under which they are used are so variable it may be of interest to examine them in detail.
The most commonly used lamps are : —
1 . Tungsten
2. High-pressure Mercury vapour
3. Sodium.
Tungsten has the advantage that no inductive surges are necessary to start them ; when working at extreme depths the inductive striking surges can easily be lost in the long outgoing cables, assuming that the ballasts are retained on surface. To overcome this the ballast could be submerged with the television equipment. The efficiency of discharge lighting can be from three to five times greater than that of tungsten, but because of the many problems involved in using discharge lighting under water, tungsten is at present used. Various lamp housings have been designed giving flood and directional lighting. To illuminate a fixed area at given range, as the first experimental assembly