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1949 TELEVISION STUDIO LIGHTING 615
Gaseous lamps are ruled out by characteristics just described and arc lamps require too much attention, discharge fumes, and are apt to be noisy.
It would be desirable to secure all of television studio illumination from fluorescent lamps, because of the lack of heat and apparent breadth of spectrum. But there are two handicaps. First, fluorescents are inherently not adaptable to directivity, because of their size. And one cannot achieve artistic illumination without directivity. Another characteristic of fluorescents is that some of the mercury spectral lines pass through the fluorescent coating and take part in the illumination. This, together with the large proportion of the fluorescent illumination in the blue end of the spectrum produces a rather hard and contrasty picture unless filters are used. A very satisfactory remedy for these shortcomings is to add some incandescent illumination. The porportion is not critical. The incandescents add red components in which the fluorescents are slightly deficient and present a concentrated source which is easily directed.
A good arrangement in line with the above points is to provide the basic illumination with fluorescents and the high lighting, back lighting, and artistic modeling light with incandescents. This combination is widely favored and is being put in by a number of stations.
A very useful fluorescent lighting unit for television is shown in Fig. 2. It uses six of the Slimline type of tubes. They operate at 300 milliamperes each and 600 volts. They have no filaments and are instant starting. The 4500-degree white-type lamp is preferred. The tubes have a long life and produce about as intense an illumination as is available from any fluorescent.
The tubes are mounted in individual reflectors to concentrate the light in the desired direction. Fig. 2 shows that a desirable degree of concentration is achieved in one plane. As the unit is mounted with the tubes horizontal, this directivity is vertical. It will be noted that the half-candle-power points are about 35 degrees off the axis. Hence a field over 10 feet high will be illuminated at a distance of 10 feet. There is very little directivity in the horizontal plane. This is good as the television field is wider than it is high by the ratio of 4 : 3 and most of the action of a performance takes place laterally.
The intensity on the axis perpendicular to the center of the unit is 7000 candle power. That means an incident illumination of 70 foot-candles on a scene at a distance of 10 feet. Where two of these