British Kinematography (1952)

188 BRITISH KINEMATOGRAPHY Vol. 20, No. 6 range of brightness was limited to 400 to 1. By increasing the size of each step a wider range could be achieved, but there was always the danger that noticeable flicker would then occur with movement of the dimmer. The minimum perceptible change, or the " Fechner fraction," when it is measured under the most critical conditions of viewing and at fairly high levels of illumination, is about 2 per cent. In practice such conditions are not met and the light on a scene is derived from a number of sources. The use of 100 steps of about 6 per cent each was quite acceptable. Using such dimmers the next problem was the size of lamp that should be used. It had to be a high loading type of lamp in order to get as much light per foot run as possible. The choice lay between the 2ft. 40W. lamp run in pairs, and the 5ft. 80W. lamp. The advantage of the 40W. lamps was that with their shorter length it was easier to initiate and maintain the discharge at low currents, and the first decision was to use these lamps. Further experience and improvement in the manufacture of the 80W. lamp showed that these lamps could meet all normal requirements and a further decision was taken to transfer to these lamps with considerable saving in cost and increase in efficiency. This was a case where in the interest of optimum technical performance we took a decision which had later to be reconsidered. The fittings used play an important part and during the application of these circuits important economies were achieved. At first sight the provision of the separate wiring to each lamp would appear to be a major item of expense. However, the low currents used and the development of very compact multiple cables has cut this cost and this now only appears as a reasonable low figure of the normal installation. Only in two cases is it a major item of expense. The first is in auditorium lighting where exceptionally long cable runs may be required. As stated in the preceding section this can be avoided by the cascade circuit and in such cases the additional cost of the matching transformers is amply repaid. The second case where difficulty arises is when it is essential to use existing wiring and, in this case also, the cascade circuit provides the answer. One other major item of expense which normally occurs in stage lighting is the provision of coloured filters which must be used for colour effects with incandescent installations. In one large theatre the expenditure on this item alone was reported to be £400 per annum. A three colour system of mixing may be used but it is relatively inefficient and it is quite common to change filters to meet the requirements of the individual producer. Filters are, in addition, fragile and particularly in front of the hot filament lamp have a short life. Blue filters, in particular, bleach very easily and a crack in such a filter may have a disastrous effect on a lighting scheme. The advantages that appear from a comparison of fluorescent and incandescent installations on grounds of efficiency are very considerable as has already been referred to in the first section and illustrated in Fig. 2. The ratio of efficiency varies between a limited increase of about 40 per cent in the red to a ratio of 27 to 1 in the blue. One other point needs to be referred to before we look at the system as a whole. In any stage production a fair proportion of the lamps are " on check" for some part of the time. An incandescent lamp still consumes half the normal wattage when its light output is reduced to blackout. In the fluorescent lamp on the other hand, the watts consumed are closely proportional to the light output, with the exception of the control gear losses. This point is made clear in Fig. 8, and it will be seen that the ratio of efficiency given here for a "White" lamp goes up by a further factor of 3 to 1 as the lamps are dimmed. To summarize these various factors in one single cost comparison is an almost impossible task. There is certainly some increase in the initial cost of a fluorescent installation. The figure of 25 per cent has been given but it will obviously vary from installation to installation. The savings are even more problematical, but in one theatre it is a matter of