British Kinematography (1952)

190 BRITISH {CINEMATOGRAPHY Vol. 20, No. 6 As well as converting the short wave ultraviolet light of the mercury discharge into visible light, we can with equal facility convert it into the long wave ultra-violet about 3600 A.U. Such lamps can, as easily as the others, be controlled in brightness which is an obvious advantage over the high pressure type of which the brightness cannot be controlled, and which needs a long period of warming up to reach full brightness. To exclude the visible light entirely necessitates the use of a black glass filter which is an expensive addition. There are probably a number of applications with or without filters and one with a particularly kinematograph application is the combination with carpets treated with a fluorescent material for gangway lighting. Surround Brightness Control Most of the applications of the fluorescent lamp which have been dealt with have been common to the lighting of auditoria and theatres as well as to kinemas. One of the few applications which is peculiar to the kinema is the control of screen surround brightness. The general problem has excited widespread interest and at the recent International Conference on "Illumination in Stockholm, in June 195 1 , it was recommended that each country represented should seek to collect information on this subject as well as on screen brightness values. The subject was recently reviewed by Pulman4 who pointed out that with the improved equipment now available it is possible to exceed the top limits of screen brightness laid down in B.S. 1404. He also noted that flicker perception can rise well above the commonly quoted figure of 32 c.p.s. at high brightness levels. The fluorescent lamp amply confirms this statement because although most of the light output from the lamp has a frequency of 100 c.p.s. on a 50 c.p.s. supply, a narrow region in front of the cathodes has a frequency of only 50 c.p.s. A proportion of people can readily perceive this and unless it is avoided by screening, or reduced by the use of long lag phosphors or careful coating control, it may be a source of discomfort. It is certain there fore that the normal frequency of 48 c.p.s. used in projection gives no assurance that flicker will be ruled out. Glare resulting from the high contrast between the picture and the surround is the main problem, but it seems probable that by reducing contrast discomfort, flicker discomfort can also be eased. The early history of kinematograph projection, with low brightness as the main problem to be met, has led to the use of black masking as one way of improving the apparent brightness. Modern work by Hopkinson, Mackenzie and Nixon,5 by Guth,6 and by Logan7 has shown that the brightness of the surround has a very important influence in controlling the amount of discomfort glare. Like all matters involving subjective judgment, it is difficult to get a clear cut answer. Great skill is needed in handling the people making the judgments, and in asking them the right questions. The latter work of Hopkinson8 demonstrates the problems involved. With some caution therefore it is worth recalling that experiments5 showed that the average luminance of the screen, the immediate surround to the screen and the general surroundings, should be in the ratio of 100 : 10 : 1. There was also another important conclusion that the angular subtense of the screen surround should not be less than three times the subtense of the screen itself, if the surround was to have its proper influence. After that digression, it may be said that this presents a difficult problem for the kinema engineer because he is limited by physical dimensions in every direction. In particular, the depth behind the screen may be restricted and he has in any case to avoid spillage of light from the surround on the projected picture itself. It is also desirable in order to avoid distraction, to have the surround uniformly lit, to be able to control the light and, if necessary, the colour. A long linear source such as the fluorescent lamp coupled with the control of brightness and colour now available obviously lends itself to this application. By choice of a suitable reflector, the main light beam can be concen