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February, 1951
STABLEFORD \ BACK PROJECTION
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a sufficiently even picture over a wide angle so that the camera could pan, zoom or track, and the actors move about in front of the back projection screen without the effect showing in the resultant film. In fact, its characteristics should provide almost the same amount of scope and freedom as given by an orthodox set.
Characteristics of Translucent Screens
Measurements of screen characteristics, which were taken in the experimental work on screens used as a datum a theoretically perfect lambert surface with a factor of 10.
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These are very high orders of efficiency and are probably beyond the scope of any generally used front projection screen material. It must be observed that the best of these screens were capable of producing a standard of quality beyond that required for entertainment purposes, since they had to be photographed by a camera, the film processed, positives made and then ieprojected in a theatre; all this tending to degrade the definition and quality.
Kinema Tests of New Screens
As a practical experiment, four kinema
Fig. I. Characteristic Curves of various Types of Translucent Screens, obtained by the use of different Grades and Quantities of Powdered Glass.
Courtesy of J. Arthur Rank Productions Ltd.
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The lambert surface readings were taken at the same time as the other curves.
Fig. 1 shows such curves, taken by Hugh McGregor Ross.10 It will be seen that the range varies from a centre brightness 15 times that of the theoretical lambert surface, to a 1-5 : 1 ratio. The latter, however, is very dense and produces an unnecessarily high loss in overall illumination. For normal work, a centre-to-side brightness ratio of between 2 and 3 : 1 was completely acceptable, and this gives a brightness, as compared with a theoretically perfect lambert surface, of 3 or 4 times.
theatres, which were using back projection, and all of which had for long dissatisfied projection engineers, were selected. No particular screen characteristic was aimed at; what was thought would be a satisfactory screen was tried. It must be said immediately that the resultant picture, from the first showing, produced an astounding effect. On a picture 18 feet wide, the quality and brightness were better than anything seen in the West End or elsewhere. There was no sensation of hot-spot, and the coverage from the widest front stall seats was quite adequate,