British Kinematography (1953)

August, 1953 43 SOME NOTES ON THE BRITISH STANDARD OF SCREEN LUMINANCE F. S. Hawkins, Ph.D., F.R.I.C. (Fellow)* ^j^HE British Standard specification for I screen brightness as it was then called, was published in August 1947. It defined the desirable limits of the brightness of the centre of the cinema screen, the measurements being made normal to this point. It also contained a note which pointed out that further work was needed to enable a more comprehensive standard to be laid down and to discover the most convenient method of measurement. Since then a great deal of work has been done on these and similar problems which has culminated in a revision of the standard. It now defines both the desirable limits for the luminance of the screen when measured from any seat in the auditorium and the optimum diversity between the luminance of the side and centre of the screen. It also describes the conditions under which the measurements should be made and recommends a type of photometer for this purpose. The work which has been done to establish the revised standard may conveniently be divided into three sections, viz. : (1) Experiments on the use of suitable types of photometer ; (2) Measurements of screen luminance in a representative selection of theatres ; (3) Experiments on the diversity between the side and centre of the screen, the diversity being defined as the ratio, side luminance/centre luminance. The experimental side of the work was carried out principally by members of the staffs of the Ship Carbon Company, the Morgan Crucible Company, the British Thomson-Houston Company and the Research Laboratories of The General Electric Company. They worked both in the laboratory and in cinemas and are indebted to many B.S. 1404 : 1953. cinema managers and projectionists for their help in making the latter measurements. Photometers The recommended type is a visual photometer. Such a photometer is usually compact and convenient to handle when compared with a photo-electric photometer and will measure the brightness of a small area of the screen but has in the past been thought unreliable except in the hands of a trained observer. Before the recommendation was made investigations were carried out on the use of this instrument by unskilled observers and it was found that the accuracy obtained was sufficient for this particular purpose. A group of twenty observers was collected, sixteen of whom were entirely unskilled in the use of a visual photometer and most of them were making such measurements for the first time. They were requested to measure the luminance of a small area of screen which was also measured at the same time by a high precision laboratory instrument. The tests were carried out in conditions similar to those found in practice in a cinema and the results obtained have been fully published in a previous issue of this journal.2 They show that eight of the observers obtained results within 5 per cent of the true value, six more were within 15 per cent, two were within 20 per cent and four, the remainder, within 22 per cent. It should be noted that the accuracy quoted for these results is the accuracy with which the absolute value of luminance in foot-lamberts can be determined. If the same photometer were used to make purely comparative measurements, e.g. to determine which of two projectors produced the greater screen luminance, then greater accuracy would be expected from its readings, for a number of sources of error, such as loss * Research Laboratories of The General Electric Company, Limited, Wembley, England,