British Kinematography (1949)

102 The conditions which led to the demand for the present -operation are the inevitable results of the gradual development in optical technique. In the early thirties the optical equipment in the majority of studios consisted of two main types of objectives : the old //3.5 lens, and the newer //2 lens. These days were before reflection reduction coating was in use, and the //3.5 lens had a transmission of about 70%, the//2 lens about 60%. Lens Blooming In the later thirties the first attempts were made to apply reflection reduction coatings. The optical manufacturers, ourselves included, adopted a conservative view, and decided not to introduce at that time an obviousl}7 immature process. Outsiders with nothing to lose stepped in, and many small shops in Hollywood and elsewhere undertook to apply the coatings. The early results were not satisfactory. Especially unsatisfactory were most of the attempts to coat old objectives. These early experiments were a natural, and indeed, a necessary step in evolution, and we know now the reasons for their relative failure. The rather drastic cleaning operations which are necessary to ensure a hard and efficient coat on the glass surfaces are altogether impossible with the lens components already mounted in their cells. Also the early vacuum apparatus was not comparable in efficiency with the present equipment. On the whole it may be fairly concluded that the early experiments on coating resulted in increasing the overall spread of transmission factors. Variations in Lens Transmission Lens users became quite vocal on the subject. During the period 1941 — 1947 there were nine papers published in the Journal of the S.M.P.E. describing methods and means for calibrating lens transmission. It was felt both widely and strongly that in order to provide the increased accurate control of exposure required not only in colour photography but also in man}' applications of black-and-white, some measure of standardisation was necessary. The demand crystallised that we should eliminate the variations in the transmission of all objectives of nominally similar aperture. This means that when a photographer sets his iris diaphragm to t/2.8 on any lens of any focal length, he must have the same exposure on his film. In order to determine the best means to ensure this result the S.M.P.E. Committee on Standards appointed, early in 1947, a Sub-Committee on Lens Transmission Calibration, under the Chairmanship of Dr. Kingslake, formerly of South Kensington, now of Rochester. Implications of Uniformity In the optical industry we were well aware of the problem and of all that it implied. In order to solve the problem we had to establish under the difficult conditions of the post-war regime a number of new techniques. The first in importance was to mature the present coating processes. It is plain that to meet the requirements of any method of standardisation of transmission we had to produce not merely an efficient coating, but in addition, the coating must be hard enough to withstand cleaning and still remain efficient. After that problem was solved we had to install the plant necessary to carry out the operation on a sufficiently large scale. It is probably not realised outside the optical industry how formidable has been the development work required to evolve the current technique of hard coating processes. The next problem was to design and build the apparatus necessary to make the measurements of transmission. On account of the import licence difficulty we decided that we had to restrict our apparatus to components then readily available in this country, and therefore this development had to start de novo.