The Cine Technician (1953-1956)

THE CINE-TECHNICIAN Jan. -Feb., 1953 PROCESSING COLOUR FILM by George Ashton, A.R.P.S., & Philip Jenkins, A.R.P.S. IN general the processes of colour photography ■*■ which are of practical use today, operate by dividing the visible light spectrum into a number of parts and recording the relative proportions of these parts throughout the picture area on photographic emulsions. The minimum number of parts that the spectrum can be divided into is clearly two, and, as is generally known, it is possible to make presentable colour pictures by two-colour separation methods. However, these methods are at best a compromise and it does not seem very likely that they will be used for very much longer. We do not propose to deal with two-colour processes for this reason. Three-colour separation methods have proved more acceptable in their colour rendition even if they are more complex in the laboratory, and at best they will give extremely satisfactory colour rendering. It is very important to realise that all the current three-colour processes which are used for motion pictures do separate the spectrum into three bands and that they do make individual records of these three bands. Where the processes differ lies in the way these three records are obtained and how they are subsequently handled. In some processes the three records — red, green and blue — are photographed on separate strips of film, in others they are recorded as successive frames on the same strip of film and in a further class the three records are all contained in three superimposed layers on the same strip of film. Unfortunately, if duplicates are needed, and in our opinion a motion picture process which does not permit the making of duplicates of satisfactory quality is of no commercial worth, it is absolutely essential that the three records should be obtainable on three separate films at some stage of the processing. So in practice today the problem of Tin i>o.sition in rvhttiun to tin framt of th< fully jittmu and the v<rtivalhi-f\tt\n<) rt-f/ister pins used in cdIiiiii photography duplication in colour motion pictures becomes the problem of separations. This separation of the three colour components on to three separate films at once introduces a major problem. Since the three images, which are, of course, three silver black and white images, are to be used to print the final combined positive in colour it is clear that they must all be exactly the same, frame for frame, both in size and position on their respective films. This problem of registration is probably the most serious which has to be met when making separations of any type. Registration is accomplished by the use of the film perforations, the film being located in the camera, in the original instance, on two register pins, one on the track side at the top of the picture which fits the perforation fully, and the other on the non-track side which fits vertically only to allow for shrinkage across the film width. The track side pin defines the position of the frame which is being exposed and the pin on the non-track side prevents the film from skewing. The shape of the perforations is of some importance and in colour work it is usual to employ the Bell-Howell, or standard negative, perforation at least on all negative and master positive. Ansco have recently proposed a slight modification of this perforation which would enable it to be used for release positive also, without the disadvantages of the combined negative-positive perforations which have hitherto been suggested as standards. This modification consists of the addition of a fillet of radius O.OlOin. to the corners of a Bell-Howell perforation. The same relative positions of the big and small pins must be held throughout a colour process on all step printers and cameras in order to maintain register. Types of Separations When a method of colour photography is used which gives three colour records as black and white silver images directly, we can conveniently call the three films obtained " direct separations." When a three-layer type of film is used, which is colour developed to give either a colour negative or positive from which silver separations have to be made in the laboratory at a later stage, we can call these separations " indirect separations." Direct separations generally give the best quality screen picture and are the least difficult for the laboratory to handle. They can be made in three' ways: 1. By photography on normal panchromatic black and white stock such as Plus X through blue, red and green filters successively. This method can only be used on static subjects or subjects such