British Kinematography (1953)

34 BRITISH KINEMATOGRAPHY Vol. 23, No. 2 optics, and the translucent screen or other media through which the light had to pass. Starting with a colour chart, which has been prepared with poster colours for tests with colour film, the first step is to make an original Ektachrome of the colour chart. Mr. Hoult then showed a series of slide which demonstrated the original Ektachrome plate and the loss in the first generation reproduction. He proceeded by a further series to demonstrate that this deficiency in copying could be overcome. The colour reproduction, while quite satisfactory in the first-generation Ektachrome, is inadequate in the second-generation Ektachrome. This represents the problem which faces the technician who photographs back projection in colour. What can be done to improve this state of affairs ? The method of achieving an improved match will now be described. Materials for Duping Colour Plates The problem is primarily one of duping a positive colour image. It is necessary to project on to the screen a positive colour image, so this part of the problem resolves itself into obtaining a positive colour image which is as faithful as possible to the original scene. The colours in the plate should match the colours of the original objects and the contrast of the projected image should also match that of the original scene : that is, the overall gamma of the reproduction should be approximately unity. For still plates it has been found that these requirements are best obtained in Eastman Kodak Ektacolor cut sheet film. Ektacolor is a negative-positive integral tripack colour process using a negative material which incorporates two degrees of automatic colour masking, and a positive material which will yield relatively lowcontrast prints of high colour saturation and high definition. The fidelity of reproduction is high and the material will reproduce satisfactorily a long range of subject tones. Any material used for making plates must be capable of yielding several positive copies. It must also be possible for the user to vary the colour balance and density of the image in making the positive. These requirements are all met by Ektacolor. For making 35mm. colour plates, similar considerations apply. A negative-positive integral tripack colour process offers the best prospects and there are in this field a wider selection of processes from which to choose. The most satisfactory results were obtained by the use of Eastman Colour Negative and Colour Print Film1 which are of similar construction to Ektacolor and possess similar characteristics in their developed images. Projectors and Screens Having a satisfactory plate, a process projector and a translucent screen, it is necessary to project the plate on to the screen with as little loss in quality as possible. This is a major problem requiring a high-grade projector and a well-designed screen. There are to be found in the literature several accounts of process projection equipment, two of which were published recently in British Kinematography.2' 3 It will be found that when projected by a process projector on to a screen, the plate will have assumed an overall cast of some unwanted colour. This may be due to the colour of the projector optics, the screen or of any other medium through which the light rays may have passed, including the glass of floor mirrors, which may be quite thick and appreciably green in colour. The cumulative effect of these different elements is that the projected image of, say, a white object will usually appear yellowgreen, compared with a white object in the foreground illumination. Correcting the Colour Balance It has been the practice of certain workers to offset unwanted colour cast by modifying the colour balance of the plate, but this is not sound practice. Consideration of this procedure will show that correction can only be applied to those tones in the plate which fall on the straight-line portion of the emulsion characteristics ; hence, both highlights and shadows become inadequately corrected. A more correct approach would be to stain