The Cine Technician (1939)

Aug. -Sept., 1937 THE CINE -TECHNICIAN 83 I I I L_L l l I I I SCALE INCHES FIGURE 2. Magnification of the Arc Crater It will perhaps be contended that the increase in the relative aperture obtained in this way is at the expense of the crater magnification at the film gate, and that the uniformity in screen brightness will be unsatisfactory. Of course, the crater of the high intensity arc is not uniform in brightness, being brighter at the centre than at the border. For this reason, and also in order to provide some tolerance in the position of the arc, present illuminating systems are made to have a higher magnification than would be necessary to just fill the aperture. However, when the lenticular colour films are projected with the above system, the corners of the picture do not appear to be more poorly illuminated than is the case with the average black and white system. The reason for this lies in a particular requirement of the camera and projector lenses used in the lenticular film process. The lenses used in black and white work, both in the camera and to a somewhat less extent in the projector, cause a falling off in the marginal illumination due to the fact that the lens aperture cannot be completely filled for oblique angles. With some of the camera lenses ordinarily used in black and white work, this becomes so bad that the corner illumination falls nearly to zero. This results in a print with a higher density at the corners than at the centre of the picture. When this print is projected, the additional density at the corners adds considerably to the deficiency of corner illumination already present in the projection system. This property of the lenses becomes objectionable in the lenticular colour film process, but for a different reason, as is seen in the diagram, Fig. 3, which shows different views of the lens and colour filters as they would appear when viewed from different points on the screen. Disproportionate areas of the colour segments are illuminated for different positions around the margin of the screen, a condition that leads to an uneven distribution of colour on the screen and cannot be tolerated. Thus, because of the choice of lenses which this makes necessary, one can afford to use a lower magnification of the crater. However, it may be desirable to have a slightly larger crater image than that used in the present system, and this could be accomplished by substituting a 9 or 10 mm. carbon for the 8 mm. one now used. The Heat Problem Considering that there are already reports from theatres when using improved black and white equipment of too much heat at the picture aperture, it was not surprising to find in the preliminary trials with this more efficient optical system that the film was badly damaged by the terrific heat. Attempts to cool the film by a jet of compressed air were insufficient. Clearly some sort of heat filter had to be used. Previous experience with water cells did not favour their use in the theatre projection booth, so heat-absorbing glass was tried. Used in a single sheet, this broke repeatedly, even though it was of the heatresisting type. Cutting the glass into f-inch strips and mounting these side by side prevented breakage, but it was found that the glass would soon melt unless subjected