The Cine Technician (1939)

82 THE CINE-TECHNICIAN An-. -Sept., 1937 of different density, a standard filter was finally adopted which was thought to have the highest transmission it was possible to get without a too noticeable loss in colour saturation. The transmission of this filter, when used with the high intensity arc system to be described later, was 22 per cent. This multiplied by the 80 per cent, transmission of the lenticular film support gives an overall transmission of 17-6 per cent. Therefore the factor by which the normal illumination needs to be increased is 5-8 times. Optical System Fundamental Conditions As shown in schematic form in Fig. 1, the essential elements of a projection system suitable for lenticular colour films are : — light source, collective element, collimator lens, film gate, projection lens, and colour filter. A detailed discussion of the optical relations involved in the use of lenticular films is not within the scope of this paper, and therefore a mere statement is made of the necessary conditions to be observed in practice : — 1 . The light source must be imaged at the film gate. 2. The collecting element must be imaged at the colour filter. 3. It is essential to have all elements centered on the optical axis. 4. The colour filter must be located at the front focus of the projection lens. It will be obvious that the first three of these are the identical conditions for the optimum screen brightness and uniformity, even in black and white projection. The fourth condition comes about as a result of a particular optical property of the lenticular colour film itself and is dependent on the optical arrangement used in printing. Projection Lenses The greatest single gain in illumination promised to come from increasing the relative aperture beyond the f/2-5 systems commonly used. In view of the successful use in the 16 mm. field of lenses having a relative aperture of f/T6 or better, it was thought that it ought to be possible to set up a 35 mm. svstem that would equal this. Two f/1-6 lenses were secured with focal lengths of 120 mm. and 160 mm. Except for the somewhat inferior definition of one of them, these lenses were entirely satisfactory for COLLECTOR ELEMENT the purpose. On account of the much larger diameter of the lens barrel, it was necessary to make a new lens mount for the Simplex projector. Illuminating Systems Before the increased relative aperture could be fully realised, it was necessary to modify existing illuminating systems so as to fill an angle of f/1-6 and at the same time to fulfil the conditions necessary for use with lenticular films. The particular lamp selected for the first experiments was the Peerless "Magnarc,"* which appeared to be a good example of a high efficiency reflector system. After a number of different optical arrangements had been tried, using reflectors of different focal lengths, it was apparent that the only change necessary was the addition at the front of the lamphouse of an inexpensive condenser lens. To avoid breakage due to the extreme heat, this lens was made of pyrex. The complete optical arrangement as it was finally used is shown in the diagram of Fig. 2, which is drawn approximately to scale. The regular "Magnarc" reflector is 14 inches in diameter, and 5| inches from the arc crater. The plane surface of the auxiliary condenser is 28 inches from the centre of the reflector, and 5J inches from the film gate. This condenser is 4^ inches in diameter and 15 inches in focal length. The addition of this condenser to the "Magnarc" brings the image of the reflector into the plane of the three-colour projection filter. The filter is located near the front focal plane of the projection lens, a necessary condition for lenticular film projection. In order to allow7 the larger cone of illumination from the modified illuminating system clear access to the film gate, it became necessary to enlarge the apertures in the shutter housing and in the masks back of the aperture plate on the Simplex projector. When the full f/1-6 relative aperture is filled, there should be 2-31 times the screen brightness that is obtained with a corresponding system of relative aperture f/2-5. The actual screen brightness obtained with this system \va> slightly less due to mild imperfections in the quality of the reflector. Certain dark zones appear on the reflector surface when viewed from the film gate. This modified "Magnarc" system was used for a great part of the experimental work and for the demonstrations which are to be mentioned presently. * There are other lamps on the market similar to this one which should be equally suitable. PROJECTION LENS COLLIMATOR LENS FILMI GATE COLOR FILTER LIGHT y SOURCE / \ / J-X — , ^ '^-^^r ^A B \ 1 "^——~~ — ^~--^\7 G R 1^ FILM (magnified) FIGURE 1. Diagram of Projection Optical System for Lenticular Colour Films.