The Cine Technician (1953-1956)

150 CINE TECHNICIAN October 1955 A Technician's Notebook We are indebted to John Gow for this report of the September Technical Lecture THE TODD-A.O. PROCESS THE new film process known as todd-a.o. was explained in a lecture to members of A.C.T. and their friends, on Tuesday, 6th September, by Mr. Brian O'Brien, Jnr, the sen of a famous American optical expert. Mr. O'Brien, Jnr, who is now employed by the American Optical Company, and has participated in the project from its very beginning, gave his listeners a fascinating account of the conception, development and present uses of the process. He explained that Mike Todd, after whom the process is named, approached his father and asked if it was possible to produce Cinerama— large curved-screen projection— " out of one hole." The question, in other words, was whether a camera could be designed to produce a single negative which, when processed and projected, would give the same answer as the three cameras and projectors used in Cinerama. Mr. O'Brien, Snr, replied that it could be done but that it would need a large team of expert technicians and opticians to produce the answer. A free hand and financial backing were guaranteed. The American Optical Company were selected as being the most suitable organisation for developing the project, and the last two letters in the name " todd-a.o. process " were added, the a.o. standing for the American Optical Company. The first step was to design a camera and optics so that the picture when projected would fill a curved screen with even light distribution and focus over its surface. It was decided to use a camera with 65mm. film and the master-lens was to have an angle of 128°. The camera and movements were based on the Mitchell with five-hole pull-down. Film stock was to be Eastman although Ansco had been tried. Registration of film and tolerances were to be one-tenth those of A.S.A. Standard. It was decided that the aperture should be 2.072" by .900" and that all stocks, both negative and positive, should have the same perforations. One of the optical problems in volved in this process occurred when projecting the image on to the curved screen. Mr. O'Brien explained that, when shooting with the 128° lens, in order to obtain an idential picture on the screen it is necessary to project with a similar Fig. 1 lens. He illustrated the ideal projector position with a blackboard diagram (see figure 1) but pointed out that in a theatre of normal dimensions this would mean having a projection box in the middle of the theatre, thus obscuring vision for a big proportion of the audience. A lens which would give exactly the same image from a greater distance therefore had to be designed. This is known as the phantom projection lens. The film used in the projector is 70mm. wide in order to allow for three sound tracks on each edge of the film. Magnetic striping has been tried but, owing to the narrowness of the stripe, it was found to be noisy. Research is still proceeding to perfect this method. At present the normal optical recording is used. The projection of an image on to a curved screen was the next problem to be solved. Light striking a flat surface is scattered in all directions, as is shown in figure 2. On a curved surface, <r Fig. 2 . Lii*' Sox«c such as that in figure 3, a certain amount of light would reflect sideways, striking the surface again and softening the projected image. However, by using millions of small optical elements in the surface of the screen it was possible to control the light and reflect it towards the audience and not to any other part of the screen surface. This effect is shown in figure 4. • This lenticular screen was found to give improved quality pictures with increased brightness but it was also found that when running the projector at the normal speed of twenty-four frames per second a stroboscopic flicker could be discerned at the edges of the screen. In order to eliminate this effect projector and camera speeds Fig. 3 have been increased to thirty frames per second. The problem of release prints was solved by making various < Fig. 4 standard reduction prints from the 65mm. film to 35mm., either squeezed or normal, so that any theatres could show the final product, but adjustments of the speed of projectors to handle the increase of speed of film had to be made. The projector designed for the