Journal of the Society of Motion Picture Engineers (1930-1949)

spends to a distance within the normal range of accommodation, the eye will attempt to accommodate for that distance. This momentarily throws the screen out of focus, so the eye then re-accommodates \ for the plane of the screen. If, by that time the image of the object — assumed to be still moving — is at a different "apparent" distance within the range of accommodation, the eye will attempt to accommodate for the new distance, thereby again throwing the screen out of focus. This cycle of events recurs with great rapidity, and is sometimes the cause of headaches amongst elderly cinema patrons, whose ocular sensory organs and muscles are, naturally, less responsive than those of younger people. With regard to the main subject discussed in Major Bernier's paper, namely, the development of alternate-frame stereo techniques, it is, perhaps, worth drawing attention to the fact that the majority of the basic problems involved were investigated in England by the writer, Edwin Wright and others several years ago. Work on such processes has been abandoned by most workers in this country, mainly owing to recognition of the fact that the disadvantages resulting from "time parallax" are inherent in all alternate-frame systems. There are several known methods of overcoming the flicker problem, that developed by Wright being as satisfactory as any; the writer considers it preferable to the use of the more complex Morgana shuttle movement. A description of Wright's method is given in the writer's book, Stereoptics — An Introduction, Macdonald & Co. Ltd., London, 1951. Major Bernier's comments concerning the flicker problem are made somewhat difficult to follow by this use of phrases such as "a flicker frequency of 72 frames/ sec, or 36 frames /sec per eye," which do not really convey what the author intended, as the important matter is the number of occultations per second rather than the number of frames. To understand the nature of the flicker problem it is essential to appreciate that with any projection system, whether stereoscopic or planoscopic, the rate of flicker per second is equal to the number of times per second that light is occulted from each eye. Thus, in ordinary planoscopic projection, the flicker rate is equal to the product of the number of frames projected per second and the number of blades in the shutter. When projecting planoscopically at 16 frames per second, for example, the flicker rate is 32 or 48 per second according to whether a 2-blade or 3-blade shutter is employed. In neither of these two cases is flicker perceptible to the eye, so the term "flicker" is really a misnomer in such instances. It is readily demonstrable that the minimum rate of occultation necessary to prevent the occurrence of objectionable flicker is about 24 frames/sec, this rate being achieved at a projection speed of 12 frames /sec with a 2-blade shutter or 8 frames/sec with a 3-blade shutter. Now, with stereoscopic systems of the type in question, light is occulted from each eye every time a picture intended for the other eye is projected. This means that in addition to the faster, imperceptible occultations produced by the shutter, there occur occultations at a slower rate numerically equal to one-half the number of frames projected per second. Accordingly, in order to provide the necessary minimum of 24 occultations per second for each eye, a projection rate of 48 frames per second must be adopted, regardless of the number of blades in the shutter. As this is generally impractical, it becomes necessary to adopt some arrangement such as those used by Wright and Major Bernier. The writer ventures, nevertheless, to express the opinion that such arrangements are not really worth while owing to the facts that "time parallax" errors are still present and that the apparatus is somewhat complex. He would like, in conclusion, to draw attention to the new single-film polarized light process some particulars of which are given in his paper "Stereoscopy in the Telekinema and in the Future," which appeared in British Kinematography, 18, No. 6: 172-181, June 1951. This would appear to be the most satisfactory polarized light process so far developed. August 30, 1951 L. Dudley Odeon Theatre Kensington High St. London, W. 8, England 71