International photographer (Jan-Dec 1941)

Record Details:

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A COMPLETE MEANS OF INSTANTANEOUS PARTICIPATION IN SIGHT AND SOUND TCLE V Reprinted from the Journal of the Society of Motion Picture Engineers, hy special permission. Summary — The Television Committee of the Society during the past year has carried out a considerable amount of work as listed : (1) Flicker and visual fatigue in television has been studied, a preliminary report on which work is presented herein. (2) A study of the most suitable type of film for television transmissions has been carried out, a report on which will be presented also. (3) More material has been added to a bibliography and glossary of terms in the field of television, which work was started more than a year ago and which still continues. Preliminary Report of the SubCommittee on Flicker and Visual Fatigue General — Since early May of this year, a sub-committee has been actively studying the problem of frame frequency in television. In this assignment, it was instructed to correlate available information on the subject as affected by three major factors, namely: (1) Flicker. ,(2) Portrayal of motion. (3) Visual fatigue. The need for such a fact-finding committee has become more apparent within the past year and it was felt at the outset that the motion picture industry as a whole was peculiarly well situated to assist television in this work. Because of its familiarity with existing experience, the ability within its ranks, and ts tools for prosecuting new experimental work when the need for such work was determined, the Society of Motion Picture Engineers is in a particularly favorable position to sponsor such work. The first task was to index and abstract as much of the existing literature as seemed pertinent and possible. Following this work, the gaps in existing knowledge would be more apparent and as the need for further work was apparent, experiments and means for performing them could be devised. This report covers the first part of this program. Bibliography — A list of the articles and books found to date relating to this subject is appended. It is not hoped that this is complete and since it is only necessary that the information obtained be comprehensive, pertinent, accurate, and descriptive of the essential facts, completeness in the bibliography was not considered vital. Summary of Findings — Since television observation, as a visual task, is not essen tially different from motion picture observation, it is possible to correlate data from the latter field for direct use in the former. One important element in such considerations is the average brightness level found in current practice. a <j S so t in U <D it a 40 <" s 8 £^A --'D .8 1.2 Log. I 1.6 2.0 Fig. 1. Critical frequency vs. log intensity for 4 degrees diameter of stimulation area for 4 subjects (P. A. Snell, J. Soc. Mot. Pict. Eng., May, 1933, p. 367). General experience shows that visual fatigue accompanies any prolonged visual task and since motion picture observation can be no exception to this, it is not to be expected that television observation will be an exception. Opthalmological research has revealed the importance of object brightness in the problem of visual fatigue; therefore, data on the present screen-brightness practice in the motion picture theater is of fundamental importance to the object of this work. The data submitted have a direct bearing on the television problem since some general knowledge obtained from practical or every-day experience is available to every Field brightness ca1dle5 per s«. meter Fig. 2. Critical frequency vs. brightness for square wave — white light illumination cycle (Fig. 94, "Principles of Optics," Hardy and Perrin, McGraw-Hill Book Co., New York). one and correlated technical data are available to the specialist from the field of motion pictures. A survey made early in 1940 and covering a group representative of the larger theaters in the United States (seating capacity from 2300 to 3500) shows a range of central screen brightness of from 6 to 10 foot-lamberts, as reported by Mr. A. C. Downes of the National Carbon Company. These measurements were made with the projector operating without film For the smaller theaters , which are in the vast majority, it has been reported that a comparative figure would be about 4Vij footlamberts under similar and favorable conditions. Since these figures are significant in the study of flicker and visual fatigue, they are included in this report in order that the present practice may be correlated with the optical requirements. Reports from foreign sources indicate that brightness levels of the order of 10 foot-lamberts are being realized. This falls within the range of 10-+-4 to 10 — 1 foot-lamberts which is the present SMPE Recommended Practice. Flicker — Since the visual apparatus does not respond instantly to a stimulus or to its removal, persistence of vision can prevent flicker from being observed. It has been shown that above the frequency at which flicker is not observable, the apparent brightness of an object viewed in interrupted illumination is the average brightness, provided the illumination is continued for more than 3 per cent of the cycle. It should be noted that under the most favorable conditions of brightness and flicker frequency, the least perceptible change in brightness is of the order of 1.5 per cent. The sensitivity of the eye to flicker has been tested by numerous investigators who agree in general that the frequency at which the phenomenon disappears, called the critical frequency, is a linear function of the logarithm of the brightness within the range of present interest. Certain authors carefully specify a constant area of stimulation (see Figs. 1 and 2). At least one authority is convinced that flicker is still apparent on the screen and, furthermore, feels that present brightness levels are so low that a change in the direction of "easiest seeing" would result in still greater flicker. It seems, however, to have been generally granted that the flicker situation has been considerably improved. 23