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Some Consideration in the Application of Tungsten Filament Lamps to Motion Picture Projectors
By L. C. Porter and W. M. States
The fact that there are today in operation approximately one thousand motion picture machines using the tungsten filament lamp as a light source proves that the incandescent lamp is a success in this field. The history of the developments which were gone through to achieve this success is very interesting. Some of the lamps and apparatus, which a short time ago seemed marvelous, today appear almost crude. However, we shall not take your time to describe the discouragements and the achievements along this line during the past three years. Nor shall we devote much time to description of the fundamental differences between the arc and the incandescent systems. This data is already more or less familiar to you, through papers presented before your Society, through articles in the trade journals, and through commercial advertising of the concerns manufacturing projection equipment.
In starting anything new there are always interesting developments which come through practice. Unforeseen difficulties arise and new methods of operation are established. It is the purpose of this paper to call attention to some of the practical considerations which have been found of material assistance in getting the best out of the incandescent projector. We wish to present data which will assist the manufacturers of motion picture equipment to realize the great advantages of accuracy in the optical system. We also hope to describe little "tricks of the trade" which will be of service to the users of projection equipment.
Few engineers or operators realize fully how closely it is possible to work with the incandescent system. In the arc system there must be more leeway. The carbon crater covers considerably greater area than is actually necessary, so far as light utilization alone is concerned. However, the arc is more or less unstable. It wanders around, and hence some allowance must be made to meet that condition. Shading by the negative carbon, and feed requirements constitute a very complex handicap. In other words, with the arc it is necessary, on account of these variations, to use larger carbons and more power than would be required if the arc could be held in a fixed position. Each optical system has a definite area over which it will pick up light. Any flux originating outside of that area adds little to the resultant screen illumination. Let us examine (Fig. I), a diagrammatic sketch of the optical system of a moving picture machine.
Assuming that the light originates from the arc A the paths of the extreme rays may be indicated by lines B and C, terminating on the motion picture screen. Suppose we have another ray of light, D, originating a considerable distance outside of the arc crater. It will be bent by the condensers similarly to rays B and C, and therefore, will not fall on the objective lens and will not reach the screen. This ray D
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