394 BELL SYSTEM TECHNICAL JOURNAL 



Many workers took up these pioneering experiments and improved 

 devices were developed representing a continuous advance in the 

 motion picture art until today we have a large industry applying the 

 knowledge gathered by these workers to the educational and entertain- 

 ment field. Paralleling commercial development of the motion picture 

 there has been a continuous advance in its application to scientific and 

 engineering problems, one phase of which relates to high speed motion 

 picture photography. 



Amateur and professional motion pictures are taken and projected 

 at the rate of 16 or 24 frames or pictures per second. If pictures are 

 taken at the rate of 48 per second and projected onto a viewing screen 

 at the rate of 24 per second a magnification of the time axis by a factor 

 of two occurs. The visual impression secured will be that of the 

 same occurrence taking twice as long. If 480 pictures are taken per 

 second and projected at the rate of 24 per second, the time magnifica- 

 tion is 20. It is this magnification of the time axis which characterizes 

 the picture as a "high speed motion picture." Cameras have been 

 developed which will, under highly specialized conditions, extend the 

 time axis by a factor of 2000 or 3000 times, although such phenomenal 

 speeds of taking impose serious restrictions upon the nature of problems 

 on which they may be used. 



Motion pictures are usually made in a camera of the so-called 

 "intermittent" type which refers to an intermittent motion given to 

 the film by the film driving mechanism. An intermittent motion is 

 employed in order that the film may be stationary during the brief 

 exposure portion of the operating cycle after which it is rapidly 

 accelerated and moved to an adjoining section for the next exposure. 

 Mechanical difficulties limit the speed of operation of intermittent 

 film moving mechanisms to a maximum taking rate of approximately 

 200 pictures per second. High speed motion pictures offering a 

 magnification of the time axis by a factor of 10 can, therefore, be 

 secured with specially constructed intermittent cameras. Higher 

 speeds require the abandonment of the intermittent mechanism and 

 the use of a non-intermittent or continuous film drive mechanism 

 together with means for securing the sharp images required. 



If film is moved continuously past an exposure aperture in which 

 lies a stationary image of an object, obviously only streaks will result. 

 Some means must be employed either to illuminate the object brightly 

 for a sufficiently short length of time to avoid blurring of the photo- 

 graphic image or some device must be incorporated in the camera 

 which will cause the image formed by the camera lens to be sharply 

 focused on the film and to move in the direction of film travel with 



