CHAPTER XXVI 

 HIGH-SPEED PHOTOGRAPHY 



By Harold E. Edgeeton 



High-speed photography is defined for this section as applying to single-exposure 

 cameras that take photographs in less than 1/10,000 sec. and to motion-picture cam- 

 eras that operate at speeds in excess of 300 frames per second. These limitations are 

 purposely set to exclude cameras of the moving-shutter type, such as the Compur and 

 focal-plane types, and high-speed motion-picture cameras of the intermittent-motion 

 types — all of which are discussed elsewhere in this book. 



By the use of high-speed photography an observer is able to obtain a picture or a 

 series of pictures which accurately record an action as a function of time, permitting a 

 detailed study which cannot be made otherwise. The pictures in a series may be 

 analyzed frame by frame, or they may be projected on the screen in ultra-slow motion, 

 enabling the eye to see vagrant actions that would not otherwise be seen. Further- 

 more, since the film records the position of objects as a function of time, velocity and 

 acceleration can be calculated. Speed photography has proved its worth in scientific 

 and industrial research and is destined to an important future in photographj^ in all 

 fields of endeavor. 



Single -exposure High-speed Cameras. — In this classification are grouped cameras 

 capable of taking single photographs in less than 10"'* sec. Both mechanical and 

 photographic factors make difficult, if not impossible, a camera of any of the usual 

 types for verj^ short exposures, and therefore they are not considered here. The 

 cameras described are of the type that rely upon an electricallj^ controlled flash of light 

 for both the required very high intensity of illumination and for the short duration of 

 exposure time. The most satisfactory electrical arrangement consists of a spark in 

 air or in a gas-filled discharge tube through which the energy stored in a condenser is 

 discharged at the desired instant. With this method the exposure time is determined 

 entirely by the duration of the flash of light and not bj' any mechanical shutter. It 

 should be mentioned that this camera may require a darkened room to prevent expo- 

 sure due to the ordinary light, though in many cases the shutter setting, the lens 

 aperture, and the type of film are such as to give no appreciable exposure with the 

 average level of illumination. 



In general there are two methods of lighting: (1) silhouette and (2) the usual 

 reflected-light type. The silhouette method takes several forms, each requiring a 

 point source of light for best results. In some cases an open spark is satisfactory. 

 The dimensions of the spark can be restricted mechanically by causing the spark to 

 occur in a small hole in an insulator. Figure 1 illustrates several methods of silhouette 

 photography, and it is to be noted that all use a point source of light — a spark in air. 

 The upper sketch shows the simplest method of silhouette photography and one that 

 }>roduces excellent results. It is to be noted that no lens is used, as the image is a true 

 shadow of the subject. Therefore the size of the film must be comparable to the size 

 of the subject. A second limitation to this method is the requirement of a darkened 

 room so that the ordinary light will not fog the film. In some cases this is incon- 

 venient, since considerable time is usually required to uncover a large sheet of film or 



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