HIGH-SPEED PHOTOGRAPHY 765 



cury-control tube and simultaneously causes the spark gap to break down. The 

 effective exposure time for silhouette spark photographs may easily be made as short 

 as 10~8 sec. and, under favorable conditions, may be as short as 10~^ sec. 



Reflected-Iight Spark Photography. — Reflected-light photography, with illumina- 

 tion from sparks or from gas-filled tubes, produces photographs similar to those 

 obtained with ordinary lighting and camera technique, except that since the exposure 

 time is not limited by a mechanical shutter, it may be made extremely short. The 

 electrical circuit for producing the short condenser discharges through the lamps is 

 similar in operation to that used for silhouette photography. However, with reflected- 

 light illumination it is not necessary to have a point source of light; in fact, it is usually 

 better to have a light source of large area to give a better distribution of illumination. 

 Reflected-light photography usually requires considerably more energy because the 

 light is not so effectively used. Therefore the energy-storage capacity of the con- 

 densers must be larger than is required for silhouette photography. A typical circuit 

 is shown in Fig. 3 with the details explained in the caption. This equipment will 

 give sufficient illumination for photographing an area approximately 6 ft. square, using 

 an //8 stop, orthochromatic film, and an effective exposure of about 1/50,000 sec. 



The effective exposure time for reflected-light photographs' is longer than for 

 silhouette photographs, principally because the energj^ required is much larger. How- 



866 

 5,000 



Same f rip circuit 

 as in l^ig.2 '' 



Fig. 3. — Diagram of electrical circuit for making high-speed photographs by reflected 



light. 



ever, a very short flash is seldom necessary, and except for certain ballistic problems a 

 flash duration of less than 1/50,000 sec. is entirely adequate and easily obtained. 

 Reflected-light photographs of small areas can be taken with an exposure time of 10""'' 

 sec. if circuit conditions are adjusted properly and if the afterglow in the gas 

 is extinguished. 



Correct timing of the flash of light is often the most important part of any particu- 

 lar problem. For example, to obtain a photograph of a golf ball compressed with a 

 driver requires that the flash of light be timed to about 1/10,000 sec; otherwise the 

 photograph will be taken either before the ball is hit, or after it has left the club. The 

 simplest method of timing is to arrange the switch marked "trip" (Figs. 2 and 3) so 

 that contact is made at the desired instant. In the case of the golf ball two small 

 wires can be arranged behind the ball so that the club will knock them together at the 

 desired part of the swing. A series of photographs with different positions of the trip 

 wires shows successive stages of the action, if the action can be repeated. Other 

 methods of tripping can be used, such as the interruption of a beam of light to a photo- 

 cell or the use of a microphone to pick up a sound impulse. A time delay can be 

 introduced electrically to delay the flash of light by a known interval of time after the 

 signal is received. 



Single-flash high-speed photography is much more convenient to use than high- 

 speed motion-picture photography, since the apparatus required is much simpler and 

 since standard cameras and lens equipment can be used. Any camera is capable of 

 taking photographs with effective exposures of 10"^ to lO"** sec, simply by using a 



