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Fig. 14. Kerr-cell shutter; left, with cover removed; right, ready for use with 35mm Foca camera.
proper moment a rectangular pulse of the required amplitude and duration. The chief disadvantage of this system is its poor transmission, less than 10%.
Our standard Kerr cell was developed by our scientific collaborator P. E. Tawil, well-known for his piezo-optical Kerr-cell chronograph.6'7 It is made of welded glass and has three electrodes (Fig. 14). It is fully open for a voltage of 22 kv. Its useful cross section is 15 X 15 mm and so does not stop down the standard SOM objective (50-mm focal length, //3.5) provided with the 35mm Foca camera.
Our pulse generator results from the combination of two RC delay networks (Fig. 15). It thus opens the Kerr cell during an adjustable time-interval, with an adjustable time-lag with respect to a timing pulse. In the steady state, there is no voltage across the cell, its two poles being grounded. The rectangular 22-kv pulse across the cell is pro
duced by triggering the spark-gaps Si and So in succession, by means of thyratrons Thi and Th2, and pulse transformers Tri and Tr2. The high timeconstant of both discharge circuits gives a perfectly rectangular pulse. The two thyratrons are fired by discharging condensers Ci and C2 through resistors RI and R2 and the master thyratron Th, triggered by the timing pulse. The two delay circuits can be set for any desired maximum delay; in a typical case, the maximum delay is 7 /zsec and the pulse duration defined with an accuracy of 0.1 jusec. Figure 16 shows the detonation of a small explosive charge, photographed with an exposure time of 0.5 jLtsec. For very short exposure times, of the order of 0.1 jtsec, spark gap 82 is simply triggered by connecting its control electrode to the anode of Si by a length of coaxial cable.
If the subject is not self-luminous and must therefore be illuminated, a very
Fayolle and Naslin: High-Speed Photography
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