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564 BERKLEY AND MANSBERG November
typical examples of halo from a screen put on by flowing, a screen put on by settling, and a screen covered by the plastic face plate just mentioned. It can be seen that the settling technique which is used on our cathode-ray tubes for photography today gives quite an improvement in the result. The use of the plastic face plate eliminates the undesirable halo completely. Another source of stray light sometimes is encountered in the case of single transient exposures, since the blanking gate causes transient changes in the direct-current level of the grid or cathode voltages of the cathode-ray tube. These changes cause an illuminated spot to appear at the edge of the pattern. The effect may be seen in Fig. 16. It can best be minimized by turning the
(a) (b)
(a) Fogging effect of transients in grid-cathode voltage.
(b) With transient eliminated.
Fig. 16
cathode-ray-tube average brightness down to such a value that no transient coming in can produce a visible stationary spot. A better solution to this problem is to use a blanking gate which is direct-current-coupled from the signal source.
Because of the high voltages used in oscillographs designed for highspeed transients, unusual difficulties are encountered with static exposures on the film caused by charge leakage. To avoid this, metal parts such as the pressure plate and film gate should be grounded to a common point with the oscillograph. The use of all-metal cameras is indicated.
Because of the prolonged development times required to render the threshold densities visible, higher fog densities have to be tolerated in cathode-ray-oscillograph photography. Preferably, experiments should be run on each new emulsion batch to determine the optimum development conditions for the best threshold density above fog.