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1949
CATHODE-RAY OSCILLOGRAPHY
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impossible to see the rise time which may contain a number of interesting peculiarities. There are several things that may be done about this. One is to make two recordings, one showing the return time with the saw considerably overexposed, the other showing the saw with the rapid rise underexposed.
Another problem frequently encountered is that of 60-cycle or other modulation in the intensity of the cathode-ray-tube beam. See Fig. 10. This is a serious difficulty only with poorly designed oscillographs. Rather than redesign the equipment, it is usually sufficient to turn up the beam intensity and slightly overexpose the film in the upper portion of the H-Siud-D curve where variations in exposure are ironed out into a uniform high density. As a general rule, it will be found that
(a) (b)
Fig. 10 — Effect of intensity setting on recordings in presence of beam modulation: (a) low intensity; (b) high intensity.
the photographic process is much more critical of the quality of a cathode-ray-tube trace than is the human eye. Recordings, being subject to closer scrutiny, show up many defects in an oscillograph that would otherwise go unnoticed.
Most of the problems encountered with stationary or recurrent phenomena are also encountered with transient phenomena. The matter of proper exposure is generally a much more critical one with transient phenomena however. In the case of transients, the problem is usually that of obtaining sufficient density for high-speed recordings. In designing equipment for photography it is desirable to know just how far to go electronically in order to obtain adequate density in the photographic image. There are a number of factors which should be considered, all of which affect the photographic density. One of these is the maximum writing rate to be encountered; another is the intrinsic unmodulated brightness of the spot measured under standard