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1949 SPLICING IN VIDEO RECORDING 247
Mechanical-shutter equipment depends upon precise construction of the shutter and exact control of its rotation rate to achieve timing accuracy. Consequently, the camera must be locked in frequency to the television signal. Even with the two systems locked the inertia of the camera may prevent adequate accommodation to rapid change in television frame rate. To prevent such effects the two systems should be given the same time constant.
The effect of a timing error in mechanical-shutter equipment will be considered in a later section. Here it is only noted that it differs from that outlined above for the electronic shutter.
PHOSPHOR PERSISTENCE
Another source of exposure error may be found in the phosphor of the cathode-ray tube. Since this factor has not received much public attention to date, it will be given closer scrutiny than has been given the preceding points.
The films which are useful in video recording are all very slow films and are sensitive only in the blue region of the spectrum. Thus a satisfactory phosphor must have a considerable blue component in its light output. This fact plus certain features of operating convenience seems to have led all workers in the field to select the type Pll phosphor as being the most acceptable. Consequently, the following discussion will be confined exclusively to the Pll phosphor.
The light emitted by an element of area of a phosphor is not restricted to the very short interval during which the electrons of the scanning beam impinge upon that area, but instead continues for an appreciable time thereafter. The rate at which the intensity of emission falls off after scanning is characteristic of the particular phosphor. For the Pll phosphor the decay is of the type shown in Fig. 2.
The figure shows the instantaneous intensity of emission plotted as a function of time. According to the present standards of phosphor designation any Pll phosphor will follow this type of curve which is simply the reciprocal of some power of time. The exponent in the expression may have any value between 1 and 2. Experimental curves supplied by various manufacturers indicate exponents ranging from about 1.1 to about 1.4. Since a large exponent is desirable, at least for electronic-shutter equipment, a value of 1.5 has been used for this diagram, a value slightly higher than is justified by available data.