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November, 1950 sandvik and veal : films for television 155
is sufficiently short to prevent '* carry-over " from one frame to another, which is dependent to some extent upon the current density in the focused spot.
Definition and Contrast.
An ever-present factor to be considered when photographing a picture from a Kinescope is the loss of definition when it is operated at a high brightness level. The decrease in definition is caused by the defocusing of the electron beam exciting the phosphor. This defocusing can arise from non-uniform fields, whether electric or magnetic, or by variations of electron velocity in the beam. The electrons emitted from the cathode in the electron gun leave the cathode at different velocities, and these differences in velocity are preserved as the electrons pass through the gun. Since the amount of deflection depends upon the velocity of the electrons, a beam composed of electrons having a range of velocities will suffer a corresponding range of deflections. The beam is spread out into an elliptical cross-section, with the long axis in the direction of the deflection, instead of remaining in a sharply defined spot.
Fig. 13. A photograph from television Kinescope 16mm. negative, Type 5373 emulsion.
A factor which affects contrast, and which must be considered when a Kinescope is to be photographed, is its signal-to-light relationship or its transfer characteristics. When its control-grid voltage is plotted against the second anode current, the curve obtained will straighten out to some extent as the second anode voltage is increased. Yet the second anode voltage cannot be increased beyond the saturation of the phosphor with beam-current density. Other losses of contrast beside phosphor saturation are scattering and internal reflections of light from the walls of the tube. However, this has been reduced to a minimum by placing a thin aluminium coating over the phosphor within the tube.
Tube Luminance and Exposure.
The highlight luminance of the 5WP-11 Kinescope, when operated at a second anode potential of 27 kv and a second anode current of 30 microamperes, will be approximately 150 foot-lamberts when adjusted for best pictures, which is adequate light for exposing slower positive and sound-recording types of