Limiting Resolution in an Image-Orthicon-Type Pickup Tube

An analysis is made of some of the factors which limit resolution in a television pickup tube of the image-orthicon type. Particular attention is given to effects of the glass-target characteristics and to departure from perfect focus in the image section, resulting from electron-emission velocities. It is shown that, as the width of lines in a resolution pattern is reduced, the signal generated tends to take the form of a small periodic modulation superimposed on a constant signal. This gives, in the reproduced image, a gradual reduction in contrast. A quantitative determination of limiting resolution, expressed as a number of lines, depends upon a physiological evaluation, not as yet available, of the minimum contrast in the reproduced image which will permit the pattern to be recognized as having a multiplicity of separate lines rather than a continuous blur. The general effects of target thickness and of color of incident light discussed herein have been confirmed experimentally.