Imaging, beam acceptance, and beam-discharge lag in camera tubes

The effect of beam interception on the initial-energy distribution of the electrons contributing to the operating beam and on the imaging in camera tubes is examined. It is shown that the imaging properties of different imaging systems can be compared mutually by calculating their figure of merit, which is defined as the product of beam angle and spot diameter. The beam acceptance by the low-potential target depends on the imaging system used in the tube. This is clearly demonstrated by calculations on three different imaging systems, the imaging properties of which are compared with one another. These systems have been chosen such that: 1) the influence of the Coulomb interactions between individual electrons on the beam acceptance is of minor importance; and 2) the initial-energy distributions of the beam electrons are different particularly in the low-energy region. This gives rise to differences in the lag for peak white in the three systems which are calculated. The importance of these differences depends on operating conditions and layer capacitance. It is shown that a space-charge minimum between mesh and target, which could affect the beam acceptance, does not occur at normal values of the mesh potential and of the distance between mesh and target. The influence of increasing the beam angle on the beam acceptance is calculated and found, to give rise to a paralell shift of the acceptance curve which looks like the effect of an increasing contact potential difference.