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Fig. 14. Lens equivalent of barrier; finite "O" points shown.
Fig. 15. Effect of finite size of "O" to avoid overlapping; "I" must be reduced to "I"', or "B"' must be reduced to "B'
molded into the film base. If one deals with electron rays instead of light, electron lenses can be used, and the apertures themselves can act as the electron lenses. Owing to the focusing action, these apertures, serving as electron lenses, can be larger, thus increasing the number of electrons reaching the image plane I. To make the apertures behave as electron lenses, it is only necessary to provide for an electric field between I and B greater than between B and O.
Finite-Area Sources. All previous discussion has been on the basis of point sources in the O plane. Actually, especially in electronic apparatus, the O points have appreciable area. In order to ensure that each O point sees only its own image plane patterns when using barriers (rather than lenses), it is necessary (see Fig. 15) to reduce either the size of the barrier plane apertures B' or the size of the image plane elements I', in either case maintaining the same separation distance between apertures and image elements. Figure 15 shows the proper geometrical
solution to achieve the nonoverlapping condition of image areas. The reduced sizes are by similar triangles:
(6)
where O' is size of O area and B' and I' are the sizes of apertures and image elements, respectively. The reduced B* or I ' size will result in loss of efficiency by reducing the number of rays from the O points which can be utilized.
Practical Application of Barriers in Cathode-Ray Tubes
The parallax barrier is particularly suitable as an internal member in cathode-ray tubes, permitting positive screen area control and ensuring that a given beam impinge only on a given portion of the fluorescent screen. Specified screen areas are associated with a desired source of electrons, and electrons from other sources are blocked from the
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July 1952 Journal of the SMPTE Vol.59