US3454820A - Image orthicon tube with improved field mesh electrode for prevention of scanning beam bending and of moire pattern production - Google Patents

Description

J y 8 1969 J. MUELLER IMAGE ORTHICON TUBE WITH IMPROVED FIELD MESH ELECTRODE FOR PREVENTION OF SCANNING BEAM BENDING AND OF MOIRE PATTERN PRODUCTION Filed ()Gt. 5, 1966 ELECTRON GUN ELECTRON GUN FIG. I

MUELLER ATTORNEYS F 3 INVENTOR,

JOHANNES BY I4! United States Patent M IMAGE ORTHICON TUBE WITH IMPROVED FIELD MESH ELECTRODE FOR PREVENTION OF SCANNING BEAM BENDING AND OF MOIRE PATTERN PRODUCTION Johannes Mueller, Elmira Heights, N.Y., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed Oct. 3, 1966, Ser. No. 584,026 Int. Cl. H01j 31/48 US. Cl. 315-11 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an improved image orthicon tube of the type having an essentially impervious electron target with an electron gun axially spaced from said target and a focussing electrode between said electron gun and said target. The improvement consists essentially of a field mesh electrode situated between said target and said electron gun and being spaced from said target in such a manner as to allow electrons from said electron gun to pass through said field mesh and contact said target so as to substantially reduce the beam bending effect and the undesirable production of moire patterns. A further improvement of the instant invention relates to biasing said field mesh electrode at a voltage that is lower than that of said focus electrode.

This invention relates to an improved field mesh for use in image orthicon tubes.

In the scanning section of an image orthicon the target is scanned by a low velocity electron beam which is produced by an electron gun. The electron beam is focused at the target by the magnetic field of an external focusing coil and the transverse electrostatic field of a focus electrode. A decelerating grid adjusts the electrostatic decelerating field between the focus electrode and the target in order to obtain uniform landing of electrons over the entire target area. The electrons stop their forward motion at the surface of the target and are turned back and focused into a multiplier section, except when they approach the positively charged portions of the pattern on the target membrane. When this condition occurs, electrons are extracted from the scanning beam in quantities sufficient to neutralize the potential pattern on the target. The returned electron beam is amplitude modulated by subtraction of electrons at the target surface in accordance with the charge pattern, the more positive areas of which correspond to the highlights of the televised scene.

Scan distortion may be caused by deflection of the low energy, scanning electron beam by highly charged local areas of the target resulting from the imaging of high intensity light spots on the photoc'athode. The low velocity scanning beam electrons approaching the target are pulled toward the highly positive spot-charge, causing a beam deflection. Beam pulling or beam bending is already in effect when the scanning beam is vertically many scanning lines away from the spot-charge. The scanning beam also remains longer on the spot-charge during its horizontal sweep. The highly positive charge of the spot is scanned with greater current density since the scanning lines fall partly one over the other in the spot region. In the monitor display, however, the scanning proceeds with constant velocity, and a larger spot is presented than that which originates at the target.

The beam bending effect, which destroys the geo metrical linearity of the picture, depends on the ratio of the transverse field strength to the strength of the decelerating field for the scanning electrons in the target vicinity. For the purpose of decreasing the transverse field 3,454,820 Patented July 8, 1969 at the target, a mesh electrode is placed in front of the target in the scanning section of the tube. Applying a relatively high potential to this field mesh increases the electrostatic decelerating field and reduces the effect of the transverse field on the scanning beam. However, this field mesh produces undesirable effects, e.g., moire pattern and increased scanning beam noise from secondary electrons.

Secondary electrons are produced by parts of the scanning and the returning beam impinging upon the field mesh. The amount of primary electrons interfering with the field mesh depends on the gauge and mesh of the screen. The secondary emission yield depends on the materials, the surface texture and the applied potential. In normal tube operation the secondary emission yield is approximately unity or slightly above. One method f reducing the effect of secondary electrons is to place a suppressor grid into the neck section of the tube. This grid prevents secondary electrons with velocities below a predetermined level from reaching the multiplier section of the tube. However, a major portion of the secondary electrons can pass this potential barrier and reach the multiplier section of the tube.

Another disadvantage arising from the use of a conventional high potential field mesh is the generation of moire patterns. The scanning beam passes twice through the field mesh and is modulated by its symmetrical structure to produce three different moire patterns:

(a) A line moire which develops by focusing the scanning beam at the mesh. This line moire is relatively strong and covers the whole scanning field.

(b) A line moire which develops if the node of the returning beam is at the mesh. This pattern covers sections of the scanning field only and is less pronounced in its appearance.

(c) A moire pattern which is produced by the relation of the scanning beam passing twice through the symmetrical mesh-structure and its own electron optical focusing field. This pattern is seen as a very coarse mesh in the video readout.

Therefore, an object of this invention is to provide an improved image orthicon field mesh which overcomes the disadvantages of the prior art.

Another object of this invention is to provide a field mesh which causes an increase in the output signal for a given scanning beam current.

A further object of this invention is to provide a field mesh which does not produce moire patterns.

Another object of this invention is to provide a field mesh which is so biased and spaced with respect to the target that returning electrons are focused through the mesh, thereby providing greater signal output and less secondary electron noise.

Another object of this invention is to provide a field mesh which is so closely spaced to the target that the effect of electron beam bending is substantially eliminated.

The above objects are realized by providing a separate low potential connection to the field mesh which is placed in the vicinity of 50 mils from the target. The beam bending effect is so effectively reduced that its effect is negligible.

In conventional tubes the field mesh is located about 0.3 inch from the target and is electrically connected to the focus electrode which is biased in the range of to 200 volts. This conventional mesh provides some reduction in the beam bending effect, but it produces moire patterns and secondary electrons. Heretofore, any attempt to reduce the beam bending effect by reducing the target to field mesh spacing has been unsuccessful due to the accompanying undesirable side effects, i.e., more intense moire patterns and increased production of secondary electrons. Additionally, the output signal decreases since scanning electrons are attracted to the field mesh.

A separate low potential electrical connection on the field mesh permits tube operation with very close mesh to target spacings, e.g., below 0.2 inch and offers additional advantages in tube performance, e.g., reduced secondary electron emission from the lower potential field mesh, less beam bending for the same field strength, and a higher return beam current to the multiplier section caused by the focusing effect of the mesh on the modulated returning beam.

Other objects and advantages of this invention will become apparent upon consideration of the embodiment illustrated in the accompanying drawings in which:

FIG. 1 is a schematic illustration of the operation of a conventional image orthicon tube in the target-field mesh area,

FIG. 2 illustrates the operation of the field mesh in accordance with this invention, and

FIG. 3 is a diagram showing the signal current change vs. the applied field mesh potential. The parameter is the focus electrode potential.

FIG. 1 schematically shows the operation of a conventional image orthicon tube in the target-field mesh area. Since this invention is primarily concerned with the scanning section of the tube, only that section is shown and described. The scanning section includes a target 11 having a collector mesh 12 located on the side thereof which faces the photocathode (not shown). The scanning electron beam, which is represented by arrows 22 and 24, is produced by an electron gun 16. The beam is focused at the target by the magnetic field of an external focusing coil (not shown) and the electrostatic field of focus electrode 13. The returning modulated beam is illustrated by an arrow 25. A field mesh 14 is internally connected to the focus electrode 13 by a lead 15.

Since the focus electrode 13 is biased at 150 to 200 volts in conventional, image orthicon tubes, the field mesh is biased at the same potential. The field mesh is located approximately 0.3 inch from the target. A field strength of approximately 200 volts per centimeter exists in the target-field mesh area. The electrostatic field pattern in this area is represented by the field lines 29.

As previously stated the purpose of the field mesh is to decrease the transverse field at the target and thereby reduce the beam bending effect. One of the above described adverse efiects resulting from the conventional field mesh is increased secondary electron noise. FIG. 1 illustrates how secondary electrons are produced from different types of primary electrons. Scanning electrons represented by an arrow 22 impinge upon the field mesh 14 and produce secondary electrons as indicated by the diverging arrows. Proportionally the same amount of secondary electrons will be produced by the returning modulated beam which is represented by an arrow 23. Several disadvantages arise due to the primary electrons striking the mesh. The signal output is decreased due to the loss of scanning and returning electrons. This necessitates the use of a scanning beam of greater current. Also the secondary electrons which reach the multiplier section of the tube constitute noise which detracts from the desired signal.

If the field mesh were located closer to the target than the distance depicted in FIG. 1 the beam bending eflect would be further reduced. However, the increased electr static field between the field mesh and the target Would create two intolerable results. More intense mOire patterns would be generated. In addition the returning beam will diverge and instead of returning through the mesh openings, many of the returning electrons will be attracted to the field mesh, further reducing the strength of the output signal.

When a separate low potential electrical connection is made to the field mesh in addition to placing it in close p x mi y 9 he t ge the b am be d g e e is stantially eliminated without the occurrence of undesirable results. FIG. 2 illustrates the operation of a field mesh in accordance with this invention. In this figure similar elements have the same reference numerals as those in FIG. 1. The field mesh 14 is provided with a low potential bias by connecting it to the movable tap of a potentiometer 31 which is connected between the focus electrode 13 and ground. Any conventional means may be used to bias the field mesh at a potential ranging from 30 to 140 volts. The graph shown in FIG. 3 illustrates the change in signal current with the applied field mesh potential. Curves 41, 42 and 43 depict tube operation for focusing electrode potentials of 110, 160 and 240 volts respectively. It can be seen that a node occurs on curve 43 at a field mesh potential in the vicinity of to volts indicating a maximum attainment of signal current. It can further be seen from FIG. 3 that the potential of field mesh electrode 14 is negative with respect to focus electrode 13 and varies between 80 and 210 volts from said focus electrode 13.

A higher focus electrode potential increases the focusing action of the field mesh on the returning beam and thereby increases the output signal current. In FIG. 2 arrows 35 and 36 illustrate the focusing action on the returning beam with a separate low potential field mesh. With the low potential biasing and small field-mesh to target spacing shown in FIG. 2, a higher amount of returning, modulated electrons will pass through the mesh. It may be expected that the focusing action of the field mesh on the returning beam will have a diverging action on the oncoming scanning beam (arrow 37); however, this does not occur. The focusing action of the mesh is so small on the fast scanning beam that no noticeable deflection will occur. For a given illumination of the photocathode there is a slight decrease in the scanning beam current for the low potential field mesh mode. However, there is a large increase in signal output.

What is claimed is:

1. In an image orthicon tube of the type having a scanning and return beam, an electron imperforate target, an electron gun axially spaced from said target, and a focus electrode situated between said electron gun and said target, the improvement consisting of:

a field mesh electrode situated between said target and said focus electrode;

said field mesh electrode being spaced from said target less than 0.2 inch so as to allow said scanning and return beams to be modulated as they pass therethrough; means negatively biasing said field mesh electrode in the range of 80 to 210 volts with respect to ground thereby producing a field strength in the area between said target and said field mesh electrode of between 400 and 1000 volts per centimeter; and,

thereby substantially eliminating the beam bending effect and moire pattern production.

2. The image orthicon tube according to claim 1 wherein the spacing between said field mesh electrode and said target is substantially 50 mils.

References Cited UNITED STATES PATENTS De Haan et al 315 -11 JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner} US, Cl. X.R. 313..67. 8.9

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