US3454819A - Field mesh electrode for improved target in image and storage tubes - Google Patents

Description

y 8, 1969 J. MUELLER FIELD MESH ELECTRODE FOR IMPROVED TARGET IN IMAGE AND STORAGE TUBES Filed Oct. 5, 1966 FIG. 1

VARIABLE NEGATIVE BIAS SOURCE SCANNING LELECTRONS OLLECTED RETURNING ELECTRONS ELECTRONS INVENTOR, JOHANNES ATTOR N E YS.

United States Patent 3,454,819 FIELD MESH ELECTRODE FOR IMPROVED TAR- GET IN IMAGE AND STORAGE TUBES 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. 584,023 Int. Cl. H01j 31/48 U.S. Cl. 315-11 2 Claims ABSTRACT OF THE DISCLOSURE An improved target for a low electron velocity image or storage tube. The target has a conductive mesh deposited thereon, which can be supplied with a low variable negative bias voltage so that only objects of a predetermined brightness appear in the output. Beam bending in also eliminated.

This invention relates to improvements in electron beam devices and has particular application to low electron velocity signal pick-up in image and storage tubes. More particularly it relates to an improved target for use in these tubes.

In conventional low electron velocity scanning devices an electron beam scans a signal plate or target on which a positive charge is produced by electron bombardment of the opposite side. By discharging these positive areas to cathode potential the scanning beam is modulated and collected, or in returns and is multiplied on special electrodes in the tube.

A few unwanted effects, i.e., target charge leakage and beam bending occur in the scanning section of the tube. As a result of these two effects a bright spot which is imaged on the photocathode appears larger than it should on a monitor and resolution is poor.

A target may be considered as being divided into many small resolution elements having an interelement or coupling capacitance between adjacent elements. The apparent interelement capacitance is largely influenced by the target resistance or target leakage and by the secondary electron redistribution. The effect of interelement capacitance can be much larger than that of the storage capacitance (that between the elements and the collector mesh), especially on borderlines of high contrast scenes. The effect of the interelement capacitance at moderate illumination-ratio levels is lowest on tubes with the highest storage capacitance. The higher the spot-illumination in a dim scene, the more the coupling capacitance affects the signal information and the less effect the storage capacitance has on the televised picture.

Thin-'film-target tubes have a very high interelement resistance and, therefore, also a very high apparent interelement capacitance. The increased spacing between target and collector-mesh of thin-film-target image orthicons decreases the storage capacitance (decreases the storage-tointerelemen-t-capacitance ratio) and causes a very pronounced blooming effect. The interelement or coupling capacitance also produces a loss of definition, as the area adjacent to a positively charged element assumes a slightly positive potential by an induced charge, whereby scanning electrons are accepted by the storage surface before the actual black-white transition is reached.

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 photocathode. 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 geometrical 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. Heretofore, a mesh electrode has been placed in front of the target in the scanning section of the tube for the purpose of decreasing the transverse field at the target. 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, increased scanning beam noise and secondary electrons.

It is therefore an object of this invention to provide a target for low velocity electron tubes which substantially eliminates scanning beam bending and lateral charge leakage over the target.

Another object is to provide a target for image and storage tubes which causes high signal-to-noise ratio and high resolution.

A further object is to provide a target mesh for image and storage tubes which can be provided with a variable negative bias so that only objects of a predetermined brightness appear in the output.

For the purpose of eliminating the beam bending effect and lateral charge leakage over the target, a fine mesh structure is evaporated on the scanning side of the target. This mesh is provided with a very low negative bias voltage. The electrostatic field from the positively charged areas of the target, which result from bombardment of electrons from the photocathode, cannot extend too far into the scanning field. The beam bending effect is therefore eliminated. The negatively charged target mesh also prevents the leakage of charge from the positively charged areas of the target to adjacent areas which are less positively charged.

The nature of the present invention along with various advantages, objects and features thereof will become more apparent upon consideration of the accompanying drawings and the following detailed description of those drawings.

In the drawings:

FIG. 1 is an illustration of the invention as well as the operation thereof, and

FIGS. 2 and 3 show two different mesh patterns.

Referring to FIG. 1 there is shown a target 11 which may be used in any of the low electron velocity image and storage tubes. A fine, high light transmission, conductive mesh structure 12 is deposited on the scanning side of the target in a pattern such as the mesh structure shown in FIG. 2 or the grid and bar structure shown in FIG. 3. This mesh structure should be very fine and have a maximum open area with sharp borderlines. The collector mesh 13 is located on the photocathode side of the target.

A negative bias on the order of a few volts is applied to the target mesh by connecting it to a source 14. Tubes have been successfully operated with target mesh potentials between 0.8 volt and l.6 volts. The colleetor potential is maintained somewhere in the range 3 of 3 volts and volts depending on the photocathode illumination level.

High velocity electrons 16 from the photocathode impinge upon the target 11 causing secondary electrons to be generated as illustrated by the diverging arrows 17..

Elements of the target are charged more positively toward the potential of the collector mesh by the removal of the secondary electrons 17. Most of these secondary electrons are collected by the collector mesh 13.

Heretofore, the electrostatic field from the positive charge on the target would attract the scanning beam of electrons 18 in the manner described hereinabove. However, the negatively charged target mesh 12 causes an electrostatic charge such as that represented by the field lines 15 to be established on the scanning side of the target. Therefore, the electrostatic field caused by the positively charged area of the target cannot extend too far into the scanning field. The effect of a small positively charged area may be limited to a single resolution element (the area between conductors). In this manner the resolution capability of the target can be increased to the range of the applied mesh structure.

The electron beam can be used fully for scanning purposes. This decreases the necessary primary beam by 30 or more over field mesh tubes. It can be seen from FIG. 1 that the target mesh rejects scanning electrons when its potential is the same or more negative than that of the cathode. Only the area between the conductive bars may collect electrons if a positive charge has been established there. Scanning electrons are locally deflected from the conductor bar 12 toward the positively charged area. These two effects additionally lower the necessary primary scanning beam by to 60% depending on the light transmission of the target mesh and the local deflection in the conductor area.

The target mesh 12 may be used in conjunction with any of the low electron velocity tubes such as image orthicon tubes, vidicon scan tubes and scan converter tubes. In all applications, the mesh is located on the scanning side of the target or signal plate which consists of an insulator or semiconductor, depending on the required application.

As a result of the target mesh high positive charges cannot leak 011 over larger areas, and the resolution of the target is increased. By making the conductive mesh more negative, only higher positively charged areas on the target can be discharged by the scanning beam, less charged sections of the target being unaffected. In this manner, if the bias source 14 were to apply the mesh 12 with a more negative voltage, only the brighter objects would be detected and appear on the monitor without background information.

The target mesh can decrease the effect of impurities which exist in the dielectrics used in the target. Thus, spots which are caused by these impurities are eliminated. Another advantage of the mesh is that it establishes an instantaneous potential over the entire signal plate.

' What is claimed is:

1. A low velocity electron tube scanning system having cathode means to provide a low velocity electron beam and a planar target situated perpendicular thereto and in the path of said electron beam, the improvement comprising: a conductive, highly electron transmissive layer consisting of a fine conductive mesh mounted in contact with the target on the side of said target facing said electron beam, and a variable negative bias source on the order of a few volts connected to said transmissive layer forthe purpose of eliminating undesirable bending of said beamand moire pattern production, said bias source being negative with respect to said cathode means and to the positively charged areas of said target.

2. A low velocity electron tube scanning system having cathode means to provide a low velocity electron beam and a planar target situated perpendicular thereto and in the path of said electron beam, the improvement comprising: a conductive, highly electron transmissive layer consisting of a conductive parallel bar structure mounted in contact with the target on the side of said target facing said electron beam, and a variable negative bias source on the order of a few volts connected to said transmissive layer for the purpose of eliminating undesirable bending of said beam and moire pattern production, said bias source being negative with respect to said cathode means and to the positively charged areas of said target.

References Cited UNITED STATES PATENTS 2,563,488 8/ 1951 Rose 31389 X 2,587,8 0 3/1952 Freeman 31389 X 2,652,515 9/1953 McGee 315-11 2,986,637 5/1961 Null 31511 X 3,204,142 8/ 1965 De Haan et a1. 315-11 3,239,766 3/1966 Manley 3 l511 X 3,259,791 7/1966 Jensen et al. 315-11 JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner.

U.S. CI. X.R. 313-67, 89

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