US3395304A - Storage tube screens - Google Patents

Description

July 30, 1968 D. D. DUGGAN STORAGE TUBE SCREENS 2 Sheets-Sheet 1 Filed Dec. 14, 1964 1 I I l I 1 I INVENTOR.

DANIEL D. DUGGAIV ATTORNEY July 30, 1968 D. D. DUGGAN 3,395,304

STORAGE TUBE SCREENS Filed Dec. 14, 1964 2 Sheets-Sheet 2 INVENTOR.

DAN/EL 0. 0066A aw/aw ATTDRNEY 3,395,304 STORAGE TUBE SCREENS Daniel D. Duggan, Roanoke, Va., assignor to International Telephone and Telegraph Corporation, a corporation of Delaware Filed Dec. 14, 1964, Ser. No. 418,251 11 Claims. (Cl. 313-89) ABSTRACT OF THE DISCLOSURE An electron beam image storage tube has a gradient transmission screen wherein a varying dimension thereof provides a greater impedance to the passage of electrons through the center portions than at the edges to achieve a relatively uniform erasure of a stored charge.

This invention relates to storage tube screens and particularly to a gradient transmission screen arrangement and method which provides varying electron transmission through different areas of the screen surface to compensate for differences in erasure time of a stored charge caused by non-uniform current density of the flood beam.

The usual electron beam storage tube of the direct viewing type contains writing and flood guns at one end and storage and display screens at the other end. The electron beam from the writing gun is intensity modulated by a signal and scanned across the storage screen, which is formed of a fine mesh metal grid and an insulator layer having secondary emission properties, to selectively charge the screen in accordance with the signal. An adjacent collector screen collects the secondary electrons emitted from the storage screen. A flood gun in one mode applies an expanded beam of electrons over the entire storage screen which controls the passage of the beam therethrough to display the charge pattern on the phosphor coated face of the tube. In the case of an electrical readout tube, an electrical output signal representing the stored image is derived from a dielectric coated metal backing plate of the screen. In a second mode, the flood beam is permitted to strike the storage screen to erase the charge. Due to inherent non-uniformities in the flood beam, such as the greater concentration or density of electrons at the center, which cannot be perfectly corrected by collimating or focus electrodes, and the variation of the radial path at which the electrons strike the different surface areas of the planar storage screen, the erasure time is not the same at all portions of the screen. The edge areas of the screen thus generally erase before the center, since the center electrons tend to pass through more readily than the outer electrons which are intercepted. In addition, a higher screen voltage is required to cut-off the more dense beam portions.

In some cases, the voltage setting may permit only the edges of the screen to erase while the center maintains the charge. A partial solution to the problem of providing a constant radial path length was the use of a curved screen, such as described in US. Patent No. 3,201,628, issued Aug. 16, 1965, and assigned to the same assignee as the instant application. This form, however, introduced structural complexities and did not compensate for the non-uniform beam current density.

It is therefore the object of the present invention to provide a simple reliable structure and method to compensate for variations in erasure time of a storage screen.

It is another object to provide a gradient electron transmission and uniform erasure for a storage surface while maintaining high writing speed brightness.

These objects are achieved by a novel arrangement which changes the electron transmission properties of the States Patent Patented July 30, 1968 mesh screen by using a graded material such as an insulator layer which varies in dimension from the center to the edges of the screen. Thus, thicker insulator layers which impede the electrons more and therefore erase faster are placed at the center to compensate for the normal lag in that area. Another uniform high secondary emission insulator coating may be applied to maintain a .high writing speed brightness. A novel mask and evaporation process provide the desired graded layer structure.

The details of the invention will be more fully understood and other objects and advantages will become apparent in the following description and accompanying drawings, wherein:

FIG. 1 shows a top view of the apparatus, mask and fragmented screen section used to form a graded thickness insulator coating;

FIG. 2 shows a partial side view and cross section of the same apparatus; and

FIG. 3 is representative fragmentary enlarged cross section of the insulator screen showing the details of the storage screen layers.

As shown in FIGS. 1 and 2, an insulator storage screen 10 is positioned in an evacuated bell jar 12 and secured on an insulated mounting ring 14 and a geared rotary support 16. The mechanism is driven by an external variable speed motor 18 having a magnetic coupling 20 and an internal follower 22 with a driver gear 24. Two idlers 26 provide additional support. The mounting ring is preferably flat to reduce edge effects during the evaporation process. The storage screen is rotated over an evaporation boat 28 containing a particular coating material. The boat is connected between a pair of electrodes to heat and evaporate the material in a known manner. A special mask 30 is positioned between the boat and screen to cause the evaporated insulator material to be deposited on the screen as a graded layer with the center of the screen having a greater thickness than the periphery or outer edges. This is achieved by the particular curve shape of the mask wherein a greater portion of the screen is blocked out at the edges than at the center to compensate for the inherent non-linearity of the flood beam electrons, with rotation of the screen providing a symmetrical coating distribution about the central axis. Other suitable masking devices such as a photographic type shutter or iris may similarly be employed.

The resultant structure is shown in an exaggerated form in FIG. 3, with coating 32 of magnesium fluoride, for example, forming the graded insulator layer on metal grid 34. Other suitable high refractory materials such as calcium or barium fluorides may similarly be employed. The meal grid is preferably formed of a suitable metal such as nickel which may be pre-aluminized to seal and protect the metal from reaction with the other coatings. The storage screen may then be further processed by providing a metal coating 36, such as gold or aluminum, on the backside to cover any stray insulation material which may have been evaporated behind the screen. An additional coating 38 of magnesium monoxide or other suitable high secondary emission material, such as silicon monoxide, may then be added with a uniform thickness to reduce the effect of the increased center thickness on the writing speed brightness. This is the speed at which the writing beam can charge the insulator surface to establish a given brightness on a display screen. This latter coating has a sutficiently high secondary emission ratio so that the writing speed increases to the extent that any increase caused by the thicker center layer is negligible. Thus the magnesium fluoride layer controls the erase time While the magnesium monoxide layer controls the writing speed brightness.

In a typical arrangement, seven fringes or incremental portions of a layer of magnesium fluoride, as measured by fringe counter 40, are evaporated onto the nickel screen, each fringe being about 0.15 microns, with a gradation of approximately 1 to 3 fringes existing between the center and outer periphery, depending upon the flood beam current characteristics. A fast binding layer is evaporated onto the screen first, with slower evaporation of the bulk of insulator material following. The gold layer on the opposite side may be about one fringe with the magnesium monoxide layer having a three fringe thickness. A detailed description of a fringe counter is found in an article entitled Control of the Thickness of Evaporated Layers During Evaporation by G. Papp, published in the Review of Scientific Instruments, October 1959, vol. 30, pp. 911-912. As a result of this arrangement, whereas prior art screens have been found to have erasure time variations in the order of between 0.8 to 3.0 second from the edges to the center, the present invention provides a substantially uniform time of about 1.5 seconds.

In addition to the graded insulating material, other variations of the instant invention which have similar effects in producing uniform erasure time, may include the provision of a graded metal backing layer, the use of a variable apertured metal grid having closer spaced apertures 0r thicker metal coatings creating smaller apertures at the center than at the edges, or a variable coating or graded mesh on the collector screen adjacent the storage screen. It may thus be seen that the present invention provides a novel screen structure and coating method which changes the electron transmission characteristics of the storage elements to compensate for variations in current density of the flood beam and achieves uniform erasure at all areas of the storage screen. While only a single embodiment has been illustrated, it is apparent that the invention is not limited to the exact form or use shown and that many other variations may be made in the particular design and configuration without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. A screen for an electron beam type storage tube including means for storing and erasing a charge image comprising a fine mesh metal grid and gradient transmission means thereon having a varying dimension providing greater impedance to the passage of electrons through the center portions of said grid than at the edges sufiic-icnt to provide a relatively uniform erasure of said charge.

2. The device of claim 1 wherein said gradient transmission means comprises a first layer of insulating ma terial on one side of said grid, said insulator layer having a greater thickness at the center portions than at the edges.

3. The device of claim 2 including a second layer of insulating material on said first layer havinga uniform thickness and high secondary emission characteristics.

4. The device of claim 2 wherein said first insulator layer is of a high refractory material.

5. The device of claim 2 wherein said grid is formed of nickel and includes an alumnum coating sealing the nickel surface.

6. The device of claim 2 wherein said insulator thickness is between 0.10 and 0.50 micron greater at the center than at the edges.

7. The device of claim 3 including a uniform thickness metallic layer on the opposite side of said grid.

8. The device of claim 4 wherein said first layer is formed of a material selected from the group consisting of magnesium fluoride, calcium fluoride and barium fluoride.

9. The device of claim 4 wherein said second layer is formed of a material selected from the group consisting of magnesium monoxide and silicon monoxide.

10. The device of claim 7 wherein said metallic layer is formed of a material selected from the group consisting of gold and aluminum.

11. The device of claim 8 wherein said second layer is formed of magnesium monoxide.

References Cited UNITED STATES PATENTS 2,979,633 4/1961 Harris 3 l389 3,089,050 5/1963 Lehrer 3l3-68 3,242,367 3/1966 Slegho 313-89 FOREIGN PATENTS 612,033 1/1961 Canada.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

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