US3249785A - Target-screen assembly for television pickup tubes - Google Patents

Description

y 3, 1965 R. w. FLOYD ETAL 3,249,785

TARGET-SCREEN ASSEMBLY FOR TELEVISION PICKUP TUBES Filed June 10, 1963 :5.--" t @j 4 Fi -4 20 INVENTORS R IN- Fl 0 Y0 W. 4/. S (/4 6' f zzmm United States Patent TARGET-SCREEN ASSEMBLY FOR TELEVISION PICKUP TUBES Robert W. Floyd, Woburn, and William J. Soule, Brookline, Mass, assignors to the United States of America as represented by the Secretary of the Air Force Filed June 10, 1963, Ser. No. 286,871 2 Claims. (Cl. 313-68) Certain television pickup tubes such as the image orthicon have a target-screen assembly consisting of a pattern on the target corresponding to the density variation over the cross section of the photoelectron current.

To produce a video signal corresponding to the charge pattern stored on the target, the target is scanned by a beam of slowly moving electrons from a cathode whose potential is so adjusted that only the fastest electrons in the beam are capable of reaching the target when its positive charge is at its lowest value. As the beam scans over the target, the number of beam electrons landing on the target at any point depends upon the target potential at the particular point. The remaining electrons, constituting the dilierence'between the total beam electrons and the number subtracted to neutralize the target charge, return toward the cathode where they enter an electron multiplier the amplified output of which constitutes the video signal. In order to allow for the collection of secondaries over the entire light intensity range, the screen is operated about 2 volts positive relative to the lowest value of target voltage for which beam electrons will land (target cutotf voltage). Since the range of target potentials is not much in excess of one volt, the screen is always positive relative to the target by a potential of from one to nearly 2 volts.

When tubes having screen-target assemblies of the above-described type are subjected to vibration, spurious video signals or microphonics are produced, making the tube unsuitable for use in aircraft or other installations where relatively high yibratory accelerations are encountered. The spurious signals are produced because the vibration produces oscillatory motions of the screen and target membranes at normally different frequencies, causing a periodic variation in the spacing of these elements. The target and screen constitute a capacitor having a charge corresponding to an average potential difference of between 1 and 2 volts. Since the charge is essentially constant and since the potential of the metallic screen is constant, the variation in capacitance due to relative motion of the target and screen produces a periodic variation in the target potential. This in turn produces a periodic variation in the number of beam electrons landing on the target and thereby introduces a spurious component into the video signal. This component usually appears as alternate light and dark bars across the screen of the reproducing tube or kinescope.

It is the purpose of this invention to prevent the generation of spurious signals due to relative motion or target and screen. Placing the target in direct contact wtih the screen has been proposed as a solution to this problem. While this prevents relative target-screen motion, it greatly increases the capacitance between screen and target which results in an objectionable time lag. The inven- 3,249,785 Patented May 3, 1966 tion provides the benefits of direct target-screen contact without increasing the capacitance by providing, in eifect, a large number of spacers between target and screen over the effective area of the target. This is accomplished by placing between and in contact with the target and screen a rectangular grid made of fine fibers of an insulating material such as glass. The diameter of the fiber is made one-half the desired target-screen spacing so that a spacer of the desired dimension is produced at each point of intersection of the fibers forming the grid.

The invention will be described in more detail with reference to the specific embodiments thereof shown in the accompanying drawings in which FIG. 1 shows the general arrangement of an image orthicon,

FIG. 2 shows a suitable method of supporting the target and screen in proper relationship,

FIG. 3 isan enlarged cross section of a target-screen assembly in accordance with the invention in which the grid fibers are interwoven,

FIG. 4 is an enlarged cross section of a target-screen assembly in accordance with the invention in which the grid is formed of two sets of parallel fibers one resting upon the other,

FIG. 5 shows the fiber grid attached to its supporting ring, and

FIG. 6 illustrates a suitable method of supporting the elements of FIGS. 3 and 4 in proper relationship.

Referring to the drawing, FIG. 1 shows the essential features of a standard image orthicon television pickup tube. The construction and operation of tubes of this type are well covered in the literature, for example, see The Image Orthicon, A Sensitive Television Pickup Tube, Rose, Weimer and Law, Proceedings of the I.R.E. (July 1946'). Briefly, a suitable lens system 1 forms an optical image of the scene to be televised on photocathode 2 causing it to emit electrons from each of its elemental areas in proportion to the intensity of the light falling on the particular elemental area. Under the influence of accelerating electrode 3 and extremely fine wire screen 4, which are at higher potential than photocathode 2, the electrons emitted from the illuminated areas of the photocathode are accelerated toward screen 4, most of them passing through the meshes of screen 4 and impinging on target 5 at an energylevel of about 400 volts. The elec trons, in traveling from photocathode to target, are constrained to move along paths to the tube axis by the axial magnetic field produced by focusing coil 6. The path of electrons from one elemental area of the photocathode is representedby line 7.

In the standard image orthicon, the target may be a glass membrane having, for example, a thickness of .002 inch and an active area diameter of about 1.4 inches. When the electrons from the photocathode strike the target, secondary electrons are emitted from the target in the ratio of about 5 to 1. These secondary elec trons are collected by screen 4 which is operated slightly positive with respect to the target. The photoelectrons thus result in the accumulation of a positive charge pattern on the target in which the more positively charged regions correspond to brighter parts of the optical image on the photocathode.

The opposite side of the target is scanned with a beam 27 of very low velocity electrons derived from a cathode hidden in FIG. 1 by electron multiplier 8 which surrounds it. The velocity of the electrons in the scanning beam is such that only the fastest are able to reach the target when it has its least positive potenial. Those which fail to land on the target are returned to the first dynode 9 of electron multiplier 8 along path 10. As the beam scans over the target, under the influence of scanning coils 11, the number of electrons landing on the target, and thereby subtracted from the return beam 10, will vary directly as the positive charge on the target varies. This modulation of the return beam, varying from point to point in response to the potential over the surface of the target as determined by the charge pattern, constitutes the video signal. This signal is amplified by the electron multiplier 8 .to produce the video output of the tube.

A standard method of mounting the target and screen 4 in proper relationship is shown in cross section in FIG. 2. There is provided a metallic screen support ring 11, having a central opening 12 and inner and outer rims 13 and 14. The screen 4 is stretched over opening 12 and inner rim 13, and secured beneath screen retaining ring 15 which is spot welded to ring 11. The target 5, which, as already explained, is a thin glass membrane of .0001 to .0002 inch thickness, is mounted on a nickel-iron alloy target mounting ring 16. To fabricate, the target is cut from a handblown glass bubble and laid across the target ring 16 which has been previously painted with solder glass. Ring and target are then placed in a furnace and heated to fuse the solder glass and soften the target. Under the influence of surface tension the thin glass membrane is pulled taut and remains so as target and ring are slowly cooled to room temperature.

The target is held in place in the target-screen assembly by hold down tabs 17 as seen in FIG. 2, the spacing between screen and target being established by an annular spacer 18 which, for example, may be made of aluminum and have a thickness of .002 inch. The target is deflected over the spacer slightly to increase its tension to the desired value. The amount of the deflection is controlled by controlling the height of the outer rim 14 of ring 11 relative to inner rim 13, taking into account the thickness of spacer 18 and the thickness of solder glass bubble 19. Higher target tension, up to the rupture point, may be attained by increasing the amount of deflection, i.e. by lowering the height of rim 14.

Target-screen assemblies constructed in accordance with the invention are shown in FIGS. 3 and 4. The target 5 is in contact with one side of a rectangular grid made of fibers 20 and the screen 4 is in contact with the other side. The fibers should have a diameter equal to one-half the desired target-screen spacing. For example, for a target spacing'of .004" the fibers should have a diameter of .002". The fibers may be interwoven as shown in FIG. 3 or the two rectangular sets of'parallel fibers may simply rest one against the other as in FIG. 4. The fibers are preferably made of glass, however, other suitable insulating materials may be used. A spacing between fibers of 0.1" is satisfactory, which places about 15 fibers across a target having an efiective diameter of about 1.5".

The fibers 20 are supported by a metallic ring 21, as shown in FIG. 5, to which they are attached in any suitable manner, such as by solder glass in the case of glass fibers. Various methods may be employed to fabricate the structure of FIG. 5. For example, the grid may first be formed on aframe larger than ring 21 by attaching individual glass fibers in the proper position and with the proper tension to the frame, using a suitable cement. Ring 21 is then painted with solder glass and the grid placed in position on it. The whole assembly is then heated in a furnace to fuse the solder glass and attach the glass fibers to the ring. After cooling, the fibers extending beyond the ring are cut away to leave the structure of FIG. 5.

FIG. 6 illustrates a suitable method of supporting the elements of the target assemblies of FIGS. 3 and 4 in their proper relationships, based on the techniques used in FIG. 2. The mounting of screen 4 is the same as described for FIG. 2. The ring 21 with the grid 20 attached is placed over screen 4 and over this the target 5 attached to ring 16 whichis slightly smaller in diameter in order to fit within ring 21. The two rings are locked in position by hold down tabs 17. The height of rim 14 of ring 11 is made such as to give the desired tensioning of grid 20 and target 5, taking into account the thickness of ring 21 and the solder glass attaching the target to ring 16 and the grid to ring 21.

The grid 20 situated between target 5 and screen 4 provides, in effect, a spacer over 0.1" over the active areas of these elements thereby preventing any change in their spacing due to vibration. This effectively holds the target-screen capacitance constant in the presence of vibration and eliminates this source of rnicrophonics.

We claim:

1. A target-screen assembly for an image orthicon or the like consisting of a rectangular grid made of fibers of insulating material, a thin membrane target electrode in contact with one side of said grid and a fine conductive screen in contact with the other side of said grid.

2. A television pickup tube having a thin membrane target electrode for converting an incident electron image into a corresponding positive charge pattern through the emission of secondary electrons, a fine conductive screen positioned parallel to and close to said target for collecting said secondary electrons, and a rectangular grid made of fibers of an insulating material situated between and in contact with said target and screen.

References Cited by the Examiner UNITED STATES PATENTS 2,403,239 7/1946 Rose 313 -68 2,879,419 3/1959 Redington 313-68 2,923,843 2/1960 Turk et al. 313-68 DAVID J. GALVIN, Primary Examiner.

R. SEGAL, Assistant Examiner.

Claims (1)

1. A TARGET-SCREEN ASSEMBLY FOR AN IMAGE ORTHICON OR THE LIKE CONSISTING OF A RECTANGULAR GRID MADE OF FIBERS OF INSULATING MATERIAL, A THIN MEMBRANE TARGET ELECTRODE IN CONTACT WITH ONE SIDE OF SAID GRID AND A FINE CONDUCTIVE SCREEN IN CONTACT WITH THE OTHER SIDE OF SAID GRID.

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