US3225237A - Photoemissive pickup tube - Google Patents

Description

Dec. 2l, 1965 A. D. coPE PHOTOEMISSIVE PICKUP TUBE 2 Sheets-Sheet 1 Filed June 1, 1961 Sm; QMN TI N SNT .Nxml

R. w... M, Na .1w I0 a W L@ Pori Dec. 21, 1965 A. D. coPE PHOTOEMSSIVE PICKUP TUBE Filed June 1, 1961 2 Sheets-Sheet 2 United States Patent Office 3,225,237 Patented Dec. 2l, 1965 3,225,237 PHOTUEMISSIVE PICKUP TUBE Appleton Danforth Cope, Hightstown, NJ., assigner to Radio Corporation of America, a corporation of Delaware Filed June 1, 1961, Ser. No. 114,056 7 Claims. (Cl. 313-65) This invention relates to pickup or camera tubes. In particular, this invention relates to photosensitive type pickup or camera tubes.

There is a type of photoemissive camera tube which is known as the Isocon or Image Isocon tube. This class of tube usually comprises an elongated envelope having au electron gun in one end thereof. Positioned in the other end of the envelope is a photoemissive cathode. Positioned between the electron gun and the photoemissive cathode is a storage target electrode which is exposed on one side to the photoelectrons from the photocathode and is exposed on the other side to the electron beam from the electron gun.

The operation of an Image Isocon type pickup tube is that a light image from a scene to be reproduced strikes the photoemissive cathode providing photoemission therefrom which, at each point, is in proportion to the amount of light form the scene. The photoelectrons land on the storage target to provide an electrical charge image thereon. The charge image is also produced on the gun side of the storage target. The charge image on the gun side is detected and read off by a low velocity electron beam from the electron gun. The electron beam reflected from the target electrode and returning toward the electron gun is composed of reflected (specularly reflected) electrons `and scattered (non-specularly reflected) electrons. The specularly reected electrons, which contain no signal information, are excess of unused electrons turned back from the storage target. The scattered electrons, which comprise the signal information, are the electrons scattered by elemental areas of the bombarded charge pattern stored on the target electrode. The number of electrons scattered from an elemental area of the charge pattern is proportional to the charge stored on that area. Between the electron gun and the storage target, a means is provided to separate the specularly reected electrons from the scattered electrons so that only the scattered electrons are directed into an electron multiplier to produce output signals from the tube.

One of the problems in the design of an Image Isocon pickup tube is that the electrostatic and magnetic elds must be adjusted with extreme accuracy so that the reected electrons may be accurately separated from the scattered electrons.

Applicant has found that it is desirable to have a pickup tube which may be operated as either an Image Isocon type tube, for selected uses, or as an Image Orthicon type tube for other uses. In the Image Orthicon type operation, both the specularly reected and the scattered electrons are fed into the electron multiplier to provide the output signals.

Applicant has also found that there is an optimum electrical steering field, for separating the reected and scattered electrons, to produce the most favorable signal-tonoise ratio in the output signals when the tube is operated as an Image Isocon.

It is therefore an object of this invention to provide an improved pickup tube.

It is another object of this invention to provide a novel pickup tube capable of being operated as either an Image Orthicon or an Image Isocon.

It is a further object of this invention to provide a new Image Isocon tube characterized in its simplicity of electrode means for producing an optimum electrical steering eld.

These and other objects are accomplished in accordance with this invention by providing a pickup tube including an electron gun and a storage target in which a steering electrode means is provided between the electron gun and the storage target. The steering electrode means is such that electric elds transverse to the tube axis, and of varying magnitude, can be easily established to efliciently separate, in space, reflected electrons from the signal modulated scattered electrons. Because of a feature of the invention involving the separation of the modulation scattered electrons at the edge of a relatively large diameter aperture, the pickup tube is adapted to operate either as an Image Orthicon or as an Image Isocon.

The invention will be more clearly understood by reference to the accompanying two sheets of drawings where- FIG. 1 is a sectional view of a pickup tube in accordance with this invention;

FIG. 2 is a view taken along line 2 2 of the novel pickup tube shown in FIG. 1;

FIG. 3 is a somewhat schmatic sectional view of another embodiment of a pickup tube in accordance with this invention; and,

FIG. 4 is a detailed sectional view of the steering electrodes shown in FIG. 3.

Referring specifically to FIG. l, there is shown a pickup tube 10 made in accordance with this invention. The tube 10 may be operated as an Image Orthicon or as an Image Isocon as will be explained. The tube 10 comprises an evacuated elongated envelope 12 having an electron gun 14 positioned in one end thereof. Any conventional electron gun having a relatively small, e.g. 0.002" has been used, exit or beam forming aperture 15, with a standard thermionic cathode 17, is suitable. Surrounding the electron gun is a plurality of electron multipliers 16 which may also be of conventional structure.

Positioned in the other end of the envelope 12 is a photoemissive cathode 18. The photocathode 18 may be any conventional, semi-transparent photocathode; for example the commercially available S-10 photocathode which includes silver, bismuth, oxygen and cesium has been utilized.

Spaced from the photocathode 1S is a storage target 20. The storage target 20 may comprise any conventional structure such as a thin membrane of glass, magnesium oxide or aluminum oxide. The thin membrane of such material is supported in any conventional manner (not shown in detail). Closely spaced from the storage target 20, on the electron gun side, is a decelerator mesh screen 2S. Closely spaced from the storage target 20 on the photocathode side is a secondary electron collector mesh screen 23. The target 20 is adapted to have a photoelectron charge image produced thereon due to photoelectrons from the photocathode 18 landing on one side thereof and dislodging secondary electrons to establish a net positive charge on the target. This photoelectron induced charge image is also produced on the gun side of the target 20 due to the thinness of the target membrane.

During operation, and with potentials applied to the tube such as those shown for example in FIG. l and with the cathode grounded, a scanning electron beam from the electron lgun 14, after passing through mesh screen 25, is decelerated to a few volts of energy as it approaches the target. The decelerated beam is scanned over the surface of the target electrode 20 facing the gun 14 by means such as a scanning yoke 26 with suitable coils. The scanning beam will establish a stable potential on the gun side of the target 20 that is close to the potential of the cathode 17 of the electron gun lll-l. Once the stable potential is established, any electron charge pattern from the photocathode I8, for example a pattern of an image focused by means of a lens 37, will be detected by the scanning beam and the charge pattern produced thereby will be neutralized through a deposition of a balancing number of electrons from the scanning electron beam. However, not all of the low energy scanning beam electrons which are incident on the target will be retained by the target 20. A fraction of these incident electrons experience scattering which is non-specular. This nonspecular scattering occurs only from the elemental areas of the target which have a charge thereon. Because of the unusued part of the scanning electron beam, i.e. the part of the beam from a non-charged area which is specularly reflected, the average energy of the scattered electrons 1.9 and the reliected electrons 21 is different. Electrons 19 and 21 constitute the return electron beam. These are illustrated in a highly schematic way in the drawing.

Positioned between the electron gun 14 and the target 20 is a separation electrode 22 which is for the purpose of separating the specularly relected electrons 21 in the return beam from the scattered electrons I9. The separation electrode 22 traps the specularly reected electrons 21 while permitting the scattered electrons 19 to pass through an aperture 32 in the separation electrode 22 to return to the rst dynode of the electron multiplier 16 and thus produce an output signal. For this purpose, separation electrode 22 has an axially extending lip to prevent secondary electrons from escaping into the electron multiplier to form a spurious signal. It should be noted that the edge of the separation electrode 22 is positioned at an anti-node of the return electron beam. The nodal points of both the incident electron beam and the return electron beam coincide since they both pass through the same magnetic field. Since the scattered electrons and the specularly reected electrons differ in their non axial velocity components it is at an anti-node position that the greatest radial displacement occurs between the scattered electrons 19 and reflected electrons 21.

More detailed description of the Image Isocon type pickup tube may be found in an article entitled The Image Isocon by P. K. Weimer, RCA Review, Volume 10, No. 3, Pages 366-368. Also, further description of the Image Isocon pickup tubes may be found in Weimer Patents 2,545,982; 2,579,351 and 2,747,133.

The Image Isocon is provided with an improved means for steering the return electron beam in the embodiments shown in FIGS. l and 2. This means comprises steering electrodes 24:l which are formed as separate metal coatings on the inner surface of the envelope wall and divided into four quadrants, i.e. four sections of a cylinder. Each of the steering electrodes 24 is separately connected to its individual lead in wire (not shown) by means of separate springs (not shown) mounted on the end of the electron gun 1d so that the proper electrostatic iields can be established between the .four steering electrodes 24 to direct the return beam in any direction to just separate the reliected electrons 21 from the scattered electrons 19. The optimum signal to noise ratio is obtained when all of the specularly reiiected electrons are trapped while all the scattered electrons pass through the separation electrode 22. The steering electrodes 24 may be formed, for example, of an evaporated metal, such as aluminum. Also, the steering electrodes may be made in substantially the same configuration and be formed of a sheet metal separate from the envelope wall. Also, the steering electrodes may comprise three electrically separate electrodes spaced 120 degrees apart. With less than three separate electrodes it has been found extremely diliicult to accurately adjust the return beam for optimum separation. It should be noted that the steering electrodes 24 are positioned throughout the entire length of the field of the scanning yoke 26. Also, the separating electrode 22 is positioned within the field of the focusing coil 28 and between the fields of the alignment coil 30 and that of the scanning yoke 26.

It should also be noted that the separating electrode 22 has a separating aperture 32 (eg. 0.04 inch) which is substantially larger than the beam forming aperture 15 (eg. 0.002 inch) in the electron gun 14. With this arrangement the tube I0 may be operated as an Image Isocon as has been described. Also, the tube may be operated as an Image Orthicon. In other words, the transverse magnetic field provided by the alignment coil 30, and the potentials applied to the four steering electrodes 24 may be adjusted to direct substantially the entire return electron beam, i.e. both the scattered and specularly reflected electrons, into the electron multiplier 16 somewhat as a conventional image orthicon operation. Also, the alignment coil field, and the potentials applied to the steering electrodes 24, may be selected to trap substantially only the reflected electrons 21 on the edge of the aperture of the separating electrode 22 while passing substantially only the scattered electrons 19 back to the dynode. This latter operation is schematically illustrated in FIG. 1. Thus, the tube shown can be operated as an Image Orthicon when the particular scene situation, such as brightness levels etc., make this type of operation desirable or as an Image Isocon in situations where this operation is more eiicient.

Because of the presence of four steering electrodes 24, each in a different quadrant, efficient separation of the electrons 19 and reflected electrons 21 may be achieved. Thus, as compared to the prior art, the necessary adjustments to perform efficient separation of scattered and retlected electrons is relatively simple and easy. As an example, if one assumes that the magnetic iield of the focusing coil is not completely uniform the desired separation can easily be obtained by over-balancing the electrostatic field provided by one side of the steering electrodes 24 to compensate for the magnetic field nonuniformity.

Also, because of the presence of the four steering electrodes, and the large size of the separating aperture 32, the reflected electrons 21 and scattered electrons I9 may be eiiiciently separated from a large target area.

Referring now to FIGS. 3 and 4, there is shown an ernbodiment of this invention in which four focus loops are used. The four focus loops, or four nodal points, are produced either by mak-ing the envelope longer and using the same magnetic ield strength, or by increasing the field of the focusing coil 28. In this embodiment, a steering electrode is provided which is made of four metallic fmger like elements or electrodes 36. Each of the steering electrodes 36 is positioned yin a different quadrant. The wall electrode 38 may, in this instance, be continuous, rather than in four separate sections as in the embodiment of FIG. 1. The steering electrodes 36 are 4all positioned in the plane of a focus node of the return electron beam and outside the deflection region of the tube. Thus, substantially only one source of rad-ial force, namely that provided by the steering electrode 36, is exerted on the return beam at this location. By being in a nodal plane, all electrons of the return beam associated with a particular element in the scene experience the same radial force. Each of the iinger-like elements 36 is separately connected to its individually adjustable energy source so that each may be separately energized, to steer the return electron beam so that the specularly reflected electrons 21 are trapped by the separating electrode 22 while the scattered electrons 19 pass through to be multiplied by the electron multiplier 16.

The embodiment shown in FIGS. 3 and 4 can also be yoperated yas `an Image Orthicon as was previously explained by passing the entire return electron beam into the electron multiplier.

It should also be understood that this invention is equally applicable to photoconductive type pickup tubes Specifically, a commercially available Vidicon type tube, e.g., see U.S. Patent Number 2,900,569, may be modified to include an electron multiplier structure, a separating electrode and two or more steering electrodes. In this modification the photosensitive storage target is scanned directly by the primary beam and the signal is taken from the amplified return beam which has passed through the multiplier.

What is claimed is:

1. A television camera tube comprising an elongated envelope, an electron gun in one end of said envelope, a photocathode in the other end of said envelope, a target electrode between said electron gun and said photocathode and in said envelope, a separation electrode between said electron gun and said target electrode and within said envelope, and at least three electrically separate steering electrodes between said separation electrode and said target electrode and within said envelope, said steering electrodes being finger-like structures extending in a plane normal to the axis of the envelope.

2. In a television pickup tube of the photoemissive type, an electron gun, a storage target, a separation electrode spaced between sa-id electron gun and said storage target, said target being adapted to produce a return beam and a plurality of electrically separate steering electrodes spaced between said separation electrode and said storage target, said separation electrode comprising a planar structure having an opening therethrough for receiving a portion only of the return beam from said target, another portion of said beam impinging upon said structure adjacent to the edge defining said opening, said structure having a tubular portion extending from said edge towards said target for preventing electrons from said another portion of the beam from entering said opening.

3. A television camera tube comprising an elongated envelope having an axis, a plurality of electrodes in said envelope, said electrodes including in the order recited a cathode, a separation electrode, four steering electrodes spaced around said axis, and a photocathode, said steering electrodes comprising linger-like structures disposed in a comomn plane transverse to the longitudinal axis of said envelope.

4. In a television camera of the type having means for producing nodal points on an electron beam, an elongated envelope having .an axis, an electron gun in one end of said evelope for producing said electron beam, a photocathode in the other end of said envelope for producing a photoelectron charge image, a target electrode in the path of said electron beam and in the path of said photoelectron charge image, a separation electrode having a control aperture and positioned at an anti-node of said electron beam, and four steering electrodes positioned around said axis, said separation electrode comprising a planar portion normal to the axis of said envelope and a lip extending from the edge defining said control aperture and towards said target for preventing entrance into said control aperture of a separated portion of said beam.

5. A television camera, as in claim 4 where-in said steering electrodes are positioned in the plane of one of said nodal points.

6. A television camera tube of the type adapted to be operated within a magnetic field, whereby an electron beam traversing said tube has a predetermined number of nodal points, said tube comprising an elongated envelope having an axis, an electron gun in one end of said envelope for producing an electron beam, a photocathode in the other end of said envelope for producing a photoelectron charge pattern, a target electrode positioned between said photocathode and said electron gun and exposed to both said electron beam and said photoelectron charge image, means for decelerating said electron beam in the space closely adjacent to said target electrode, means including said photocathode for establishing a charge pattern on said target electrode, whereby electrons of said electron beam approaching an area of said target electrode having a charge thereon `are given a non-axial velocity component, four steering electrodes positioned between said target electrode and said electron gun and around said axis whereby electrons from said target may vbe directed in a direction transverse to said axis said steering electrodes being positioned in the plane of one of said nodal points, and means between said steering electrodes and said electron gun for passing the electrons from said target which include said non-axial velocity component.

7. A television camera tube comprising an elongated envelope, an electron gun positioned in one end portion of said envelope, a photocathode disposed on the inner face of the other end of said envelope and adapted to emit photoelectrons in a pattern corresponding to a light pattern directed onto the outer face of said other end, a storage target adjacent to said photocathode and positioned to receive said photoelectrons for producing a charge image on said storage target corresponding to said pattern of photoelectrons, means outside of said envelope for scanning a beam from said electron gun across said storage target, whereby a portion of said beam is specularly reflected from non-signal portions of said storage target and another portion of said beam is reflected in a non-specular scattering fashion from signal portions of said target, an electron multiplier system adjacent to said electron gun, an apertured electrode adjacent to said multiplier system for separating said specularly and nonspecularly reilected electrons, said apertured electrode having an edge defining the aperture therein and including a lip extending from said edge and towards said storage target for preventing migration of said specularly reflected elect-rons in the aperture of said apertured electrode, and electrode means between said apertured electrode and said storage target for steering said reflected electrons to cause said specularly reflected electrons to be collected by said apertured electrode and to permit said non-specularly reflected electrons to` pass into said multiplier system.

References Cited by the Examiner UNITED STATES PATENTS 2,365,006 12/1944 Ricketts 131-65 X 2,747,133 5/1956 Weimer 313-65 X GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, Examiner.

Claims (1)

1. A TELEVISION CAMERA TUBE COMPRISING AN ELONGATED ENVELOPE, AN ELECTRON GUN IN ONE END OF SAID ENVELOPE, A PHOTOCATHODE IN THE OTHER END OF SAID ENVELOPE, A TARGET ELECTRODE BETWEEN SAID ELECTRON GUN AND SAID PHOTOCATHODE AND IN SAID ENVELOPE, A SEPARATION ELECTRODE BETWEEN SAID ELECTRON GUN AND SAID TARGET ELECTRODE AND WITHIN SAID ENVELOPE, AND AT LEAST THREE ELECTRICALLY SEPARATE STEERING ELECTRODES BETWEEN SAID SEPARATION ELECTRODE AND SAID TARGET ELECTRODE AND WITHIN SAID ENVELOPE, SAID STEERING ELECTRODES BEING FINGER-LIKE STRUCTURES EXTENDING IN A PLANE NORMAL TO THE AXIS OF THE ENVELOPE.

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