US 3432711 A
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Description (OCR text may contain errors)
March 11, 1969 H. CLAYTON 3,432,711
HYBRID DEFLECTION IMAGE DISSECTOR HAVING CONCAVE DEFLECTION PLATES CONVERGING AT HORIZONTAL EDGES 0F RESOLVING APERTURES Filed July 5, 1966 adx l 32 Til i/ .9 J
R/l. CLAYTON BY W A TTORNEY United States Patent 3,432,711 HYBRID DEFLECTION IMAGE DISSECTOR HAVING CON CAVE DEFLECTION PLATES CONVERGING AT HORIZONTAL EDGES OF RESOLVING APERTURES Robert H. Clayton, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Delaware Filed July 5, 1966, Ser. No. 562,773 US. Cl. 313-79 Int. Cl. H0lj 29/76 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a novel image dissector tube and particularly to such a tube which utilizes a combination of magnetic focusing and electrostatic deflection for line scanning to provide a more compact structure with high resolution and fast response.
Image dissector tubes, providing an electrical signal representing successively scanned areas of an image or scene of varying light intensity, are being increasingly utilized for space surveillance and for optical input information to computers. It is generally desirable to employ magnetic focusing of the tube to obtain high resolution. However, the conventional accompanying magnetic deflection coils add excessive weight and require high power for operation, so that an all magnetic tube is not satisfactory for use in many instances. In addition, electrostatic deflection provides faster response where high rates of line scan are required. It is therefore the primary object of the present invention to provide an image dissector tube utilizing both magnetic focusing and electrostatic deflection for line scanning to provide a more efficient structure with improved operating characteristics.
These results are achieved by a novel tube structure which utilizes magnetic focusing coils extending axially around the tube envelope between the photocathode at one end and electron multiplier at the other end, one set of magnetic deflection coils around the front portion of the tube between the photocathode and a first transverse apertured plate to provide vertical deflection for the slower frame scan, and a pair of horizontally extending electrostatic deflection plates between the first apertured plate and a second transverse apertured plate adjacent the electron multiplier to provide horizontal deflection for high speed line scan. The effects of the focusing coil establishing an axial magnetic field and the horizontally positioned electrostatic plates forming an electrostatic field in the vertical direction combine to cause a cycloidal trajectory of the electrons which provides the horizontal scanning of the electron image across the second aperture. The first aperture is in the form of a horizontal slit t provide sharp resolution in the vertical dimension for the image scanned by the vertical deflection coils. The second aperture is in the form of a vertical slit to provide sharp resolution in the horizontal dimension and to compensate for phase shift or displacement in the vertical direction caused by the length of the electrostatic plates not corre- "ice sponding exactly to an integral number of focusing loops between the two apertures. 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 three-dimensional sectional schematic view of the novel tube structure;
FIG. 2 shows a three-dimensional view of a curve of the electron path under the influence of magnetic focusing and electrostatic deflection fields, and
FIG. 3 shows a schematic of the relationship of the electron image from the first aperture as scanned across the second aperture.
As shown in FIG. 1, an external image 10 or scene of varying light intensity is projected onto a photoeathode 12 on the inner surface of a face plate at one end of a tubular envelope 14. The photocathode emits electrons in accordance with the light image. A set of electromagnetic deflection coils 16 provides repetitive relatively slow frame scanning of the electron image in the vertical direction across a first aperture 18 in a transverse plate 20. Aperture 18 is in the form of a narrow horizontal slit with a small vertical dimension to provide a fine limit for resolution in the vertical direction. Electrons passing through aperture 18 are subjected to an electrostatic field E as indicated in FIG. 2, in the vertical direction, established by a pair of horizontal preferably concave deflection plates 22 extending along and outwardly curved from the longitudinal axis of the tube. At the same time the electrons are affected by the axial magnetic field B from focusing coils 24. As is well known, the cross field relationship established by the two interacting orthogonal fields in the Y and Z directions, causes the electrons to follow a cycloidal curve having a resultant vector along the X axis which provides scanning in the X direction. For integral scanning cycles the X axis deflection is at right angles to the electrostatic deflection field B The basic theory and mathematical relationships of this electron trajectory may be found in the text entitled Electron Optics, by O. Klemper, published in 1953 by Cambridge University Press. The usual sawtooth scanning voltage varying about a fixed direct voltage level is applied to the deflection plates and suitable potentials between the photocathode and first apertured plate provide initial electron acceleration along the tube length. A field mesh (not shown) spaced closely to the cathode may also provide acceleration.
The length of the plates along the tube axis represents a compromise between plate capacitance, which increases with size and limits the rate of scan, and deflection sensitivity, which decreases with shorter plates since higher voltages are required. The plates may extend only a short distance from the first aperture 18 but, for maximum sensitivity and resolution, preferably extend to a second aperture plate 26 adjacent an electron multiplier 28. The separation of the plates in the vertical dimension along each point of the tube axis is determined by the same cycloidal curve of FIG. 2 which provides the scan in the horizontal or X direction. The Y dimension between the plates is thus preferably twice the intantaneous maximum displacement from the axis to the curve at each point to form a pair of curved plates symmetrically positioned about the longitudinal axis. The plates diverge from a small dimension adjacent the horizontal edges of the first aperture to a larger separation at the central portion and then converge at the other end. For best deflection sensitivity and to avoid fringe field problems, it is preferable to converge the deflection plates at both ends as shown, with an outwardly curved structure opposite to that of conventional deflection plate curvature.
The separation of the apertured plates and cathode is determined by the focusing requirement that there be an integral number of focusing loops between the photocathode and first apertured plate and between the two apertures. The two apertured plates are preferably connected to the same potential. Due to inherent nonuniformities in the deflection fields and the physical limitation that the deflection plates cannot extend exactly to the full length between apertures, a phase shift or displacement in the vertical direction occurs, as illustrated in FIG. 3. The electron image 30 from aperture 18, shown in dashed lines, thus scans across aperture 32 in both the horizontal and vertical directions. In order to compensate for this phase shift or vertical displacement, aperture 32 is extended to form a vertical slit with a narrow dimension a'x along the edge in the horizontal direction to define X axis resolution. The Y dimension of aperture 32 must be long enough to accommodate the entire image of the first aperture as it is scanned in the nonorthogonal direction at an angle with respect to the horizontal line. In addition, the curved deflection plates must have a wider separation at the end adjacent transverse plate 26 and the electron multiplier to accommodate the full length of aperture 32. The first dynode of the electron multiplier must also be of sufficient length and positive voltage with respect to the photocathode to receive all of the electrons passing through aperture 32. The output signal is further amplified in the multiplier, with suitable potentials between the various dynodes and then applied to any suitable utilization device.
In some cases where vertical scan is provided externally, such as by movement of the vehicle carrying the image tube, the vertical deflection coils may be omitted. The first horizontal aperture is then replaced by a limited area photocathode with electrons being scanned and accelerated across the second aperture in the same manner as previously described.
The present invention thus provides a novel, compact, fast acting image dissector of high resolution. While only a single embodiment has been illustrated, the invention is not be considered as limited to the exact form or use shown and many variations may be made in the particular configuration without departing from the scope of the invention as set forth in the appended claims.
What is claimed is:
1. An image dissector tube comprising:
a tubular envelope;
a photocathode at one end of said envelope;
means projecting an image of variable light intensity onto said photocathode, said photocathode providing an electron image corresponding to said light image;
a first apertured plate spaced from said photocathode along the longitudinal axis of said tube and positioned transversely with respect to said axis, said first plate having an aperture with a narrow dimension in the vertical direction and a long dimension in the horizontal direction;
means scanning the electron image across said first aperture in the vertical direction;
a second apertured plate spaced from said first plate along said longitudinal axis and having a second aperture therein;
a pair of horizontally positioned electrostatic deflection plates extending longitudinally between said first and second apertured plates providing an electrostatic deflection field in the vertical direction, said electrostatic deflection plates being concave to said longitudinal axis and converging at each end adjacent the horizontal edges of said apertures;
electromagnetic means extending axially around said tube envelope providing a magnetic field for focusing said electron image along the longitudinal axis, said electrostatic and magnetic fields providing horizontal line scanning of said electron image across said second aperture; and
an electron multiplier positioned at the other end of said tube adjacent said second aperture providing an output signal in accordance with the electrons passing through said second aperture.
2. The device of claim 1 wherein said second aperture has a narrow dimension in the horizontal direction and a long dimension in the vertical direction.
3. The device of claim 1 wherein the curved plates conform to a cycloidal electron trajectory resulting from the interaction of the magnetic focusing and electrostatic deflection fields.
4. The device of claim 1 wherein said electromagnetic focusing means extends from said photocathode to said second apertured plate.
5. The device of claim 4 wherein said means scanning said electron image in the vertical direction comprises electromagnetic coils extending axially around said tube envelope between said photocathode and first apertured plate.
6. The device of claim 5 wherein said first and second apertured plates are positioned at integral numbers of focusing loops along said tube.
References Cited UNITED STATES PATENTS 2,152,820 4/ 1939 Schlesinger 313-79 X 2,213,176 8/1940 Rose 313-79 2,266,621 12/1941 De Vore 31379 X 2,377,972 6/ 1945 Schade 31367 X 3,225,237 12/ 1965 Cope 313- ROBERT SEGAL, Primary Examiner.
US. Cl. X.R. 313-80