EP0461205A4

EP0461205A4 - Electron gun with reduced-movement of cross-over point at increased beam current levels, and methods of operating same.

Info

Publication number: EP0461205A4
Application number: EP19900908899
Authority: EP
Inventors: Thomas A Dunbar
Original assignee: Imaging and Sensing Technology Corp
Current assignee: Mirion Technologies IST Corp
Priority date: 1989-12-28
Filing date: 1989-12-28
Publication date: 1991-08-19
Also published as: DK152091D0; KR920702896A; WO1990009675A1; EP0461205A1; JPH04504631A; DK152091A

Abstract

An electron gun (40) has a cathode (41) and three sequentially-spaced plate-like electrodes (42, 43, 44). Each electrode has an aperture (45, 56, 48) aligned with a line (x-x) extending normally away from a point of the emitting surface (58) of the cathode. A power supply (60) is arranged to supply respective voltages (Vc, V1, V2, V3) to the cathode and the three electrodes. Electrons issue from the emitting surface as a substantially-laminor flow, and are focused on a cross-over (59) beyond the third electrode after the electrons have been accelerated to substantially their maximum velocity. The velocity of the electrons at the cross-over is substantially greater than in prior art guns. The cross-over is of greater resolution due to a diminished space charge effect. The cross-over remains at a substantially fixed location from the emitting surface at all beam current levels. The invention also provides an improved method of operating an electron gun, which method includes dynamic switching of the various voltages applied to the cathode and the three electrodes, so as to maintain the resolution of a reformed image point on the screen within a predetermined bandwidth throughout the entire range of possible beam currents.

Description

ELECTRON GUN WITH REDUCED-MOVE¬ MENT OF CROSS-OVER POINT AT IN¬ CREASED BEAM CURRENT LEVELS, AND METHODS OF OPERATING SAME

Technical Field

The present invention relates generally to the field of electron guns, such as used in cathode ray tubes and the like, and, more particularly, to an improved electron gun having a smaller cross-over or object point, and in which movement of such cross¬ over point relative to the cathode as a function of beam current is reduced, all with the object of improving the resolution of a reformed image point on a dfeplay screen.

Background Art

Electron guns are in common use. Such guns are typically used in cathode ray tubes (CRT) to create a beam of electrons which is then focused on, and selectively deflected relative to, a display screen. The screen typically has a phosphor coating on its inner surface, which is illuminated by the incident electron beam.

In many applications, such as a conventional television, the ambient or back¬ ground light in the room in which the CRT is located, varies within a relatively-small bandwidth. In these applications, the electron gun may be designed to produce a, .reason¬ ably well-defined spot on the display screen within a range of anticipated beam currents. In other applications, however, the level of ambient light may vary significantly. Consider, for example, a CRT or heads-up display (HUD) in the cockpit of an aircraft. On the one hand, the aircraft may be flying at an altitude of, say, 30,000 feet [9.144 kilometers] in brilliant sunshine. If the pilot sits beneath a transparent canopy, the intensity of such ambient or background light may literally "wash out" the readability of the screen. On the other hand, the aircraft may be flying in conditions of darkness. This problem is amplified in projection-type devices in which the beam current required to produce a screen image of acceptable relative intensity, varies with the area of the projected display.

To compensate for changes in ambient light intensity, the pilot typically adjusts the brightness and contrast of the CRT in an attempt to produce a readable image under the existing light conditions. At night, the pilot would typically reduce the brightness and contrast of the CRT. During bright conditions, however, he would normal¬ ly increase the brightness and contrast. By making such adjustments, the pilot is adjusting the beam current of the electron gun within the CRT. While this sounds reasonably innocuous, in such electron guns as have been developed heretofore, the axial position of the cross-over point is believed to have changed as a function of the magnitude of the operator-selected beam current. Thus, if one particular beam current would produce an image point on the screen of acceptable resolution, the fineness or quality of that resolu¬ tion might well be affected if the beam current were to be changed. In an extreme case, such as an aircraft in which the pilot sits beneath a transparent canopy, the level of ambient light may change drastically (Le., for conditions of darkness to conditions of brilliance). Thus, assuming that the brightness and contrast are adjusted, either manually or automatically, in an attempt to accommodate such changes in the ambient lighting, the CRT might produce an image point of acceptable resolution under one condition, but an unacceptable "fuzzy" image point under another condition. Accordingly, it would be generally desirable to provide an improved electron gun having a more precisely defined cross-over or object point, and in which axial move¬ ment of the cross-over in response to varying beam currents, is reduced, with the object of improving the resolution of the reformed image point on the display screen under various background lighting conditions.

Disclosure of the Invention

The present invention provides an improved electron gun, such as for use in a CRT of either the display- or projection-type having a physically-smaller cross-over point, and in which axial movement of the cross-over as a function of changing beam current is reduced. With parenthetical references to the corresponding parts, portions or surfaces of the disclosed embodiment for purposes of illustration, the improved gun (e.g., 40) broadly includes a cathode (e.g., 41) having an emitting surface (e.g., 58); a first electrode (e.g., 42) arranged in spaced facing relation to the emitting surface and having a first aperature (e.g., 45) axially aligned with an imaginary line (e.g., x-x) extending normally (Le., perpendicularly) from a point on the surface; a second electrode (e.g., 43) arranged in spaced facing relation to the first electrode and having a second aperture (e.g., 46) axially aligned with the imaginary line, the distance from the emitting surface to the second electrode (le., distance x₂) being greater than the distance from the emitting surface to the first electrode (Le., distance Xj); a third electrode (e.g., 44) arranged in spaced facing relation to the second electrode and having a third aperture (e_g., 48) axially aligned with the imaginary line, the distance from the emitting surface ^"to fhe third elec- trode (Le., distance x₃) being greater than the distance from the emitting surface to the second electrode; and control means (e.g., 60) for selectively controlling the respective voltages of the emitting surface and the three electrodes such that the flow of electrons between the cathode and the first electrode will be substantially laminar and parallel to the imaginary line, for causing such electrons to be progressively accelerated as they leave the emitting surface, and for focusing the electrons on a cross-over point after the elec¬ trons have substantially reached their maximum velocity attributable to the controlled voltages provided by the control means. Thus, by creating the cross-over dqwnstream of the point of maximum velocity, the electrons exhibit less of a space charge effect at the cross-over point, and the cross-over point will therefore be of higher resolution (Le., finer or of smaller diameter) and will be less susceptible to axial movement relative to the emitting surface in response to changes in the beam current.

In another aspect, the invention provides an improved method of operating an electron gun (e.g., 40) having a cathode (e.g., 41) and having three sequentially-spaced electrodes (e.g., 42,43,44) associated with the cathode. Each of the electrodes has an aperature (e.g., 45,46,48) aligned with an imaginary line (e.g., x-x) extending away from a point on the emitting surface (e.g., 58) of the cathode. The improved method broadly includes the steps of: applying a first voltage (V_j) to a first of the electrodes (e.g., elec¬ trode 42); applying a second voltage (V₂) to a second of the electrodes (e.g., electrode 43); applying a third voltage (V₃) to a third of the electrodes (e.g., electrode 44); and selectively controlling the respective voltages to cause a desired flow of electrons to be emitted from the cathode to cause the electrons to be accelerated to a velocity at the cross-over greater than in prior art electron guns, and to focus the emitted electrons on a cross-over point which remains substantially stationary along the imaginary line (Le., at a substantially fixed distance from the emitting surface), independent of the magnitude of the electron flow.

Accordingly, the general object of the invention is to provide an improved electron gun.

Another object is to provide an improved electron gun in which the elec¬ trons are accelerated to a velocity at the cross-over greater than in prior art guns. Another object is to provide an improved electron gun in which the cross¬ over is smaller and more precisely defined than in prior art guns.

Another object is to provide an improved electron gun in which axial move¬ ment of a focused cross-over is less a function of the beam current. Another object is to provide an improved electron gun suitable for use in a display- or projection-type CRT, and which affords a high-resolution reformed image point on the screen substantially independently of changes in the brightness or contrast.

Still another object is to provide an improved method of operation an elec¬ tron gun so as to reduce axial movement of an object point in response to changing beam currents.

These and other objects and advantages will become apparent from the foregoing and ongoing written specification, the drawings, and the appended claims.

Brief Description of the Drawings

Fig. 1 is a fragmentary view, partly in section, partly in elevation and partly schematic, of a prior art display-type cathode ray tube.

Fig. 2 is an enlarged fragmentary vertical sectional view of the electron gun used in the prior art CRT shown in Fig. 1, this view depicting the cross-over as being spaced from the cathode emitting surface along line x-x by a horizontal distance x_c when one set of voltages is applied to the grids. Fig. 3 is a view similar to Fig. 2, but showing the cross-over as being spaced from the emitting surface along line x-x by a greater horizontal distance x_c when another set of voltage conditions is applied to the grids.

Fig. 4 is a fragmentary vertical sectional view of the improved electron gun, showing the three electrodes arranged in sequentially-spaced relation to the cathode, and showing the control means for supplying individual voltages to the cathode and the grids. Fig. 5 is a plot of resolution (ordinate) vs. beam current (abscissa), compar¬ ing the general shapes of the curves of the prior art and improved guns.

Fig. 6 is a plot of resolution (ordinate) vs. beam current (abscissa) of the improved gun, for different voltages applied to the grids, and demonstrates how the resolution may be confined within a desired bandwidth by dynamically switching the grid voltages at different desired beam currents. Modefsl of Carrying Out the Invention

At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g. arrangement of parts, mounting, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms "horizontal", "vertical", "left", "right", "up" and "down", as well as adjectival and adverbial derivatives thereof (e.g., "hori¬ zontally", "rightwardly", "upwardly", etc.) simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Unless otherwise indicated, the terms "inwardly" and "outwardly" refer to the orientation of a surface relative to its axis of elongation, or axis or rotation, as appropriate. Referring now to the drawings, this invention broadly provides an improved electron gun which is particularly adapted for, but not limited to, use in a display-type or projection-type CRT, where the intensity of the background light is widely variable. The improved gun affords the desirable capabilities of forming a physically-smaller object point or cross-over, and maintaining the cross-over at a substantially-fixed distance from the emitting surface, with accompanying improvement in the resolution of a reformed image point on the screen, notwithstanding widely-varying beam currents.

However, before proceeding to a discussion of the improved gun, it is deem¬ ed desirable to first review the structure and operation of a prior art gun. With such background, it is felt that the improved gun may be appreciated in context. Hence the prior art and improved guns will be described seriatim herebelow.

Prior Art Gun (Figs. 1-3.

* »_,

Referring now to Fig. 1, a prior art CRT, generally indicated at 20, is shown as broadly including a horizontally-elongated evacuated envelope having a leftward_, neck portion 21, an intermediate funnel-shaped portion 22, and a rightward face-plate or dis- play screen 23. This type of CRT is generally disclosed in U.S. Patent No. 4,334,170, the aggregate disclosure of which is hereby incorporated by reference. A plurality of cathodo- luminescent phosphor targets (not shown) are provided on the inner surface of the screen. A shadow mask, represented by dashed line 24, is positioned within the envelope in close proximity to the face-plate.

An electron gun 25 is positioned within the neck portion and is arranged to produce or generate a rightwardly-directed beam of electrons along the horizontal center- line x-x of the tube. A power supply 26 is arranged to supply various voltages to gun 25, and to other parts of the tube. This power supply may be operated to cause the gun to produce a stream of electrons of variable beam current, and to selectively deflect the beam either on-axis or off-axis to different desired parts of the face-plate. In Fig. 1, an off-axis deflected beam of electrons is schematically represented by line 27.

Fig. 2 is a schematic vertical sectional view of pertinent portions of the electron gun 25 shown in Fig. 1. The gun is similarly elongated along tube axis x-x, and is shown as including a leftward thermionic cathode 28 and two sequentially-spaced verti¬ cal plate-like electrodes 29,30 having apertures 31,32, respectively, aligned with axis x-x. Plates 29,30 are shown as having annular vertical surfaces 33,34 and 35,36 facing toward and away from the cathode, respectively. Plate 29 is sometimes referred to as grid G , and plate 30 as grid G₂. Power supply 26 is arranged to supply various individual volt¬ ages to the cathode and to grids G₇,G₂, inter alia, for varying the beam current issuing from the rightwardly-facing vertical emitting surface 38 of the cathode. However, grids G ,G₂ also form an electrostatic lens for focusing such electrons on a cross-over or object point, indicated at 39. Thus, with this prior art embodiment, the various voltages applied to the cathode and grids G^G, would be selectively varied to controllably alter the beam current. However, variations in the desired beam current would also affect the axial distance (Le., distance x_c) of the cross-over from the emitting surface, as comparatively illustrated in Figs. 2-3.

Fig. 2 illustrates the cross-over as occurring at one distance from the emit¬ ting surface along line x-x when one set of voltages is applied to the cathode and grids G^G^ whereas Fig. 3 depicts the cross-over as occurring at a greater distance from the emitting surface along line x-x when another set of voltages is applied to the cathode and grids. In this prior art CRT, the cross-over typically occurred somewhere between grids G_j and G The position of the cross-over relative to the emitting surface, however, was a function of the voltages applied to the cathode and the two grids, inter alia. At the same time, the beam current was also a function of these voltages. Hence, if the viewer attempted to increase the beam current dramatically, the cross-over or object point would move along line x-x from one point to another. This translation or movement of the object point resulted in diminished resolution (Le., a physically-larger spot) of the reform¬ ed image point on the display screen at higher beam current levels. Thus, whereas the image point might be reasonable well-defined at lower current levels, it would be "fuzzy" at high currents.

In addition to this, even at lower beam currents, the cross-over was relatively large. Upon information and belief, the size (Le., diameter) of the cross-over was a function of the velocity of the electrons at the cross-over. Persons skilled in this art will readily appreciate that such electrons have a zero velocity at emitting surface 38, but are quickly accelerated as they leave this surface and pass through the grid apertures en route to their impingement on the phosphor targets. However, in the prior art gun, the cross- over point typically occurred between the first and second grids, as shown fct IJΪgs. 2 and 3. Since the electrons were still accelerating in this region and had not reached their maximum velocity, it is felt that the space charge effect (Le., the fact that negatively- charged electrons tend to repel one another) limited the formation of a* precisely-defined point-like cross-over. The resolution of the image point reformed on the display screen was, of course, dependent upon the size ofthe object point. Therefore, upon information and belief, the prior art electron gun suffered from three principal drawbacks: (1) the maximum velocity of the electrons at the cross-over was relatively slow, (2) the space charge effect had a greater influence where the cross-over was formed before the elec¬ trons had reached their maximum velocity, and (3) the axial position of the cross-over from the emitting surface varied with changes in the magnitude of the beam current.

The Improved Gun (Figs. 4-7)

The present invention overcomes these deficiencies by providing an improved electron gun in which the electrons are accelerated to a greater velocity prior to forma¬ tion of the cross-over, in which the faster moving electrons are less susceptible to the space charge effect at the cross-over, and in which the cross-over point remains substan¬ tially-stationary on line x-x despite changing beam currents.

Referring now to Fig. 4, the improved gun, generally indicated at 40, is fragmentarily illustrated as being elongated along horizontal axis x-x, and as having a thermionic cathode 41. Three vertical plate-like electrodes 42,43,44, provided^* ith aper¬ tures 45,46,48, respectively, are sequentially spaced along axis x-x. More particularly, electrodes 42,43,44 are shown as having annular vertical surfaces 49,50, 51,52 and 53,54, facing toward and away from the cathode, respectively. Surfaces 49,51,53 are shown as being arranged at distances x , x₂, x₃ from the cathode, respectively. Electrode 42 may be regarded as grid G₇, electrode 43 as grid G_j, and electrode 44 as grid G₃. Grids G_j.G^G_j form an electrostatic focusing lens. To this end, an annular groove is shown as extending leftwardly into grid G₂ from its right face 52, immediately about aperture 46. This groove is bounded by an inwardly-facing cylindrical surface 55 extending leftwardly into the grid from right surface 52, and a rightwardly-facing annular vertical surface 56 extending radial¬ ly inwardly therefrom to join aperture 46. A suitable power supply or control means, generally indicated at 60, is arranged to supply respective voltages to the cathode and to the three grids, possibly inter alia. More particularly, the control means or power supply is arranged to supply selected voltages (V_C,V₇,V₂,V₃) to the cathode and to grids G₇,G₂,G₅, respectively. For example, if the cathode is grounded (Le., V_c = 0), grid G may be at -40 volts, grid G₂ at +1000 volts, and grid G₅ at +7000 volts. Thus, the third grid serves to accelerate the electrons to a velocity not obtained with the prior art triode.Thus, electrons issue from the rightwardly-facing vertical emitting surface 58 ofthe cathode as a substantially-laminar flow parallel to axis x-x, and are focused by the G₂-G₅ lens on cross-over 59. However, this cross-over is located farther from emitting surface 58 than in the prior art. In fact, such cross-over occurs farther from the emitting surface than grid G₃. Hence, the electrons are accelerated to a velocity greater than with prior art guns, and the cross-over occurs after the electrons have substantially reached their maximum velocity attributable to the voltages supplied to the cathode and the three grids. Since the velocity of the electrons is greater at the cross-over than in prior art guns, the space charge effect has less of an effect on the fineness or preciseness (Le., smallness of diameter) of the focused cross-over.

Fig.5 is a graph of resolution or screen spot size (ordinate) vs. beam current (abscissa), showing the general forms of the curves for the prior art and improved elec¬ tron guns. The prior art curve appears to fit the general equation:

(1) = __r^? + bx² + c + d This equation can be rewritten as:

(2) R = ύ³ = br + ά + d where R is the spot size at the screen (measured in terms of its diameter), and i is the beam current. Equation (2) has a first derivative:

(3) dR/di = 3az^'2 + 2b. + c and a second derivative:

(4) d²R/df = 6ai + 2b

From Fig. 5, it can be seen that, with the prior art gun, spot size is relatively constant between i = 0 and i * but increases sharply between i « Si^^. and i = i^^. Thus, while resolution was gener¬ ally between R₂ and R₄ at lower beam currents (Le., I < "0.751,,,^, spot size increased dramatically at higher beam currents and exceeded the R₂- ₄ bandwidth at beam currents greater than about i = Q.751^^ reaching a maximum value of _ ₇ at i = i^^

Still referring principally to Fig. 5, the curve of the improved gun, on the other hand, is believed to generally follow the equation:

(5) y = ax⁴ + bx³ + cx² + άx + e. This equation can be rewritten as:

(6) R = ai⁴ + b_⁵ + ά² + di + e having a first derivative: (7) dRldi = 4ai³ + 3bi² + 2d + d and a second derivative:

(8) d²R/di² = 12a.² + 6bi + 2c

Thus, the resolution of the improved gun is shown as having greater amplitude swings between i — 0 and i « 0.5/,^ than with the prior art gun. However, between i « 0.50/^^. and i = i_mαx, the improved gun has a resolution falling generally within R_j-R₄ bandwidth, and between i ∞ 0.75/,^ and i = i_mαx, resolution is generally within the R₂- R₄ bandwidth. Thus, whereas the resolution of the prior art gun falls within the R₂-R₄ bandwidth at beam currents of less than about i = 0.75/,,,^, the spot size increases dra¬ matically between i » 0.75i_mαx and i = i_mαxr Thus, at higher beam currents (Le., i > »0.75_: _β ), resolution suffered. The improved gun appears to be the converse, namely, that resolution is relatively poor at lower beam currents, but appears to fall within the R₂- R₄ bandwidth at higher beam currents (Le., between i = ^Q.75i_mαx and i = _H.)_l!

This characteristic of the improved gun can be overcome by dynamically switching the voltages applied to the cathode and the three grids to maintain the screen resolution within a predetermined bandwidth. Fig. 6 is a plot of screen resolution (ordi¬ nate) vs. beam current (abscissa) for the improved gun, showing two different curves representing different voltage conditions applied to the cathode and the grids'. Curve A was obtained by holding the cathode at ground, by holding G₂ and G_j at +550 and +7000 volts, respectively, and by varying G₂ from -140 to 0 volts. Curve B, cm the other hand, was obtained by holding the cathode at ground, by holding G₂ and G₃ at +450 and +7000 volts, respectively, and by varying G₇ from -110 to 0 volts. Curve./! is shown as having a maximum current of i^,^, and curve B is shown as having a maximum current Curved is shown as having a resolution of R₅ at i = 0, of rising quickly to a resolution of about R₇ at i « 2/22i_(A)max, of thereafter falling sharply to a resolution of R_j at i « 8/22i_(A)max, and as thereafter rising more slowly to a resolution of about R₄ between i « 8/22i_(A)max and i^_razr Curve B, on the other hand, is seen as rising from a resolution of R₃ at i — 0 to a resolution of R₄ at i = 5/18i_(B^_max, as increasing more quickly to a resolution of R₆ at i ∞ \QI\8i^_max> and as thereafter falling to a resolution of R₂ at i = i_'B)m_aχ- Th^us _> the resolution may be maintained within a preselected band¬ width by selectively varying the voltages applied to the cathode and the grids, for different beam current levels. For example, if it were desired to hold the resolution between R and R₄, the control means could be operated to apply selected voltages to the cathode and the grids at different desired beam currents to use the portion of curve B between i = 0 and i = 5/22i/_A>max = 5/lSif_{B)n x}, and to use the portion of curved between i = 522i_(A)max and i = i_(A)max- In this manner, the screen resolution may be maintained within a predetermined bandwidth throughout the entire operating range of possible beam currents. Of course, the foregoing example is merely illustrative, and persons skilled in this art will readily appreciate that the control means may be selectively operated to dynamically switch between other voltage conditions as well.

It should also be noted that the prior art curve shown in Fig. 5 has a single inflection point /, whereas the curve of the improved gun has two, J₇ and I_? These inflection points occur when the second derivative of the general equation for each curve is made equal to zero.

Modifications

The present invention expressly contemplates than many changes and modifi¬ cations may be made.

For example, the emitting surface of the cathode need not necessarily be planar, but may be convex or have some other shape if desired. The relative spacings of the grids may be changed. The sizes and shapes of the apertures may also be changed as well. Similarly, the power supply or control means may be operated to supply desired voltages to the cathode and the three grids. If desired, the control means may be opera- tively arranged to dynamically switch the respective voltages applied to the cathode and the three grids, to maintain the resolution of a reformed image point on the screen within a predetermined bandwidth. Therefore, while the presently-preferred form of the improved electron gun, and the method of operating same, has been shown and described, and several modifica¬ tions and changes thereof discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit of the invention, as defined and differentiated by the following claims.

Claims

1. An electron gun, comprising: a cathode having an emitting surface; a first electrode arranged in spaced facing relation to said emitting surface and having a first aperture axially aligned with an imaginary line extending normally from a point on said surface; a second electrode arranged in spaced facing relation to said first electrode and having a second aperture axially aligned with said imaginary line, the distance from said emitting surface to said second electrode being greater than the distance from said emitting surface to said first electrode; a third electrode arranged in spaced facing relation to said second electrode and having a third aperture axially aligned with said imaginary line, the distance from said emitting surface to said third electrode being greater than the distance from said emitting surface to said second electrode; and control means for selectively controlling the respective voltages of said sur¬ face and said three electrodes such that the flow of electrons between said cathode and said first electrode will be substantially laminar, for causing electrons to be progressively accelerated as they leave said emitting surface, and for causing said electrons to cross¬ over at a point after said electrons have substantially reached their maximum velocity attributable to said voltages.

2. An electron gun as set forth in claim 1 wherein said emitting surface is planar.

3. An electron gun as set forth in claim 1 wherein each of said electrodes is a plate-like element.

4. An electron gun as set forth in claim 3 wherein said plate-like electrodes are substantially parallel to one another. 5. An electron gun as set forth in claim 1 wherein said cross-over point occurs along said imaginary line at a distance from said emitting surface greater than the distance from said emitting surface to said third electrode.

6. An electron gun as set forth in claim 1 wherein each of said apertures is circular.

7. An electron gun as set forth in claim 1 wherein said second and third elec¬ trodes form a lens for directing the flow of electrons toward said cross-αver point.

8. An electron gun as set forth in claim 1 wherein the emission of electrons from said surface is a function of the voltage differential between said first electrode and said cathode.

9. An electron gun as set forth in claim 1 wherein the emission of electrons from said surface is a function of the voltage differential between said secoad electrode and said cathode.

10. An electron gun as set forth in claim 1 wherein the emission of electrons from said surface is a function of the voltage differential between said third electrode and said cathode.

11. An electron gun as set forth in claim 1 wherein said control means is oper¬ ated to cause said cross-over point to remain substantially stationary on said imaginary line independently of the flow of electrons emitted from said surface. 12. In a cathode-ray tube including a cathode having an emitting surface from which electrons are adapted to be emitted and including a screen toward which a beam of electrons is adapted to be directed, the improvement which comprises: an improved gun for causing electrons emitted from said emitting surface to converge at an object point, said gun including a first electrode having one surface arranged in spaced facing relation to said emitting surface, having a first aperture aligned with an imaginary straight line extending outwardly from a point on said emitting surface, and having another surface facing away from said emitting surface; a second electrode having one surface arranged in spaced parallel facing relation to said first electrode other surface, having a second aperture aligned with said line, and having another surface facing away from said emitting surface; a third electrode arranged in spaced parallel facing relation to said second electrode other surface, having a third aperture aligned with said line, and having another surface facing away from said emitting surface; said second and third electrodes framing an adjustable lens for focusing electrons emitted from said surface toward said object point; and control means for applying respective voltages to said electrodes for varying the flow of electrons emitted from said source and for causing emitted electrons to be focused on said object point; whereby said object point will remain substantially stationary notwithstanding variations in the flow of electrons emitted from said source.

13. The improvement as set forth in claim 12 wherein said contact means is arranged to selectively vary each of said voltages independently of one another.

14. The improvement as set forth in claim 12 wherein said third electrode other surface faces toward said object point.

15. The improvement as set forth in claim 12 wherein said object point is sub¬ stantially on said line. 16. The improvement as set forth in claim 12 wherein said object point is form¬ ed after said electrons have substantially reached their maximum velocity attributable to said voltages.

17. The improvement as set forth in claim 12 wherein said control means is arranged to cause the flow of electrons emitted from said emitting surface to be substan¬ tially laminar between said emitting surface and said first electrode.

18. The improvement as set forth in claim 17 wherein said control means is arranged to cause the flow of electrons emitted from said emitting surface to be substan¬ tially laminar between said emitting surface and said second electrode.

19. The improvement as set forth in claim 12 wherein each of said apertures has a substantially-circular transverse cross-section.

20. The method of operating an electron gun said gun having a cathode and having three sequentially-spaced electrodes associated with said cathode, each of said electrodes having an aperture aligned with an imaginary line extending away from a point on said emitting surface, comprising the steps of: applying a first voltage to a first of said electrodes; applying a second voltage to a second of said electrodes; applying a third voltage to a third of said electrodes; and controlling said voltages for causing a desired flow of electrons to be emitted from said cathode and for causing said emitted electrons to converge at a cross-over point which remains substantially stationary independent of the magnitude of said flow.

21. The method of operating a cathode ray tube having an electron øm opera- tively arranged to generate a beam of electrons toward a display screen said gun having a cathode and having three sequentially-spaced electrodes, each of said electrodes having an aperture aligned with an imaginary line extending normally away from a point on said cathode, which method comprises the steps of: supplying voltages to each of said electrodes to cause a beam of electrons to issue from said cathode and for focusing said electrons on a cross-over point after said electrons have substantially reached their maximum velocity attributable to said voltages; reforming the image of said cross-over point on said screen; and controlling the relative polarities and magnitudes of said voltages to cause said beam of electrons to have a desired current and to maintain the resolution of said reformed image within a predetermined bandwidth; thereby to cause the resolution of said reformed image to be within said bandwidth at all operating beam current levels.