GB2102258A - Closed-loop correction system for CRT-type display - Google Patents

Abstract

A closed-loop system employing feedback techniques to accomplish automatically both convergence adjustment and geometric alignment within a CRT-type display device. A plurality of feedback elements disposed within the CRT produce, upon excitation by a passing electron beam, feedback signals indicative of the beam's position in at least two dimensions. A processor converts the signals into correction factors for subsequent application to the convergence and deflection waveforms controlling the beam's movement. Inclusion of the feedback elements within the tube enclosure permits the convergence and geometric adjustment processes to be performed without apparent system interruption and without operator intervention. <IMAGE>

Description

SPECIFICATION Closed-loop correction system for CRT-type display Background of the invention The subject matter of the present invention pertains to means for controlling the deflection of an electron beam within a cathode-ray tube, and especially within a tube of such kind employing multiple electron guns and a shadow mask.

The general construction of a conventional three-gun, shadow-mask type, cathode-ray tube and the manner in which it is operated to produce a raster-scan color image are well known to the art. Equally known to the art is that, absent dynamic correction, the image produced by such a tube will contain certain inherent distortions.

Primary among these are pincushion distortion caused by the center of deflection of the three electron beams being located apart from the center of curvature of the tube's display screen (present in monochromatic as well as color CRT's), trapezoidal distortion caused by at least two of the electron guns being located off the longitudinal axis of the tube envelope, and misconvergence of the beams at the tube's shadow-mask caused by the guns being displaced from one another laterally. With a delta-gun configuration, all three guns are spaced about the longitudinal axis of the gun assembly; with an inline configuration, one gun is located on axis and the other two are spaced at either side.

The usual method of correcting geometric distortion is to impress certain analog correction factors onto the deflection signals used to deflect the beam or beams back and forth across the display screen to produce the image raster.

Misconvergence is usually corrected by a similar impression of different analog correction factors onto the magnetic fields used to converge the three beams at center screen. Of the two distortions, the most difficult to correct accurately and uniformly, and one which requires periodic adjustment, is that of misconvergence.

Basic schemes for accomplishing beam convergence include the production of individual vertical and horizontal signals for each of the beams within the tube. Approximately somewhat the form of slightly skewed parabolas, the correction signals provide zero correction at center screen and increasing correction as the beams are deflected away from center. Such a basic approach is usually adequate for a home television environment where viewers are not overly critical and viewing distances are on the order of 6 to 10 ft. In the field of information display, however, where viewers are more critical and viewing distances much shorter, and more importantly, where resolution requirements are much more strict, the amount of misconvergence left uncorrected by such a basic approach is unacceptable.

An improvement over the basic scheme described above is exemplified by the 4027 color graphics terminal produced by Tektronix, Inc., the assignee of the present invention, wherein the display screen in divided into several sub-areas and different correction signals, independently adjustable, are generated for each such division.

Such an approach permits a more accurate convergence of the three beams over the entire area of the screen. In the 4027, the display screen is divided into nine sub-areas and the beams may be converged in each such area by the adjustment of three potentiometers, one for each beam.

Although providing increased correction, such a scheme still requires the somewhat timeconsuming adjustment of 27 different potentiometers, three for each of the nine subareas. Other known schemes divide the display screen into an even greater number of sub-areas (the Tektronix 690 color monitor, for example, uses thirteen) and require the attendant adjustment of an even greater number of potentiometers. A common disadvantage of such schemes is the requirement for a human operator to assume full control of the display system for the time necessary to perform the several adjustments at each individual sub-area.

More recent developments include digital convergence schemes wherein correction information may be entered digitally, via a keyboard or other similar means, for conversion into analog signals producing the desired amount of beam adjustment. Examples of such schemes includes those disclosed by Hallett et al. U.S.

Patent No. 4,203,051 and its companion Sowter U.S. Patent No. 4,203,054, both of which are assigned to IBM, and the SRL Model 382 color display developed by Systems Research Laboratories, Inc., of Dayton, Ohio. The IBM scheme is also described in an article by J. S.

Beeteson et al. entitled "Digital System for Convergence of Three-Beam High-Resolution Color Data Display" appearing at page 598 of the September 1980 issue of IBM J. Res. Develop.

Vol. 24, No. 5. A description of the SRL convergence scheme may be found in a paper entitled "A 25%in. Precision Color Display for Simulator Visual Systems" by R. E. Holmes and J.

A. Mays of System Research Laboratories. A common characteristic of both the IBM and SRL systems is the use of a keyboard permitting operator entry of digital information representing the degree of movement necessary for each of the three beams to accomplish their convergence or other geometric adjustment. The IBM system permits the beams to be individually adjusted at 1 3 different points over the display area, while the SRL system permits adjustment at 256 different points.

A semi-automatic scheme for performing deflection adjustments only is disclosed in Bristow U.S. Patent No. 4,099,092. In that scheme, a photodiode array or solid-state imaging camera positioned in front of a CRT display, and a digital computer, are employed to generate correction factors for later application, via a programmable read-only memory, to the conventional deflection waveforms.

A common disadvantage of all known prior art schemes is that a human operator is still required to assume full control of the system during the time necessary to perform the convergence or geometrical correction operation.

Summary of the invention The present invention is directed to a system and method of same employing closed-loop feedback techniques to accomplish automatically both convergence and geometric alignment within a cathode-ray tube (CRT). (As used in this specification, the term "geometric alignment" includes those beam adjustments necessary to affect the size, position, linearity orthogonality, and the like, of a displayed image, as well as those beam adjustments necessary to correct such image disorders as pincushion distortion and trapezoidal distortion.) Although associated more frequently with shadow-mask type color CRT's, convergence (or controlled misconvergence) is an important consideration in the operation of any CRT, monochromatic or color, having multiple electron guns and a common, shared deflection system.

A functional requirement of the system of the present invention is a CRT capable of providing during operation, feedback signals indicative of the position in two dimensions of a scanning electron beam. Several versions of a CRT particularly suitable for use with the system of the present invention are disclosed in copending U.S.

Application No. [to be supplied later] filed concurrently herewith by Ronald C. Robinder, David J. Bates and Dan F. Denham and assigned to Tektronix, Inc., the assignee of the present invention. For completeness of disclosure, the Robinder, Bates, and Denham application is incorporated herein by this reference; however, other CRT's capable of providing the feedback signals indicated above may be used without departing from the invention as disclosed.

More specifically, the system of the present invention comprises means associated with a CRT of the type described for causing a signal to be produced that is indicative of the position within the tube of a scanning electron beam, means for detecting such signal, and means responsive to such detection for producing a correction signal representative of a desired amount of beam adjustment. For convergence adjustments, signals indicative of the position of each electron beam are compared with those of the others so as to produce selected difference indications representative of the degree and direction of any misconvergence.Responsive to the difference indications, appropriate correction signals are produced and applied to conventional convergence circuitry in a manner bringing the beams into a desired spatial relationship at the tube's shadow mask or, in the case of a monochromatic tube, at the tube's display surface. For geometric adjustment, signals indicative of the position of a representative beam are compared against a reference so as to produce an indication representative of the degree and character of any geometric distortion.

Responsive to the distortion indications, appropriate correction signals are produced and applied to conventional deflection circuitry in a manner producing the desired amount of beam adjustment.

Except for the use of a special CRT and the closed-loop manner in which the beam or beams within the tube are controlled, operation of the system of the present invention is essentially as described earlier with reference to the cited prior art; that is, the display area of the cathode-ray tube is divided into a number of sub-areas and the convergence and geometric adjustment processes is performed, in gross steps or iteratively, for each sub-srea.

The primary advantages of the system and method as disclosed are the elimination of the need for human intervention in the geometric and convergence correction processes, and the speed and efficiency with which those processes can be performed, all without seeming to interrupt the normal operation of the surrounding display system.

It is, therefore, a principal objective of the present invention to provide an improved closedloop system for accomplishing and maintaining a predetermined spatial relationship between the electron beams within a multi-beam type cathode-ray tube.

It is an additional principal objective of the present invention to provide an improved closedloop system for accomplishing and maintaining beam convergence within a shadow-mask type color cathode-ray tube.

It is an additional principal objective of the present invention to provide an improved closedloop system for accomplishing and maintaining geometric alignment within a cathode-ray tube.

It is a further principal objective of the present invention to provide a system for performing beam convergence and beam alignment adjustment in a cathode-ray tube type display without operator intervention.

It is a still further principal objective of the present invention to provide a system for performing beam convergence and beam alignment adjustments in a cathode-ray tube type display without apparent interruption of the display's normal operation.

The foregoing objectives, features, and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings.

Brief description of the drawings Fig. 1 is a symbolic representation of the beam adjustment available in a conventional delta-gun, shadow-mask type cathode-ray tube.

Fig. 2 is a symbolic representation of the procedure usually followed to converge the beams of Fig. 1.

Fig. 3 is a block-diagram representation of a prior art correction system.

Fig. 4 is a block-diagram representation of the convergence and geometric adjustment system of the present invention.

Fig. 5 is a sectional view of a cathode-ray tube suitable for use with the system of Fig. 4.

Fig. 6 is an enlarged representation of a feedback element associated with the tube of Fig. 5.

Fig. 7 is a representation of the cathode-ray tube of Fig. 5 showing a suggested array of feedback elements.

Fig. 8 is a simplified schematic representation of a position detector circuit forming a part of the system of Fig. 4.

Fig. 9 is a timing chart showing certain signals produced within the circuit of Fig. 8 during its operation.

Fig. 10 is a second representation of the feedback element of Fig. 6 showing certain timing relationships developed during the operation of the system of Fig. 4.

Fig. 11 is a representation of two raster line groupings developed during a particular operation of the system of Fig. 4.

Fig. 1 2 is a symbolic representation of the beam adjustment available in a conventional inline gun shadow-mask type cathode-ray tube.

Detailed description of the preferred embodiment In the discussion that follows, a basic understanding of cathode-ray tube (CRT) type display devices, and especially those employing shadow-mask type color cathode-ray tubes, is assumed.

As is known to the art, the three electron beams of a conventional shadow-mask type cathode-ray tube must converge at the shadow mask apertures in order to produce an image of acceptable color fidelity. With a delta-gun arrangement, such convergence is usually achieved by the modulation of certain electromagnetic fields through which the beams are constrained to pass on their way to the mask apertures. With an in-line gun arrangement, the process is somewhat different, but the basic concept still applies. For convenience, this disclosure assumes a delta-gun type cathode-ray tube. Such assumption is not means to be limiting in any way. Moreover, the system of the present invention is not limited to applications employing a shadow-mask type CRT; many of the principles to be discussed apply as well to multi-beam monochromatic tubes employing a common, shared deflection system.

In Fig. 1, there is indicated the four degrees of beam adjustment usually available, and necessary, to converge a delta-gun cathode-ray tube. Each beam position, as seen from the operator's side of the cathode-ray tube display screen, is represented in Fig. 1 by a circle enclosing the letter R, G, or B to indicate the particular color, red, green, or blue, respectively, produced by the beam. The arrows indicate the available adjustment directions. As is the convention, the red and green beams may be moved in one diagonal dimension each toward or away from a common convergence point, represented in Fig. 1 by a small cross, while the blue beam may be moved both horizontally and vertically.In practice, as suggested in Fig. 2, the red and green beams are converged first to form a yellow indication, and the blue beam is then moved into spatial coincidence to form the white indication of complete convergence. The symbol actually displayed on the cathode-ray tube screen to facilitate the convergence process is a matter of choice.

A basic prior art system for performing the convergence process is shown schematically in Fig. 3 as including a cathode-ray tube 20 with its three-element gun assembly 22 and shadow mask 23, a convergence assembly 24, a deflection yoke (or plates) 26, a source 28 of Zaxis or image signals, a source 30 of horizontal and vertical synchronization signals, a waveform generator 32 for producing the gross convergence and deflection waveforms, and some means 34 for permitting a human operator 36 to select manually certain correction factors for application to the gross waveforms of the generator 32.

Examples of such a prior art system include the Tektronix 4027 color graphics terminal and the IBM and SRL systems cited in an earlier part of this specification. In the Tektronix 4027, the waveform generator 32 and the manual adjustment means 34 are analog; in the SRL system, the waveform generator is analog and the adjustment means is digital; and in the IBM system, both functions are digital. A common feature of each system is the requirement for the human operator 36 and complete dedication of the system during the time necessary to perform the correction operation. As suggested earlier, the primary function of the prior-art system is to permit the operator 36 to manually adjust certain correction waveforms in a manner causing the three electron beams 38a,b,c, first, to converge at the shadow mask 23, and second, to define a geometrically acceptable image.In a fourth system, that of Bristow also cited earlier the human operator 36 is augmented by an external sensing device and the correction waveforms are generated automatically; however, the requirement for the operator's presence and the dedicated system still remains.

Referring now to Fig. 4, there is shown in similar schematic form the system of the present invention. As disclosed in the figure, the system includes, again, a cathode-ray tube 40 with its three-element gun assembly 42 and shadow mask 43 (described in more detail below), a convergence assembly 44, a deflection yoke (or plates) 46, a source 48 of Z-axis signals, a source 50 of horizontal and vertical synchronization signals, and a waveform generator 52.Also included are a detector circuit 60 for detecting, via a suitable interface 62, certain feedback signals present during operation of the tube 40 and for producing in response thereto second signals indicative of the position of each beam 64a,b,c, within the tube; and a processor 66 responsive to such second signals for producing certain correction factors for application, as before, to the gross convergence and deflection waveforms of the generator 52. The term "processor" is not meant to be limiting, and may be interpreted where appropriate to include sufficient control logic and storage facilities to perform any procedure, calculations, or other operation identified herein. As with the prior art, the waveform generator may be either analog or digital. The primary function of the system of Fig.

4 is to perform the convergence and geometric correction operations of the prior art system of Fig. 3 without intervention by a human operator and without seeming to interrupt the otherwise normal operation of any system of which the system of Fig. 4 forms a part.

The tube 40 may be any cathode-ray tube capable of providing an indication representative of the horizontal and vertical position of a scanning electron beam. Several versions of a suitable shadow-mask tube are disclosed in the earlier-referenced copending application of Ronald C. Robinder, and David J. Bates, and Dan F. Denham; however, other tubes capable of providing the required indication may be employed without departing from the invention as disclosed. For convenience, the tubes of the Robinder, Bates, and Denham application will serve as the basis of the instant specification.

As disclosed in the Robinder, Bates, and Denham application, and as shown in Fig. 5 of the present drawings, one embodiment of a suitable cathode-ray tube 40 comprises a shadow-mask type cathode-ray tube having a plurality of distinct feedback elements 70 arrayed over the gun-side surface of its shadow mask 43. The feedback elements 70 may be realized in several forms.

One such form is a rapid-decay phosphorescent material, for example P47 phosphor, deposited at selected locations over the shadow mask surface.

Other forms are suggested below. With elements 70 of a phosphorescent material, the appropriate interface 62 is a photomultiplier tube positioned external to the tube 40 and adjacent a transparent viewing port formed in the tube's envelope. The use of a small circle in Fig. 4 to depict the interface 62 is symbolic only and is not meant to specify a particular mechanical, optical, or electrical connection. As disclosed below, the interface 62 may assume several different forms.

An example of a feedback element configuration that has been found to be particularly advantageous is shown in Fig. 6 and forms the basis of the following discussion. As seen in the figure, the element comprises two disjointed legs 80, 82 of a right triangle, the first or leading leg 80 being oriented vertically and the second or trailing leg 82 being inclined at an angle 84 of 300 from the horizontal (600 with respect to leg 80). The terms "leading" and "trailing" are defined with respect to the direction of beam travel which, in the figure, is assumed to be from left to right. (Recall that the element is shown in Fig. 6 as it would appear, if observable, from the operator's side of the display screen.

From the gun side of the shadow mask, the element would appear reversed and the beam movement would be from right to left.) The terms "horizontal" and "vertical" have their usual meanings with respect to electron beam deflection within a cathode-ray type display tube.

Typical dimensions for the element of Fig. 6 are a height 86 of about 0.3" to about 1", an overall length 88 of about 0.6" to about 1.75", a leg width 90 in the horizontal direction of about 0.01" to 0.1", and a leg separation 92 substantially equal to the horizontal leg width.

Other dimensions may also be suitable depending on the other characteristics of the system, the important criteria being that the element be predictably locatable by a scanning electron beam and that the signals produced by such scanning be both recognizable and distinct. Moreover, the height 86 should be sufficient to permit scanning beams to be adjusted vertically the distances required to obtain their convergence without leaving the leg 80, and the leg separation 92 should.be such as to ensure that two distinct signals will be produced no matter where in the vertical plane the element is scanned. The constant or equal horizontal width 90 of the two legs 80, 82 is chosen so that the two signals will be of similar amplitude and duration. The suggested angle 84 of 300 is chosen to align the leg 82 with the perforations of the shadow mask 43.This latter characteristic is not critical, however, as the relationships between the perforation diameters, the beam diameters, and the overall size of the element tend to minimize any nonuniformities caused by misalignment of the element components with the aperture array.

The same element configuration may also be used with a shadow-mask tube of the in-line gun type and with monochromatic tubes.

In Fig. 7, there is shown the gun-side surface of the shadow mask 43. As indicated therein, and the sectional side view of Fig. 5, the feedback elements 70 (depicted as small crosses in Fig. 7) are disposed over the shadow mask surface in a manner forming a regular spaced array of such elements. Depending on the choice of the designer, the elements may be located wholly within a predefined quality area, indicated by the dashed rectangle 94, or partially without. As each element defines the center of sub-area of display space over which the three beams of the cathoderay tube may be accurately converged and aligned, the number and location of elements used is largely a matter of correction resolution.

The pattern shown in Fig. 7 permits convergence and geometry correction to be accomplished at 1 7 different locations; center, top and bottom, left and right, the four corners, and at similar points therebetween. For systems employing totally digital convergence, larger arrays (i.e., 30 to 256 members) of equally spaced, but proportionately smaller, elements are more desirable. As discussed more fully below, each feedback element 70 may be interrogated individually by a scanning electron beam and the convergence and correction operations may be performed on a point-by-point basis or over the whole display area at one time.

An exemplary position detector circuit for use with the system of Fig. 4 is disclosed in Fig. 8. For purposes of illustration, the feedback elements 70 are assumed to be formed of a phosphorescent material as suggested earlier, and the interface 62 is therefore depicted symbolically as a photomultiplier tube. As disclosed, the circuit of Fig. 8 includes a toggle-type fiip-flop 110 for switching between alternate output states in response to a predetermined series of switched input signals, a ramp generator 112 for converting one output state of the toggle 110 into an analog magnitude, and an analog-to-digital converter 114 for producing a digital representation of the analog magnitude.The ramp generator 11 2 includes an amplifier 11 6 and two transistor switches Q1 and Q2 for controlling the charge developed across a.

capacitor C. The converter 114 includes a digital oscillator 11 8 and counter 120 for deveioping a digital count, and a digital-to-analog converter 122 and comparator 1 24 for stopping the counter 120 and resetting the ramp generator 112 when the count in the counter is equivalent to the charge across the capacitor. Also included in the circuit of Fig. 8 are an amplifier 126 for controlling the amplitude of the photo-multiplier signal. The function of the circuit is to provide digital timing signals representative of the horizontal and vertical position of a given raster line segment, or scan line, relative to a selected feedback element. (The use in this specification of the term "raster line segment" is not meant to limit the disclosed invention to a raster-scan environment.As will be appreciated by those persons skilled in the art, the invention is equally applicable and useful in a directed-beam (e.g., caligraphic) environment.) Operation of the circuit of Fig. 9 is best understood with reference to the signal chart of Fig. 10. To determine the position of a given raster line, for example a red line passing through the center of the display area, with respect to a selected feedback element, for example an element located at the left center edge of the display area, it is necessary only to produce a scan line of sufficient length and spatial displacement to traverse both legs of the selected element. The first traversal produces signals representative of the line's horizontal position, and the second produces signals representative of its vertical position.By repeating the procedure for each of the three color components of the same scan line, the adjustments necessary to produce their convergence or other spatial relationship can be readily determined. By knowing the physical location of the feedback element relative to the display area, the adjustments necessary to accomplish geometric corrections can also be readily determined.

In Fig. 9 there are shown the Z-axis (Z) and feedback (PMT) signals received by the circuit of Fig. 8, and the toggle states (TOGGLE) and capacitor charge (C) produced in response thereto. Note that there are two sets of TOGGLE and C signals, one for a first traversal of the feedback element and one for a second. Also shown in the chart of Fig. 9 is the scan line (LINE) produced by the Z-axis signal. Super imposed over the scan line signal is a feedback element of Fig. 6 to indicate symbolically the position of the generated line relative to the two legs 80, 82 of the element. The horizontal ordinate of the LINE curve is therefore measured in both time and distance.

At a time to before the chosen scan line is generated, suitable control signals are issued by the processor 66 to set the toggle 110 to its high state, and set the counter 120 to a preselected initial value, e.g. zero. Under these conditions, the input to the amplifier 116 is grounded and the capacitor C is discharged. Horizontal deflection is then initiated and the three beams within the cathode-ray tube begin their movement from left to right across the display area. At a predetermined time t, before reaching the selected feedback element 70, the Z-axis signal to one of the electron beams, for example red, is maintained at a constant amplitude so as to produce a trace of uniform intensity. Concurrently, the Z-axis signals applied to the green and blue electron guns are set to zero.A suitable control signal is also issued by the processor 68 to set the toggle 110 to its low state as indicated by the upper TOGGLE curve. With the toggle 110 in its low state, transistor Q1 is biased nonconductive, and the capacitor C begins to charge. This is indicated by the upper C curve of Fig. 9. At time t2, the red beam encounters the leading leg 80 of the element 70, causing the material of the element to phosphoresce and produce a first feedback indication. This latter indication is detected by the photo-multiplier tube 100 to produce the first PMT pulse 140. Receipt of the PMT pulse 140 causes the toggle 110 to switch back to its high state, and thereby restore the ground connection to the input of the amplifier 116 and terminate the charging cycle of the capacitor C.A suitable control signal is then issued to prevent the toggle from reacting to the next PMT pulse 142 discussed more fully below.

At this point, the charge on the capacitor C is an analog representation of both the horizontal distance and the elapsed time between the start of the constant intensity line segment 144 and its encounter, or intersection, with the leading leg 80 of the element 70.

Note that the portion of the Z-axis signal defining the scan line 144 need be at an amplitude just sufficient to produce a usable feedback signal. Such amplitude may be less than that necessary to produce a trace visible to the system operator.

Before the next operation of the circuit of Fig. 8, a suitable control signal is be issued to cause the counter 120 to begin counting. When the digital count thus produced, converted to an analog signal by the converter 122, reaches a value equivalent to the magnitude of the charge developed within the capacitor C, the comparator 1 24 issues a signal stopping the count and discharging the capacitor. When so stopped, the count in the counter 120 is a digital representation of the horizontal distance and time mentioned above. This digital representation locates the scan line 144 with respect to the vertical plane represented by the leading leg 80 of the element 70.

To locate the scan line 144 relative to a horizontal reference, the circuit of Fig. 8 is reinitialized and the horizontal scanning operation repeated. This time, at time t1 when the constant intensity scan line 144 begins, no signal is issued to the toggle 110 and the toggle remains in its high state as indicated by the lower TOGGLE curve of Fig. 9. Now, at time t2 when the scanning beam encounters the leading leg 80 of the element 70, the resultant PMT pulse 140 causes the toggle 110 to switch to its low state and start anew the charging of the capacitor C. This is indicated by the lower C curve of Fig. 9. At time t3, when the beam encounters the trailing leg 82 of the element 70, the resultant second PMT pulse 142 causes the toggle 110 to return to its high state and terminate the charging operation.The digital signal produced in the counter 120 by the renewed operation of the converter 11 4 is now a representation of the horizontal distance and time elapsed between the points of intersection of the scan line 144 and the two element legs 80 and 82. Because of the inclined nature of the trailing leg 82, the signal is also a representation of the vertical position of the line segment 144 relative to the element 70. If the precise physical location of the element 70 is known, this vertical information may be used to accomplish geometric correction, such as, for example, by causing the scan line 144 to move in a direction causing the difference between the detected elapsed time and the elapsed time representing the known physical location of the element 70 to be reduced below a predetermined limit.It is not necessary, however, that the physical location of the element be known in order to achieve convergence.

Practice in a non-interlaced, raster-scan environment has shown that the position detector circuit of Fig. 8 and the processor 66 can be made sufficiently fast to permit the information gained during a first horizontal scan of a selected feedback element 70 to be digitized and stored before the next succeeding scan of the same element. Thus, it is possible to obtain horizontal and vertical positional information for a given scan line in less than the time required to produce two successive raster lines. After the red beam information has been obtained and stored for a given line segment and feedback element, the process is repeated to obtain the corresponding information for the green and blue beams. As the horizontal position information is the same for both scans of a particular color, it matters not which of the two operations described above is performed first.It is only necessary that the line segment employed to obtain the vertical information be the same for each of the three colors.

Analysis of the information thus obtained is fairly straight-forward. In Fig. 10, there is shown again the feedback element of Fig. 6 together with three displaced scan line labeled 144R, 144G, and 144B, the letters indicating the respective color assigned to the beam producing each segment. For convenience of reference, the center of each scan line is indicated in the figure by a small circle. The positional information, i.e., elapsed time, obtained via the above described operation of the position detector 60 and processor 66 are represented by the brackets labeled Rh for red horizontal, Rv for red vertical, G h for green horizontal, etc.In the discussion that follows, the term "increase" means to correct the convergence waveforms so as to cause the respective beams to move in the directions indicated by the arrows positioned near the center of the red and green line segments 144R and 1 44G in Fig. 11. The term "decrease" means to cause the beams to move in the opposite directions.

From Fig. 11, the following relationships are apparent: 1. If Rh > Gh, red is to the left of green; increase both to converge horizontally 2. If Rh < Gh, red is to the right of green; decrease both to converge horizontally 3. If RV > Gv, red is below green; increase red and decrease green to converge vertically 4. If RV < Gv, red is above green; decrease red and increase green to converge vertically.

And, assuming red and green to be converged: 1. If Bh > Rh, blue is to the left of the convergence point 2. If Bh < Rh, blue is to the right of the convergence point 3. If BV > Rv, blue is below the convergence point 4. If BV < Rv, blue is above the convergence point.

Note that the direction of each inequality suggests the direction of beam movement necessary to accomplish convergence; the magnitude of the inequality indicates the amount of such movement. Note also that, once the red and green scan lines are converged, it is only necessary to compare the blue line values with those for one or the other of the converged lines, but not both.

The particular process or algorithm employed to calculate the correction factors to be applied to the convergence waveforms is somewhat of a design choice and well within the skill of the experienced designer. The choices include a purely iterative solution wherein the beams are repeatedly moved in unit steps until convergence is obtained, a purely mathematical solution wherein the required correction is calculated and the beams are moved in one step, and a hybrid or intermediate solution wherein the beams are moved interatively, but in steps related to the degree of their misconvergence. For practical purposes, convergence is obtained when the differences between the respective elapsed times have been reduced below a predetermined design limit.The solution most suitable for a particular system will depend upon such factors as the amount of time available to perform the necessary calculations and the speed and sophistication of the calculating entity. An example of a hybrid solution is as follows: 1. Scan the selected feedback element as indicated above to obtain Rh, Rv, Gh, Gv, Bh,Bv 2. Calculate Ah=RhGh, Av=RvGv 3. Analyze per cited relationships (see Fig.

10) i.e., from Rh > Gh, set R=R0+A h' G=G0 +Ah from Rv < Gv, set R=RoAv, G=GO+v combining, set R=Ro+(AhAv)/2, G=Go+(Ah+v)/2 4. Repeat steps 1 through 3 until Ah, åv < predetermined limit, indicating red and green convergence.

5. Calculate new Ah=BhRh, åv=BvRv using latest Rh, Rv 6. Analyze as before (still with reference to Fig. i.e., from BV < Rv move B down Av from Bh < Rh, move B to left Ah 7. Repeat steps 5 and 6 until Ah, h, < predetermined limit, indicating red and blue convergence.

Recall that, as mentioned earlier, once the red and green beams are converged, it is only necessary to compare the blue beam alignment with one of the other two beams. Whether the red or green beam is selected is a matter of choice.

Because of cross-talk within the convergence assembly 44, and also between the CRT gun elements, adjustment of each beam will affect previous adjustment of the others. It may therefore be advisable to perform the entire process more than once to obtain complete convergence of the three beams. The process is then repeated for each feedback element 70 and for as many times as necessary to obtain acceptable convergence over the entire display area. Assuming an element array such as that shown in Fig. 7, a suggested order of convergence adjustment is as follows: center, top center, left center, right center, bottom center, the eight central positions, and then the upper left corner, upper right corner, lower left corner, lower right corner. It is anticipated that the time required to converge completely the three beams of a color cathode-ray tube at the 1 7 points of Fig.

7, assuming a 60 Hz non-interlaced raster, will be considerably less than two seconds. Any suitable scheme may be employed to make the actual waveform corrections once their character has been determined as disclosed herein. Several such schemes are known to the art and include that used in the Tektronix 4027 color graphics terminal, as well as those disclosed in the IBM and SRL references cited earlier.

The correction process described above may be performed automatically according to a predefined schedule or on operator command.

When performed automatically, the line segments 144 would appear so quickly and infrequently as to be practically unnoticable by the system operator. Assuming a 60 Hz image raster, each line segment tests for less than 1/60 second and, after initial warm up, correction intervals are measured in hours. Even so, there are applications, such as display photography, where any interruption is intolerable and manual disable or initiation of the correction process may be desirable. The circuitry necessary to effect such manual control is well within the skill of the art.

In the preceding discussion, the feedback elements 70 have been assumed to comprise a fast-decay phosphor material applied to the gunside surface of the shadow mask 43. The elements may also be formed of a material capable of emitting secondary electrons upon bombardment by a passing electron beam. In this case, the interface 62 would comprise a suitable collector or plurality of collectors of secondary electrons positioned inside the tube enclosure and accessible from the outside via a suitable conductor. A known emitter of secondary electrons is magnesium oxide (MgO). The configuration and placement of secondary collectors is well known to the art and well within the skill of those persons familiar with the design and manufacture of, for example, monoscopetype character generator tubes.

The size and placement of the feedback elements 70 over the surface of the shadow mask 48, while subject to certain restrictions, are almost a matter of design choice. As mentioned earlier, the primary consideration is that the signals produced by scanning the elements be clear and distinct. Thus, the elements must be large enough to produce a usable signal over a limited range of beam adjustment, yet small enough to permit adequate separation. If the elements are located too close to one another, more intelligence is required in the processor 68 to distinguish between signals produced by the various components of a single element and those that might be produced by one component of a first element and another component of a next adjacent element. The latter set of signals might occur if the beams where grossly misaligned at the start of a convergence or other correction process.As the amount of beam adjustment usually necessary to obtain convergence is on the order of +1/8" for a display area of about 10" by 7 > ", a feedback element of about " by 1" should be adequate for both convergence and geometry correction. The element spacing is, of course, dependent on element size and number, as well as the size of the display area itself.

As a third alternative, the feedback elements 70 may be formed as apertures in a conductive/insulative coating deposited over the gun-side surface of the shadow mask 43. A process for forming such elements is disclosed in the cited application of Robinder and Bates. As so formed, the feedback elements 70 comprise specially configured apertures extending through the deposited layers of conductive and insulative material, but not through the shadow mask itself.

(The original, much smaller apertures forming the shadow-mask perforations are, of course, carefully maintained.) Via external electrical connection (a third embodiment of the interlace 62) to both the shadow mask 43 and the conductive layer, it is possible to detect both positive and negative indications of a passing electron beam. When the beam is in the area defined by an element aperture, a beam current will be induced in the shadow mask; when the beam is elsewhere in the display area, a beam current will be induced in the conductive overlay. The first condition may be considered a positive indication, and the second, a negative indication.Some beam current will, of course, be induced into the shadow mask 43 as the beam passes through the smaller dot-defining perforations extending through the entire sandwich; however, this latter current should be readily distinguishable from the feedback currents. Processing of the feedback currents to produce the desired correction signals is accomplished via the procedure outlined earlier.

The feedback element 70 may assume a number of different forms. For reasons indicated, the element configuration of Fig. 6 is preferable when the convergence process is performed using single line segments. In some instances, however, it may be advantageous to employ a smaller, simpler element configuration, such as a single dot, and perform the convergence process using a raster of line segments. Such a process is indicated in Fig. 11. In the left portion of that figure there is shown a raster of eight red line segments 1 44R superimposed over a feedback dot 70'. For convenience, the even line segments 0 through 8 have been numbered along the left side of the raster. As the raster is generated, suitable counters or other means are activated to record the line number and time t1 at which a first feedback signal is detected.The process is then repeated, as indicated in the right portion of Fig.

11, with a raster of eight green line segments 1 44G to obtain a new line number and time t2.

The amount of correction necessary to move the rasters into convergence may now be determined essentially as outlined earlier.

The disclosure thus far has been made primarily with reference to a delta-gun type CRT.

As indicated earlier, this is not meant to be limiting as the concept of the present invention is equally applicable to other gun arrangements as well as to CRT's having more or less three electron guns. In Fig. 12 there is shown the three horizontally algined beams of an inline type CRT together with an indication of the four degrees of beam adjustment usually available to obtain convergence. As before, each beam is represented by a circle enclosing the letter R, G, or B to indicate the particular color, red, green, or blue, respectively produced by the beam. Note that the center beam is fixed, i.e., movable only via deflection, while the two outboard beams may each be moved in the two dimensions necessary to obtain convergence.The process to be followed by the system of the present invention to obtain and maintain such convergence should be apparent to those persons comprehending the earlier discussion relative to Fig. 10. Now, instead of converging the red and green beams and then moving the blue beam into coincidence, the two outboard beams are moved toward the center.

The theory behind the signal detection and timing evaluation process remains substantially the same.

Although the preceding disclosure has been directed primarily toward a shadow-mask type color CRT, it will be recognized by those persons skilled in the art that, as mentioned at the beginning of the disclosure, most of the principles regarding beam convergence in a color CRT apply equally well to controlled beam misconvergence in a multi-beam monochromatic CRT having a single shared deflection system. For example, monochromatic display systems are known wherein two or more electron beams are deflected in parallel across a display medium to produce an image raster having an increased number of raster lines or a decreased frame rate.

In such systems, it is precise beam misconvergence that is important rather than precise convergence. Given the system of the present invention, it is a simple matter to develop and apply the continuous correction necessary to maintain the desired degree of beam separation.

For example, rather than causing the scan lines to move until certain elapsed times are equal, or nearly equal, the lines would be caused to move until their respective elapsed times, primarily those relating to vertical displacement, differ by a predetermined amount. If the beams share a convergence assembly and a common deflection system, the necessary correction factors are applied to the convergence assembly in a manner similar to that described for a shadow-mask type CRT. If the beams are deflected individually, the convergence assembly is omitted and the correction factors are applied to the individual deflection systems.

For those systems, such as that just described, where the CRT has no shadow mask, the feedback elements 70 are most conveniently spaced around the periphery of the image quality area. Several means for accomplishing such spacing without adversely affecting the image quality are disclosed in the cited application of Robinder, Bates, and Denham.

While the preceding disclosure has been directed primarily to a discussion of automatic convergence, or controlled misconvergence, the system of the present invention is not so limited and applies as well to the automatic correction of known types of geometric distortion. To perform such correction, it is only necessary to know the desired position of each scan line or converged set of scan lines relative to the known physical location of its respective feedback element. That physical location may be determined by physical measurement during the manufacturing process or it may be obtained via the correction system itself.Once the display raster is converged and adjusted to geometric acceptance, it is a simpie matter to store the detected position (in terms of t,, t2, and t3) of a given line relative to its feedback element and then maintain that position via future automatic detection, comparison, and adjustment. The IBM references cited earlier elude to an open-loop system for accomplishing both beam convergence and distortion correction.

As stated earlier, the primary function of the system of the present invention is to close that loop with the special tube 40, interface 62, position detector 60, and processor 66.

It is also recognized that the concept behind the system of the present invention can be employed as well to converge the two or more projected beams of a projection-type color display system. In such a system, feedback elements such as those disclosed above could be applied to the projection screen in the form of transparent photoconductors. Feedback signals produced as the photoconductors are scanned by the moving light beams can be processed as described to produce the correction signals necessary to provide the desired degree of adjustment.

The terms and expressions which have been used in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims (67)

Claims

1. A closed-loop correction system for controlling the movement of an electron beam within a cathode-ray tube, said system comprising: (a) a cathode-ray tube having a display screen and an electron gun; (b) means for causing an electron beam to be produced by said electron gun and directed toward said display screen; (c) deflection means for causing said electron beam to be deflected across said display screen in a manner defining a scan line and for displacing said scan line to a predetermined position relative to the center of said display screen; (d) feedback means associated with said cathode-ray tube for producing a signal indicative or the displacement in two dimensions of said scan line; and (e) means responsive to said signal for causing said scan line to assume a preferred displacement relative to said center of said display screen.

2. The correction system of claim 1 wherein said means (e) includes means for producing a digital signal representative of the displacement of said scan line.

3. The correction system of claim 2 wherein said means (e) further includes means for comparing said digital signal against a reference and for causing said scan line to move in a direction causing the difference between said digital signal and said reference to be reduced.

4. The correction system of claim 1 wherein said feedback means (d) includes a feedback element located within said cathode-ray tube, said element being capable of providing an indication upon being struck by a passing electron beam, said system further comprising means for causing said scan line to intersect said feedback element.

5. The correction system of claim 4 wherein said means (e) includes means for determining the time elapsed between the beginning of said scan line and its intersection with said element.

6. The correction system of claim 5 wherein said means (e) further includes means for causing said scan line to move in a direction causing said elapsed time to approach a predetermined value.

7. The correction system of claim 6 wherein said means (e) further includes means for causing said scan line movement to continue until the difference between said elapsed time and said predetermined value is less than a predetermined limit.

8. The correction system of claim 4 wherein said feedback element comprises two distinct members, said system further comprising means for causing said scan line to intersect both said members.

9. The correction system of claim 9 wherein said means (e) includes means for determining the time elapsed between the intersection of said scan line with said first member and the intersection of said scan line with said second member.

1 0. The correction system of claim 9 wherein said means (e) further includes means for causing said scan line to move in a direction causing said elapsed time to approach a predetermined value.

11. The correction system of claim 10 wherein said means (e) further includes means for causing said scan line movement to continue until the difference between said elapsed time and said predetermined value is less than a predetermined limit.

1 2. The correction system of claim 1 wherein said deflection means (c) includes means for causing said electron beam to be deflected across said screen in a manner defining a raster of said scan lines, and wherein said feedback means (d) includes a feedback element located within said cathode-ray tube, said element being capable of providing an indication upon being struck by a passing electron beam, said system further comprising means for causing at least one of said scan lines in said raster to intersect said element.

1 3. The correction system of claim 12 wherein said means (e) includes means for determining the time elapsed between the beginning of said intersecting scan line and its intersection with said element.

14. The correction system of claim 1 3 wherein said means (e) further includes means for causing said raster to move until said elapsed time assumes a predetermined value.

1 5. The correction system of claim 12 wherein said means (e) includes means for determining which of said scan lines in said raster is caused to intersect said element.

1 6. The correction system of claim 1 5 wherein said means (e) further includes means for causing said raster to move until a preselected raster line is caused to intersect said element.

1 7. The correction system of claim 1 wherein said cathode-ray tube further includes a second electron gun, wherein said means (b) further includes means for causing a second electron beam to be produced by said second electron gun and directed toward said display screen, wherein said deflection means (c) includes means for causing both said electron beams to be deflected across said display screen in a manner defining first and second scan lines, wherein said feedback means (d) includes means for producing a second said signal with respect to said second scan line, and wherein said means (e) includes means for causing said second scan line to assume a second preferred position relative to the center of said display screen.

1 8. The correction system of claim 1 7 wherein said deflection means (c) includes means for causing said first and second electron beams to be deflected across said display screen in a manner defining, respectively, first and second rasters of said scan lines, each said raster containing a like plurality of said scan lines, and wherein said feedback means (d) includes a feedback element located within said cathode-ray tube, said element being capable of providing an indication upon being struck by a passing electron beam, said system further comprising means for causing at least one of said scan lines in each said raster to intersect said element.

1 9. The correction system of claim 1 8 wherein said means (e) includes means for determining the times elapsed between the beginning of each said intersecting scan line and its respective intersection with said element.

20. The correction system of claim 1 9 wherein said means (e) further includes means for causing both said rasters to move until said elapsed times assume a common value.

21. The correction system of claim 1 8 wherein said means (e) includes means for determining which of said scan lines in each said raster is caused to intersect said element.

22. The correction system of claim 21 wherein said means (e) further includes means for causing both said rasters to move until a mutually corresponding scan line in each said raster is caused to intersect said element.

23. The correction system of claim 17 wherein said feedback means (d) includes a feedback element located within said cathode-ray tube, said element being capable of providing an indication upon being struck by a passing electron beam, said system further comprising means for causing both said scan lines to intersect said feedback element.

24. The correction system of claim 23 wherein said means (e) includes means for determining the times elapsed between the beginning of each said scan line and its respective intersection with said element.

25. The correction system of claim 24 wherein said means (e) further includes means for causing said first scan line to move in a direction causing its respective elapsed time to approach a first predetermined value and said second scan line to move in a direction causing its respective elapsed time to approach a second predetermined value.

26. The correction system of claim 25 wherein said means (e) further includes means for causing said scan line movement to continue until the difference between said first elapsed time and said first predetermined value and the difference between said second elapsed time and said second predetermined value are both reduced below a predetermined limit.

27. The correction system of claim 25 or claim 26 wherein said first and second predetermined values are equal.

28. The correction system of claim 24 wherein said means (e) further includes means for causing said first and second scan lines to move in directions causing their respective elapsed times to approach a common value.

29. The correction system of claim 23 wherein said feedback element comprises two distinct members, said system further comprising means for causing both said scan lines to intersect both said members.

3U. The correction system of claim 29 wherein said means (e) includes means for determining a first time elapsed between the intersection of said first scan line with said first and second members, and for determining a second time elapsed between the intersection of said second scan line with said first and second members.

31. The correction system of claim 30 wherein said means (e) further includes means for causing said first and second scan lines to move in directions causing their respective elapsed times to approach a common value.

32. The correction system of claim 4 or claim 23 wherein said feedback means (d) includes a plurality of said elements spaced at different locations within said cathode-ray tube, said system further comprising means for causing at least one of said scan lines to intersect selectively each of said elements.

33. A closed-loop correction system for controlling the movement of an electron beam within a cathode-ray tube, said system comprising: (a) a cathode-ray tube having a display screen and a plurality of electron guns; (b) means for causing an electron beam to be produced by each of said electron guns and directed toward said display screen; (c) deflection means for causing each said electron beam to be deflected across said display screen in a manner defining a separate scan line and for permitting said scan lines to be displaced relative to the center of said display screen; (d) feedback means associated with said cathode-ray tube for producing a first signal indicative of the displacement of a first scan line and a second signal indicative of the displacement of a second scan line; and (e) means responsive to said signals for causing said first and second scan lines to assume a substantially common displacement relative to the center of said display screen.

34. In a cathode-ray tube display system of a type including means for receiving a corrective input representative of a desired change in beam displacement, the improvement comprising: feedback means associated with said cathode-ray tube for providing an indication representative of said beam displacement in two dimensions, means for detecting said indication, and means responsive to said detection for providing said corrective input.

35. The display system of claim 34 wherein said input-producing means includes processor means for recognizing a difference between a desired beam displacement and a detected beam displacement, and for producing a corrective signal sufficient to reduce said difference below a predetermined limit.

36. A process for controlling the movement of an electron beam within a cathode-ray tube, said process comprising the steps of: (a) providing a cathode-ray tube having a display screen and an electron gun; (b) causing an electron beam to be produced by said electron gun and directed toward said display screen; (c) causing said electron beam to be deflected across said display screen in a manner defining a scan line; (d) producing a signal indicative of the displacement in two dimensions of said scan line relative to the center of said display screen; (e) responsive to said signal, causing said scan line to assume a preferred displacement relative to said center of said screen.

37. The process according to claim 36 wherein said step (e) includes comparing said signal against a reference and causing said scan line to move in a direction causing the difference between said signal and said reference to be reduced below a predetermined limit.

38. The process according to claim 36 wherein said cathode-ray tube includes a feedback element capable of providing an indication upon being struck by a passing electron beam, said process comprising the further step of causing said scan line to intersect said feedback element.

39. The process according to claim 38 wherein said step (e) includes determining the time elapsed between the beginning of said scan line and its intersection with said element.

40. The process according to claim 39 wherein said step (e) includes causing said scan line to move in a direction causing said elapsed time to approach a predetermined value.

41. The process according to claim 40 wherein said step (e) further includes causing said scan line movement to continue until the difference between said elapsed time and said predetermined value is reduced below a predetermined limit.

42. The process according to claim 38 wherein said feedback element comprises two distinct members, said system comprising the further step of causing said scan line to intersect both said members.

43. The process according to claim 42 wherein said step (e) includes determining the time elapsed between the intersection of said scan line with said first member and the intersection of said scan line with said second member.

44. The process according to claim 43 wherein said step (e) further includes causing said scan line to move in a direction causing said elapsed time to approach a predetermined value.

45. The process according to claim 44 wherein step (e) further includes causing said scan line movement to continue until the difference between said elapsed time and said predetermined value is reduced below a predetermined limit.

46. The process according to claim 36 wherein said cathode-ray tube includes a feedback element capable of providing an indication upon being struck by a passing electron beam, and wherein said step (c) includes causing said electron beam to be deflected across said screen in a manner defining a raster of said scan lines, said process comprising the further step of causing at least one of said scan lines in said raster to intersect said element.

47. The process according to claim 46 wherein said step (e) includes determining the time elapsed between the beginning of said intersecting scan line and its intersection with said eiement.

48. The process according to claim 47 wherein step (e) includes causing said raster to move until said elapsed time assumes a predetermined value.

49. The process according to claim 48 wherein said step (e) includes determining which of said scan lines in said raster is caused to intersect said element.

50. The process according to claim 49 wherein said step (e) further includes causing said raster to move until a preselected raster line is caused to intersect said element.

51. The process according to claim 36 wherein said cathode-ray tube further includes a second electron gun, wherein said step (b) further includes causing a second electron beam to be produced by said second electron gun and directed toward said display screen, wherein said step (c) includes causing both said electron beam to be deflected across said display screen in a manner defining first and second scan lines, wherein said step (d) includes producing a second said signal with respect to said second scan line, and wherein said step (e) includes causing said second scan line to assume a second preferred displacement relative to the center of said display screen.

52. The process according to claim 36 wherein said cathode-ray tube includes a feedback element located therewithin, said element being capable of providing an indication upon being struck by a passing electron beam, and wherein said step (c) includes causing said first and second electron beams to be deflected across said display screen in a manner defining, respectively, first and second rasters of said scan lines, each said raster containing a like plurality of said scan lines, said system comprising the further step of causing at least one of said scan lines in each said raster to intersect said element.

53. The process according to claim 52 wherein said step (e) includes determining the times elapsed between the beginning of each said intersecting scan line and its respective intersection with said element.

54. The process according to claim 53 wherein said step (e) further includes causing both said rasters to move until said elapsed times assume a common value.

55. The process according to claim 52 wherein said step (e) includes determining which of said scan lines in each said raster is caused to intersect said element.

56. The process according to claim to 55 wherein said step (e) further includes causing both said rasters to move until a mutually corresponding scan line in each said raster is caused to intersect said element.

57. The process according to claim 56 wherein said cathode-ray tube includes a feedback element located therewithin, said element being capable of providing an indication upon being struck by a passing electron beam, said process comprising the further step of causing both said scan lines to intersect said feedback element.

58. The process according to claim 57 wherein said step (e) includes determining the times elapsed between the beginning of each said scan line and its respective intersection with said element.

59. The process according to claim 58 wherein said step (e) further includes causing said first scan line to move in a direction causing its respective elapsed time to approach a first predetermined value, and causing said second scan line to move in a direction causing its respective elapsed time to approach a second predetermined value.

60. The process according to claim 59 wherein said step (e) further includes causing said scan line movement to continue until the difference between said first elasped time and said first predetermined value and the difference between said second elapsed time and said second predetermined value are both reduced below a predetermined limit.

61. The process according to claim 60 or claim 60 wherein said first and second predetermined values are equal.

62. The process according to claim 58 wherein said step (e) further includes causing said first and second scan lines to move in directions causing their respective elapsed times to approach a common value.

63. The process according to claim 57 wherein said feedback element comprises two distinct members, said process comprising the further step of causing both said scan lines to intersect both said elements.

64. The process according to claim 63 wherein said step (e) includes determining a first time elapsed between the intersection of said first scan line with said first and second members and then determining a second elapsed time between the intersection of said second scan line with said first and second members.

65. The process according to claim 64 wherein said step (e) includes causing said first and second scan lines to move in directions causing their respective elapsed times to approach a common value.

66. A closed loop correction system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

67. A process for controlling the movement of an electron beam within a cathode-ray tube substantially as hereinbefore described and exemplified.