Schwartz, James W. (Glenview, IL)
Chiodi, Wayne R. (Northbrook, IL)
Rowe, William A. (Palatine, IL)
Wilson, Iva M. (Glenview, IL)

05/408720

07/15/1975

10/23/1973

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Zenith Radio Corporation (Chicago, IL)

315/14

315/16, 313/414, 313/449

H01J29/50; H01J29/48

313/69C,7C,82DF,85R,409,414,444,449,460 315/13CG,16

Mullins, James B.

Coult, John H.

We claim

1. In combination:

2. The lens defined by claim 1 herein the applied potentials progress upwardly from said central electrode to said second and fourth intermediate electrodes and from said second and fourth electrodes to said first and fifth end electrodes in approximately geometrically progressive fashion.

3. The lens defined by claim 2 wherein the progression factor is approximately 2.

4. The lens defined by claim 1 wherein said relatively intermediate potential is in a range from about 30 to 70% of said relatively high potential, said relatively low potential is in a range from about 15 to 35% of said relatively high potential, and the progression from said relatively low potential to said relatively intermediate potential to said relatively high potential is monotonic.

5. The lens defined by claim 1 wherein said central electrode has an axial length of about 0.5 to about 2.0 times its inner diameter.

6. The lens defined by claim 1 wherein all of said electrodes have an equal inner diameter.

7. The lens defined by claim 1 wherein each of said intermediate electrodes has axial dimension which is less than 1.5 times its inner diameter.

8. In combination:

9. The lens as defined in claim 8 wherein said third electrode has an axial length of about 0.5 to 2.0 times its inner diameter.

10. The lens defined by claim 8 wherein each of said second and fourth electrodes has axial dimension which is less than 1.5 times its inner diameter.

11. In combination:

12. The lens as defined by claim 11 wherein said third electrode has an axial length of about 0.5 to 2.0 times its inner diameter.

13. The lens defined by claim 11 wherein each of said second and fourth electrodes has axial dimension which is less than 1.5 times its inner diameter.

14. For use in a television cathode ray tube having associated therewith a power supply for developing discrete supply voltages, an electron gun for receiving supply voltages from the power supply to produce a focused beam of electrons, comprising:

15. For use in a color television cathode ray tube having associated therewith a power supply for developing discrete supply voltages, an electron gun of the type constituting one of a delta or three-in-line gun cluster for receiving supply voltages from the power supply to produce a focused beam of electrons, comprising:

16. The lens defined by claim 15 wherein said relatively intermediate supply voltage is in a range from about 30 to 70% of said relatively high supply voltage, wherein said relatively low supply voltage is in the range from about 15 to 35% of said relatively high supply voltage but always lower than said relatively intermediate supply voltage, and wherein the progression from said relatively low supply voltage to said relatively intermediate supply voltage to said relatively high supply voltage is monotonic.

17. The lens defined by claim 15 wherein said central electrode has an axial length of about 0.5 to 2.0 times its inner diameter.

18. The lens defined by claim 15 wherein all of said electrodes have an equal inner diameter.

19. The lens defined by claim 15 wherein each of said intermediate electrodes has an axial dimension which is less than about 1.5 times its inner diameter.

BACKGROUND OF THE INVENTION

Notwithstanding the present state of maturity of color television cathode ray tubes, higher quality image reproduction remains a constant objective. Specific goals of the color television industry today include pictures with improved brightness and resolution. To provide improved resolution, especially at high beam currents, requires improved means for focusing the phosphor-exciting electron beams.

Conventional color reproducing cathode ray tubes include a multi-color image screen having interspersed groups of red-emitting, blue-emitting and green-emitting phosphor elements. Excitation of these elements is provided by an in-line or delta cluster of three electron guns which emit three electron beams, each of which is focused into a beam spot on the tube screen by means of an electrostatic electron lens. The size of the electron spots focused on the screen, and thus the picture resolution, is a result of many factors. An important factor is the aberrations, particularly spherical aberration, introduced by the focus lens. In the presence of spherical aberration, all electrons emanating from an object point do not, after focusing, recombine at a common point.

Commercially available electron guns for color cathode ray tubes have focus lenses of two basic types. One type is the so-called "bi-potential" lens comprising a relatively low voltage electrode followed by a second electrode which is maintained at a relatively high voltage -- typically the phosphor screen voltage.

This invention is concerned with an improvement on a second basic lens type, commonly termed the "uni-potential"-type lens comprising three electrodes, the first and third of which are maintained at the same potential, typically the screen voltage, and the second (intermediate) of which is maintained at a much lower potential.

Designers of prior art uni-potential-type lenses have reduced spherical aberration primarily by decreasing the ratio of beam diameter to lens diameter. Generally, in order to decrease this ratio the lens diameter is increased. However, this approach conflicts with the space limitations imposed by the neck diameters of standard color tube bulbs which are deliberately made small in order to minimize the yoke driving power required to deflect the beams, to minimize convergence power requirements and to minimize residual convergence errors. Neck size constraints are perhaps most severe in color tubes of the "small-neck" type having an inline cluster of three guns. For three-in-line gun clusters, the maximum diameter of the focus lens in each gun must necessarily be less than one-third of the neck inner diameter.

It has been suggested in the electron gun literature that spherical aberration of a uni-potential focus lens of the type having three closely spaced tubular electrodes may be reduced by lengthening the lens. See "Electron Optics," O. Klemperer, University Press (1953), Chapters 4, 6. However, it is recognized that there is a limit to the amount of electrode elongation which can be made in any such lens before inactive or drift regions are created in the focusing field of the lens which prevent further reduction in spherical aberration.

By increasing the separation of lens electrodes, or by using thin disc electrodes, somewhat more success may be had in reducing spherical aberration in a three electrode uni-potential lens. However, the increased gap lengths subject the focusing field to disturbance from external or stray fields. Good isolation from external fields is especially important in a plural lens color cathode ray tube because the focus field of each gun in the cluster is influentially near the other focus fields in the gun cluster.

Other techniques have been used in attempting to reduce the spherical aberration of a uni-potential-type lens. For example, U.S. Pat. No. 3,652,896 to Miyaoka discloses a three-beam electron gun having a single focus lens for all three beams. The focus lens may be described as a modification of a three electrode uni-potential lens in which a large diameter, low potential center electrode is split into two parts and an additional higher potential electrode is inserted therebetween. The result is asserted to be a lens having substantially diminished spherical aberration. Calculations have shown the Miyaoka lens, however, to lack the performance capabilities required to generate pictures having resolution meeting today's demanding standards. The Miyaoka gun also requires additional beam deflecting means between the focus lens and the cathode ray tube screen because the beams emerge from the lens along divergent, rather than convergent, paths.

As previously mentioned, improved picture resolution requires improved beam focusing means, but to provide improved resolution on a commercial basis requires not only that improved focusing means be available, but also requires that the means be commercially practicable. Uni-potential-type lenses having large numbers of electrodes may be found in the prior art but these appear to be commercially impractical. For instance, U.S. Pat. No. 2,859,378 to Gundert et al. discloses a lens comprising a plurality (21 in one embodiment) of individual, electrically conducting plates mounted in spaced parallel relationship. The plates are apertured and are impressed with voltages such that a center group of plates is excited at a relatively low potential and the remaining plates progressing from the center toward both ends are excited at successively higher voltages.

The Gundert disclosure appears mute on the subject of diminishing spherical aberration and would appear directed to solutions of other problems. Its attractiveness appears limited by such difficulties as interference between lens fields, susceptibility to stray-fields, reliability, fabrication difficulties and cost. Furthermore a very large number of discrete potentials must be supplied to the individual electrodes. This requires the additional circuitry for generating the potentials. The problem of introducing many closely spaced high voltage leads into the neck of a cathode ray tube while eliminating possible arcing is not insignificant.

OBJECTS OF THE INVENTION

It is a general object of this invention to provide an electron gun for use in television cathode ray tubes having an electrostatic focus lens which provides greatly reduced spherical aberration for a lens of given diameter and thus which provides smaller focused spot sizes and improved television picture resolution.

It is a less general object to provide a focus lens for an electron gun which has the capability described, and yet which is susceptible of economical mass production.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further objects, features and advantages of the invention will be more apparent upon reference to the following specification, claims and appended drawings wherein:

FIG. 1 is a sectional fragmentary side view of a cathode ray tube including a prior art three-beam electron gun having a single five-electrode focus lens;

FIG. 2 is a sectional fragmentary side view of a cathode ray tube including an in-line array of focus lenses constructed in accordance with this invention;

FIG. 3 is a schematic perspective view showing a lens of the invention arranged in delta array;

FIG. 4 is a sectional fragmentary side view of a color cathode ray tube including a gun which has as a part thereof a focus lens representing a preferred embodiment of the invention;

FIG. 5 is an enlarged sectional side view of a general representation of the electron lenses illustrated in FIGS. 2 and 4;

FIG. 6 is a sectional side view of an electron focus lens representing an alternative embodiment of the invention;

FIG. 7 shows an axial potential distribution V ₀ and the second derivative thereof, V ₀ ^II , of a prior art lens and of an embodiment of this invention. ##EQU1## lens and of an embodiment of this invention; and

FIG. 9 shows computer-calculated equipotential lines and electron trajectories for an embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings in detail and initially to FIGS. 1 and 2 thereof, it is seen that in each Figure a color cathode ray tube comprises a glass envelope (only partially shown) having a neck 11 and a cone 12 extending from the neck 11 to a color screen 13 provided with the usual arrays of red-emissive, blue-emissive and green-emissive phosphors 14R, 14B, 14G and with an apertured beam selecting mask 15. The color cathode ray tubes of FIGS. 1 and 2 each include means disposed within the neck 11 for generating and focusing three electron beams.

FIG. 1 represents a prior art approach employing a single lens 20 to focus three electron beams emanating from cathodes 21, 22 and 23 and grids 24 and 25. Also shown are conventional mounting beads 26 and 27 for supporting the electron gun. The prior art lens of FIG. 1 comprises five consecutively disposed electrodes, 31, 32, 33, 34 and 35 impressed with a distribution of potentials which is high at end and center electrodes 31, 35, 33 and which is low at intermediate electrodes 32, 34. Electrodes 32, 33 and 34 consume a substantial fraction of the radial dimension within the neck 11. The three electron beams exit the lens along divergent paths and are reconverged by the beam converging system 36.

As will be pointed out below, a number of structures are contemplated for implementing the principles of the invention; FIG. 2 depicts a preferred embodiment thereof. For the sake of clarity in describing the invention, certain necessary structures are omitted from FIG. 2, such as support beads, voltage supply and leads and cathode and grid structural details.

In FIG. 2 the system of beam generation and focusing according to this invention is illustrated as comprising an in-line array of three separate electron guns 41, 42, 43. Each gun includes a cathode and grid system 44 (shown schematically) and a novel lens 45 constructed according to the teachings of this invention. The three guns of FIG. 2 are appropriately tilted to effect the desired convergence. Each lens 45 includes five electrodes 72, 73, 74, 75, 76 impressed with voltages V _a , V _b , and V _c as schematically represented. In contrast to the FIG. 1 prior art lens, V _c >V _b >V _a . As will be described in much greater detail later in this specification, in accordance with this invention, the lenses 45 each constitute a unipotential-type extended field lens having an essentially saddle-shaped distribution of axial potential. By the application of this invention, a cluster of electron guns can be constructed, which is small enough to fit in the neck of a standard color cathode ray tube, each of which guns is capable of focusing a picture of improved resolution in spite of the small focus lens diameter permitted.

Whereas FIG. 2 illustrates an execution of this invention in a color cathode ray tube having an in-line cluster of guns, the principles of the invention may be equally utilized in delta gun arrays. FIG. 3 depicts a color cathode ray tube having in the neck thereof a delta cluster of electron guns 46, 47 and 48. As in the FIG. 2 embodiment, the electron beams generated by the guns are converged at a shadow mask (not shown) which serves to shadow a phosphor screen (not shown) from the beams, all as is well known. The guns 46, 47 and 48 each include a novel focus lens constructed in accordance with this invention, shown in detail in FIG. 4 and discussed at length below.

FIG. 4 shows an electrically connected, beaded gun 49 comprising one of a delta array of guns mounted in the neck of a color cathode ray tube. Reference will also be made to FIG. 5 which depicts a general representation of preferred lens embodiments typified by the FIG. 2 guns 41, 42 and 43 and the FIG. 4 gun 49.

The gun 49 of FIG. 4 includes a cathode and grid system 50 and a preferred focus lens 51. Lens 51 includes first, second, third, fourth and fifth hollow cylindrical electrodes 52, 53, 54, 55 and 56. The electrodes are aligned along a common gun axis X--X by means of glass beads, one of which is shown at 28. Each electrode is sufficiently axially separated from adjacent electrodes to preclude arcing therebetween upon application of appropriate operating potentials (to be described hereinafter), and yet the gaps are maintained small to provide good field isolation. Typically, electrode separation is about 0.03 inch.

As is commonly practiced, the electrode ends are curled to reduce occurrences of arcing between electrodes, for example, as shown at 58. All electrodes are preferably constructed from a standard tubular 0.008 inch thick electrode material. The electrodes are impressed with potentials which progressively increase from a relatively low potential on the center (third) electrode 54 to a relatively high potential on the end (first and fifth) electrodes 52, 56.

As is shown in FIG. 4, appropriate connections are made between the electrodes 52-56 and an external power supply 60. As depicted in FIG. 4, center electrode 54 is supplied with a potential V ₁ , a relatively low potential, via a tube pin 61 and lead 62. The end electrodes 52 and 56 receive a potential V ₃ from supply 60, a relatively high potential which is preferably substantially equal to anode (screen) potential, by way of the envelope-implanted high voltage button 59, the internal coating of colloidal graphite 63, snubber spring 64, and convergence cage 65. Lead 66 connects electrodes 52 and 56. Intermediate electrodes 53 and 55 receive a relatively intermediate potential V ₂ , via an additional intermediate voltage button 67 in the neck 11 of the tube and lead 68. Other appropriate approaches of supplying and applying the potentials will be apparent to those skilled in the art. For example, rather than using a button on the neck of the tube, the described relatively intermediate voltage may be supplied to the focus lens through an intermediate voltage pin at the base of the tube.

In employing the principles of the invention, appropriate selections are made for lens and electrode lengths, electrode diameter and electrode potentials. Lens 51 of FIGS. 4-5 has been constructed with the following dimensions (see FIG. 5 for a definition of the dimensional designators).

FIGS. 4-5 lens of delta gun application

λ ₁ = .265 inch λ ₄ = .165 inch λ ₂ = .165 inch λ ₅ = .300 inch λ ₃ = .460 inch d ₁ = .350 inch s ₁ = .030 inch

Generally speaking lens diameter is chosen as the maximum value which is spatially permitted by a particular application in a particular tube neck. For example, and as earlier mentioned, for three-in-line gun clusters, the maximum diameter of the focus lens of each gun must necessarily be less than one-third of the neck inner diameter. Analogous constraints exist for delta applications. Furthermore, it is preferred that, in any particular embodiment, all electrode diameters be equal.

In the particular lens 51 depicted in FIGS. 4 and 5, electrodes 53 and 55 are equal in axial length (i.e., using FIG. 5 definitions, λ ₂ =λ ₄ ). Also the axial length of each of electrodes 53, 55 is about 0.47 times its inner diameter. Generally speaking the length equality is not necessary but the axial length of each of these intermediate electrodes is preferably less than 1.5 times its diameter.

The axial length λ ₃ of central electrode 54 of FIGS. 4-5 lens 51 consumes about 31% of total lens length L and is about 1.3 times its inner diameter. As a general rule it is preferable for central electrode axial length λ ₃ to equal about 30% to 40% the total lens length and to range from about 0.5 to 2.0 times its diameter.

With regard to potentials selected for application to the electrodes, it has been determined that best results are obtained with voltages V ₁ , V ₂ , V ₃ which progress upwardly from center electrode 54 toward end electrodes 52, 56 in approximately geometrically progressive fashion. In other words, it is preferred that V ₁ V ₂ and V ₃ correspond approximately to the first three terms of a sequence of the form α, αβ, αβ ² , αβ ³ . . .

Thus, using this expression with V ₁ =α, V ₂ =βV ₁ , and V ₃ =β ² V ₁ , each voltage differs from the preceding voltage in the sequence by a progression factor (i.e., constant multiplier) of β.

A progression factor of approximately β=2 has been experimentally determined to be preferred.

The following list describes the preferred voltage ranges:

Designated Voltage Applied % of Anode Voltage Voltage to ______________________________________ V ₃ End electrodes 52, 56 Approx. 100 V ₂ Intermediate electrodes Approx. 50 ± 20 V ₁ Central electrode 54 Approx. 25 ± 10 Operating Condition: Although the preferred voltage ranges listed for V ₂ and V ₁ slightly overlap, for any particular operating set of voltages (V ₃ , V ₂ , V ₁ ), V ₁ is never greater than V ₂ . In other words, the progression from V ₁ to V ₂ to V ₃ is always monotonic. ______________________________________

It is to be borne in mind that the lens 51 depicted in FIGS. 4-5 is only one of the many possible embodiments which may be constructed to employ the principles of the invention. For instance, referring back to FIG. 2, each lens 45 adapted for in-line application as shown includes first, second, third, fourth and fifth hollow cylindrical electrodes 72, 73, 74, 75 and 76 analogous to the lens 51 of FIGS. 4-5. The dimensions for lens 45 of FIG. 2 are listed below. Refer to FIG. 5 for definition of dimensional designators.

FIG. 2 lens of in-line gun application:

λ ₁ =.230 inch λ ₄ =.160 inch λ ₂ =.100 inch λ ₅ =.290 inch λ ₃ =.490 inch s ₁ =.030 inch d ₁ =.270 inch

Comparison of the FIG. 2 and FIGS. 4-5 embodiments highlights some of the variations possible within the scope of the invention. First it is seen that lens diameter for the FIG. 2 in-line application is substantially smaller than the FIGS. 4-5 delta application. Lens and electrode lengths for the two embodiments are different. It is also seen that the axial length of the second and fourth electrodes 73 and 75, unlike the FIGS. 4-5 embodiments, are unequal. Otherwise the FIG. 2 and FIGS. 4-5 embodiments share many similarities. Intermediate and center electrode ratios of axial length to inner diameter fall within the previously mentioned preferred ranges. Also the voltages V _a , V _b , V _c applied to individual electrodes of the FIG. 2 lens 45 are selected by the same criteria as potentials V ₁ , V ₂ and V ₃ described in connection with FIG. 4.

As a general rule in the embodiments such as FIG. 2 lens 45 wherein λ ₂ is unequal to λ ₄ , it is preferred that λ ₂ be the lesser of the two. The underlying rationale will be presented below in connection with a discussion of the theory of operation of focus lenses according to this invention.

Continuing briefly with other variations permissible within the principles of the invention, FIG. 6 depicts yet another alternative lens embodiment whose dimensions (in inches) are as follows:

X ₁ =.044 inch X ₆ =.390 inch X ₁₁ =.400 inch X ₂ =.330 inch X ₇ =.370 inch X ₁₂ =.050 inch X ₃ =.060 inch X ₈ =.165 inch X ₁₃ =.460 inch X ₄ =.200 inch X ₉ =.020 inch X ₁₄ =.300 inch X ₅ =.300 inch X ₁₀ =.045 inch X ₁₅ =.050 inch X ₁₆ =.045 inch

Operationally, the embodiments of FIGS. 5 and 6 are substantially the same. However, due to the axially varying inner diameter, the structure of FIG. 6 provides some additional control over the internal field established.

The greatly reduced spherical aberration provided by the extended field lens may be explained in terms of electric field theory and more particularly in terms of axial potential distribution established. (See Maloff and Epstein, Electron Optics in Television, McGraw Hill Book Company, 1938.) Theoretically, the vector force on electrons and resultant trajectories can be determined from a knowledge of the spatial potential distribution within the lens. Potential distribution for an axially symmetric field may be determined by solving the equation ##EQU2## where V= V(r,z) = potential function of an axially symmetric electrostatic field, z= lens axial coordinate, r=lens radial coordinate, subject to the boundary conditions that V assume the given values of potential on the electrodes.

In general it is not possible to obtain simple analytic solutions of the above equation subject to the actually existing boundary conditions. However, the spatial potential distribution within an axially symmetric lens can also be determined from a knowledge of axial potential V ₀ (z) [where V ₀ (z)= V(O,z)], and its even order derivatives with respect to z. ##EQU3## where ##EQU4##

It can be shown that lens aberrations depend largely on the value of the line integral of the quantity ##EQU5## Therefore, it follows that large values of V ₀ " are particularly harmful in regions where the axial potential V ₀ is low or where beam radius is large. FIGS. 7 and 8 illustrate certain findings and compare an embodiment of the present invention with a well designed conventional three electrode unipotential lens having the same diameter. As is seen from FIG. 7 the axial potential distributions V ₀ of both lenses are generally saddle-shaped. It is also seen, however, that for the extended field lens of this invention V ₀ ^II is substantially less over the entire lens length and is especially low in regions of low axial potential. Furthermore, the maximum values of V ₀ ^II are substantially reduced. ##EQU6## lens. Each curve is normalized with respect to the maximum value of The substantial reduction is this quantity is an indication of the substantial reduction in spherical aberration provided in accordance with the principles of the invention. It should be noted at this time that the focusing field of the extended field lens is axially continuously active. For instance, a reduction in V ₀ ^II alone especially in regions of low axial potential might be achieved with a "complete lens" formed by placing two bi-potential lenses essentially back to back separated by some axial distance. However, any reduction in V ₀ ^II would also likely be accomplished by the establishment of a drift or inactive focusing region at the composite lens center due to the axial separation of said bi-potential lenses.

For some idealized embodiments significant equi-potential lines and electron trajectories from an on-axis object point have been computed. FIG. 9 graphically represents such computations for a lens having the following dimensions and applied potentials:

λ ₁ =.450 inch V ₁ =7.6 × 10 ³ volts λ ₂ =.165 inch V ₂ =15 × 10 ³ volts λ ₃ =.460 inch V ₃ =30 × 10 ³ volts λ ₄ =.165 inch λ ₅ =.450 inch s ₁ =.030 inch d ₁ =.350 inch

Alpha is the electron trajectory angle from the object point. E is the value of the designated equi-potential line in kilovolts. Object distance is 1 inch.

FIG. 9 summarizes much of what has been said above. The small gap (30 mils) is large enough to preclude arcing, yet is small enough such that the focusing field is well shielded from any external field disturbances. It is also apparent that the focusing field contains no drift regions and provides a greatly reduced disc of least confusion, and thus reduced spherical aberration, for a lens of its diameter.

FIG. 9 also assists in explaining an earlier made statement that when λ ₂ is unequal to λ ₄ it is preferred that λ ₂ be the lesser of the two. Momentarily returning to theory of operation, in addition to our findings that large values of V ₀ ^II are particularly harmful in regions where V ₀ is low, we have observed that large values of V ₀ ^II are also particularly harmful in regions where the beam diameter is large. FIG. 9 shows that the tendency of the beam is to have a large diameter in regions of the fourth electrode than in regions of the second electrode. This means that in some instances, values of V ₀ ^II are less critical in regions of the second electrode than in regions of the fourth electrode and thus the second electrode may be shorter in axial length than the fourth.

Although certain preferred embodiments hereinabove described have included five electrodes impressed with three voltages as shown, it is presently contemplated that the principles of the invention may be otherwise embodied. For instance, in accordance with the above rationale regarding inequality of second and fourth electrode axial length, it is presently contemplated that for some applications the second electrode could be eliminated altogether, leaving in substance a four electrode extended field type lens. Also it is contemplated that substitution of plural electrodes for any one of the five electrodes could be made. For example, any electrode could be divided and the segments impressed with only slightly different voltages from those described above but in the same basic axial scheme or direction as those of the preferred embodiments.

Moreover, although many of the preferred lens embodiments have been described herein as being constructed from standard tubular electrode material, it may be desirable in certain instances of plural gun application to construct the corresponding electrodes from a common piece of electrically conductive material. For instance, in a delta or three-in-line configuration, all three first electrodes could be stamped, machined, or otherwise constructed from a single block of material. In other words, instead of having three individually distinct first electrodes, one block of material would have three cylindrical channels appropriately dimensioned, appropriately spaced from each other, appropriately axially converging, and appropriately arranged either in delta or in-line fashion. All three second electrodes could likewise be made from a single block of material, and so forth for the remaining electrodes. Other structural modifications may be employed to achieve focusing fields equivalent to those achieved by preferred embodiments herein. It should also be appreciated that although the foregoing description has presumed an application of the principles of the invention in a color cathode ray tube environment, the same principles are applicable and the advantages likewise enjoyed in black and white cathode ray tubes. Thus it is to be understood that changes and variations according to the principles of this invention may be made on the above-described embodiments by one skilled in the art without departing from the true spirit and scope of this invention.