JP2000340129A - Color cathode-ray tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the plane vibration of a shadow mask with lowcost lightweight constitution by bending the edges, corresponding to the lateral ends of an image plane, of a shadow mask at a nearly right angle, and making the bent parts abut on a pair of parallel plates, corresponding to the lateral ends of the image plane, of a mask frame. SOLUTION: The lateral ends of a shadow mask 11 is bent at a nearly right angle before assembly to form bent parts 14, and after assembly, the bent parts 14 abut elastically on the inner abutting parts of lateral plates of a mask frame 12. When external vibration from a speaker or the like is transmitted to the shadow mask 11 through peripheral air and the mask frame 12, the shadow mask 11 vibrates a moment, and its vibrational energy is transmitted to the abutting parts in an extremely short time and changed into frictional energy by the mutual rubbing of the bent parts 14 and mask frame 12, and the vibration of the shadow mask 11 is converged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube having a shadow mask having a vibration damping function.

[0002]

2. Description of the Related Art A color CRT widely used at present is a so-called shadow mask color CRT. As is well known, a shadow mask generally has a large number of circular or rectangular electron beam passage holes in iron or an iron-nickel alloy, a shadow mask having a circular electron beam passage hole is replaced with a dot type shadow mask, and a rectangular electron beam passage hole. Such a shadow mask is called a slot type shadow mask. Conventionally, both dot-type shadow masks and slot-type shadow masks have often been press-molded into spherical shapes. This is because the spherical shadow mask has a high rigidity and can hold its shape independently, and thus has an advantage that a special shape holding mechanism is not required.

However, recently, there has been a tendency that the screen of a color cathode ray tube is made to be a part of a cylindrical surface close to a plane (hereinafter, a part of the cylindrical surface is called a "cylindrical surface") or a complete plane to make the screen more easily viewable. is there. For this purpose, the shape of the shadow mask must be changed from a spherical surface to a cylindrical surface or a flat surface. Requires a simple shape holding mechanism.
One example is an attempt to apply tension to a shadow mask and hold the shadow mask on a mask frame, as disclosed in Japanese Patent Application Laid-Open No. 8-77936.

If the shadow mask shape is not a cylindrical surface but a flat surface, the flat surface has a lower rigidity than the cylindrical surface, so that not only the shadow mask holding mechanism becomes extremely complicated but also the focus and convergence deterioration around the screen are solved. It is necessary to greatly improve the deflection yoke, the electron lens, the focus circuit, and the like, and this is not always advantageous compared to the case of forming a cylindrical surface. Rather, the cylindrical surface shape is often more practical with a better balance between cost and performance.

[0005]

Since the current shadow mask has a very small thickness and a low rigidity of the material itself to increase the resolution, the shadow mask has a cylindrical surface shape (hereinafter referred to as a cylindrical shadow mask). Tension can be applied in the direction along the axis of the cylindrical surface (the vertical direction of the color CRT screen, hereinafter referred to as the vertical direction), but in the direction perpendicular thereto (the horizontal direction of the color CRT screen, hereinafter referred to as the horizontal direction) It is technically difficult to apply tension to the wire.
If tension is forcibly applied in the left-right direction, wrinkles will occur in the shadow mask, causing large color misregistration and impractical.

However, if the left and right ends of the cylindrical shadow mask are set as free ends, no matter how strong the tension is applied in the vertical direction, the shadow mask will resonate with external vibration from a speaker or the like, causing color shift. Not practical.

As means for preventing the surface vibration of the cylindrical shadow mask described above, JP-A-8-77936, JP-A-9-284767, and JP-A-9-274868.
The publication discloses a structure in which a vibration attenuator is brought into contact with the left and right ends of a cylindrical shadow mask.

However, in the structure disclosed in the above publication, it is necessary to manufacture a vibration damping body having a complicated shape and fix it to the mask frame with high accuracy, so that a considerable increase in material and processing costs is required. , Weight increase is inevitable.

An object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a high-quality color cathode-ray tube having a cylindrical shadow mask capable of preventing surface vibration with an inexpensive and lightweight structure.

[0010]

In order to achieve the above object, the present invention provides a color cathode ray tube having a cylindrical shadow mask, wherein the ends of the cylindrical shadow mask corresponding to the left and right edges of the screen are bent at substantially right angles, The bent portion is brought into contact with the mask frame to exhibit a vibration damping effect.

According to a first aspect of the present invention, there is provided a mask frame formed by connecting and holding a pair of parallel flat plates corresponding to upper and lower ends of a screen of a color CRT and a pair of parallel flat plates corresponding to left and right ends of the screen. A shadow mask is fixed by applying tension between a pair of parallel flat plates corresponding to upper and lower ends of the screen of the mask frame, and the shape of the shadow mask is a cylindrical surface of an axis parallel to the vertical direction of the screen of the color cathode ray tube. In the color cathode ray tube which forms a part of the above, the ends corresponding to the left and right screen edges of the shadow mask are bent at substantially right angles, and the bent portions abut against a pair of parallel flat plates corresponding to the left and right edges of the screen of the mask frame. It is characterized by the following.

According to a second aspect of the present invention, the bent portion at the end corresponding to the left and right edges of the screen of the shadow mask contacts the inside of a pair of parallel flat plates corresponding to the left and right edges of the screen of the mask frame. It is characterized by having been done.

According to a third aspect of the present invention, the tip of the bent portion at the end corresponding to the left and right ends of the screen of the shadow mask is bent further inward.

According to a fourth aspect of the present invention, the distal end of the bent portion corresponding to the left and right ends of the screen of the shadow mask is further bent outward.

According to a fifth aspect of the present invention, the bent portion at the end corresponding to the left and right ends of the screen of the shadow mask is corrugated.

According to a sixth aspect of the present invention, the inside of a pair of parallel flat plates corresponding to the left and right ends of the screen of the mask frame is subjected to corrugation processing.

According to a seventh aspect of the present invention, the bent portion at the end corresponding to the left and right edges of the screen of the shadow mask contacts the outside of a pair of parallel flat plates corresponding to the left and right edges of the screen of the mask frame. It is characterized by having been done.

According to an eighth aspect of the present invention, the tip of the bent portion at an end corresponding to the left and right ends of the screen of the shadow mask is further bent inward.

According to a ninth aspect of the present invention, the tip of the bent portion at the end corresponding to the left and right ends of the screen of the shadow mask is further bent outward.

According to a tenth aspect of the present invention, the bent portion at the end corresponding to the left and right ends of the screen of the shadow mask is subjected to a corrugated processing.

According to an eleventh aspect of the present invention, the outer sides of a pair of parallel flat plates corresponding to the left and right edges of the screen of the mask frame are corrugated.

According to a twelfth aspect of the present invention, the shadow mask is a slot type shadow mask having a rectangular electron beam transmitting hole.

According to a thirteenth aspect of the present invention,
The shadow mask is a dot type shadow mask having a circular electron beam transmitting hole.

[0024]

With the above configuration, even if the cylindrical shadow mask is welded and fixed to the mask frame by applying tension only in the vertical direction as in the related art, the left and right ends of the shadow mask contact the mask frame. Since the vibration damping effect is exerted, the resonance of the shadow mask due to external vibration is suppressed, and the perceived color shift does not occur.

Further, in the above-described structure, the existing mask frame or shadow mask is simply subjected to simple mechanical processing with low processing accuracy, so that the weight increase is negligible and the cost increase is slight.

[0026]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a perspective view before assembling for explaining the structure of a shadow mask structure 10 mounted on a first embodiment of a color cathode ray tube of the present invention, and FIG. 2 is a perspective view and a cross section after assembling of the shadow mask structure 10. In the figure, 11 is a shadow mask,
12 is a mask frame, 13 is a welding point between the shadow mask 11 and the mask frame 12, and 14 is a shadow mask 11.
The bent portion 15 is a contact portion between the bent portion 14 of the shadow mask 11 and the mask frame 12. The X direction indicates the horizontal direction of the screen of the color CRT, and the Y direction indicates the vertical direction of the screen of the CRT. The material of the shadow mask 11 is an Invar alloy (36% nickel-iron alloy), and the material of the mask frame 12 is 13% chrome stainless steel.

The left and right ends of the shadow mask 11 are bent at substantially right angles before assembly (FIG. 1) to form a bent portion 14, and after assembly (FIG. 2), the bent portions 14 It is in elastic contact with the contact portion 15. Shadow mask 11 and mask frame 12
Is fixed by spot welding a welding point 13 in a state where a vertical tension is applied to the shadow mask 11 as is known. As described above, since only the upper and lower ends of the screen of the shadow mask 11 are fixed by welding and the left and right ends are only elastically abutted, no wrinkling occurs when the shadow mask 11 is fixed to the left and right ends. The invention of this embodiment is realized only by bending the left and right ends of the shadow mask 11 to form the bent portion 14. Since there is no need for special parts and no processing accuracy, the weight increase of the shadow mask structure 10 is ignored. It is possible, and the cost increase is slight.

External vibration from a speaker or the like causes shadow mask 11 through surrounding air or mask frame 12.
The shadow mask 11 vibrates momentarily,
The vibration energy is transmitted to the contact portion 15 in a very short time, and when the bent portion 14 and the mask frame 12 rub against each other, the vibration energy is changed to frictional energy and the shadow mask 1 is changed.
The vibration of 1 converges. Since the vibration convergence time of the shadow mask 11 is very short, a human cannot perceive a screen color shift due to the vibration of the shadow mask 11. In this way, the problem of vibration of the shadow mask 11 is solved. Hereinafter, other embodiments of the present invention will be described. However, any of the materials of the shadow mask 11, the mask frame 12, and the mechanism for reducing the vibration of the shadow mask 11 in a short period of time that cannot be perceived by a human are the same as those in the first embodiment. Since they are the same, the description thereof will be omitted, and the characteristic features of each embodiment will be described.

FIG. 3 is a perspective view showing the structure of a shadow mask structure 20 mounted on a color cathode ray tube according to a second embodiment of the present invention before assembly. FIG. 4 is a perspective view showing the shadow mask structure 20 after assembly. FIG. 3 is a diagram and a sectional view, wherein 21 is a shadow mask, 22 is a mask frame, 2
3 is a welding point between the shadow mask 21 and the mask frame 22, 24 is a bent portion of the shadow mask 21, and 25 is a bent portion 24 of the shadow mask 21 and the mask frame 2.
The contact portion 26 is a portion where the tip of the bent portion 24 is further bent inward.

According to this embodiment, since the tip of the bent portion 24 of the shadow mask 21 is bent further inward, the contact portion 25 between the shadow mask 21 and the mask frame 22 is replaced by the contact portion of the first embodiment. Rigidity is higher than that of No. 15, and the contact is more reliable. For this reason, shadow mask 2
The vibration convergence of 1 is performed in a shorter time.

FIG. 5 is a perspective view showing a structure of a shadow mask structure 30 mounted on a color cathode ray tube according to a third embodiment of the present invention before assembly, and FIG. 6 is a perspective view showing the shadow mask structure 30 after assembly. FIG. 3 is a diagram and a sectional view, wherein 31 is a shadow mask, 32 is a mask frame, 3
Reference numeral 3 denotes a welding point between the shadow mask 31 and the mask frame 32, reference numeral 34 denotes a bent portion of the shadow mask 31, reference numeral 36 denotes a portion obtained by further bending the tip of the bent portion 34 outward, and reference numeral 35 denotes a portion between the bent portion 34 of the shadow mask 31 and the mask frame 32. This is the contact part.

According to this embodiment, since the tip of the bent portion 34 of the shadow mask 31 is further bent outward, the contact portion 35 between the shadow mask 31 and the mask frame 32 is the contact portion of the first embodiment. Rigidity is higher than that of No. 15, and the contact is more reliable. Therefore, shadow mask 3
The vibration convergence of 1 is performed in a shorter time.

FIG. 7 is a perspective view showing a structure of a shadow mask structure 40 mounted on a color CRT according to a fourth embodiment of the present invention before assembling, and FIG. 8 is a perspective view showing the shadow mask structure 40 after assembling. FIG. 4 is a view and a sectional view, wherein 41 is a shadow mask, 42 is a mask frame,
Reference numeral 3 denotes a welding point between the shadow mask 41 and the mask frame 42, reference numeral 44 denotes a portion where the bent portion of the shadow mask 41 is corrugated, and reference numeral 45 denotes a contact portion between the portion 44 where the bent portion of the shadow mask 41 is corrugated and the mask frame 42. is there.

According to this embodiment, since the bent portion of the shadow mask 41 is corrugated, the contact portion 45 between the shadow mask 41 and the mask frame 42 is several times as large as in the first to third embodiments. With area. Therefore, the convergence of the vibration of the shadow mask 41 is performed in a shorter time.

FIG. 9 is a perspective view showing a structure of a shadow mask structure 50 mounted on a color cathode ray tube according to a fifth embodiment of the present invention before assembly, and FIG. 10 is a perspective view showing the shadow mask structure 50 after assembly. Figures and sectional views, 51 is a shadow mask, 52 is a mask frame,
53 is a welding point between the shadow mask 51 and the mask frame 52; 54 is a bent portion of the shadow mask 51; 56 is a portion obtained by corrugating the inside of a pair of parallel flat plates corresponding to the left and right ends of the screen of the mask frame 52; 5
2 and a contact portion of the shadow mask 51.

According to this embodiment, the mask frame 52
Of the bent portion of the shadow mask 51
6 is subjected to the waveform processing, so that the contact portion 55 between the shadow mask 51 and the mask frame 52 is in a fitted state, and the contact state is more reliable than in the first to third embodiments.
Therefore, the convergence of the vibration of the shadow mask 51 is performed in a shorter time.

FIG. 11 shows a sixth embodiment of the color cathode ray tube of the present invention.
FIG. 12 is a perspective view and a cross-sectional view of the shadow mask structure 60 before assembly for explaining the structure of the shadow mask structure 60 mounted in the embodiment. FIG. Is a mask frame; 63 is a welding point between the shadow mask 61 and the mask frame 62; 64 is a bent portion of the shadow mask 61;
Denotes a contact portion between the bent portion 64 of the shadow mask 61 and the mask frame 62.

According to this embodiment, the vibration energy of the shadow mask 61 quickly changes to frictional energy at the contact portion 65 with the mask frame 62, and the shadow mask 6
The vibration of 1 subsides in a very short time. The vibration prevention mechanism of this embodiment is the same as that of the first embodiment, except that the bent portion 64 and the contact portion 65 of the shadow mask 61 are located outside the mask frame 62. Is suitable when the assembly of the shadow mask structure 60 is easy.

FIG. 13 shows a seventh embodiment of the color cathode ray tube of the present invention.
FIG. 14 is a perspective view and a cross-sectional view before assembling for explaining the structure of the shadow mask structure 70 mounted in the embodiment. FIG. Is a mask frame, 73 is a welding point between the shadow mask 71 and the mask frame 72, 74 is a bent portion of the shadow mask 71, 75
Denotes a contact portion between the bent portion 74 of the shadow mask 71 and the mask frame 72, and denotes a portion where the tip of the bent portion 74 is further bent inward.

According to this embodiment, since the tip of the bent portion 74 of the shadow mask 71 is bent further inward, the contact portion 75 between the shadow mask 71 and the mask frame 72 is replaced by the contact portion 75 of the sixth embodiment. Rigidity is higher than in the case of No. 65, so that the contact is more reliable. Therefore, the shadow mask 7
The vibration convergence of 1 is performed in a shorter time.

FIG. 15 shows an eighth embodiment of the color cathode ray tube of the present invention.
FIG. 16 is a perspective view and a cross-sectional view before assembling for explaining the structure of the shadow mask structure 80 mounted in the embodiment. FIG. Is a mask frame, 83 is a welding point between the shadow mask 81 and the mask frame 82, 84 is a bent portion of the shadow mask 81, 85
Denotes a contact portion between the bent portion 84 of the shadow mask 81 and the mask frame 82, and 86 denotes a portion obtained by further bending the tip of the bent portion 84 outward.

According to this embodiment, since the tip of the bent portion 84 of the shadow mask 81 is further bent outward, the contact portion 85 between the shadow mask 81 and the mask frame 82 is replaced by the contact portion 85 of the sixth embodiment. Rigidity is higher than in the case of No. 65, so that the contact is more reliable. Therefore, the shadow mask 8
The vibration convergence of 1 is performed in a shorter time.

FIG. 17 shows a ninth color cathode ray tube of the present invention.
FIG. 18 is a perspective view before assembling for explaining the structure of a shadow mask structure 90 mounted in the embodiment, FIG. 18 is a perspective view and a sectional view after assembling of the shadow mask structure 90, and 91 is a shadow mask, 92. Is a mask frame, 93 is a welding point between the shadow mask 91 and the mask frame 92, 94 is a portion where the bent portion of the shadow mask 91 is corrugated, and 95 is a portion where the bent portion of the shadow mask 91 is corrugated 94 and the mask frame 92. This is the contact part.

According to this embodiment, since the bent portion of the shadow mask 91 is corrugated, the contact portion 95 between the shadow mask 91 and the mask frame 92 is several times as large as in the sixth to eighth embodiments. With area. Therefore, the convergence of the vibration of the shadow mask 91 is performed in a shorter time.

FIG. 19 shows a first embodiment of the color cathode ray tube of the present invention.
FIG. 20 is a perspective view before assembling for explaining the structure of the shadow mask structure 100 mounted in the embodiment, FIG. 20 is a perspective view and a sectional view after assembling the shadow mask structure 100, and 101 is a shadow mask. 102 is a mask frame, 103 is a welding point between the shadow mask 101 and the mask frame 102, 104 is a shadow mask 101
A bent portion 106 is a portion where a pair of parallel flat plates corresponding to the left and right ends of the screen of the mask frame 102 are corrugated, and 105 is a shadow mask 101 and the mask frame 1.
02 is a contact portion.

According to this embodiment, the mask frame 102
Since the portion of the shadow mask 101 in contact with the bent portion 104 is corrugated, the shadow mask 10
The contact portion 105 between the first and the mask frame 102 is the sixth to eighth
It has several times the area as compared with the embodiment. Therefore, the convergence of the vibration of the shadow mask 101 is performed in a shorter time.

[0047]

As described above, according to the present invention, it is possible to provide a high-quality color cathode-ray tube which realizes prevention of vibration of a cylindrical shadow mask to which a tension is applied in the vertical direction of a screen with an inexpensive and lightweight configuration. it can.

[Brief description of the drawings]

FIG. 1 is a perspective view before assembling a shadow mask structure mounted on a first embodiment of a color cathode ray tube of the present invention.

FIG. 2 is a perspective view and a sectional view after assembling of a shadow mask assembly mounted on the first embodiment of the color cathode ray tube of the present invention.

FIG. 3 is a perspective view before assembling a shadow mask structure mounted on a color cathode ray tube according to a second embodiment of the present invention;

FIG. 4 is a perspective view and a sectional view after assembling a shadow mask structure mounted on a color cathode ray tube according to a second embodiment of the present invention;

FIG. 5 is a perspective view before assembling a shadow mask structure mounted on a color cathode ray tube according to a third embodiment of the present invention;

FIG. 6 is a perspective view and a sectional view after assembling a shadow mask structure mounted on a third embodiment of the color cathode ray tube of the present invention.

FIG. 7 is a perspective view before assembling a shadow mask structure mounted on a fourth embodiment of the color cathode ray tube of the present invention.

FIG. 8 is a perspective view and a sectional view of a shadow mask assembly mounted on a fourth embodiment of the color CRT according to the present invention after assembly.

FIG. 9 is a perspective view of a color CRT according to a fifth embodiment of the present invention before assembling a shadow mask structure mounted on the CRT.

FIG. 10 is a perspective view and a cross-sectional view of a fifth embodiment of a color cathode ray tube according to the present invention after assembling a shadow mask structure mounted thereon.

FIG. 11 is a perspective view of a shadow cathode ray tube assembly mounted on a color cathode ray tube according to a sixth embodiment of the present invention before being assembled.

FIG. 12 is a perspective view and a sectional view of a shadow mask assembly mounted on a sixth embodiment of the color CRT according to the present invention after assembly.

FIG. 13 is a perspective view of a color CRT according to a seventh embodiment of the present invention before assembling a shadow mask structure mounted on the CRT.

FIG. 14 is a perspective view and a sectional view of a shadow mask assembly mounted on a color cathode ray tube according to a seventh embodiment of the present invention after assembly.

FIG. 15 is a perspective view of a color cathode ray tube according to an eighth embodiment of the present invention before assembling a shadow mask structure mounted on the color cathode ray tube.

FIG. 16 is a perspective view and a sectional view after assembling a shadow mask structure mounted on an eighth embodiment of the color cathode ray tube of the present invention.

FIG. 17 is a perspective view of a color CRT according to a ninth embodiment of the present invention before assembling a shadow mask structure mounted thereon;

FIG. 18 is a perspective view and a sectional view of a shadow mask assembly mounted on a ninth embodiment of the color cathode ray tube of the present invention after assembly.

FIG. 19 is a perspective view of a shadow mask assembly mounted on a color cathode ray tube according to a tenth embodiment of the present invention before assembly.

FIG. 20 is a perspective view and a sectional view of a shadow mask assembly mounted on a color cathode ray tube according to a tenth embodiment of the present invention after assembly.

[Explanation of symbols]

10, 20, 30, 40, 50, 60, 70, 80, 9
0, 100 Shadow mask structure 11, 21, 31, 41, 51, 61, 71, 81, 9
1, 101 shadow mask 12, 22, 32, 42, 52, 62, 72, 82, 9
2, 102 mask frame 13, 23, 33, 43, 53, 63, 73, 83, 9
3, 103 welding points 14, 24, 34, 54, 64, 74, 84, 104
Folded parts 15, 25, 35, 45, 55, 65, 75, 85, 9
5, 105 abutment portions 26, 36, 76, 86 bent portions 44, 94 at the distal end portions, wavy portions of bent portions 56, 106 portions, wavy portions of mask frames

Claims (13)

[Claims] 1. A mask frame comprising a pair of parallel flat plates corresponding to upper and lower ends of a screen of a color cathode ray tube and a pair of parallel flat plates corresponding to left and right ends of a screen connected and held, wherein a shadow mask is formed of the mask frame. In a color cathode ray tube, wherein a tension is applied between a pair of parallel flat plates corresponding to the upper and lower ends of the screen and the shadow mask is formed as a part of a cylindrical surface of an axis parallel to a vertical direction of the screen of the color cathode ray tube. An end corresponding to the left and right ends of the screen of the shadow mask is bent at substantially right angles, and the bent portion is in contact with a pair of parallel flat plates corresponding to the left and right ends of the screen of the mask frame. 2. The shadow mask according to claim 1, wherein the bent portions at the ends corresponding to the left and right edges of the screen of the shadow mask are in contact with a pair of parallel flat plates corresponding to the left and right edges of the screen of the mask frame. Color CRT. 3. The color cathode ray tube according to claim 2, wherein the end of the bent portion at the end corresponding to the left and right ends of the screen of the shadow mask is further bent inward. 4. A color cathode ray tube according to claim 2, wherein the tip of said bent portion at the end corresponding to the left and right ends of the screen of said shadow mask is further bent outward. 5. The color cathode ray tube according to claim 2, wherein the bent portions at the ends corresponding to the left and right edges of the screen of the shadow mask are subjected to corrugated processing. 6. The color cathode ray tube according to claim 2, wherein the inside of a pair of parallel flat plates corresponding to the left and right ends of the screen of the mask frame is subjected to waveform processing. 7. The shadow mask according to claim 1, wherein the bent portions at the ends corresponding to the left and right edges of the screen of the shadow mask are in contact with the outside of a pair of parallel flat plates corresponding to the left and right edges of the screen of the mask frame. Color CRT. 8. The color cathode ray tube according to claim 7, wherein the tip of the bent portion at the end corresponding to the left and right ends of the screen of the shadow mask is bent further inward. 9. The color cathode ray tube according to claim 7, wherein a tip of said bent portion at an end corresponding to a left and right end of the screen of said shadow mask is further bent outward. 10. A color cathode ray tube according to claim 7, wherein said bent portions at the ends corresponding to the left and right edges of the screen of said shadow mask are corrugated. 11. The color cathode ray tube according to claim 7, wherein the outside of a pair of parallel flat plates corresponding to the left and right ends of the screen of the mask frame is processed. 12. A color cathode ray tube according to claim 1, wherein said shadow mask is a slot type shadow mask having a rectangular electron beam transmitting hole. 13. A color cathode ray tube according to claim 1, wherein said shadow mask is a dot type shadow mask having a circular electron beam transmitting hole.