WO2003081630A1

WO2003081630A1 - Getter device and cathode ray tube

Info

Publication number: WO2003081630A1
Application number: PCT/IB2003/000703
Authority: WO
Inventors: Henricus G. M. Beckers
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-03-25
Filing date: 2003-02-21
Publication date: 2003-10-02
Also published as: AU2003202799A1; TW200305183A

Abstract

A getter device for a cathode ray tube has a container (11) releasing getter material during flashing and a screen shield (25) at a first predetermined minimum distance (d1) to the container (11). The screen shield (25) extends, at least partially, over the base (12) of the container (11). According to the invention, the first predetermined minimum distance (d1) is between 0.25 and 2 times a diameter of the container (11), and a surface area of the screen shield (25) is between 3 and 20 times a surface area of the base (12). The amount of getter material being deposited on the display screen (20) is reduced thereby, while the getter material covers a portion of the cone area (22) that is as large as possible. The getter device is particularly suitable for use in a beam index type cathode ray tube incorporating an optical beam tracking system.

Description

Getter device and cathode ray tube

The invention relates to a getter device for a cathode ray tube, comprising: a container having a base and a side wall extending from said base in a first direction, said container releasing a getter material during flashing, and a screen shield having a first predetermined minimum distance to said container in the first direction and at least partially extending over the base of the container.

The invention further relates to a cathode ray tube provided with such a getter device.

An embodiment of such a getter device is known from US-A-3,996,488. Cathode ray tubes commonly incorporate getter devices for removing residual gases present or formed after evacuation of the cathode ray tube. Residual gases form ions because of electrons colliding with the atoms of the gas. These ions are accelerated towards the cathode area of the cathode ray tube, where they inflict damage on the cathodes. This reduces the operational lifetime of the cathode ray tube. The getter device comprises a getter container, which usually holds a compound comprising the getter material, the latter being, for example, barium. During manufacture of the cathode ray tube, the getter material is activated by flashing. An external RF magnetic field causes induction currents in the container, which is heated thereby. When the temperature of the compound reaches an activation value, an exothermic reaction is initiated. The compound is activated, thereby releasing the getter material. Because of the high temperature occurring during the exothermic reaction, for example, 1100 °C, the getter material is released from the container in gaseous or vaporous form.

The known getter device has an annular container provided with a screen shield. This screen shield reduces the amount of getter material directed towards the screen area of the cathode ray tube, since it is desirable that in the screen area the amount of deposited getter material is as small as possible.

The presence of getter material on the display screen is undesired. Among other effects, it causes reduced image brightness, since the electrons now have to pass through a layer of getter material before impinging on the phosphors. Such a layer partially absorbs the electrons.

It has been found that the amount of deposited getter material on the screen may be too large for certain applications in the known getter device. For example, a cathode ray tube of the beam index type, incorporating an optical beam tracking system, is particularly affected. Such a cathode ray tube is known, for example, from patent application JP-A-2001-143635. In such a tube, the getter material is deposited on the tracking phosphors, whereby a sensitivity of the tracking system is reduced.

It is therefore an object of the invention to provide a getter device of the type as described in the opening paragraph, wherein the amount of getter material that is deposited in the screen area during getter flashing is reduced further.

For this purpose, a getter device according to the invention is characterized in that the predetermined minimum distance is between 0.25 and 2 times a diameter of the container, and the screen shield has a surface area of between 3 and 20 times a surface area of the base of the container.

The invention is based on the recognition that the efficiency of a screen shield as known in the prior art is not high enough. A screen shield of the type as known in the prior art will be referred to as a "deflector" hereinafter. Getter flashing is a highly energetic process, which causes a relatively high pressure of the gaseous getter material flow between the container and the deflector. In the prior art, the deflector is positioned at a relatively small distance from the container. In particular, the deflector is in close contact with the getter container.

The relatively high pressure causes turbulence to occur in the getter material flow, so that the getter material is able to partially circumvent the deflector. Getter material will still be deposited on the screen area.

This can be counteracted by increasing the size of the deflector, but this negatively affects the amount of getter material being deposited on the cone of the cathode ray tube. The area of the cone that is covered with getter material should be as large as possible, because this improves the pump speed, i.e. the speed at which residual gases can be absorbed by the getter material, and the capacity of the deposited getter material.

It has been found that the screen area is shielded more efficiently by a combination of a suitable increase in size of the screen shield with a suitable increase in minimum, while the part of the cone area covered by getter material is relatively large. Preferred values for the size of the screen shield and the minimum distance between the screen shield and the getter container are defined in the independent claim 1.

Preferably, the screen shield has a convex shape bulging towards the base of the container. Because of the convex shape, a particularly homogeneous layer of getter material is deposited on the cone area of the inner surface of the cathode ray tube. The amount of getter material deposited on the screen shield is relatively small here, and the amount of getter material deposited in the cone area is as large as possible.

Preferably, the first predetermined distance is approximately 10 mm.

Fluorescence occurs relatively often in the case of smaller distances because electric discharges between the screen shield and the container are formed relatively easily. An electric discharge during flashing may ignite the gaseous getter material flow, thereby negatively affecting the efficiency of the flashing process and introducing more residual gases. In this case, the amount of getter material deposited on the cone area of the cathode ray tube is reduced. This embodiment provides a particularly efficient way to implement the invention, because the occurrence of fluorescence is reduced.

The getter device according to the invention may further be provided with a deflector as known from the prior art for reducing the amount of getter material deposited on the screen shield. However, more advantageously, the container is covered by means of a lid substantially parallel to the base, while the side wall is provided with a slit adjacent to the lid, said slit releasing the getter material.

The amount of getter material deposited on the screen shield is further reduced thereby. The container may then comprise two parts such that the side wall extends over a first height so as to contact said lid in the first part of the container, and in the second part of the container the side wall extends over a second height smaller than the first height, so that a slit is formed between the side wall and the lid in the second part of the container.

The first part of the container is closed and can be made to face the yoke area of the cathode ray tube, so that substantially no getter material is deposited in the yoke area and on the electron gun.

Generally, the yoke area is covered with a so-called soft flash coating for limiting the current that flows in the cathode ray tube when a flashover occurs. If metallic getter material is deposited on top of the soft flash coating, its resistance decreases, usually by a factor often or more, and a relatively strong current flows in case of a flashover. Such a current may damage the driving circuitry of the cathode ray tube.

Moreover, no getter material is deposited on the electrodes of the electron gun of the cathode ray tube. This is an advantage because deposited getter material in the gun may be a source of field emission leading to electron leakage between electrodes of the gun. Furthermore, since the getter material is ejected from the slit, the screen shield does not need to cover the closed first part of the container. The dimensions of the screen shield may therefore be reduced, so that the getter device has a reduced weight and is cheaper to manufacture, and is less likely to block or interfere with the electron beam in a cathode ray tube.

The size of the second part, and thus the size of the slit, should be large enough to cover substantially the entire cone area of the cathode ray tube with getter material.

For this purpose, preferably, the slit extends over half the circumference of the container. In this case, the slit is large enough for allowing an unrestricted flow of the gaseous getter material to cover substantially the entire cone area, while at the same time the yoke area and the screen area are shielded efficiently. The first part and the second part in this case both correspond to a half of the container.

Preferably, the screen shield measures approximately 65 x 35 mm. This screen shield has relatively small dimensions while providing an efficient shielding of the display screen from getter material released from the slit in the second part of the getter container.

In a preferred embodiment, the getter device comprises a detector shield, positioned at a second predetermined distance from a center of the base of the container, such that said detector shield shields a predetermined portion of a cone area of the cathode ray tube from released getter material. Although generally it is desired to maximize the portion of the cone area that is covered with getter material, it may be advantageous for certain applications to exclude predetermined parts of the cone area from the getter material.

For example, in a beam index type cathode ray tube incorporating an optical beam tracking system, the detector windows advantageously remain uncoated. The light originating from the tracking phosphors passes through these detector windows. A getter material film on the detector windows reduces the intensity of this light and thus the sensitivity of the optical beam tracking system.

A preferred embodiment may be characterized in that the getter device comprises fixation means for fixing the container to the cathode ray tube, said fixation means comprising a main support and a side arm extending from said main support at a given angle thereto, an end of said side arm supporting the detector shield.

Said given angle and the size of the deflector shield depend on, amongst other factors, tube size, and the position and the size of the portions of the cone area that are to remain uncovered.

In a particularly advantageous embodiment, the fixation means comprise two side arms, the screen shield being supported by the side arms.

This embodiment has two detector shields, because generally two detector windows are to be shielded. The screen shield is supported either directly by the side arms, or by means of a screen shield support, the ends of which are fixed to the side arms.

This is a particularly efficient construction, in which relatively few elements are required for constructing the getter device.

Preferably, the second predetermined distance is at least 4 cm. For smaller distances, turbulence effects may enable the getter material flow to partly circumvent the detector shield.

These and other aspects of the invention will now be elucidated with reference to the following Figures, in which:

Fig. 1 is a cathode ray tube of the beam index type; Fig. 2 is a front elevation of the cone area and yoke area of the cathode ray tube with a preferred embodiment of the getter device according to the invention; and

Fig. 3 shows some constructional features of the preferred embodiment of the getter device in more detail.

In a cathode ray tube, the electron gun 30 generates a beam 32 of electrons which are accelerated to the display screen 20. The display screen 20 is provided with phosphor tracks 28 separated by black matrix material 27. The phosphors light up when the electron beam 32 impinges thereon.

The deflection unit 40 deflects the electron beam 32 for altering its landing position on the display screen 20. The entire display screen 20 can thus be scanned by the electron beam 32. The brightness of the phosphors 28 is changed through modulation of the beam cuπent of the electron beam 32 during the scanning. The beam current is modulated in dependence on image information being supplied to the cathode ray tube. Thus, an image is displayed on the display screen 20. The cathode ray tube as depicted in Fig. 1 is of the beam index type. Such a cathode ray tube is provided with a beam tracking system to keep the electron beam 32 aligned with the phosphor track 28 during scanning of the phosphor track 28.

If, under whatever influence, an electron beam 32' is offset in a direction perpendicular to the phosphor tracks 28, so that it no longer lands entirely on a phosphor track, part of the electrons will impinge on a tracking phosphor 31. This causes light 33 to be emitted towards the cone area 22 of the cathode ray tube. The tracking phosphors 31 and the phosphor tracks 28 are separated by a layer 26 of aluminum.

The cone area 22 is provided with a detection window 34 (as shown in Fig. 2) for passing the tracking light 33. On the outside of the cathode ray tube, a photodetector (not shown) is aligned with a detection window 34 for receiving the tracking light 33. The electric signal from the photodetector is entered into a feedback loop, so that the offset in the path of electron beam 32' may be corrected.

Such a cathode ray tube is usually a color cathode ray tube. Because of the beam tracking system, no shadow mask is required for color separation. The cathode ray tube is cheaper to manufacture, while good color separation is obtained by means of the beam tracking system.

Generally, separate phosphor tracks 28 are provided for each of the primary colors green, red and blue, each receiving their own corresponding electron beam 32. The tracking phosphors 31 are provided in two colors, each of said colors corresponding to a separate detector window 34 and photodetector. Such an optical beam tracking system is particularly efficient in a color cathode ray tube.

It has hitherto been difficult to construct a getter device for such a color cathode ray tube. Because of the optical beam tracking system, the display screen 20, the detection window 34, and furthermore preferably the yoke area 24 and electron gun 30, should receive the smallest amount of the getter material possible, while the part of the cone area 22 covered with getter material is desirably as large as possible.

A preferred embodiment of a getter device 10 according to the invention, which is particularly advantageous for use in a color cathode ray tube incorporating an optical beam tracking system, is shown schematically in Fig. 2. The main constructional features of the getter device 10 are shown in more detail in Fig. 3.

The getter device 10 is an anode-type getter. It comprises an annular getter container 11 holding the compound from which the getter material is released during flashing. The getter material is usually barium. The getter device 10 is preferably constructed from stainless steel which does not have a measurable influence on the magnetic field of the deflection yoke 40. If the distance between the getter device 10 and the deflection yoke 40 is large enough, cheaper generic iron material can be used as well. At a first distance dl from the container 11, a screen shield 25 is provided.

Preferably, this screen shield 25 is convex, bulging towards the container 11. Its surface area is between 3 and 20 times the surface area of the base 12 of the container 11. If a smaller screen shield is used, the amount of getter material deposited on the display screen 20 is still too large, while a larger screen shield will also shield an undesirably large portion of the cone area 22 from getter material. Also, a larger screen shield could block or interfere with the electron beam 32 in a cathode ray tube in operation, and thereby cause a shadow in the displayed image.

The diameter dc of the base 12 is, for example, 19 mm, so that its surface area is 2,8 cm². The screen shield 25 has, for example, a length of 65 mm and a width W of 35 mm, so that its surface area is approximately 22.8 cm². The screen shield 25 is positioned, for example, at a first distance dl from the container 11 of 10 mm.

Thus, the amount of getter material that circumvents the screen shield 25 during flashing is relatively small, and fluorescence hardly occurs.

The getter container 11 is fixed to one end of a main support 16 which supports the construction of the getter device 10 and fixes it to the cathode ray tube. The other end of the main support 16 is provided with a spring construction, which is compressed when the display screen 20 is fritted on the cone area 22 during manufacturing of the cathode ray tube. The spring pressure fixes the getter device.

From the main support 16, two side arms 17 extend at an angle of, for example, 70 ° to the main support 16. A screen support 18 is mounted between the side arms 17 for supporting the convex screen shield 25 over the container 11. The main support 16, side arms 17, and screen support 18 have, for example, a width of 10 mm and a thickness of 0.35 mm.

At the end of each side arm 17 a detector shield 36 is positioned. These detector shields 36 keep the detector windows 34 substantially free of barium during getter flashing. Thus, in operation, the tracking light 33 can pass the detector windows 34 with minimum absorption, so that the intensity of the tracking light 33 impinging on the photodetector is as high as possible and the optical beam tracking system has a relatively high sensitivity. The detector shields 36 are positioned at a second distance d2 from the center of the base 12 of the container 11, said second distance d2 being, for example, 8 cm. Smaller values of the second distance d2 reduce the efficiency of the detector shields 36, while for larger values the side arms 17 and the detector shield 36 may partially block the electron beam 32 and thereby cause a shadow in the displayed image during tube operation.

In Fig. 3, a so called semi-closed getter container 11 is shown, which is covered by means of a lid 14. This container comprises a first part 11 A and a second part 1 IB. In the first part 11A, the side wall 13 has a first height hi and contacts the lid 14, so that the first part 11 A is closed. In the second part 1 IB, the side wall 13 has a second height h2 that is smaller than the first height hi .

Thus, in the second part 1 IB, a slit 15 is formed between the side wall 13 and the lid 14. During flashing, the getter material is ejected from this slit 15. Each part 11 A, 1 IB constitutes about one half of the getter container 11, so that the slit 15 extends over half the circumference of the getter container 11. The closed first part 11 A faces the yoke area 24 and electron gun 30 of the cathode ray tube. Because of this, getter material released from the slit 15 hardly reaches the yoke area 24, so that the yoke area and electron gun are kept substantially free of getter material.

By virtue of the semi-closed getter container 11, the amount of getter material that is lost by deposition on the screen shield 25 is as small as possible. Thus, a maximum amount of getter material is deposited on the cone area 22 of the cathode ray tube. Therefore, the cathode ray tube has a particularly high pump speed, and is able to remove residual gases particularly efficiently during operation.

Alternatively, instead of a semi-closed getter container, a conventional combination of an open annular getter container with a deflector may be applied. Generally, this construction requires the use of a larger screen shield covering the entire base of the container. The dimensions of the screen shield are then, for example, 88 x 46 mm.

However, this alternative embodiment is less efficient in preventing getter material from being deposited on the screen shield and in the yoke area of the cathode ray tube.

The drawings are schematic and were not drawn to scale. The cathode ray tube is, for clarity reasons, shown with only a limited number of phosphor tracks, whereas an actual display device would have, for example, 600 phosphor tracks and a corresponding number of tracking phosphors. While the invention has been described in connection with preferred embodiments, it will be understood that modifications thereof, within the scope of the appended claims, will be evident to those skilled in the art. Thus, the invention is not limited to the preferred embodiments but is rather intended to encompass all such modifications.

Claims

CLAIMS:

1. A getter device (10) for a cathode ray tube, comprising: a container (11) having a base (12) and a side wall (13) extending from said base (12) in a first direction, said container (11) releasing a getter material during flashing, and a screen shield (25) having a first predetermined minimum distance (dl) to said container (11) in the first direction and at least partially extending over the base (12) of the container (11), characterized in that said first predetermined minimum distance (dl) is between 0.25 and 2 times a diameter (dc) of the container (11), and said screen shield (25) has a surface area of between 3 and 20 times a surface area of the base (12) of the container.

2. A getter device (10) as claimed in claim 1, characterized in that the screen shield (25) has a convex shape bulging towards the container (11).

3. A getter device (10) as claimed in claim 1, characterized in that the first predetermined distance (dl) is approximately 10 mm.

4. A getter device (10) as claimed in claim 1, characterized in that the container (11) is covered by means of a lid (14) which is substantially parallel to the base (12) such that the side wall (13) is provided with a slit (15) adjacent to the lid (14), said slit (15) releasing the getter material.

5. A getter device (10) as claimed in claim 4, characterized in that the slit (15) extends over half a circumference of the container (11).

6. A getter device (10) as claimed in claim 5, characterized in that the screen shield (25) measures approximately 65 x 35 mm.

7. A getter device (10) as claimed in claim 1, characterized in that the getter device (10) comprises a detector shield (36) positioned at a second predetermined minimum distance (d2) from a center of the base (12) of the container (11), such that said detector shield (36) shields a predetermined portion (34) of a cone area (22) of the cathode ray tube from released getter material.

8. A getter device as claimed in claim 7, characterized in that the getter device (10) comprises fixation means for fixing the getter device (10) to the cathode ray tube, said fixation means comprising a main support (16) and a side arm (17) extending from said main support at a given angle thereto, an end of said side arm (17) supporting the detector shield (36).

9. A getter device as claimed in claim 8, characterized in that the fixation means comprise two side arms (17), the screen shield (25) being supported by the side arms (17).

10. A getter device as claimed in claim 7, characterized in that the second predetermined distance (d2) is at least 6 cm.

11. A cathode ray tube provided with a getter device (10) comprising: a container (11) having a base (12) and a side wall (13) extending from said base (12) in a first direction, said container (11) releasing a getter material during flashing of the cathode ray tube, and a screen shield (25) having a first predetermined minimum distance (dl) to said container (11) in the first direction and at least partially extending over the base (12) of the container (11), characterized in that said first predetermined minimum distance (dl) is between 0.25 and 2 times a diameter (dc) of the container (11), and said screen shield (25) has a surface area of between 4 and 20 times a surface area of the base (12) of the container.

12. A cathode ray tube as claimed in claim 11 , characterized in that the cathode ray tube is a beam index tube incorporating an optical beam tracking system.