JP4671036B2 - Cold cathode fluorescent discharge tube and surface light source device - Google Patents

Description

  The present invention relates to a cold cathode fluorescent discharge tube which improves luminance uniformity and emits light in a short time, and a surface light source device using the cold cathode fluorescent discharge tube.

Cold cathode fluorescent lamps (CCFLs) are widely used as liquid crystal backlight light sources for notebook personal computers.
For example, a conventional cold cathode fluorescent discharge tube has a glass tube having a closed space filled with a discharge gas, a pair of electrodes fixed to both ends of the glass tube, and an inner surface of the glass tube and an electrode. And a light emitting layer formed of a fluorescent film that emits visible light upon receiving irradiation of ultraviolet rays generated by the discharge. A rare gas such as argon and mercury are accommodated in the glass tube as a discharge gas. The electrode includes an introduction line and a cup-shaped discharge part fixed to the introduction line, and the lead-out part of the introduction line is connected to an external lead outside the glass tube. When a voltage is applied between a pair of electrodes, electrons are emitted from one electrode, and the emitted electrons collide with mercury in the glass tube to generate ultraviolet rays. The ultraviolet light is wavelength-converted by a light emitting layer made of a fluorescent film formed on the inner wall of the glass tube, and visible light is emitted to the outside of the glass tube.

In recent years, the number of cold cathode fluorescent discharge tubes that supply light to the liquid crystal screen has been increased to cope with the increase in the brightness of the liquid crystal screen. However, if the number of cold cathode fluorescent discharge tubes is increased, the cold cathode fluorescent discharge tube of the surface light source device Since the installation space shape (occupancy ratio) increases and the cost of parts also rises, further enhancement of the brightness of the cold cathode fluorescent discharge tube is desired.
Patent Document 1 is a hot cathode fluorescent discharge tube having a double tube structure. As shown in FIG. 8, a fluorescent lamp (hot cathode fluorescent discharge tube) having a double tube structure includes a glass outer tube (1) that forms a closed space (4) for enclosing a discharge gas, and an outer tube ( A pair of electrodes (3) hermetically fused and fixed to both ends of 1), and disposed in the closed space (4) of the outer tube (1) and communicated with the closed space (4) through the end (2d) The inner tube (2) made of glass having a hollow portion (2c) and the inner peripheral surface (1a) of the outer tube (1), the inner peripheral surface (2a) or the outer peripheral surface (2b) of the inner tube (2) The coated fluorescent film (5), the second phosphor (13) applied to the outer peripheral surface (2b) of the inner tube (2), and a thin glass body that does not impede discharge, and the inner glass tube And a support (14) for fixing (2) to the glass tube (2). Supports (14) provided near both ends of the inner pipe (2) hold the inner pipe (2) to the outer pipe (1). In the fluorescent lamp of Patent Document 1, the path of the discharge plasma is confined to the cavity (2c) of the inner tube (2), and the electron density increases and the brightness of the lamp increases.

JP 11-339724 A

  In the hot cathode discharge tube as shown in Patent Document 1, since the outer tube (1) having an outer diameter of 10 mm or more is generally used, the inner and outer tubes (2c) of the inner tube (2) and the outer tube ( A sufficient amount of mercury is supplied to the gap between 1) and the inner tube (2), but in a cold cathode fluorescent discharge tube having an outer diameter of 5.0 mm or less, as shown in Patent Document 1, the outer tube ( the provision of the inner tube (2) in the closed space (4) of 1), the inner diameter becomes very small cavities of the inner tube (2) in (2c). In recent years, along with the screen size of liquid crystal displays, there is a tendency that the longitudinal dimension of the cold cathode fluorescent discharge tube becomes increasingly longer. Generally, as shown in the background art, the double tube structure can increase the current density by increasing the electron flow to the cavity (2c) of the inner tube (2), thereby increasing the luminance. . Therefore, the inner pipe (2) is also made as long as possible in the longitudinal direction.

  In addition, when a predetermined voltage is applied to the pair of electrodes (3), the cold cathode fluorescent discharge tube having a double-tube structure has an increase in the wall temperature of the outer tube (1) than that of the inner tube (2). Therefore, mercury tends to stay relatively on the inner peripheral surface (1a) of the outer tube (1). Furthermore, unless the mercury gas is evenly supplied to the hollow portion (2c) of the inner tube (2), the cold cathode fluorescent discharge tube cannot start light emission or maintain light emission.

When the double tube structure as in Patent Document 1 is applied to a thin and long cold cathode fluorescent discharge tube, when a voltage is applied to the pair of electrodes (3), it stays on the inner peripheral surface (1a) of the outer tube (1). When the mercury that has been diffused into the closed space (4) as a gas and filled with mercury gas to the vicinity of the center in the longitudinal direction in the cavity (2c) of the inner tube (2), a cold cathode fluorescent discharge tube Emits light. Therefore, it takes time until the cold cathode fluorescent discharge tube emits light well after a voltage is applied to the pair of electrodes (3). If the mercury gas is not sufficiently diffused to the vicinity of the center in the longitudinal direction in the cavity (2c) of the inner tube (2), the brightness near the center in the longitudinal direction in the cold cathode fluorescent discharge tube is There is also a problem of lowering than (3) side).
Accordingly, an object of the present invention is to provide a cold cathode fluorescent discharge tube that improves the uniformity of luminance in the longitudinal direction of the cold cathode fluorescent discharge tube and emits light in a short time, and a surface light source device using the cold cathode fluorescent discharge tube. .

The cold cathode fluorescent discharge tube of the present invention comprises a glass outer tube (1) forming a closed space (4) for enclosing a discharge gas, and a pair of electrodes (3) fixed to both ends of the outer tube (1). And a glass inner tube (2) having a cavity (2c) disposed in the closed space (4) of the outer tube (1) and communicating with the closed space (4) through the end (2d). I have. The outer peripheral surface (1c) of the outer tube (1) has a pair of linear depressions (1b) facing inward, and a pair of linear depressions (1b) forms a hollow elliptical cross-section with a circular cross-section. The outer tube (1) is formed, and the connecting portion (6) is formed to fuse the inner peripheral surface (1a) of the outer tube (1) and the outer peripheral surface (2b) of the inner tube (2).
The outer peripheral surface of the outer tube (1) has a pair of linear recesses (1b) facing inward, and the recess (1b) of the outer tube (1) is an inner peripheral surface (1a) of the outer tube (1). When the voltage is applied to the pair of electrodes (3), the wall surface of the inner tube (2) in the vicinity of the connection portion (6) In this case, mercury is retained in the vicinity of the connecting portion (6) of the inner pipe (2) having a relatively low temperature. As a result, mercury stays in the vicinity of the connecting portion (6) of the inner pipe (2) as compared with the conventional double pipe structure, so that the longitudinal direction in the hollow portion (2c) of the inner pipe (2) is shortened in a short time. The cold cathode fluorescent discharge tube can emit light in a short time after a voltage is applied to the pair of electrodes (3). Further, the mercury gas can be sufficiently diffused to the vicinity of the center of the hollow portion (2c) of the inner tube (2), and the luminance near the center in the longitudinal direction in the cold cathode fluorescent discharge tube is reduced as compared with the peripheral portion. The problem can be solved and the uniformity of luminance in the longitudinal direction can be improved.

  Holding the inner tube (2) a short time and more uniform mercury gas in the longitudinal direction of the cavity inner tube in (2c) (2) of the. This makes it possible to light emission in a short time the cold cathode fluorescent discharge tube, and can improve the uniformity of brightness in the longitudinal direction of the cold cathode fluorescent discharge tube. In addition, the surface light source device using this cold cathode fluorescent discharge tube can display in a short time and achieve uniform luminance.

  Hereinafter, embodiments of the cold cathode fluorescent discharge tube according to the present invention will be described with reference to FIGS. In these drawings, the same parts as those shown in FIG.

  As shown in FIGS. 1 and 2, in the cold cathode fluorescent discharge tube of the present invention, the outer peripheral surface (1c) of the outer tube (1) is on the inner side (center axis (7) connecting the pair of electrodes (3)). It has a pair of linear recesses (1b) facing each other, and each recess (1b) fuses the inner peripheral surface (1a) of the outer tube (1) and the outer peripheral surface (2b) of the inner tube (2). Thus, the connecting portion (6) is formed, and mercury is retained on the inner peripheral surface (2a) of the inner tube (2). The outer diameter of the outer tube (1) is 5.0 mm or less, for example, 4.0 mm. In the embodiment of the present invention, the diameter of the outer peripheral surface (1c) and the inner peripheral surface (1a) of the outer tube (1) is a recess (1b) that decreases near the center of the recess (1b), and the outer tube ( A plurality are formed in the longitudinal direction of 1). Recesses (1b) are heated outer tube (1) is pressed against the inner deformed and fused to the recess (1b) the inner tube (2). Therefore, as shown in FIG. 2, in the cross section of the connecting portion (6) cut at a plane perpendicular to the central axis (7) and including the recess (1b), the outer tube (1 ) Tube wall thickness is substantially equal to the tube wall thickness of the outer tube (1) other than the connecting portion (6).

Further, the connecting portion (6) holds the inner tube (2) at a predetermined position in the outer tube (1) and, as shown in the cross-sectional view of FIG. 2, the inner tube (2) and the outer tube (1 ) On both sides of the hollow (1b) where the inner tube (2) is fused, an arc-shaped gap (2e) is formed between the outer peripheral surface (2b) of the inner tube (2) and the inner peripheral surface (1a) of the outer tube (1). It is formed. The angle (θ) shown in FIG. 2 around the central axis (7) in the cross section of the connecting portion (6) is provided in a limited region and is preferably 90 degrees or less, preferably 45 degrees or less. When the gap (2e) is formed, the gap between the electrode (3) and the inner pipe (2) from the gap between the outer pipe (1) and the inner pipe (2), the cavity (2c) of the inner pipe (2) ) And the circulation of the discharge gas returning to the gap between the outer tube (1) and the inner tube (2) is not hindered, so that the life of the cold cathode fluorescent discharge tube can be kept good.
The electrode (3) is hermetically fused to both ends of the outer tube (1) and led out to the outside of the outer tube (1), and a lead (3a) disposed inside the outer tube (1) And a discharge part (3b) connected to 3a).

  In the embodiment shown in FIG. 1, the fluorescent film (5) is applied to the inner peripheral surface (2a) of the inner tube (2). In particular, a fluorescent film (5) is partially applied to the inner peripheral surface (1a) of the outer tube (1) that covers the space between the inner tube (2) and the electrode (3), and then radiated from mercury (15). All the light that passes through the fluorescent film (5) converts the light wavelength, so that a long effective light emitting region is obtained. In this case, if the two fluorescent films (5) applied to the inner tube (2) and the outer tube (1) overlap in the optical path, they will be shaded by the fluorescent film (5) of the outer tube (1). it is desirable that as much as possible reduce the overlapping portion of the fluorescent film (5) of the outer tube (1) and the inner tube (2), may partially overlap.

  The discharge gas filled in the closed space (4) of the outer tube (1) includes an inert gas such as argon or a rare gas or a mixed gas of neon and argon, and a gas such as mercury vapor. The pair of electrodes (3) is made of a nickel lead (3a), a discharge part (3b) made of nickel and formed in a cup shape, and made of tungsten that connects the lead (3a) and the discharge part (3b). And a buried portion (not shown). The lead (3a) connected to the discharge part (3b) may be provided with a tungsten coating having glass affinity. Further, instead of the cup shape, a cylindrical (sleeve-shaped) discharge part (3b) may be provided.

  Mercury in the cold cathode fluorescent discharge tube tends to collect at the lowest temperature part of the tube wall temperature of the cold cathode fluorescent discharge tube. When a voltage is applied to the pair of electrodes (3), the temperature of the cold cathode fluorescent discharge tube rises, and mercury that has accumulated on the wall surface becomes mercury gas due to the temperature rise of the wall surface and fills the cold cathode fluorescent discharge tube. The cold cathode fluorescent discharge tube emits light. In the case of a general double tube structure, the wall surface temperature of the inner tube (2) is higher than the wall surface temperature of the outer tube (1). In the embodiment of the present invention, as shown in FIG. 3, a recess (1b) directed inward is provided on the wall surface of the outer tube (1), and the recess (1b) includes the outer tube (1) and the inner tube (2). Are connected to each other to form the connecting portion (6) .In the connecting portion (6), the wall temperature of the outer pipe (1) is easily propagated to the inner pipe (2), and the inner pipe (2) connecting portion (6) the tube wall temperature of the surrounding is reduced in the circumferential surface (2a), easily relatively residence mercury in the recess (1b) around. Furthermore, the outer peripheral surface (2b) of the inner tube (2) and the inner peripheral surface (1a) of the outer tube (1) on both sides of the recess (1b) where the inner tube (2) and the outer tube (1) are fused. If the arc-shaped gap (2e) is formed between the inner pipe (2a) and the inner peripheral surface (2a) of the inner pipe (2), the area where the inner peripheral surface (2a) is relatively retained can be limited. .

  In a conventional cold cathode fluorescent discharge tube having a double tube structure, mercury accumulates on the inner peripheral surface (1a) of the outer tube (1), and the inner tube (2) has a small diameter and a long longitudinal dimension. Therefore, it takes time for the mercury gas to be diffused well to the vicinity of the center in the longitudinal direction of the inner pipe (2). In some cases, the luminance near the center of the cathode fluorescent discharge tube in the longitudinal direction is lower than that in the periphery.

  However, the cold cathode fluorescent discharge tube of the present invention reduces the tube wall temperature around the connecting portion (6) on the inner peripheral surface (2a) of the inner tube (2), and the inner peripheral surface of the inner tube (2) ( By relatively retaining mercury around the connecting part (6) of 2a) and providing a recess (1b) at a position (center side) separated to some extent from the opening (2d) of the inner pipe (2), short time by filling the mercury gas in the longitudinal direction near the center of the inner tube (2), it is possible in a short time to emit light a cold cathode fluorescent discharge tube. Further, to suppress the reduction in luminance due to depletion of the mercury gas in the vicinity of the center portion of the inner tube (2), it is possible to suppress the uneven brightness. Therefore, the length from the opening (2d) of the inner pipe (2) to the center of the nearest connecting part (6) is not less than 1/8 and less than 1/2 of the length of the inner pipe (2). It is desirable to be.

  Further, a gripping member (11) supported on a surface light source device (not shown) can be provided in the recess (1b) of the outer tube (1) shown in FIG. 4 to fix the cold cathode fluorescent discharge tube to the surface light source device. . When a voltage is applied to the pair of electrodes (3), the temperature of the gripping member (11) does not rise much compared to the tube wall temperature of the outer tube (1) and the inner tube (2). The tube wall temperatures of the outer tube (1) and the inner tube (2) in the vicinity of the attached location are lower than the other tube wall temperatures of the respective tubes. In the embodiment of the present invention, the heat of the gripping member (11) propagates from the tube wall of the outer tube (1) to the inner tube (2) through the connecting portion (6) provided in the recess (1b). The temperature of the tube wall around the connecting portion (6) on the inner peripheral surface (2a) of the tube (2) is further lowered, and mercury is more easily collected around the connecting portion (6). Therefore, it is possible to emit light from the cold cathode fluorescent discharge tube in a short time, and further suppress the decrease in luminance near the center of the inner tube (2), and uneven luminance between the periphery and the center in the longitudinal direction of the cold cathode fluorescent discharge tube. Can be further suppressed.

  The outer tube (1) made of glass is formed in a cylindrical shape with a circular cross section forming a closed space (4) as a whole, but in the recess (1b), the radius of the outer peripheral surface of the outer tube (1) is recessed. It decreases from the periphery of (1b) toward the center and is formed into a hollow elliptical cross-section with a crushed circular cross-section. Note that radius radius substantially the same as the outer peripheral surface of the inner peripheral surface of the outer tube (1) (1a), it decreases from the periphery of the depression (1b) towards the center. The glass inner tube (2) is formed in a circular cross section that is contained in the outer tube (1) and has a cavity (2c). Alternatively, the entire outer tube (1) or the inner tube (2) may be formed in a cylindrical shape with an elliptical cross section. The discharge gas in the closed space (4) circulates well in the gap (2e) due to the temperature difference, and the closed space (4) of the outer tube (1) and the cavity (2c) of the inner tube (2). The temperature inside can be kept substantially constant, and the cold cathode fluorescent discharge tube can emit light well.

  Referring to FIG. 7 showing the manufacturing method of the cold cathode fluorescent discharge tube according to the present invention, as shown in FIG. 7 (1), first, the inner tube (102) which is fired and the phosphor is coated on the inner peripheral surface is used as the outer tube. Insert into (101). Next, as shown in FIG. 7 (2), a part of the glass outer tube (101) is heated and melted by a burner (103) to grip a plurality of locations on the outer tube (101). The pipe (101) is welded to the inner pipe (102). Thus, it is formed connecting portion (6) and recesses (1b) is, the outer tube (101) is held by the inner tube (102) in place. Next, as shown in FIG. 7 (3), the discharge part (3b) of one electrode (3) with the glass beads (3c) attached to the lead (3a) is arranged at both ends of the outer tube (101). Then, the glass beads (3c) are fused to the outer tube (101) and the leads (3a). Subsequently, a mercury getter was inserted from the other end of the outer tube (101) and evacuated.In the same manner as before, the inside of the outer tube (101) was filled with a discharge gas, and then the glass beads (3c ) electrode (3) having disposed on the other end of the outer tube (101) to be fused glass beads (3c) in the outer tube (101) and the lead (3a).

  Further, in the embodiment of the present invention, as shown in FIG. 1, the depression (1b) is not limited to two places. In FIG. 6, which shows a cross-sectional view of a short cold cathode fluorescent discharge tube according to the second embodiment of the present invention, three depressions (1b) in the central axis (7) direction (longitudinal direction) are formed in the outer tube (1). The holding member (11) is attached only to the middle depression (1b) of the three depressions (1b), and the surface light source device (30) and the cold cathode fluorescent discharge tube (20) are attached by the holding member (11). May be fixed. Also, the inner pipe (2) and the outer pipe (1) are fused only at the middle depression (1b) of the three depressions (1b), and the outer circumference of the inner pipe (2) is fused at the other depressions (1b). The surface (2b) and the inner peripheral surface (1a) of the outer tube (1) may not be fused.

  In the present invention, the inner tube (2) is brought into contact with the connecting portion (6) formed integrally with the outer tube (1) so as to be able to transfer heat, and is separated from the opening (2d) of the inner tube (2). A connecting portion (6) is provided near the center in the longitudinal direction of the tube (2). When a voltage is applied to the pair of electrodes (3), a current flows in the cavity (2c) of the inner tube (2), but heat of the outer tube (1) is transferred through the connecting portion (6). connection (6) to suppress the temperature rise in the vicinity of the inner tube (2), the connecting portion of the inner tube (2) (6) relatively to stay mercury around. When a voltage is applied to the electrode (3) again, mercury gas is filled relatively quickly to the vicinity of the center of the inner tube (2), light can be emitted early, and the luminance near the center of the cold cathode fluorescent discharge tube is increased. It can suppress that it falls compared with the peripheral part of a longitudinal direction. Further, by attaching the gripping member (11) to the recess (1b) of the outer tube (1), the temperature near the connecting portion (6) of the inner tube (2) is further lowered, and mercury is retained, The effect can be further improved.

  The length from the opening (2d) of the inner tube (2) to the center of the nearest connecting portion (6) is 1/8 or more of the length of the longitudinal dimension of the inner tube (2) and less than 1/2, Preferably, it is not less than ¼ but less than ½ so that a certain length is secured from the opening (2d) to the connecting portion (6) of the inner tube (2), and the inner tube (2) The mercury staying on the inner peripheral surface of the tube becomes vapor and moves to the outside of the inner tube (2) to suppress the deficiency of mercury in the center of the inner tube (2) and to achieve the above effect better. Can do. Also, the length of the connecting portion (6) closest to the end of the outer tube (1) should be less than half the length of the outer tube (1), and the position of the connecting portion (6) should be asymmetrical Thus, as shown in FIG. 5, by installing the cold cathode fluorescent discharge tubes (20) adjacent to each other installed in the surface light source device (30) in the left and right directions (longitudinal directions) in reverse directions, Even if a cold cathode fluorescent discharge tube is used, there is an advantage that the holding member (11) can be arranged in a zigzag manner. Even if a part of the light generated from the cold cathode fluorescent discharge tube is blocked by the gripping member (11) and becomes a shadow, the stripes due to the brightness of the light are not formed as a whole. It can be made uniform.

  In the embodiment of the present invention, the inner tube (2) is arranged inside the outer tube (1), and electrons flowing between the electrodes (3) are concentrated in the inner tube (2), so that the inner tube (2) The electron density increases, and the brightness of the lamp increases. Moreover, by moving the mercury gas in the vicinity of the central portion of the inner tube (2), it is possible to reduce the density of the electrode (3) near the mercury gas. As a result, contact between the electrode spatter generated from the electrode (3) and mercury can be prevented, and the formation of mercury amalgam can be suppressed, so the mercury consumption rate is suppressed and the operating life of the cold cathode fluorescent discharge tube is extended. be able to.

  An edge-light type that emits light collected from the end portion of the light guide plate directly from the end of the light guide plate by arranging a plurality of cold-cathode fluorescent discharge tubes directly under the light emitting surface or reflected by the reflector sheet. In any case, the present cold cathode fluorescent discharge tube can be constructed.

  Various modifications can be made to the embodiment of the cold cathode fluorescent discharge tube of the present invention. For example, to facilitate electrons flows into the opening of the inner tube (2) (2d), may be larger inside diameter to the outside like a horn. The inner pipe (2) is not a single shape, and a plurality of inner pipes (2) may be arranged in series.

  Emits light in a short time, the cold cathode fluorescent discharge tube and a surface light source device of the present invention having an improved longitudinal luminance can be utilized in a liquid crystal television or a liquid crystal display or the like.

Sectional view of a cold cathode fluorescent discharge tube according to the present invention, shortened by a broken line Sectional view of a cold cathode fluorescent discharge tube according to the invention along a plane perpendicular to the central axis and including a depression Partial sectional view showing the behavior of mercury at the joint shown in FIG. Partial sectional view when a gripping member is attached to the connecting portion of FIG. The top view of the surface light source device by this invention Sectional drawing which shows 2nd Embodiment of the cold cathode fluorescent discharge tube of this invention Process drawing which shows the manufacturing method of the cold cathode fluorescent discharge tube of this invention Cross section of conventional hot cathode discharge tube

Explanation of symbols

  (1) ・ ・ Outer tube, (1a) ・ ・ Inner surface of outer tube, (1b) ・ ・ Recess of outer tube, (2) ・ ・ Inner tube, (2a) ・ ・ Inner surface of inner tube, (2b) ・ ・ Outer surface of inner tube, (2c) ・ ・ Cavity of inner tube, (2d) ・ ・ Opening, (3) ・ ・ Electrode, (3a) ・ ・ Lead of electrode, (3b) ・・ Electrode discharge part, (4) ・ ・ Closed space, (5) ・ ・ Fluorescent membrane, (6) ・ ・ Connecting part, (7) ・ ・ Center shaft, (11) ・ ・ Gripping member, (20) ・・ Cold cathode fluorescent discharge tubes, (30) ・ ・ Surface light source devices,

Claims (7)

A glass outer tube that forms a closed space that encloses the discharge gas; and
A pair of electrodes fixed to both ends of the outer tube;
An inner tube made of glass having a cavity portion disposed in a closed space of the outer tube and communicating with the closed space through an end portion;
The outer peripheral surface of the outer tube has a pair of linear depressions facing inward,
A pair of linear recesses form the outer tube in a hollow elliptical cross-section with a circular cross-section, and constitute a connecting portion that fuses the inner peripheral surface of the outer tube and the outer peripheral surface of the inner tube A cold cathode fluorescent discharge tube.   The cold cathode fluorescent discharge tube according to claim 1, wherein radii of an outer peripheral surface and an inner peripheral surface of the outer tube decrease near a center of the recess.   The cold cathode according to claim 1 or 2, wherein the length from the opening of the inner tube to the center of the nearest connecting portion is 1/8 or more and less than 1/2 of the length of the inner tube. Fluorescent discharge tube.   The cold cathode fluorescent discharge tube according to any one of claims 1 to 3, wherein a length of the connecting portion closest to an end portion of the outer tube is less than ½ of a length of the outer tube.   5. The cold cathode fluorescent discharge tube according to claim 1, wherein an angle of the connecting portion around a central axis connecting the pair of electrodes is 90 degrees or less in a cross section of the connecting portion.   The cold cathode fluorescent discharge tube according to any one of claims 1 to 5, wherein an arcuate gap is formed between an outer peripheral surface of the inner tube and an inner peripheral surface of the outer tube on both sides of the recess.   A surface light source device, wherein a holding member for holding the cold cathode fluorescent discharge tube according to any one of claims 1 to 6 is provided in a recess of the outer tube.

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