JP2008292581A - Backlight for direct type liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a backlight with which unevenness of brightness on a picture is suppressed. <P>SOLUTION: The backlight 100 for direct type liquid crystal display comprises fluorescent lamps 10 and a housing 20. The number of the fluorescent lamps 10 is an odd number of three or more and, among the number (m) of the fluorescent lamps, length (h) of picture longitudinal direction, length (L) of inner circumference of a valve 12 and depth (d) of the housing, the following relations (1), (2) are established. the odd numbers of fluorescent lamps 10 are arrayed in an equal interval (p) and the central fluorescent lamp 10 among the odd numbers of fluorescent lamps are located on the position of h/2 which is the central position in the screen longitudinal direction. Therein, the relations (1), (2) are presented as follow: m≤15.1h/(28.4d+4L) (1), m≥15.1h/(28.4d+19L) (2). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a backlight for a direct type liquid crystal display.

Currently, cold cathode fluorescent lamps are mainly used as the light source of the backlight of the liquid crystal display. Since the cold cathode fluorescent lamp is suitable for reducing the diameter, it is used as a light source for a backlight that is required to be thin (see, for example, Patent Document 1).
JP 2002-116704 A

In recent years, the screen of liquid crystal displays has been increased, and the backlight has also increased in size. When the cold cathode fluorescent lamp is used as a light source due to the increase in the size of the backlight, the lighting circuit becomes complicated and there is a concern that the power consumption increases due to an increase in the number of lamps used.
More specifically, a cold cathode fluorescent lamp requires a higher voltage (drive voltage) for driving than other lamps, and it is necessary to use a high-voltage power supply. In particular, since a large-screen liquid crystal display having a screen size of 32 inches or more has recently appeared, the lamp length becomes longer, and the drive voltage tends to be further increased accordingly.

In addition, since cold cathode fluorescent lamps require a small amount of electric power to be supplied per lamp, it is necessary to increase the number of lamps in order to ensure screen brightness. Therefore, the cost of parts increases and the number of assembly steps increases. There is a high possibility that the problem will become apparent.
Under such circumstances, it has begun to consider adopting a hot cathode fluorescent lamp having higher efficiency and higher output than a cold cathode fluorescent lamp as a light source of a backlight. By adopting a hot cathode fluorescent lamp, it is possible to reduce power consumption and reduce the number of lamps due to the above-mentioned features, and it can be expected that the lighting circuit can be simplified, parts costs can be reduced, and assembly man-hours can be reduced.

The present inventor solves the problem of the backlight which becomes more and more apparent with the enlargement of the screen of the liquid crystal display by using a hot cathode fluorescent lamp instead of solving the current mainstream cold cathode fluorescent lamp. Trying to do.
According to the inventor's study, a backlight using a hot cathode fluorescent lamp can certainly reduce the number of lamps compared to a cold cathode fluorescent lamp, but there are still disadvantages when the number of lamps is reduced. I knew it would turn. That is, if the number of lamps in the backlight is small, the luminance unevenness of the screen becomes severe and the use as a backlight for a liquid crystal display becomes impossible. In particular, it has been found that luminance unevenness and luminance reduction near the center of the screen are immediately unsuitable because they immediately lead to image quality degradation.

  The present invention has been made in view of such points, and its main object is to suppress luminance unevenness in a backlight using a low number of light sources.

The backlight according to the present invention is a backlight for a direct liquid crystal display including a fluorescent lamp and a casing for storing the fluorescent lamp. The number of the fluorescent lamps stored in the casing is 3 The number of the fluorescent lamps is an odd number, and the fluorescent lamp is composed of a substantially linear bulb and a phosphor layer formed on the inner periphery of the bulb. h, the relationship of the following formulas 1 and 2 is established between the inner circumferential length L in the cross section perpendicular to the longitudinal direction of the bulb and the depth d of the casing:
m ≦ 15.1h / (28.4d + 4L) (Formula 1)
m ≧ 15.1h / (28.4d + 19L) (Formula 2)
Here, the odd-numbered fluorescent lamps are arranged at equal intervals, and the center fluorescent lamp of the odd-numbered fluorescent lamps is located at the position h / 2 which is the center position in the vertical direction of the screen. Has been placed.

In a preferred embodiment, the fluorescent lamp is a hot cathode fluorescent lamp, and the hot cathode fluorescent lamp has a filament that emits thermoelectrons in the bulb.
In a preferred embodiment, the valve has a circular cross section.
The cross section of the bulb may be substantially elliptical.
In a preferred embodiment, the backlight is a direct liquid crystal display backlight having a screen size of 32 inches to 65 inches.

  According to the present invention, the number of fluorescent lamps housed in the casing is an odd number of 3 or more, and the odd number of fluorescent lamps are arranged at equal intervals, and the central fluorescent lamp is the center in the vertical direction of the screen. The relationship of Equations 1 and 2 is established among the number m of fluorescent lamps, the length h in the vertical direction of the screen, the length L of the inner circumference in the bulb cross section, and the depth d of the housing. By doing so, luminance unevenness can be suppressed. In addition, since it is possible to realize a low number of backlights while suppressing luminance unevenness, the cost of the light source member in the backlight can be reduced.

  The inventor of the present application does not use the current mainstream cold cathode fluorescent lamp (CCFL), but one that is suitable for a backlight for a liquid crystal display whose screen size is increasingly accelerated. R & D was undertaken in the light of a shift to a hot-cathode fluorescent lamp (HCFL) capable of supplying high output power. The reason for this transition is that the contrast ratio of LCD TVs can be increased by taking advantage of the “high output” feature of hot cathode fluorescent lamps, enabling high image quality including moving images. This is because the number of lamps used as the backlight can be greatly reduced and the cost can be reduced as compared with the cold cathode fluorescent lamp. In such development, the present inventor has made various studies on how to suppress luminance unevenness without increasing the cost in a hot cathode fluorescent lamp capable of reducing the number compared with a cold cathode fluorescent lamp. The present invention achieved the present invention by suppressing luminance unevenness using a method different from the above idea.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, components having substantially the same function are denoted by the same reference numerals for the sake of brevity. In addition, this invention is not limited to the following embodiment.
FIG. 1 schematically shows a configuration of a backlight 100 according to an embodiment of the present invention. FIG. 1A is a top view schematically showing the backlight 100 of the present embodiment, and FIGS. 1B and 1C are schematic cross-sectional views corresponding to FIG.

The backlight 100 of this embodiment is a direct-type liquid crystal display backlight including a fluorescent lamp 10 and a housing 20 that houses the fluorescent lamp 10. And in the structure of this embodiment, the number of the fluorescent lamps 10 accommodated in the housing | casing 20 is one of the characteristics that it is an odd number of 3 or more.
In the example shown in FIG. 1, three fluorescent lamps 10 are accommodated in the housing 20. The odd-numbered (three) fluorescent lamps 10 shown in FIG. 1 are arranged at equal intervals (p), and the center fluorescent lamp 10 of the odd-numbered fluorescent lamps 10 has a length in the vertical direction of the screen. Is set at h / 2, which is the center position in the vertical direction of the screen.

The fluorescent lamp 10 includes a substantially linear bulb 12 and a phosphor layer (not shown) formed on the inner periphery of the bulb 12. The fluorescent lamp 10 in the present embodiment is a hot cathode fluorescent lamp. The hot cathode fluorescent lamp 10 has a filament (not shown) that emits thermoelectrons in the bulb 12. Details of the configuration of the hot cathode fluorescent lamp 10 will be described later.
An opening 20a is formed in the housing 20 that houses the fluorescent lamp 10, and the optical sheet 30 is disposed in the opening 20a. A liquid crystal display panel is disposed above the optical sheet 30 (in the direction of the arrow 40). The direction in which the liquid crystal display panel is arranged may be referred to as the screen direction 40. In other words, the direction in which the light of the backlight 100 as the planar light source should go (the direction in which the liquid crystal display panel exists) is the screen direction 40. It becomes.

  The optical sheet (or optical film) 30 is configured by laminating a plurality of layers, and includes, for example, a diffusion sheet, a lens sheet, and a polarizing sheet. The main surface (here, the bottom surface) 20 b of the housing 20 is located on the surface facing the optical sheet 30. The main surface (bottom surface) 20 b of the housing 20 functions as a reflector, and has a function of directing light emitted from the fluorescent lamp 10 in the screen direction 40. Specifically, the light radiated from the fluorescent lamp 10 is reflected toward the main surface (bottom surface) 20b of the housing 20 and passes through the optical sheet 30 toward the liquid crystal display panel. Go.

In FIG. 1, an example in which three fluorescent lamps 10 are accommodated in the casing 20 is shown, but in FIG. 2, an example in which five (odd number) fluorescent lamps 10 are accommodated in the casing 20 is shown. Yes. As in FIGS. 1A to 1C, FIG. 2A is a top view schematically showing the backlight 100 of the present embodiment, and FIGS. 2B and 2C are FIGS. It is a schematic sectional drawing corresponding to a).
FIG. 3A schematically shows a cross-sectional configuration of the bulb 12 of the fluorescent lamp 10, more specifically, a cross section perpendicular to the longitudinal direction 50 of the bulb. The bulb 12 of the present embodiment is made of a glass tube having an inner surface 12a and an outer surface 12b, and a phosphor layer 19 is formed on the inner surface 12a of the bulb 12. The cross section of the valve 12 shown in FIG. Here, as shown in FIG. 3B, the inner periphery (peripheral length with respect to the inner surface 12a) of the valve cross section is represented as “L”. As can be seen from the fact that the phosphor layer 19 is formed on the inner surface 12a of the bulb 12, the inner circumference L of the bulb cross section means the length of the light emitting portion (inner circumference).

The fluorescent lamp 10 shown in FIGS. 1 to 3 shows a bulb 12 having a circular cross section, but as shown in FIG. 4, the cross section of the bulb 12 is not limited to a circle but is substantially oval (elliptical). , A shape such as an ellipse or a flat shape). Also in this case, the inner periphery (peripheral length based on the inner surface 12a) of the cross section of the valve 12 is expressed as “L”.
In the backlight 100 of the present embodiment, the relationship of the following formulas 1 and 2 among the number m of fluorescent lamps, the length h in the vertical direction of the screen, the inner peripheral length L of the bulb 12, and the depth d of the housing. Is established.

m ≦ 15.1h / (28.4d + 4L) (Formula 1)
m ≧ 15.1h / (28.4d + 19L) (Formula 2)
The depth d of the housing 20 is a distance from the bottom surface of the optical sheet 30 to the bottom surface 20b of the housing 20. The pitch p is a distance between the centers of two adjacent valves 12. The screen vertical length h is the length of the image display area in the direction perpendicular to the longitudinal direction 50 of the bulb. The technical significance of the above formulas (1) and (2) will be described later.

Next, the hot cathode fluorescent lamp 10 mounted on the backlight 100 according to the embodiment of the present invention will be described with reference to FIG. FIG. 5 schematically shows a cross-sectional configuration of the hot cathode fluorescent lamp 10 of the present embodiment.
Since the hot cathode fluorescent lamp 10 of the present embodiment is used for a backlight, a long-life lamp is used. Preferably, the hot cathode fluorescent lamp 10 is a lamp having a nominal life of 12,000 hours or more, more preferably a lamp having a nominal life of 20,000 hours or more, or 30,000 hours or more. Since the lifetime of a CRT device that has been widely used as a display is about 20000 hours, it is desirable that the lamp has a longer lifetime.

  There are two major elements in the definition of the lifetime, and one is the lamp brightness reduction rate (so-called luminance maintenance rate) and non-lighting. Assuming the use as a backlight, the life of the hot cathode fluorescent lamp is limited by the non-lighting due to the exhaustion of the thermoelectron emitting material (emitter) formed on the electrode filament. In order to estimate the lifetime, a plurality of lamps are subjected to a life test under a predetermined lighting condition (continuous lighting test at the rated current of the lamp), and are performed for a certain time (for example, 100 hours, 500 hours, 1000 hours, 2000 hours, 5000 hours). Time) The remaining amount of the emitter is measured at any time by sequentially destroying the lamp after lighting or nondestructing, and the amount of consumption (consumption rate) from the beginning is measured. Based on these results, the lifetime can be estimated by plotting the relationship between the lighting elapsed time and the amount of consumed emitter (or the remaining amount of emitter) and fitting with a linear function. The nominal life is determined in consideration of consumption variation, measurement variation, and manufacturing variation (3 times standard deviation: 3 sigma as a standard) based on the acquired data.

The illustrated hot cathode fluorescent lamp 10 is composed of a straight tubular glass bulb 12 and a pair of electrodes 11 disposed at both ends of the glass bulb 12.
The glass bulb 12 is made of soda-lime glass or barium strontium silicate (soft glass having a softening point of 675 ° C.). As an example of the dimensions of the bulb 12, for 32 inches, the bulb 12 has an outer diameter of 12 mm, a wall thickness of 0.8 mm, and a length of 730 mm. For 45 inches, the bulb 12 has an outer diameter of 12 mm, a wall thickness of 0.8 mm, and a length of 1010 mm. For 65 inches, the outer diameter of the bulb 12 is 25.5 mm, the wall thickness is 0.8 mm, and the length is 1499 mm. For 105 inches, the bulb 12 has an outer diameter of 38 mm, a wall thickness of 0.9 mm, and a length of 2367 mm. The wall thickness of the valve can be 1.0 mm.

A phosphor (not shown) is applied to the inner surface 12 a of the glass bulb 12. More specifically, a protective film made of alumina is formed on the inner surface 12a of the glass bulb 12, and a phosphor layer is laminated on the protective film. The phosphor constituting the phosphor layer emits, for example, each color of red (Y ₂ O ₃ : Eu), green (LaPO ₄ : Ce, Tb ₃ ), and blue (BaMg ₂ Al ₁₆ O ₂₇ : Eu, Mn). A mixture of rare earth phosphors can be used. As the phosphor, other rare earth phosphors can be used. For example, as red, (Y, La) ₂ O ₃ : Eu, 3.5MgO.0.5MgF ₂ .GeO ₂ : Mn, and as green, CeMgAl ₁₁ O ₁₉ : Tb, GdMgB ₂ O ₁₀ : Ce, Tb, blue (Sr, Ca) ₁₀ (PO ₄ ) ₆ C _l2 : Eu can be mentioned.

  Mercury and a rare gas are enclosed in the glass bulb 12. In this embodiment, about 5 mg of mercury (not shown) and argon (Ar) at a pressure of 500 Pa at room temperature are enclosed as a buffer rare gas in the glass bulb 12. The mercury sealed in the bulb 12 can be sealed in the form of amalgam such as zinc mercury, tin mercury, bismuth, indium mercury, etc. in addition to mercury alone.

As the rare gas, a mixture of argon (Ar) and a mixture ratio of krypton (Kr) to argon (Ar) can be used in addition to the argon (Ar) mixture ratio of 100%. The mixing ratio (partial pressure ratio) of krypton (Kr) is, for example, 20% to 60%. As an example, a mixed gas (gas pressure 600 Pa) of argon: krypton = 50%: 50% can be given.
The electrode 11 in this embodiment includes a filament 14, a pair of lead wires 13 that hold the filament 14, and a bead glass 15 that holds the pair of lead wires 13. The bead glass 15 is also referred to as a bead mount. The illustrated electrode 11 is of a so-called glass bead mount type.

  The filament 14 is made of tungsten. In this embodiment, the filament 14 has a complicated coil shape so as to increase the amount of emitter applied in order to obtain a long-life lamp. That is, a long tungsten-like structure is formed by winding a thin tungsten wire so as to loosely cover the periphery of a thick tungsten wire, and a structure in which this structure is spirally wound is called a double coil. The filament 14 is one in which the double coil is spirally wound to form a triple coil, or the triple coil is further spirally wound to form a quadruple coil. When the filament 14 is a triple coil, the triple coil is an electrode coil of 5 to 7 turns. Moreover, when the filament 14 is a quadruple coil, it is an electrode coil of 2-4 turns.

  The emitter applied to the filament 14 is, for example, an oxide of strontium, calcium, or barium. In this embodiment, in order to realize a long-life lamp, the amount of emitter applied to the filament 14 is increased. In this embodiment, one of the pair of electrodes is used for each hot cathode fluorescent lamp 10. An emitter of 5.0 mg or more is applied to one filament 14. In addition, when the composition of the rare gas is not 100% argon and krypton having an atomic weight larger than argon is mixed at a predetermined mixing ratio, the emitter is difficult to scatter from the filament 14, and the lamp life is extended in the technical sense. Can do.

  The illustrated electrode 11 is pinch-sealed at the sealing portion 16 of the glass bulb 12. An exhaust pipe 17 is sealed at at least one end of the glass bulb 12. The exhaust pipe 17 is used when exhausting the inside of the valve 12 or enclosing a rare gas, and is sealed after exhausting and enclosing. If the exhaust pipe 17 is provided at both ends of the valve 12 instead of one end, there is an advantage that gas exhausting / sealing can be performed efficiently. Thereby, the ratio of impurities inside the bulb 12 can also be reduced.

  A base 18 is provided at the end of the glass bulb 12 so as to cover the sealing portion 16 and the exhaust pipe 17. Note that the connection method between the lead wire 13 extending outward from the sealing portion 16 and the base 18 may be appropriately determined according to the specifications of the lamp 10. For example, pins for attachment to the backlight unit can be arranged on the end surfaces (left and right end surfaces of the paper surface) of the base 18, and the pins can be connected to the lead wires 13, or It is also possible to dispose the mounting pin on a part of the side surface of the base 18 (for example, a part of the cylinder on the front side of the paper surface) and connect the pin.

  The hot cathode fluorescent lamp 10 is a lamp to which low-pressure mercury vapor discharge is applied. The principle of light emission is that the electrode to which the electron-emitting substance is applied is maintained at a temperature at which only hot electrons are emitted by discharge (and another means for heating the electrode), whereby electrons are supplied and arc discharge. (This is a significant difference from the cold cathode). Of the transition spectrum of mercury atoms obtained by this discharge, the ultraviolet light of 254 nm is mainly used by converting it into visible light by using it as the excitation line of the phosphor.

  As described above, the phosphor is applied to the inner surface 12a of the glass bulb 12, but a protective film (alumina oxide, silica powder, etc.) for preventing deterioration of characteristics due to a chemical reaction between the phosphor and the glass. Is given. The filament used as an electrode is generally a tungsten double or triple coil, and an emitter, which is an electron-emitting material, is applied to the filament. Liquid mercury (or mercury amalgam, alloy) and a rare gas as a buffer are enclosed in the tube. Generally, argon is often used as the rare gas, but a mixed gas such as krypton or neon may be used depending on the structure and type of the lamp.

Further, an example of the configuration of the backlight 100 of the present embodiment will be described in detail with reference to FIGS. 6 and 7. 6 and 7 are an exploded perspective view and a cross-sectional view, respectively, showing the configuration of the backlight 100 of the present embodiment.
In the illustrated configuration, three hot cathode fluorescent lamps 10 shown in FIG. 1 are arranged. In the configuration shown in FIG. 2, five hot cathode fluorescent lamps 10 are arranged. The reflection plate 21 which is a part of the housing 20 in this example is made of a metal plate (for example, iron or aluminum made by plating), and has a thickness of 1.5 mm. The reflection plate 21 includes a main surface (or bottom surface) 20b of the housing 20 and a side surface 20c extending from the main surface.

  A reflection sheet 23 is formed on the upper surface of the reflection plate 21 (main surface 20b of the housing). The reflection sheet 23 is composed of a resin layer of polyethylene terephthalate (PET) in which white titanium oxide (or calcium carbonate) is dispersed, and the thickness thereof is 2.0 mm. In addition, the depth d (height from the upper surface of the reflecting plate 21 to the surface on which the optical sheet 30 is located) of the housing 20 is, for example, 40 to 75 mm.

  A lighting circuit (ballast circuit or ballast) 70 is disposed below the reflecting plate 21 of the backlight 100. In this example, one lighting circuit 70 is provided for each lamp 10, but one lighting circuit 70 may be provided for two lamps 10. The lighting circuit 70 is electrically connected to the lamp 10 and has a dimming function. A lower cover 72 is provided under the reflecting plate 21 so as to accommodate the lighting circuit 70. The lower cover 72 is made of a metal plate having a thickness of 1.5 mm. For example, wiring is disposed in the space between the lower cover 72 and the reflection plate 21. Note that the backlight 100 does not need to be provided with the lower cover 72, and in that case, the lighting circuit 70 may be disposed in a housing of a liquid crystal display (for example, a liquid crystal television).

  A lamp holder 75 for holding the lamp 10 is provided at the end of the reflecting plate 21. The lamp holder 75 is made of, for example, a white resin. In addition, an optical sheet 30 is disposed in the opening 20 a of the casing 20 of the backlight 100. In this example, the optical sheet 30 includes, in order from the top, a polarizing sheet 31 (DBEF (Dual Brightness Enhancement Film manufactured by Sumitomo 3M), thickness 0.440 mm), a lens sheet 32 (thickness 0.155 mm), and a diffusion sheet. 33 (thickness 0.113 mm) and a diffusion plate 34 (thickness 2.0 mm). It is also possible to provide a lens sheet on the lower surface of the diffusion plate 34.

Further, a liquid crystal display panel (for example, about 2 mm thick) 60 is disposed on the optical sheet 30, and an upper cover 62 is disposed so as to cover the liquid crystal display panel 60 and the optical sheet 30. Yes. The upper cover 62 is made of a metal plate having a thickness of 1.5 mm, for example.
Note that the backlight 100 in this example is a direct-type liquid crystal display backlight having a screen size of 20 inches or more and 42 inches or less, which satisfies the relationship of the expressions 1 to 3 described above, and has a longitudinal length h and an inner circumference. As long as the ratio (h / L) to the length L is 2 or more and 10 or less, screens with other screen sizes may be used. Further, the periphery of the sealing portion 16 of the lamp 10 is covered as a frame region in order to hide the non-lighting portion of the lamp 10, and the non-lighting portion is not visible to the outside.

Next, with reference to FIGS. 8A to 8C, the luminance distribution of the even number optical system (for example, four lines) and the odd number optical system (for example, three lines) will be described.
FIG. 8A shows the vertical direction 90 at the center of the screen. FIG. 8B shows the luminance distribution of the even number of optical systems (four) along the longitudinal direction 90 in the center of the screen shown in FIG. 8A, while FIG. The luminance distribution of odd-numbered optical systems (three) along the vertical direction 90 is shown. In FIG. 8B and FIG. 8C, the lamp 10 is clearly shown so that the correspondence of the lamp 10 (bulb 12) can be easily understood. Unlike the layout of three or more odd numbers such as the backlight 100 of the present embodiment, in the backlight of the typical example, the number of the lamps 10 is for evenly distributing the current to each lamp in the lighting device. The circuit (balancer) is an even number (preferably 2 to the nth power).

  If the total power is the same in the even number (four) optical system shown in FIG. 8B and the odd number (three) optical system shown in FIG. 8C, the odd number optical system is better. The power per one can be increased. Therefore, in the odd-numbered optical system shown in FIG. 8C, the luminance directly above the lamp 10 can be increased, and the backlight is constructed with the high luminance portion as the center of the screen.

Next, the lamp image will be described with reference to FIG. FIGS. 9A to 9C are diagrams for explaining lamp images based on the luminance distribution in the vertical direction (90) in the center of the screen, and the pitch p of the two adjacent lamps 10 is changed. In this case, the change in the luminance pattern is shown.
FIG. 9A is a configuration example in which the pitch p of the two adjacent lamps 10 is small, FIG. 9C is a configuration example in which the pitch p of the two lamps 10 is large, and FIG. Between them. The luminance unevenness at the center of the screen is an important index for the liquid crystal display because it immediately leads to image quality degradation. Here, the ratio of the peaks and valleys of brightness in the vertical direction at the center of the screen was evaluated using an indicator called a lamp image.

The lamp image is represented by a ratio (Im / Ip) of the lowest luminance (Im) between the lamps 10 to the highest luminance (Ip) between the lamps 10 as described on the right side of FIG. It should be noted that the luminance change in the direction along the longitudinal direction 50 of the lamp is at a level that hardly causes a problem, and therefore no special evaluation is performed here.
First, when the pitch p as shown in FIG. 9 (a) is small, the distance between the two lamps 10 is too short, resulting in a luminance distribution in which a large mountain exists, which is not preferable. That is, the lamp image (Im / Ip; the ratio of the brightness and the valley) is a value larger than 1.

On the other hand, as shown in FIG. 9C, when the pitch p is large, the distance between the two lamps 10 is too long, and an image of uneven brightness remains strongly. That is, the lamp image (Im / Ip) is smaller than 1, which is bad evaluation.
As shown in FIG. 9B, when the pitch p is just a good distance, the lamp image (Im / Ip) is approximately 1 (or a value close to 1), which is a good evaluation.

  FIG. 10 is a graph in which the value of (p / L) is plotted with the lamp image (min / max between lamps) [%] on the vertical axis and (d / L) on the horizontal axis. It can be seen that the smaller the p / L (p / L = 2.11), the better the lamp image result. It can also be seen that the result of the lamp holder is improved as d / L is increased, that is, as the depth d of the housing 20 is increased. FIG. 10 also shows a straight line obtained by linear approximation of the plot (regression straight line by the method of least squares).

From the result (regression line) shown in FIG. 10, the ramp image can be expressed by the following equation.
Lamp image = 104 + 28.4 (d / L)-15.1 (p / L)
Further, in the odd-numbered optical system, since the luminance between the lamps 10 is desired to be a valley, the premise is that the lamp image ≦ 100. Therefore, the expression of the lamp image is as follows.

104 + 28.4 (d / L) -15.1 (p / L) ≦ 100
28.4 (d / L) + 4 ≦ 15.4 (p / L)
p ≧ 1.9d + 4L
Here, when the lamps 10 are equally arranged in the screen vertical length h and the number of lamps m, p = h / m can be obtained, and the following formula 1 is derived.

m ≦ 15.1h / (28.4d + 4L) (Formula 1)
Next, if the allowable lower limit of the lamp image is 85%, the expression of the lamp image is as follows.
104 + 28.4 (d / L) -15.1 (p / L) ≦ 85
Further, when the equation is derived in the same manner as described above, the following equation 2 is obtained.

m ≧ 15.1h / (28.4d + 19L) (Formula 2)
If the boundary conditions of the above Formula 1 and Formula 2 are represented on a graph, it will become as shown in FIG. That is, the area sandwiched between the straight line S expressed based on Formula 1 and the straight line T expressed based on Formula 2 is an odd number of three or more fluorescent lamps, and the vertical direction 90 in the center of the screen is 90. Is a mountain range (that is, the luminance between the lamps 10 is a valley), and is a range in which luminance unevenness can be suppressed while securing screen luminance. Within this range, an acceptable backlight for a liquid crystal display can be realized.

The graph shown in FIG. 11 is based on an actual measurement value in which the horizontal axis is the display size [inch] and the vertical axis is the number of lamps, and the region sandwiched between the straight line S and the straight line T is shown in FIG. The backlight 100 with the above conditions is established, and the conditions are plotted in the graph of FIG.
In the equal arrangement of the present embodiment, the center lines of the lamps at both ends of the odd number of lamps 10 (center lines of the uppermost lamp 10 and the lowermost lamp 10 in FIGS. 1A and 2A). The distance from the wall surfaces of the casing 20 adjacent to the lamps 10 at both ends thereof is set to be approximately p / 2. This is because, when the wall surface of the housing 20 is regarded as a mirror surface, it is easy to maintain the tendency of substantially uniform arrangement as a whole. When the distance between the lamps 10 at both ends and the wall surface of the housing 20 is exactly p / 2, p = h / m as described above. However, in consideration of the fact that the wall surface of the housing 20 is not actually a mirror surface and actual dimensions or conditions (for example, an example in which the side surface 20c in FIG. It may be preferred that the lamps 10 at both ends are disposed close to the wall surface of the housing 20 so that the distance between the wall and the wall surface of the housing 20 is smaller than p / 2. In such a case, that is, in the case of an equal arrangement in which the lamps 10 at both ends are brought closer to the wall surface of the housing 20 at a distance smaller than p / 2, the straight line S expressed based on Equation 1 is the upper side in FIG. It is only meant that the region sandwiched between the straight line S and the straight line T is enlarged, and therefore the region sandwiched between the straight line S and the straight line T expressed based on Equation 1 is a suitable region. It does n’t change.

  As described above, according to the backlight 100 of the present embodiment, the number of fluorescent lamps 10 housed in the housing 20 is an odd number of three or more, and the odd number of fluorescent lamps 10 are arranged at equal intervals p. The fluorescent lamp 10 in the center is arranged at the center position (h / 2) in the vertical direction of the screen, the number m of fluorescent lamps, the vertical length h of the screen, the inner peripheral length L of the bulb cross section, and The luminance unevenness can be suppressed by establishing the relationship of the above formulas 1 and 2 between the depths d of the casing. In addition, since it is possible to realize a low number of backlights while suppressing luminance unevenness, the cost of the light source member in the backlight can be reduced.

As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible. For example, the present invention is not limited to a hot cathode fluorescent lamp but can be applied to other lamps (cold cathode fluorescent lamp).
Further, the “substantially oval” that is the shape of the valve 12 of the present embodiment shown in FIG. 4 is a circular shape in which the valve 12 that is generally manufactured with a circular cross section is displaced due to an error (tolerance) in the manufacturing process. However, what is referred to as “circular” is not intended to include a geometrically non-circular shape, but is displaced by an error (tolerance) in the manufacturing process. The substantially elliptical valve may be manufactured from a circular valve by increasing the flattening ratio using a typical manufacturing process. In the substantially elliptical lamp in the present embodiment, for example, the value of the major axis L1 / minor axis L2 is 1.6, but typically, the one in the range of 1.2 ≦ (L1 / L2) ≦ 1.8. Can be used.

  In order to produce the substantially elliptical bulb 12 in the present embodiment, the following may be performed. First, if a bulb with a circular cross section (glass bulb) is prepared, the bulb is heated and placed between approximately elliptical hollow molds (molds), and the bulb is sandwiched and deformed by the mold, it is approximately elliptical. Can be obtained. In addition, what is necessary is just to form the alumina and fluorescent substance apply | coated to the inner surface of the bulb | ball 12 in a suitable stage suitably. Alternatively, a substantially elliptic bulb 12 can be manufactured by producing a circular lamp and then applying heat to the lamp to soften and press the glass of the lamp.

  According to the present invention, it is possible to provide a backlight using an odd number of light sources in which luminance unevenness is suppressed.

(A) is a top view which shows typically the backlight 100 which concerns on embodiment of this invention, (b) and (c) are sectional drawings corresponding to (a). (A) is a top view which shows typically the backlight 100 which concerns on embodiment of this invention, (b) and (c) are sectional drawings corresponding to (a). (A) is a figure which shows typically the cross section of the bulb | bulb of the hot cathode fluorescent lamp 10, (b) is a figure showing the length L of the bulb | ball inner periphery. Diagram showing a cross section of a substantially oval valve Sectional drawing which shows the structure of the hot cathode fluorescent lamp 10 which concerns on embodiment of this invention. 1 is an exploded perspective view for explaining a configuration of a backlight 100 according to an embodiment of the present invention. Sectional drawing which shows the structure of the backlight 100 which concerns on embodiment of this invention. (A) to (c) are diagrams for explaining the luminance distribution of the even numbered optical system and the odd numbered optical system. (A)-(c) is a figure for demonstrating the lamp image based on the luminance distribution of the vertical direction of the screen center. Graph of (p / L) where the vertical axis is the lamp image [%] and the horizontal axis is (d / L) The graph showing the straight line S and straight line T which prescribe | regulate the boundary condition in embodiment of this invention The figure showing the conditions of the backlight in embodiment of this invention

Explanation of symbols

10 Fluorescent lamp (hot cathode fluorescent lamp)
DESCRIPTION OF SYMBOLS 11 Electrode 12 Glass bulb 13 Lead wire 14 Filament 15 Bead glass 16 Sealing part 17 Exhaust pipe 18 Base 19 Phosphor layer 20 Case 21 Reflector plate 23 Reflective sheet 30 Optical sheet 31 Polarizing sheet 32 Lens sheet 33 Diffusing sheet 34 Diffusing plate 40 Screen direction 60 Liquid crystal display panel 62 Upper cover 70 Lighting circuit 72 Lower cover 75 Lamp holder 100 Backlight (backlight for direct liquid crystal display)

Claims (5)

A backlight for a direct type liquid crystal display, comprising a fluorescent lamp and a housing for storing the fluorescent lamp,
The number of the fluorescent lamps housed in the housing is an odd number of 3 or more,
The fluorescent lamp is composed of a substantially linear bulb and a phosphor layer formed on the inner periphery of the bulb,
Between the number m of the fluorescent lamps, the length h in the vertical direction of the screen, the length L of the inner circumference in the cross section perpendicular to the longitudinal direction of the bulb, and the depth d of the casing, The relationship of 2 is established,
m ≦ 15.1h / (28.4d + 4L) (Formula 1)
m ≧ 15.1h / (28.4d + 19L) (Formula 2)
Here, the odd number of fluorescent lamps are arranged at equal intervals, and
Of the odd number, the center fluorescent lamp is arranged at a position of h / 2 which is a center position in the vertical direction of the screen. The fluorescent lamp is a hot cathode fluorescent lamp,
The backlight according to claim 1, wherein the hot cathode fluorescent lamp has a filament that emits thermoelectrons in the bulb.   The backlight according to claim 1, wherein a cross section of the bulb is circular.   The backlight according to claim 1, wherein a cross section of the bulb is substantially elliptical.   The backlight according to any one of claims 1 to 4, wherein the backlight is a direct liquid crystal display backlight having a screen size of 32 inches to 65 inches.

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