JP2005251437A - Backlight unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a backlight unit not generating unevenness of brightness on the whole part of a light emitting surface, especially in upper and lower direction in the case of increasing the size and the brightness. <P>SOLUTION: The direct backlight is formed by arranging fluorescent lamps L1 to L16 in vertical direction with a prescribed distance in an envelope 20. In the case that more than two pieces of fluorescent lamps adjacent to each other including the fluorescent lamp L16 at the uppermost step is designated as a first group, and more than two pieces of fluorescent lamps at the part lower than the first group, including the lamp L6 at the lowermost step of the first group is designated as a second group, the distance between adjacent fluorescent lamps of the first group is made narrower than that of the second group. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a backlight unit, and more particularly to a direct backlight unit incorporating a plurality of fluorescent lamps.

The backlight unit is mainly attached to the back surface of the liquid crystal panel and used as a light source of the liquid crystal display device.
FIG. 7 is a diagram showing a configuration of a conventional backlight unit disclosed in, for example, Patent Document 1, in which a part of a diffusion plate, a diffusion sheet, or the like is cut out so that the internal structure can be easily understood. Shown in a missing state.

In the backlight unit 200, eight straight tubular cold cathode fluorescent lamps 110 extending in the horizontal direction are arranged in parallel at equal intervals in the vertical direction in an envelope 120 having a bottom plate 122 and a side plate 124. A diffusion plate 140 is attached so as to cover the front surface of the envelope 120, and has a sealed structure in which foreign matters such as dust do not enter the envelope 120. A diffusion sheet 142 and a lens sheet 144 are further laminated on the surface of the diffusion plate 140, so that the light from each cold cathode fluorescent lamp 110 is on the front surface of the backlight unit 200 (hereinafter referred to as “light emitting surface”). It is comprised so that the front may be irradiated uniformly over the whole.
JP 2002-116704 A

However, along with the recent demand for larger liquid crystal display devices and higher image quality, there is a strong demand for larger backlight sizes and higher brightness.
Accordingly, the present inventors have increased the size of the envelope 120 and provided a backlight unit in which a larger number of cold cathode fluorescent lamps than those described in FIG. 7 are arranged at equal intervals in the envelope. It was manufactured and installed so that the front surface of the backlight unit was in a substantially vertical direction, similar to the conditions used for the backlight of an actual liquid crystal panel.

Then, it became clear that the brightness | luminance fell, so that it went to the upper part of the front surface of the said large sized backlight unit. If such a backlight unit with nonuniform luminance on the light emitting surface is attached to a liquid crystal panel, the display screen of the panel becomes difficult to see, and the purpose of improving image quality cannot be achieved.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a backlight unit that does not cause uneven brightness even on the entire light emitting surface, particularly in the vertical direction, even when the size is increased and the brightness is increased. And

  In order to achieve the above object, a backlight unit according to the present invention is a direct type backlight unit in which a plurality of three or more fluorescent lamps are arranged in parallel at predetermined intervals in a vertical direction. A light unit, wherein two or more adjacent fluorescent lamps including the uppermost fluorescent lamp among the plurality of fluorescent lamps are set as a first group, and the lowermost fluorescent lamp of the first group is included. When two or more adjacent fluorescent lamps below the first group are used as the second group, the interval between the adjacent fluorescent lamps in the first group is the adjacent fluorescent lamp in the second group. It is characterized by being narrower than the interval between lamps.

The interval between the adjacent fluorescent lamps in the first group is gradually narrowed toward the topmost fluorescent lamp.
Furthermore, the interval between the adjacent fluorescent lamps in the first group becomes narrower in an arithmetic progression as it approaches the uppermost fluorescent lamp.
Furthermore, among the plurality of fluorescent lamps, when the fluorescent lamp having the highest luminous efficiency is in the Nth stage from the lowest stage to the uppermost stage of the plurality of fluorescent lamps, The first group includes fluorescent lamps from the Nth stage to the uppermost stage.

  In the backlight unit according to the present invention, two or more fluorescent lamps are arranged in parallel in the envelope at predetermined intervals in the vertical direction, and a current for supplying current to the fluorescent lamps is provided. A direct-type backlight unit having a supply means, wherein a predetermined number of fluorescent lamps including the uppermost fluorescent lamp among the plurality of fluorescent lamps is defined as a first group, and is lower than the first group. When the predetermined number of fluorescent lamps is the second group, the current value supplied to each fluorescent lamp in the first group is compared with the current value supplied to each fluorescent lamp in the second group. It is characterized by being large.

Further, the current value supplied to each fluorescent lamp in the first group gradually increases as it approaches the uppermost fluorescent lamp.
Furthermore, the current value supplied to each fluorescent lamp in the first group increases in an arithmetic progression as it approaches the uppermost fluorescent lamp.
Further, among the plurality of fluorescent lamps, when the fluorescent lamp having the maximum luminous efficiency is in the Nth stage from the lowest stage to the uppermost stage of the plurality of fluorescent lamps, The first group includes fluorescent lamps from the (N + 1) th stage to the uppermost stage.

  The backlight unit according to the present invention includes two or more adjacent lamps including the uppermost fluorescent lamp among the plurality of fluorescent lamps arranged in the envelope. When two or more adjacent fluorescent lamps below the first group are included in the second group including the lowermost fluorescent lamp, the interval between the adjacent fluorescent lamps in the first group is the second It is narrower than the interval between adjacent fluorescent lamps in the group. For this reason, it is possible to compensate for the decrease in luminous efficiency of individual lamps in the upper part of the envelope, and to suppress the decrease in brightness in the upper front part of the backlight unit. When the backlight unit is increased in size and brightness However, it is possible to suppress a change in luminance in the vertical direction.

Further, according to the backlight unit of the present invention, the predetermined number of fluorescent lamps including the uppermost fluorescent lamp in the envelope are set as the first group, and the predetermined number of fluorescent lamps below the first group are set. When the lamp is a second group, the current value supplied to each fluorescent lamp in the first group is larger than the current value supplied to each fluorescent lamp in the second group. This also compensates for the decrease in luminous efficiency of the lamps in the upper part of the envelope, makes it possible to make the luminous flux radiated from each lamp substantially equal, and suppresses the decrease in luminance in the upper front part of the backlight unit. it can. For this reason, even when the backlight unit is increased in size and brightness, it is possible to suppress a change in brightness in the vertical direction.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 is a schematic perspective view showing a configuration of a backlight unit 100 for a liquid crystal television of 32 inches (aspect ratio 16: 9) according to the present embodiment. In the figure, a part of the diffusion plate 40, the diffusion sheet 42, and the lens sheet 44 are notched so that the internal structure can be easily understood. Further, the cold cathode fluorescent lamp itself is shaded so that the interval between adjacent cold cathode fluorescent lamps can be easily understood, and the mounting structures at both ends thereof are omitted.

  As shown in the figure, the direct-type backlight unit 100 includes 16 cold cathode fluorescent lamps L1 to L16 arranged in parallel at a predetermined interval inside the envelope 20, and the envelope 20 The front opening is covered with a translucent front panel 45 formed by laminating a diffusion plate 40, a diffusion sheet 42, and a lens sheet 44, and foreign matter such as dust and dust enters the lamp L. Etc. are sealed to prevent damage.

The envelope 20 is made of, for example, polyethylene terephthalate (PET) resin, and a reflective surface is formed by depositing a metal such as silver on the inner surface (inside the bottom plate 22 and the side plate 24). Further, the inner dimensions of the envelope 20 are set to be 728 mm in the horizontal direction (X-axis direction), 408 mm in the vertical direction (Y-axis direction), and the depth of the outer shape is 19 mm.
The reflecting surface of the bottom plate 22 is separated from the inner surface of the front panel 45 by about 14 mm, and 16 cold cathode fluorescent lamps L1 to L16 (hereinafter referred to as cold cathode fluorescent lamps) extending in the horizontal direction are positioned in the vicinity of the bottom plate 22. These are simply referred to as “lamps”, which are referred to as “lamps L” when calling unspecified lamps among the 16 lamps) and are arranged in parallel at predetermined intervals in the vertical direction. The distance between the lamps L is gradually narrower from the predetermined number of stages, so that the luminance at the upper part of the light emitting surface is not lowered below the luminance at the central part or the lower part. Details will be described later.

Further, the diffusion plate 40 and the diffusion sheet 42 in the front panel 45 are for scattering and diffusing the light emitted from the lamp L, and the lens sheet 44 is for collecting the light in a direction normal to the sheet 44. Thus, the light emitted from the lamp L is irradiated in the forward direction in a state where it is made as uniform as possible over the entire irradiation surface.
FIG. 2 is a cross-sectional view showing the configuration of the lamp L. As shown in FIG. The straight tubular arc tube 10 is made of, for example, hard borosilicate glass, and has a tube outer diameter of 4.0 mm, a tube inner diameter of 3.4 mm, and a total length of the tube of about 705 mm. Cylindrical hollow electrodes 4 a and 4 b made of nickel are disposed inside both ends of the arc tube 10, and electrode lead wires 2 a and 2 b connected thereto are sealed at both ends of the arc tube 10. As a result, it is held at a predetermined position.

In the hermetically sealed arc tube 10, about 1500 μg of mercury (not shown) and neon / argon mixed gas (Ne 95% -Ar 5%) having a cooling pressure of 10 kPa are enclosed. In addition, on the inner peripheral surface of the arc tube 10, three types of red (Y ₂ O ₃ : Eu), green (LaPO ₄ : Ce ₃ , Tb ₃ ) and blue (BaMg ₂ Al ₁₆ O ₂₇ : Eu ² , Mn) are used. ) A rare earth phosphor 6 mixed with a luminescent phosphor is applied.

When a predetermined voltage is applied to the hollow electrodes 4a and 4b via the electrode lead wires 2a and 2b, a discharge occurs in the arc tube 10, and ultraviolet rays are emitted from the excited mercury atoms. This ultraviolet light is converted into visible light by the phosphor 6.
FIG. 3 is a block diagram showing the cold cathode fluorescent lamp L and its lighting circuit 50. As shown in the figure, the lighting circuit 50 includes an inverter unit 52, a control unit 54, and a lamp current detection unit 56, and a DC voltage is supplied to the lighting circuit from a power supply device (not shown).

  The inverter unit 52 generates a predetermined high-frequency AC voltage and applies it to the lamp L, thereby lighting the lamp L. The lamp current detection unit 56 detects the value of the current flowing through the lamp L and sends the detection signal to the control unit. The control unit 54 controls the current value to a predetermined value by changing the duty ratio of the PWM signal. One lighting circuit 50 is provided for each lamp, and the lamp current of each of the lamps L1 to L16 (see FIG. 1) is kept constant.

FIG. 4 is an enlarged perspective view for showing a mounting portion of the left end portion of the lamps L1 to L4 of the envelope 20 of FIG. In the figure, the side plate 24, the front panel 45, and the like are removed so that the holding structure of the lamps L1 to L4 can be easily understood.
As shown in FIG. 4, insulating rubber 60 is fitted into the ends of the four lamps L1 to L4, respectively, and lead wires 2a (see FIG. 2) led out from the ends of the lamps L1 to L4. The printed circuit board 62 is fixed with solder or the like. Both ends of the printed circuit board 62 are fixed by being fitted into grooves of U-shaped support members 66 a and 66 b fixed to the bottom plate 22 so as to face each other.

A member denoted by reference numeral 64 is a connector, and is electrically connected to a lead wire of each lamp L via a wiring pattern (not shown) formed on the printed circuit board 62, and power from an external lighting circuit is It is supplied to each lamp L via a connector 64.
In particular, although not shown, the connecting portions at the other ends of the lamps L1 to L4 are configured in the same manner. Similarly, the other lamps L5 to L16 are connected to another printed circuit board four by four. It is held in the envelope 20.

As described above, in this embodiment, the interval between adjacent lamps is narrower in the upper part of the envelope 20 than in the lower part of the envelope 20. This is due to the following reason.
Initially, the inventors set the distance between 16 adjacent lamps to be equal to 25.7 mm (the distance between the uppermost lamp of the envelope 20 and the distance between the lowermost lamp and the side plate 24 is approximately 11 mm, respectively). Made the backlight unit. This is because, in order to make the luminance of the light emitting surface uniform, it is naturally considered that the plurality of lamps L should be evenly arranged.

However, it has been found that if the lighting is continued for a while, the luminance at the upper part of the light emitting surface is lower than the luminance at the central part and the lower part.
FIG. 5 is a graph showing the relationship between the position of the front panel 45 surface (lens sheet 44 surface) in the vertical direction and the luminance on the surface. The backlight unit having the above-described conventional configuration was turned on at a lighting frequency of 58 kHz, a lamp current of 5.5 mA, and room temperature (25 ° C.), and the luminance of each part was measured 60 minutes after the lighting. As a result. The brightness value in this experiment was measured by changing the position in the vertical direction at the center in the horizontal direction on the surface of the front panel 45.

  As shown by the broken line in the figure, the luminance slightly increases as the vertical position Y increases from the position of 0 mm, and the vicinity of the position where the lamp L6 (vertical position Y is 11 + 25.7 × 5 = about 140 mm) is arranged. In FIG. 4, the luminance is maximum, and the luminance is greatly decreased as the vertical position Y is increased. In the vicinity of the upper end portion of the front panel 45, the luminance was reduced by about 10% compared to the maximum luminance, and it was confirmed that the luminance change could be clearly confirmed with the naked eye in the vertical direction.

In order to investigate this cause, the temperature of the lamp at each vertical position was measured and found to be 58 ° C., 65 ° C., and 85 ° C. at the positions of the lamps L2, L6, and L15, respectively.
In general, it is known that the luminous efficiency of a cold cathode fluorescent lamp varies depending on the mercury vapor pressure in the arc tube, and this mercury vapor pressure depends on the coldest spot temperature on the inner wall of the arc tube.
The atmosphere in the envelope in which the cold cathode fluorescent lamps are arranged increases in temperature as it moves from the bottom to the top in the envelope due to thermal convection. As a result, the coldest spot temperature of each lamp decreases. It goes higher as you move up. And as it moves on the envelope, the coldest spot temperature rises and deviates from the optimum range, so that the luminous efficiency decreases.

  In particular, in lamps having a tube inner diameter of 1.8 mm to 3.4 mm, it is known that the coldest spot is around 65 ° C. and the luminous efficiency of the lamp is the best (“Latest trend of fluorescent lamps for backlights” “Takagi Masami, 2002.7 Light Bulb Industry Bulletin No. 449, page 40) In the above-described conventional configuration, the brightness greatly decreased as the vertical position of the front panel 45 increased. This is because the coldest spot temperature exceeds 65 ° C. and the luminous efficiency of the lamp decreases.

In the conventional backlight unit for small liquid crystal panels, the number of lamps is small and the lamp power of each lamp is also small. Therefore, the temperature distribution in the envelope is not so different between the upper part and the lower part, and the above problems are obvious. It was not converted.
In order to eliminate the temperature difference between the upper and lower sides, it may be sufficient to provide a ventilation hole or the like in the upper part of the envelope 20, but it becomes impossible to form a reflective surface only in that part, which may cause uneven brightness. As described above, since the envelope 20 needs to be kept sealed so that dust does not enter the inside of the envelope 20, such a method is difficult to select.

Therefore, in the present embodiment, the distance between the central axes (tube axes) of adjacent lamps L (hereinafter simply referred to as “lamp interval”) is smaller than that of the lower part of the envelope 20. The upper part of the vessel 20 is made narrower so that the upper and lower luminances are uniform.
Specific values of the lamp interval are as shown in Table 1 below. The distance between the lamp L1 arranged at the lowermost stage of the envelope 20 and the side plate 24 and the distance between the lamp L16 arranged at the uppermost stage and the side plate 24 are about 11 mm.

同表に示すように、最上段のＬ１６から発光光束が最大のＬ６までの隣り合うランプ間隔は、Ｌ１からＬ６までの隣り合うランプ間隔に比べて狭く、Ｌ１６に近付くに連れて次第に狭くなっている。すなわち、外囲器２０の上部に移るにつれて、前面パネル４５表面の単位垂直方向長さ当たりのランプの本数が次第に多くなるので、外囲器２０の上部に移るに伴う各ランプの発光効率の低下を埋め合わせることができ、垂直方向での輝度変化を抑制することが可能となる。

As shown in the table, the adjacent lamp interval from the uppermost L16 to the maximum luminous flux L6 is narrower than the adjacent lamp interval from L1 to L6, and gradually becomes narrower as it approaches L16. Yes. In other words, the number of lamps per unit vertical length on the surface of the front panel 45 gradually increases as it moves to the upper part of the envelope 20, so that the luminous efficiency of each lamp decreases as it moves to the upper part of the envelope 20. Therefore, it is possible to suppress the luminance change in the vertical direction.

In this embodiment, the distance between the n-th lamp and an (n + 1) th ramp When P _n, as from P ₆ to P ₁₅ for narrowing the arithmetic sequence manner apart by 0.3 (mm) from the bottom (That is, P _n = P ₆ −ΔP (n−6), where ΔP is a tolerance of 0.3 mm and n is an integer from 6 to 15). In the conventional configuration, when the position at which L6 is arranged is a peak, it decreases as it moves to the upper part of the envelope 20, and if the luminance reduction rate is approximated by a straight line M, the luminance is almost linear (that is, It can be considered that it is lowered (like an arithmetic progression) (see the broken line in FIG. 5). For this reason, as in the present embodiment, if the interval between the lamps L6 to L16 is narrowed in an arithmetic progression as it approaches L16, a change in luminance in the vertical direction can be effectively suppressed. .

  In addition, in a short period at the beginning of lighting (for example, about 180 seconds), there is not much difference in the internal temperature distribution, so the luminous efficiencies of the lamps L are substantially equal. Although it should be slightly higher than the brightness, as described above, the lamp interval is narrowed to an arithmetic progression, so the change in luminance is considered to gradually change in an almost arithmetic progression, and is conspicuous to the naked eye. There is also an advantage that it is difficult to recognize the brightness change and does not give a sense of incongruity.

Actually, when the luminance of the surface of the front panel 45 of the backlight unit 100 according to the present embodiment was examined by the same method as described above, good results as shown by the solid line in FIG. 5 were obtained.
As shown by the solid line graph in the figure, in the present embodiment, the luminance is maximum in the vicinity where the lamp L6 (vertical direction position Y is 11 + 26.8 × 5 = about 145 mm) is arranged, and the front panel 45 Although the luminance slightly decreases as the vertical position Y increases, the difference between the maximum luminance and the minimum luminance is only 2.5%, the luminance distribution is substantially uniform in the vertical direction, and an average value of 7750 [cd / m ² ] and high luminance surface emission could be obtained.

In the present embodiment, among the lamps L1 to L16 that are lit at room temperature (25 ° C.), the coldest point of the lamp L6 is about 65 ° C., which is the optimum. , The ramp interval from L6 to L16 is made narrower than the ramp interval from L1 to L6.
Since the coldest spot temperature of the lamp above L6 deviates from the optimum value and the luminous efficiency is lowered, if the lamp interval is narrowed with L6 as the boundary, the brightness at the position above L6 on the surface of the front panel 45 decreases. Can be suppressed.

Although the present embodiment has been described using an example in which the shape of the lamp is a straight tube, the shape of the lamp may be, for example, U-shaped.
FIG. 6 is a plan view of the envelope 20 showing a schematic configuration of such a modification. Near the bottom plate 22 of the envelope 20 (the size is 728 mm × 408 mm as in FIG. 1), eight U-shaped lamps L1 to L8 are arranged in parallel in the horizontal lighting direction. . In the U-shaped lamp, the number of lamps that cross the envelope 20 in the horizontal direction is 16, but the number of lamps is eight because the number of lamps has the same discharge path length.

The dimensions of the U-shaped lamps L1 to L8 are, for example, a tube outer diameter of 4.0 mm, a tube inner diameter of 3.0 mm, a length in the longitudinal direction of 705 mm, and a total length in an unbent state of 1435 mm. is there.
In this modified example, when the lamp is lit at room temperature, the coldest spot temperature of the lamp L3 is the optimum temperature. With this L3 as a boundary, the lamp interval above the L3 is narrowed in an arithmetic progression. (In other words, the magnitude relationship between the lamp intervals is P ₁ = P ₂ > P ₃ > P ₄ > P ₅ > P ₆ > P ₇ ).

Also in this modification, the same effect as described above can be obtained.
(Second Embodiment)
This embodiment is basically the same as the first embodiment except that the lamp interval, the current value supplied to the lamp, and the like are different. Therefore, description of common parts is omitted, and different parts will be mainly described.

  In the present embodiment, the value of the current supplied to the lamps L <b> 1 to L <b> 16 is increased in the upper part of the envelope 20 compared to the lower part of the envelope 20. Specific current values of the lamps L1 to L16 are as shown in Table 2 below. The lamp intervals of the lamps L1 to L16 are equal (25.3 mm), and are set at the same positions as in the conventional configuration.

同表に示すように、最上段のＬ１６からＬ７までのランプに供給される電流値は、Ｌ１からＬ６までのランプに供給される電流値より大きく、Ｌ１６に近付くに連れて次第に大きくしている。なお、電流値は、例えば前述のように、それぞれのランプＬの点灯回路５０（図３参照）の制御部５４におけるＰＷＭ信号のデューティー比等を設定することによって所望の値に制御されている。

As shown in the table, the current value supplied to the uppermost lamps L16 to L7 is larger than the current value supplied to the lamps L1 to L6, and gradually increases as it approaches L16. . For example, as described above, the current value is controlled to a desired value by setting the duty ratio of the PWM signal in the control unit 54 of the lighting circuit 50 (see FIG. 3) of each lamp L.

  In the present embodiment, the coldest spot temperature of the lamp L6 is about 65 ° C. (see FIG. 5), so the lamp current value is increased with this lamp L6 as a boundary. For this reason, it is possible to make the luminous flux radiated from one lamp substantially equal by making up for the decrease in the luminous efficiency of the lamp above L6, so that it is possible to suppress the occurrence of a luminance change in the vertical direction. Become.

Note that the current value and the luminous flux of the lamp are in a substantially proportional relationship in a narrow range where the current value is about 5.30 mA to 6.00 mA.
Further, when the lamp current supplied to the n-th light and I _n, is from I ₇ to I ₁₆ are increasing the current value by arithmetic sequence manner 0.07mA [I _n = I ₇ +0.07 (N-7), where ΔI is a tolerance of 0.07 mA and n is an integer from 7 to 16.

Thus, by increasing the lamp current in an arithmetic progression, the same effect as in the first embodiment can be obtained.
The measurement result for examining the luminance change in the vertical direction on the surface of the front panel 45 of the backlight unit 100 according to the present embodiment is as shown by the one-dot chain line in FIG. In this measurement, the lamps L1 to L16 were turned on at a lighting frequency of 58 kHz and a predetermined lamp current shown in Table 2 and turned on at room temperature (25 ° C.), and the luminance was measured on the surface of the front panel 45 60 minutes after the lighting. .

As shown in the one-dot chain line graph in the figure, also in this embodiment, the difference between the maximum luminance and the minimum luminance is small, a substantially uniform luminance distribution is obtained in the vertical direction, and the average value of luminance is also 7700 [ cd / m ² ] and high brightness.
Note that this embodiment can also be applied to a U-shaped lamp in the same manner as described in the first embodiment.

(Other)
1. In each of the above-described embodiments, it has been described that the lamp interval or the lamp current is changed at the room temperature of 25 ° C. with the lamp having the coldest spot temperature of 65 ° C. as a boundary. However, the luminous efficiency of this lamp is maximized. The optimum cold spot temperature is not limited to this value, and may vary depending on various conditions such as the inner diameter of the lamp tube, the amount of rare gas to be sealed, and the ambient temperature. The optimum cold spot temperature of the lamp is determined by experimentation based on the above conditions, and the lamp interval can be set by considering the optimum cold spot temperature and the temperature distribution in the envelope. In addition, the change in the lamp interval or the lamp current is not limited to the above-mentioned change in the arithmetic progression.

  In the first embodiment, the upper lamp interval from the lamp L6 having the highest luminous efficiency is made narrower than the lower lamp interval. However, a plurality of lamps including at least the uppermost stage are used. If the lamp interval of the lamp is narrower than the lamp interval of the lamp below it, the effect of uniforming the luminance distribution can be obtained to some extent as compared with the prior art. The same applies to the second embodiment.

  Further, the number of lamps arranged in the envelope is not limited to the one described in each of the above-described embodiments. In the first embodiment, if there are at least three lamps, the uppermost stage is provided. The present invention can be applied by making the distance between the lamp and the lamp located at the lower stage narrower than the distance between the lamp and the lowermost lamp. In the second embodiment, if there are two lamps, the present invention can be applied by increasing the current value supplied to the uppermost stage as compared with the current value supplied to the lowermost stage. It can be applied to a backlight unit having a lamp.

  2. In each of the above-described embodiments, the backlight unit 100 has been described with respect to a case where the vertical axis (Y axis) of the light emitting surface is used in a posture that substantially matches the vertical axis that is the direction of gravity. The present invention is not limited to the one used in such a posture. That is, even if the vertical axis of the light emitting surface is used with a slight inclination (for example, about 30 degrees) with respect to the vertical axis, the temperature rise in the upper part of the envelope 20 due to thermal convection is the same. Therefore, the present invention can be applied.

3. In each of the above-described embodiments, an example in which a cold cathode fluorescent lamp is used as a fluorescent lamp has been described. However, in a backlight unit that uses another fluorescent lamp whose luminous efficiency decreases as the coldest spot temperature increases. Is also applicable.
4). The present invention is preferably applied to a large backlight unit in which the temperature difference between the upper part and the lower part in the envelope is large. Specifically, it is preferable to apply to a large liquid crystal display device of 27 inches or more. The upper limit of the screen size of the liquid crystal display device is appropriately determined to a value that is practically used (for example, 54 inches).

  Further, in a backlight unit applied to a horizontally long liquid crystal display device (for example, an aspect ratio of 16: 9), the luminance of one lamp L is deteriorated on the display screen because the light emitting surface is long horizontally. Since the influence exerted is large, the effect obtained by the present invention is also increased.

  The backlight unit according to the present invention has a large size and high luminance, and can suppress a change in luminance in the vertical direction of the light emitting surface, and thus can be applied to a liquid crystal display device such as a liquid crystal television.

FIG. 2 is a schematic diagram showing a configuration of a backlight unit 100. 2 is a cross-sectional view showing a configuration of a cold cathode fluorescent lamp L. FIG. 2 is a block diagram showing a cold cathode fluorescent lamp L and its lighting circuit 50. FIG. It is the perspective view which expanded partially the edge part vicinity of L1-L4 of the envelope 20 of FIG. It is a graph which shows the relationship between the position of the perpendicular direction of a lens sheet, and the brightness | luminance on the said sheet | seat surface. It is a figure which shows the modification which concerns on 1st Embodiment. It is the schematic which shows the structure of the conventional backlight unit.

Explanation of symbols

20 Envelope 50 Lighting circuit 100 Backlight unit L Cold cathode fluorescent lamp

Claims (8)

A direct-type backlight unit in which a plurality of three or more fluorescent lamps are arranged in parallel at a predetermined interval in the vertical direction in the envelope,
Among the plurality of fluorescent lamps, two or more adjacent fluorescent lamps including the uppermost fluorescent lamp are set as a first group,
When including the lowermost fluorescent lamp of the first group and two or more adjacent fluorescent lamps below the first group as the second group,
The interval between adjacent fluorescent lamps in the first group is:
The backlight unit characterized in that it is narrower than the interval between adjacent fluorescent lamps in the second group.   2. The backlight unit according to claim 1, wherein an interval between adjacent fluorescent lamps in the first group is gradually narrowed toward the uppermost fluorescent lamp. 3.   3. The backlight unit according to claim 1, wherein an interval between adjacent fluorescent lamps in the first group becomes narrower in an arithmetic progression as the uppermost fluorescent lamp is approached. 4. Among the plurality of fluorescent lamps, when the fluorescent lamp having the highest luminous efficiency is in the Nth stage from the lowest stage to the uppermost stage of the plurality of fluorescent lamps,
The backlight unit according to any one of claims 1 to 3, wherein the first group includes fluorescent lamps from the Nth stage to the uppermost stage. A direct-type backlight having a current supply means for supplying current to the fluorescent lamp, in which two or more fluorescent lamps are arranged in parallel at predetermined intervals in the vertical direction in the envelope. A unit,
When a predetermined number of fluorescent lamps including the uppermost fluorescent lamp among the plurality of fluorescent lamps is a first group, and a predetermined number of fluorescent lamps below the first group is a second group. ,
The current value supplied to each fluorescent lamp in the first group is:
The backlight unit characterized in that it is larger than a current value supplied to each fluorescent lamp in the second group.   6. The backlight unit according to claim 5, wherein a current value supplied to each fluorescent lamp in the first group gradually increases as it approaches the uppermost fluorescent lamp. 7.   7. The backlight according to claim 5, wherein a current value supplied to each fluorescent lamp in the first group increases in an arithmetic progression as it approaches the uppermost fluorescent lamp. unit. Among the plurality of fluorescent lamps, when the fluorescent lamp having the highest luminous efficiency is in the Nth stage from the lowest stage to the uppermost stage of the plurality of fluorescent lamps,
The backlight unit according to any one of claims 5 to 7, wherein the first group includes fluorescent lamps from the (N + 1) th stage to the uppermost stage.

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