JP4532346B2 - Surface light source device - Google Patents

Description

  The present invention relates to a surface light source device suitably used for an electric signboard, a lighting fixture, a display backlight, a lighting device, and the like.

  In recent years, many displays using liquid crystals have been used as display devices for personal computers, televisions, mobile phones and the like. Since these liquid crystal displays themselves are not light emitters, they can be displayed by irradiating light from the back side using a backlight. In general, the backlight has a structure of a surface light source called a side light type or a direct type in order to meet the requirement that the entire screen should be irradiated uniformly, not just irradiating light.

  Also, as a lighting device for a liquid crystal screen, a so-called edge light system using a cold cathode ray tube provided at the edge of a light guide plate as an illumination light source is widely used (see Patent Document 1). In this type of illumination method, in order to use light more efficiently, a reflector is provided around the cold cathode ray tube, and the light guide plate is used for the purpose of efficiently reflecting the light diffused from the light guide plate to the liquid crystal screen side. A reflector is provided underneath. Thereby, the loss of light from the cold cathode ray tube is reduced, and the function of brightening the liquid crystal screen is provided.

  Furthermore, for large screens such as liquid crystal televisions, since the edge light method cannot be expected to increase the screen brightness, the direct light method has been adopted. In this method, a cold cathode ray tube is provided in parallel at the lower part of the liquid crystal screen, and the cold cathode ray tubes are arranged in parallel on the reflector. As the reflecting plate, a flat plate or a cold cathode ray tube formed into a semicircular concave shape is used (see Patent Document 2).

  Since a liquid crystal display is required to have high brightness and space saving, a reflector and a reflector used for a surface light source for a liquid crystal screen are required to have a high reflection function. Conventionally, as the reflector or reflector, a film added with a white dye or a white pigment or a film containing fine bubbles inside, or a film obtained by laminating these films with a metal plate, a plastic plate, etc. Have been used. In particular, a polyester film containing fine bubbles inside is widely used because it is excellent in luminance improvement effect and uniformity. Such a film containing fine bubbles inside is disclosed in Patent Document 3, Patent Document 4, and the like.

  However, there is a limit to improving the brightness of the liquid crystal display with the reflector. Therefore, for the purpose of improving the luminance, it is conceivable to increase the output of the illumination light source. In addition, as a higher-definition liquid crystal screen image is required, improvements have been made to increase the brightness of the liquid crystal screen to make the image clearer and easier to view. For example, an illumination light source (for example, a cold cathode ray tube: CCFL) has also become higher brightness and higher output. However, by increasing the output of the CCFL, the luminance unevenness directly above the CCFL and between the CCFLs is increased, and thus improvement of the luminance unevenness is demanded.

  On the other hand, the liquid crystal display is required to save space, and for this reason, the backlight portion is also becoming thinner. However, since the liquid crystal panel and the CCFL approach each other by making the backlight portion thinner, uneven brightness occurs on the screen. Therefore, various optical films are installed for the purpose of eliminating the uneven brightness. For example, in order to increase the in-plane uniformity and obtain a high-quality image, a light diffusing film is installed on the light guide plate to make the light uniform. As an example of such a light diffusing film, there is a diffusion sheet obtained by forming a transparent thermoplastic resin into a sheet shape and then subjecting the surface to physical unevenness (see Patent Document 5).

  However, the diffusion effect of the light diffusing film depends on the degree of unevenness on the surface. Moreover, the damage of the film at the time of uneven | corrugated processing becomes a problem. Further, such an optical film is very expensive, and an assembly process for manufacturing a liquid crystal display is increased, resulting in an increase in cost. Therefore, in order to make the liquid crystal display as cheap as possible, it has been desired to eliminate unevenness in luminance by a reflector.

JP 63-62104 A JP 2002-182579 A JP-A-6-322153 JP-A-7-118433 JP-A-4-275501

  The present invention has been made in view of the above-mentioned circumstances, and is made thin by using a synthetic resin foam excellent in processability and shape retention as a light reflecting plate, and by giving the reflecting plate a diffusion effect. An object of the present invention is to provide a surface light source device that not only has a high reflectance, but also can improve luminance unevenness.

In order to achieve the above object, the present invention includes a plurality of linear light sources and a light reflection plate that reflects light from the plurality of linear light sources, and a surface of the light reflection plate that reflects light is the plurality of light sources . In the surface light source device facing the linear light source, the light reflection plate is a synthetic resin light reflection plate made of a white sheet containing bubbles therein, and the bubbles contained therein are XY planes. When viewed in a cross section of the XZ plane perpendicular to the XY plane , both are substantially elliptical, and the linear light source has a longitudinal direction of the X- Provided is a surface light source device which is installed so as to be parallel to a minor axis direction in a cross section of a Y plane.

As shown in FIG. 1, the light reflecting plate made of a synthetic resin used in the present invention is a light reflecting plate 10 made of a synthetic resin made of a white sheet containing bubbles (in the present invention, the sheet includes a film). The bubbles 12 contained therein are substantially elliptical when viewed in the cross section of the XZ plane perpendicular to the XY plane and the XY plane . In the present invention, “substantially elliptical when viewed in the cross section of the XY plane” means that the surface of the light reflecting plate that reflects light (the surface facing the linear light source) is the XY plane (see FIG. When it is defined as (sheet upper surface in the example of 1), it means that the shape of the bubble in this cross section is substantially elliptical.

  In the present invention, the cross-sectional shape of the bubble on the light reflecting surface (XY plane) of the light reflecting plate is anisotropic, and the short diameter direction of the bubble and the longitudinal direction of the linear light source in the cross section are The effect of this invention can be acquired by arrange | positioning in parallel. On the other hand, when the bubble shape is substantially spherical, since the cross-sectional shape of the bubble in the XY plane is substantially circular, the cross-sectional shape has a small anisotropy, so that the effect of eliminating luminance unevenness can be obtained. Can not.

  The manufacturing method of the light reflecting plate that is substantially elliptical when the bubbles contained therein as viewed in the cross-section of the XY plane are not particularly limited depending on the mechanical method and composition, for example, It can be obtained by stretching a white sheet containing bubbles inside at least in a uniaxial direction, particularly only in a uniaxial direction. More specifically, the light reflecting plate made of a synthetic resin used in the present invention includes a step of holding the thermoplastic resin sheet in a pressurized inert gas atmosphere and causing the thermoplastic resin sheet to contain an inert gas; The thermoplastic resin sheet containing an inert gas can be produced by a production method comprising a step of heating and foaming under normal pressure and a step of stretching the foamed thermoplastic resin sheet in a uniaxial direction.

In addition, as shown in FIG. 1, the light reflecting plate made of a synthetic resin used in the present invention has a bubble diameter in the major axis direction X of Xd and a bubble diameter in the minor axis direction Y of Yd in the cross section of the XY plane. The following formula (1a) is preferably satisfied, and further preferably the following formula (1b) is satisfied.
Yd / Xd ≦ 0.6 (1a)
0.05 ≦ Yd / Xd ≦ 0.6 (1b)

Furthermore, the light reflecting plate made of synthetic resin used in the present invention, as shown in FIG. 1, when the bubble diameter in the minor axis direction in the cross section of the XZ plane perpendicular to the XY plane is Zd, Together with the above formula (1a) or (1b), it is preferable to satisfy the following formula (2a) and further (2b)
Zd / Xd ≦ 0.6 (2a)
0.05 ≦ Zd / Xd ≦ 0.6 (2b)

  The surface light source device of the present invention includes the above-described light reflection plate and a linear light source. As shown in FIG. 1, the linear light source 14 has an XY direction of bubbles 12 in the light reflection plate 10 in the longitudinal direction Q. It is installed so as to be parallel to the minor axis direction Y in the plane cross section. As the linear light source, a CCFL, a fluorescent lamp, or the like can be used.

  The surface light source device of the present invention is arranged so that the longitudinal direction of the linear light source is parallel to the minor axis direction in the cross section of the bubble of the light reflecting plate in the XY plane, while maintaining the luminance. It is an excellent surface light source device with almost no uneven brightness between the light source and the light source. Therefore, the surface light source device of the present invention can be suitably used for an electric signboard, a lighting fixture, a display backlight, a lighting device, and the like.

  Hereinafter, the present invention will be described in more detail. The material of the light reflector used in the present invention is not limited, but for example, general-purpose resins such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, polychlorinated biphenyl, polyethylene terephthalate, and polyvinyl alcohol, polycarbonate, and polybutylene. Terephthalate, polyamide, polyacetal, polyphenylene ether, ultrahigh molecular weight polyethylene, polysulfone, polyethersulfone, polyphenylene sulfide, polyarylate, polyamideimide, polyetherimide, polyetheretherketone, polyimide, polytetrafluoroethylene, liquid crystal polymer, fluorine Examples thereof include engineering plastics such as resins, and copolymers or mixtures thereof. Among these, polyester, polycarbonate, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, and cyclopolyolefin are preferable because of good heat resistance and impact resistance, and polyester is particularly preferable. Furthermore, among polyesters, polyethylene terephthalate is particularly preferable.

  The light reflecting plate used in the present invention preferably has an average reflectance of light in the wavelength range of 400 nm to 700 nm of 90% or more. More preferably 95% or more, still more preferably 97% or more, and particularly preferably 98% or more.

  The thickness of the light reflector used in the present invention is not particularly limited, but is preferably 200 μm or more. More preferably, it is 300 micrometers or more, More preferably, it is 350 micrometers or more. When the thickness of the light reflecting plate is 200 μm or more, it exhibits high reflectivity and is excellent in shape retention, secondary workability, and assembly workability.

  The light reflecting plate used in the present invention is not particularly limited, but the surface glossiness (incident angle 60 °, light receiving angle 60 °) is preferably 50% or more. More preferably, it is 60% or more, More preferably, it is 65% or more. When the surface glossiness is 50% or more, the diffuse reflection component is reduced, and particularly when used for a backlight, the amount of light reflected on the front surface increases, and the luminance tends to be further improved.

  In the light reflection plate used in the present invention, a crystallization nucleating agent, a crystallization accelerator, a bubbling nucleating agent, an antioxidant, an antistatic agent, an ultraviolet ray are added to the synthetic resin before foaming as long as the properties are not affected. Various additives such as inhibitors, light stabilizers, fluorescent brighteners, pigments, dyes, compatibilizers, lubricants, reinforcing agents, flame retardants, crosslinking agents, crosslinking aids, plasticizers, thickeners, thickeners May be blended. Moreover, the resin containing the said additive may be laminated | stacked on the obtained white sheet, and the coating material containing the said additive may be coated.

  When the light reflecting plate used in the present invention is produced by, for example, stretching, there is no limitation on the stretching method. For example, the unstretched sheet is led to a heated roll group, and the longitudinal direction (longitudinal direction, that is, the traveling direction of the sheet) is The method of extending | stretching and then cooling a sheet | seat with a cooling roll group is mentioned.

  Further, the both ends of the unstretched sheet can be guided to a heated tenter while being gripped by clips, and stretched in a direction perpendicular to the longitudinal direction (lateral direction or width direction). This stretching method has an advantage that the sheet surface does not come into contact with the heated roll, so that the sheet surface is not damaged as an optical defect.

  In the case of the sequential biaxial stretching method in which the stretching in the longitudinal direction and the stretching in the width direction are separated or the simultaneous biaxial stretching method in which the stretching in the longitudinal direction and the stretching in the width direction are performed simultaneously, the longitudinal and lateral stretching ratios are changed. Thus, although it is possible to make a desired bubble shape, uniaxial stretching is more preferable in order to produce a light reflecting plate capable of obtaining the effects of the present invention with high productivity.

  The stretching is preferably performed at a temperature not lower than the glass transition temperature of the thermoplastic resin and not higher than the melting temperature. For example, when the white sheet is polyethylene terephthalate, the stretching is preferably performed at 70 to 220 ° C, preferably 80 to 200 ° C. Alternatively, the white sheet may be heat-treated after stretching by heat treatment. In general, the heat treatment is performed at a temperature not lower than the glass transition temperature and not higher than the melting point of the thermoplastic resin. Conditions are appropriately set so as to finally satisfy various characteristics required as a product.

  As an example of a specific stretching method, there is a method of stretching by a predetermined ratio by a tenter method. Although the draw ratio at this time is not particularly limited, for example, when the white sheet is polyethylene terephthalate, it is preferably 1.1 times or more and less than 5 times in a uniaxial direction. More preferably, it is 1.2-4 times, More preferably, it is 1.2-3 times. If the draw ratio is 1.1 times or more and less than 5 times, a sufficient drawing effect can be obtained, and problems such as an increase in thermal shrinkage and a decrease in tear propagation resistance do not occur, and a higher reflectivity can be obtained. A white sheet can be obtained.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to the following examples. In addition, the measurement and evaluation of various characteristics of the obtained thermoplastic resin foam were as follows.

(density)
The density (ρf) of the thermoplastic resin foam was determined by a water displacement method using an electronic balance (manufactured by METLER: AE240).

(Foaming ratio)
The density (ρf) of the thermoplastic resin foam was measured by an underwater substitution method and calculated as a ratio ρs / ρf to the density (ρs) of the resin before foaming. However, when the thermoplastic resin was polyethylene terephthalate, ρs was calculated as 1.34.

(Thickness)
The thickness of five points in the width direction of the obtained thermoplastic resin foam was measured using a thickness gauge (manufactured by Teclock: SM-112, probe shape Φ10 mm, measuring force 2.5 N or less), and the average value was measured. It was set as the thickness of the plastic resin foam.

(Reflectance)
The reflectance at 550 nm was measured using a spectrophotometer (manufactured by Shimadzu Corporation: UV-3101PC). In Table 1, the diffuse reflectance of each thermoplastic resin foam is shown as a relative value, assuming that the diffuse reflectance of the white plate in which the fine powder of barium sulfate is hardened is 100%.

(Surface gloss)
The surface gloss was measured with a gloss meter (Minolta: GM-268) at an incident angle of 60 ° and a light receiving angle of 60 °. All measurements were performed with the number of samples n = 5, and the average value was obtained.

(Brightness of surface light source, uneven brightness)
A thermoplastic resin foam is set on the liquid crystal television, and brightness (cd / m ² ) is obtained using a digital luminance meter (TOPCOM: BM-9) and a digital luminance meter receiver (TOPCOM: BM-910D). Was measured. The luminance was measured three times and expressed as an average value. Further, the luminance unevenness is indicated by the difference between the maximum value and the minimum value of the five measurement points between the CCFL and the CCFL. The formula for calculating the luminance unevenness is as follows.
Brightness difference average value = {(ab) + (ac) + (dc) + (de)} / 4
a: Brightness right above the third CCFL b: Brightness between the second and third CCFLs c: Brightness between the third and fourth CCFLs d: Brightness right above the fourth CCFL e: Fourth And the brightness between the fifth CCFL

(Comparative Example 1)
0.5 mm thickness x 300 mm width x 60 m length polyethylene terephthalate sheet (Unitika Ltd .: C-0312 grade) roll and 160 μm thickness x 290 mm width x 60 m length as separator and olefin based weight 55 g / m ² A roll of non-woven fabric (manufactured by Japan Vilene Company: FT300 grade) was prepared. Both rolls were overlapped and wound so that there was no portion where the surfaces of the polyethylene terephthalate sheet were in contact with each other, and a new roll was created.

  Thereafter, the roll was put in a pressure vessel, pressurized to 6 MPa with carbon dioxide gas, and carbon dioxide gas was infiltrated into the polyethylene terephthalate sheet. The carbon dioxide gas permeation time into the polyethylene terephthalate sheet was 72 hours.

  Next, the roll is taken out from the pressure vessel, and only the polyethylene terephthalate sheet infiltrated with carbon dioxide gas is continuously supplied to a hot-air circulating foaming furnace set at 220 ° C. so that the foaming time is 1 minute while removing the separator. Foamed. The thickness of the obtained foam was 760 μm, and the average cell diameter was about 10 μm. Table 1 shows the properties of the obtained sample.

Example 1
The foam prepared in Comparative Example 1 was stretched in a uniaxial direction at 116 ° C. using a tenter method stretching apparatus so that the stretching ratio was 1.3 times, and heat-treated at 220 ° C. Slowly cooled. The stretched foam was thinned to about 67% of the thickness before stretching. Table 1 shows the properties of the obtained sample.

(Example 2)
The foam was stretched in the same manner as in Example 1 except that the stretch ratio was 2.0. The stretched foam was thinned to about 38% of the thickness before stretching. Table 1 shows the properties of the obtained sample.

(Comparative Example 2)
The foam prepared in Comparative Example 1 was stretched in the biaxial direction by a simultaneous biaxial stretching method at a stretch ratio of 1.4 times in length and 1.4 times in width. The stretched foam was thinned to about 33% of the thickness before stretching. Table 1 shows the properties of the obtained sample.

  Comparative Example 3 in Table 1 is obtained by making the stretching direction and the longitudinal direction of the linear light source parallel to each other in Example 2 (lateral direction).

(Measurement)
The stretched foam obtained in Example 2 was set on a liquid crystal television, and the luminance was measured. A case where the stretching direction and the longitudinal direction of the fluorescent lamp are parallel to each other is referred to as a “lateral direction”, and a case where the stretching direction and the longitudinal direction of the fluorescent lamp are perpendicular to each other is referred to as a “longitudinal direction”. FIG. 2 is a graph of luminance when the stretched foam is placed in the horizontal direction and the vertical direction, respectively. The positions of 0 mm, 3 mm, 6 mm, 9 mm, 12 mm, and 15 mm on the graph are directly above the fluorescent lamp. It can be seen that when the stretched foam is placed in the longitudinal direction, the luminance unevenness is small, and it is excellent as a reflector.

  The graph of FIG. 3 is a graph of luminance when the foams obtained in Comparative Example 1, Example 1, and Example 2 are placed in the vertical direction. It can be seen that the stretched foam has a small luminance difference between the fluorescent lamp and the fluorescent lamp. It can also be seen that the brightness difference decreases as the draw ratio increases. That is, the stretched foam was excellent as a reflector because the stretched direction was set perpendicular to the longitudinal direction of the fluorescent lamp, and the brightness unevenness was small.

  The graph of FIG. 4 is a graph of luminance when the foams obtained in Comparative Example 1, Example 1, and Example 2 are placed in the horizontal direction. It can be seen that there is a large luminance difference between the fluorescent lamp and the fluorescent lamp. This tendency does not change even when the draw ratio increases.

  The graph of FIG. 5 is a graph of luminance when the foams obtained in Comparative Example 1, Example 2, and Comparative Example 2 are placed in the vertical direction. In Comparative Example 2, fluorescent lamps are arranged in a certain arbitrary direction, the longitudinal direction thereof is set as the A direction, and the direction rotated by 90 degrees with respect to the A direction is set as the B direction. It can be seen that the uneven brightness is eliminated by the uniaxially stretched foam used in the present invention, but this effect cannot be obtained by the biaxially stretched foam of the comparative example.

It is explanatory drawing which shows the light reflection board and linear light source which are used for the surface light source device of this invention. It is a graph of the brightness | luminance at the time of putting the foam obtained in Example 2 in the horizontal direction and the vertical direction. It is a graph of the brightness | luminance at the time of putting the foam obtained by the comparative example 1, Example 1, and Example 2 to the vertical direction. It is a graph of the brightness | luminance at the time of putting the foam obtained by the comparative example 1, Example 1, and Example 2 in the horizontal direction. It is a graph of the brightness | luminance at the time of putting the foam obtained by the comparative example 1, Example 2, and the comparative example 2 in the vertical direction.

Explanation of symbols

10 light reflector 12 bubble 14 linear light source

Claims (6)

In the surface light source device including a plurality of linear light sources and a light reflecting plate having a light reflecting surface facing the plurality of linear light sources, the light reflecting plate is made of a white sheet containing bubbles therein. A light reflecting plate made of a synthetic resin, in which bubbles contained therein are substantially elliptical when viewed in a cross section of an XY plane and an XZ plane perpendicular to the XY plane. The surface light source device is characterized in that the linear light source is installed such that its longitudinal direction is parallel to the minor axis direction in the cross section of the XY plane of the bubbles of the light reflecting plate. The light reflecting plate made of synthetic resin satisfies the following formula (1a) when the bubble diameter in the major axis direction is Xd and the bubble diameter in the minor axis direction is Yd in the cross section of the XY plane. The surface light source device according to claim 1.
Yd / Xd ≦ 0.6 (1a) The synthetic resin of the light reflection plate is, when the cell diameter of the minor axis in the cross section of the prior Symbol X -Z plane as Zd, according to claim 1 or 2, characterized by satisfying the following formula (2a) Surface light source device.
Zd / Xd ≦ 0.6 (2a)   The surface light source device according to claim 1, wherein the light reflecting plate made of synthetic resin is obtained by stretching a white film containing bubbles therein in at least a uniaxial direction.   The surface light source device according to claim 4, wherein the light reflecting plate made of synthetic resin is obtained by stretching a white film containing bubbles in a uniaxial direction.   The light reflecting plate made of the synthetic resin includes a step of holding the thermoplastic resin sheet in a pressurized inert gas atmosphere and containing the inert gas in the thermoplastic resin sheet, and a thermoplastic containing the inert gas. A thermoplastic resin foam produced by a production method comprising a step of heating and foaming a resin sheet under normal pressure and a step of stretching the foamed thermoplastic resin sheet in a uniaxial direction. Item 6. The surface light source device according to any one of Items 1 to 5.

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