JP3584057B2 - Backlight - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a panel backlight for irradiating a transmissive or transflective panel from the back.
[0002]
[Prior art]
Recently, a liquid crystal display device having a thin and easy-to-see backlight mechanism has been used as a display device such as a laptop or book type word processor or computer. As such a backlight, an edge light system in which a linear light source such as a fluorescent tube is provided at one end of a light-transmitting light guide plate as shown in FIG. 1 is often used.
[0003]
In the case of this edge light method, as shown in FIG. 2, a material containing particles such as TiO ₂ , SiO ₂ , and BaSO ₄ having a property of diffusing light is provided on one surface of the light guide plate. A light diffusing element is formed in the form of dots or stripes in the form of a small concave or convex shape on the surface of the light guide plate in an optically connected state, and substantially the entire surface is mirror-reflected or diffused. Cover (3 in the figure), and the opposite surface (light emitting surface) of the light guide plate is coated on a light-transmitting base film such as polyethylene terephthalate (PET) with TiO ₂ , SiO ₂ , BaSO ₄ or the like. A light diffusing material is applied, or a light transmissive sheet such as polycarbonate (PC) is embossed and arranged so as to be covered with a light diffusing sheet (2 in the figure) provided with light diffusing property. Many.
[0004]
In particular, in order to reduce the weight of the backlight, as shown in FIG. 3, a light guide plate having a shape in which the thickness decreases as the distance from the linear light source increases is also used. The reason that the thickness of the light guide plate on the side of the linear light source must be thicker than other parts is that a linear light source such as a fluorescent tube has a certain size of a light emitting point ( (For example, 3 mmΦ), so that the light emitted from the linear light source efficiently enters the end face of the light guide plate, the thickness of the end face of the light guide plate is close to or more than the thickness of the linear light source (for example, 3 mm). 4 mm).
[0005]
In the case of a so-called flat plate having a uniform thickness of the light guide plate, as shown in FIG. 4 (a), the light incident on the end face of the light guide plate is in the light guide plate unless the light diffusing element applied to the light guide plate is irradiated. 4B, only the light rays hitting the light diffusing element are transmitted or reflected, and consequently are emitted from the light exit surface of the light guide plate, as shown in FIG.
[0006]
On the other hand, in the light guide plate in which the light diffusing element is not formed on the light guide plate and the thickness becomes thinner as the distance from the linear light source increases, as shown in FIG. As the total reflection is repeated at the light guide plate interface, the direction gradually deviates from the total reflection condition, and finally emerges from the light exit surface of the light guide plate. However, the light emitted from the light-emitting surface of this light guide plate has a variation in intensity, and appears in a stripe shape with high-luminance and low-luminance portions spaced apart in a state almost parallel to the central axis of the light source, resulting in a uniform planar shape. Not only cannot it be used as a light source, but it is not preferable from the viewpoint of light use efficiency. In particular, when the sides of the cross section perpendicular to the two large surfaces of the light guide plate are linear, the ratio of the luminance of the high luminance portion of the stripe to the luminance of the other portions becomes 100: 1 or more. Was a very serious problem.
[0007]
In order to solve this problem, as shown in FIG. 6, it has been proposed to form a light diffusing element on a wide surface of a light guide plate whose thickness gradually decreases as the distance from the linear light source increases. However, in this method, light is emitted from portions other than the stripe-like high-luminance portion, and the light use efficiency is improved. However, the light intensity of the stripe-like high-luminance portion is still lower than other portions. He was extremely dissatisfied with the appearance of stripes to the human eye. In particular, when the sides of the cross section perpendicular to the two wide surfaces of the light guide plate are linear, even if the light diffusing element is formed as described above, the luminance of the high luminance portion of the stripe and the other The luminance ratio between the portions was 10: 1 or more, which was not a preferable state.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a backlight using a light guide plate that becomes thinner as the distance from the linear light source increases, and particularly to a backlight using a light guide plate whose cross section perpendicular to two wide surfaces of the light guide plate is straight. An object of the present invention is to provide a backlight which can obtain uniform planar light emission without stripes when used.
[0009]
[Means for Solving the Problems]
The present inventors have conducted various studies on the above points, and as a result, the state of the wide surface of the backlight using the light guide plate that becomes thinner as the distance from the linear light source increases, the state of the striped pattern The present invention was found to be a backlight capable of obtaining an even and uniform planar light emission, and completed the present invention.
[0010]
That is, the present invention has a linear light source close to at least one side end, and roughens at least one wide surface of a light guide plate made of a light-transmitting material whose thickness decreases as the distance from the linear light source increases. The present invention relates to a backlight in which dot-shaped or stripe-shaped light diffusing elements are formed on a surface opposite to a light-emitting surface. Next, the present invention will be described in more detail with reference to the drawings.
[0011]
FIG. 7 is a perspective view of one embodiment of the present invention, and FIG. 8 is a cross-sectional view of one embodiment of the present invention. In the figure, reference numeral 1 denotes a light guide plate, which becomes thinner as it gets away from the linear light source, has a so-called wedge-shaped cross section, and may be any substance that allows light to pass therethrough efficiently, such as quartz, glass, or a transparent natural or synthetic resin. , For example, an acrylic resin.
[0012]
It is essential that the shape of the light guide plate used in the present invention is such that the thickness becomes thinner as the distance from the linear light source increases. The thickness of the portion farthest from the linear light source, i.e., the thickness of the thinnest portion of the effective surface of the light guide plate is not particularly limited, but is usually 0.5 mm or more, preferably from the viewpoint of the mechanical strength of the light guide plate. Is 1 mm or more, and the thickness is about 25% to 75% of the thickness of the thickest part of the effective surface on the side of the linear light source. The thickness of the thickest part on the linear light source side of the light guide plate is appropriately selected depending on the diameter of the linear light source used, but is preferably equal to or larger than the diameter of the linear light source in order to improve light use efficiency. In order to make the backlight thinner and lighter, it is preferable that the thickness is as thin as possible. However, the diameter is usually 0.5 to 1.6 times the diameter of the linear light source. By making the shape of the light guide plate such that its thickness becomes thinner as the distance from the linear light source increases, the weight of the light guide plate can be reduced while maintaining the efficiency of light rays entering the light guide plate end face from the linear light source. You can do it.
[0013]
Such a light guide plate thickness reduction state may be either continuous or stepwise, but the effect of the present invention is most remarkably exhibited when the light guide plate thickness is continuously linearly changed at a constant rate. In particular, the side of the cross section perpendicular to two wide surfaces of the light guide plate is linear. The light guide plate having such a shape is preferable also in that the production of the light guide plate is easy. Also, the surface on the light emitting surface side of the light guide plate has a substantially straight side in a cross section perpendicular to the surface, and is substantially parallel to the sheet surface via a light diffusible sheet described later and a slight air layer. It is preferable that
[0014]
At least one wide surface of the light guide plate is a rough surface (9 in FIG. 8: an example in which the opposite side to the light exit surface of the light guide plate is a rough surface), but the method is not particularly limited, and sand blasting, etching, and the like may be used. There are methods such as a chemical treatment, a portion corresponding to a wide surface of the light guide plate of the injection molding die is roughened in advance, or a rough surface is formed when the light guide plate is injection-molded. The range of the portion where the light guide plate is to be roughened is a range where the above-described stripe pattern is observed at least when the light guide plate surface is not roughened, that is, the portion of the light guide plate near the linear light source can be roughened. Required.
[0015]
Further, it is more preferable that the entire surface corresponding to the light emitting surface of the light guide plate is made substantially uniform and rough from the viewpoint of easy manufacturing and obtaining uniform brightness. Although the degree of the rough surface depends on the light diffusing performance of the light diffusing element described later, the surface is generally in a translucent milky white color, more preferably a very light milky white color. If the degree of the rough surface is too strong, the brightness near the linear light source of the light guide plate becomes extremely high as compared with other portions, and a uniform planar light emission state cannot be obtained. By setting the wedge-type light guide plate in such a state, the above-mentioned striped pattern disappears. It is considered that the reason is that the regularity of the light beam totally reflected in the light guide plate can be disturbed.
[0016]
In order to form the light diffusing element (6 in FIG. 8) on the back surface of the light guide plate, a substance having a function of diffusing light, for example, silica, barium sulfate, calcium carbonate, titanium white, glass bead, resin bead. A method of printing a light-diffusing substance such as paint, printing ink containing bubbles, etc. on the light guide plate surface in a dot or stripe form by a method such as screen printing, or a dot or stripe form on the surface of the light guide plate There is a method of forming a rough surface, a method of forming a small hole or forming a small projection on the surface of the light guide plate, and the like. The dot-shaped light diffusing element referred to here has, for example, a circular or square shape when it is screen-printed, and the striped light diffusing element is formed in a straight line. Things.
[0017]
FIG. 9 shows an example of forming a light diffusing element on a light guide plate. The distance between the light diffusing elements formed on such a light guide plate is preferably in the range of 0.01 mm to 5 mm. However, when the distance is 3 mm or more, the shape of the light diffusing element itself is easily seen through. If the size is reduced, the production yield is extremely deteriorated. Therefore, it is preferably in the range of 0.03 mm to 2 mm.
[0018]
Reference numeral 4 denotes a linear light source, which is preferably in the form of a reflective sheet of a mirror surface such as Ag or Al or a polyethylene terephthalate (PET) having a gap (slit) for allowing light to enter the end of the light guide plate. It is covered with a light reflector 5 formed of a light diffuse reflection sheet provided with light diffusibility by BaSO ₄ , TiO ₂ , bubbles or the like with a gap of a certain width from the light source surface of the linear light source. For example, one light type (one side), two light type (for example, as shown in FIG. 10), or a light guide plate of three or four sides is close to at least one end face portion, and the central axis thereof is in contact with the end face of the light guide plate. They are installed so that they are almost parallel. Examples of the linear light source include a fluorescent tube, a tungsten incandescent tube, an optical rod, an LED array, and the like. However, a fluorescent tube is preferable. It is preferable that the length of the light guide plate is substantially equal to the length of the end portion.
[0019]
A light reflecting plate having a light diffusing element (3 in the figure) is a mirror reflecting sheet made of Ag, Al or the like or a light diffusing reflecting sheet obtained by adding light diffusing properties to PET with BaSO ₄ , TiO ₂ , bubbles or the like. Is arranged so as to cover almost the entire surface.
[0020]
On the opposite side (light emitting surface) of the light guide plate, a light diffusing substance such as TiO ₂ , SiO ₂ , BaSO ₄ is applied on a light-transmitting base film such as polyethylene terephthalate as in the conventional case. Alternatively, a light-transmitting sheet such as polycarbonate is embossed and covered with a light-diffusing sheet (2 in the drawing) provided with a light-diffusing property.
[0021]
The main part of the present invention has such a configuration and is used as a backlight of a panel, particularly a liquid crystal panel.
[0022]
【The invention's effect】
The present invention is directed to a backlight using a light guide plate which becomes thinner as the distance from the linear light source increases, and particularly to a backlight using a light guide plate having a light guide plate having a wide cross section of two straight lines, without stripes. It can be used as a light-weight backlight that can obtain a good planar light emission.
[0023]
【Example】
Next, the present invention will be described in more detail with reference to Comparative Examples and Examples. As shown in FIG. 7, the transparent light source has a thickness of 3 mm on the side of the linear light source and a thickness of 1.5 mm on the farthest part, and the thickness decreases linearly from the end face on the side of the linear light source toward the center. A cold cathode fluorescent tube (manufactured by Harrison Electric Co., Ltd.) having a diameter of 3 mm is arranged at the short end of a light guide plate (outside dimensions 210 mm × 155 mm) made of natural polymethyl methacrylate (PMMA), and is in contact with the light guide plate. An Ag film having a slit of 3 mm in the portion was arranged in an elliptical shape so that the reflection surface faced the light source, and the light emitted from the slit was arranged to enter the light guide plate from the end of the light guide plate.
[0024]
On the other hand, one surface of the light guide plate surface (the side opposite to the light output surface) was roughened by sandblasting so that the entire surface became light milky white. Further, on this rough surface, a light-diffusing substance (white paint including titanium white) is placed on the light guide plate at intersections (grids) each having an interval of 1 mm, at a point where the coverage is the minimum (light source side). ) Is set to 10%, and is sequentially increased, and is formed by printing with a circular dot pattern so as to become 85% at the maximum point (the farthest part from the light source).
[0025]
In addition, the coverage of the light diffusing substance on the light guide plate surface in a direction parallel to the central axis of the linear light source is determined from the light source side with reference to a line perpendicular to the axis at the central portion in the longitudinal direction of the linear light source. Printing was performed so that the value gradually increased toward both sides as a minimum near the vertical line up to substantially the center of the light plate.
[0026]
The surface of the light guide plate on which the light diffusing element was printed was covered with a 0.125 mm thick light diffusing reflection sheet (Melinex 329, manufactured by ICI). Further, a light diffusion sheet (8B36, manufactured by Sansho Bussan Co., Ltd.) having a thickness of 0.18 mm and made of polycarbonate having a rough surface on one side is provided on the light emitting surface side of the light guide plate. One sheet was arranged on the side. The average luminance (average value of 100 points equally divided in the light emitting surface) when a 30 KHz alternating voltage is applied to the cold cathode tube from an inverter (CDKAM-10L manufactured by TDK) and driven at a constant current (5 mA). was 1300 cd / ^{m 2} was measured with respect to the normal direction down to the exit surface at a viewing angle of 2 degrees by a luminance meter (Topcon BM-8). At this time, no light stripe pattern was observed on the light guide plate surface, and a uniform planar light emission state was obtained (Example).
[0027]
Except that the surface of the light guide plate was not roughened, the operation was performed under the same apparatus and conditions as in the example, and the measured luminance was 1300 cd / m ² . However, light stripes were observed on the light guide plate surface, and a uniform planar light emission state could not be obtained. Such a striped portion (high-brightness portion) had a brightness 10 times or more higher than other portions, and was difficult to use as a backlight (Comparative Example 1).
[0028]
The light guide plate used in the example was operated under the same apparatus and conditions as in the example except that the surface was not roughened and the light diffusing material was not applied, and the measured luminance was 900 cd / m. It was ² . However, light stripes were observed on the light guide plate surface, and a uniform planar light emission state could not be obtained. The luminance of other portions of such a striped portion (high-luminance portion) was more than 100 times higher and was difficult to use as a backlight (Comparative Example 2).
[0029]
The average luminance measured was 1000 cd / m ^{2 ,} operating under the same apparatus and under the same conditions as in the example except that the light guide plate used in the example was not subjected to a light diffusing substance on its surface. . However, the brightness gradually decreased as the distance from the light source increased (Comparative Example 3).
[Brief description of the drawings]
FIG. 1 is a perspective view of a conventional backlight example. FIG. 2 is a cross-sectional view of a conventional backlight example. FIG. 3 is a cross-sectional view of a backlight example using a conventional wedge-type light guide plate. FIG. 5 shows how light rays travel in a flat light guide plate of a light. FIG. 5 shows how light rays travel in a conventional wedge light guide plate. FIG. 6 is a cross section of a backlight using a conventional wedge light guide plate. FIG. 7 is a perspective view of a backlight according to one embodiment of the present invention. FIG. 8 is a cross-sectional view of a backlight according to one embodiment of the present invention. FIG. 9 illustrates a light diffusing element of a light guide plate used in the present invention. FIG. 10 is a cross-sectional view of a backlight according to another embodiment of the present invention.
1: light guide plate 2: light diffusion sheet 3: light reflection sheet 4: linear light source 5: light reflection sheet 6 covering the linear light source: light diffusion element 7: light ray 8: light guide plate viewed from the front (light emitting surface) 9: rough surface

Claims (3)

A light guide plate made of a light-transmitting material having a linear light source close to at least one side end thereof, the thickness of which decreases as the distance from the linear light source increases , and the thinnest portion of the effective surface of the light guide plate thick in the 0.5mm or more, at least one lateral broad aspect of the linear light source side thickest portion of the thickness of 25% to 75% in a light guide plate of the effective surface of the fringe when no roughened A backlight in which a pattern observation range is a rough surface, and a dot-shaped or stripe-shaped light-diffusing element is formed on a surface of the light guide plate opposite to a light-emitting surface. 2. The backlight according to claim 1, wherein a light guide plate having a cross section perpendicular to two wide surfaces of the light guide plate is straight. 3. The backlight according to claim 1, wherein the degree of the rough surface on the side opposite to the light exit surface of the light guide plate is substantially uniform.

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