JP7572479B2 - A direct-type point light source backlight unit and a liquid crystal device equipped with the direct-type point light source backlight unit. - Google Patents

Description

The present invention relates to a direct-type point light source backlight unit and a liquid crystal device equipped with the direct-type point light source backlight unit.

Liquid crystal devices are widely used in various information devices, and based on their basic structure they can be broadly categorized into direct-light types, in which the light source is placed on the rear side of the liquid crystal panel mounted on the liquid crystal device, and edge-light types, in which the light source is placed near the side of the liquid crystal panel.

In recent years, there has been an increasing demand for thinner and lower-cost liquid crystal devices such as notebook PCs, tablets, car navigation systems, and smartphones. There is also a demand for improved image quality and HDR (High Dynamic Range: wider range of brightness) compatibility. As a method for HDR compatibility, consideration has been given to adjusting the amount of light in the backlight in addition to the shutter function using liquid crystal by turning on and off the LEDs and adjusting the light amount for each individual LED (local dimming) in a direct backlight. In the case of a direct type that uses a point light source such as an LED (Light Emitting Diode), there is a demand to increase the amount of light on the light-emitting surface while at the same time increasing the uniformity of the luminance within the light-emitting surface, and there is also a demand for reducing costs by avoiding an increase in the number of parts.

To summarise, in order to respond to the multifaceted social needs of recent years, such as lower costs, lighter weight, thinner dimensions, higher brightness and better appearance (reduced unevenness in the brightness of the display screen), for notebook PCs, tablets, car navigation systems, smartphones and the like, the functions of direct-type backlight units using LEDs must be improved more than ever before.
1) Reducing manufacturing costs by adopting simple manufacturing methods. 2) Reducing manufacturing costs by reducing the number of parts and types of materials and by utilizing common parts and materials. 3) Reducing the thickness of the parts themselves, as well as the overall thickness by making each part multifunctional. 4) Uniformizing and optimizing the brightness on the light exit surface (reducing uneven brightness).
5) Improvement and development efforts from various perspectives are required, such as increasing the luminance of the front luminance at the light exit surface. Note that some of the above factors affect each other in a complex way, and in particular, the demand for uniform luminance (elimination of unevenness in the front luminance at the light exit surface) and the demand for high luminance of the front luminance (increasing the amount of light emitted from the light exit surface) tend to be mutually contradictory, so it is important to achieve an excellent balance between them rather than attempting to improve only one side.

The basic configuration of a direct-type point light source backlight unit in the prior art is generally as disclosed in Patent Document 1. The configuration is shown in Figure 1 of the document. In terms of functionality, the functional configuration is such that 1) the light emitted from the LED is 2) first diffused in all directions by a light diffusion plate or light diffusion sheet that exhibits a light diffusion function (also called light guide), and then the light that has been diffused there and comes out from the emission surface is collimated by a laminate of separately arranged prism sheets.

When searching the Japan Patent Office's online database using the keyword "direct-type point light source backlight unit," one can see that various proposals have been made to date to meet the above-mentioned social and technical demands for direct-type point light source backlight units, which are among the backlight units of this basic configuration. Among such proposals, recent improvements have focused mainly on reducing unevenness in brightness while maintaining or improving the brightness at the light exit surface. These proposals can be broadly divided into those that focus on improving the LED light source itself (e.g., Patent Document 2) and those that focus on controlling the propagation direction of light emitted from the LED (e.g., Patent Document 3). In the latter case, development efforts are focused on reducing the amount of light incident on the light diffusion sheet (light diffusion film) and on increasing the diffusion function to diffuse the light that has entered the light diffusion sheet (light diffusion film) as much as possible.

In recent years, the market has demanded LCD display screens that are brighter than ever before, and to meet this demand, some direct-type point light source backlight units use color conversion films (color conversion sheets) such as QD films (quantum dot films) as their color generation source, which has become more common in recent years as the color display on LCD display screens is brighter than ever before. In such direct-type point light source backlight units, the blue light emitted from an LED point light source excites particles contained in a color conversion film such as a QD film, which produces red and green (RG) colors out of the three primary colors of RGB, and the blue (B) color is produced by using the light of the blue LED itself. The tiny LED package used as a point light source in such a direct-type point light source backlight unit is formed by sealing a blue-emitting LED element with a sealing material and further covering it with a microlens, but in the first place, it is necessary to apply the blue light itself to a color conversion film such as a QD film, and for that reason, this sealing material does not contain fluorescent fine particles that are contained in the LED sealing material of an LED package that becomes a general white point light source. Therefore, the emitted light has a relatively high directivity and a narrow luminous flux width. In addition, the general configuration of such a direct-type point light source backlight unit equipped with a color conversion film such as a QD film is not a white LED point light source as a point light source, in which a fluorescent material is mixed into the sealing material of the LED package and a light diffusion functional member (which may be a single film or may be composed of multiple films) is placed on the upper side of the package, but a light diffusion functional member to be improved by the present invention is placed on the light emission side of a point light source composed of an LED package that does not have a fluorescent material in the sealing material, and a color conversion film such as a QD film is further placed on the light emission surface side.

JP 2013-073875 A JP 2013-004408 A International Publication No. WO 11/030594

As mentioned above, the invention disclosed in Patent Document 2 focuses on improvements to LED light sources, and is an invention related to a direct-type point light source backlight that uses a point light source with a light peak on the high-angle side. The basic premise is that a special LED with a light peak angle of ±50 to 80° is used as the point light source, and the negative factor of increased manufacturing costs is inevitably a problem.

Furthermore, the invention disclosed in Patent Document 3 cited in the same document focuses on improving the light diffusion performance of the light diffusion film by making the upper surface (light exit surface) of the light diffusion film into a waffle shape with an inverted pyramid-shaped recess, but Patent Document 2 points out that this alone is insufficient, and there is no choice but to use an LED with controlled light directivity of the light source itself (a special LED with a light peak angle of ±50 to 80°).

Some of the various proposals made so far involve inserting a separate light diffusion functional component between the LED point light source and the light diffusion film (or sheet) to change the directionality of the directional light from the LED. However, from the perspective of reducing the number of parts and the types of parts, reducing manufacturing costs by using common parts and components, or from the perspective of making the whole thinner by making one part multifunctional, as well as making the parts themselves thinner, it is difficult to say that these fully meet the social and technical needs mentioned above.

In addition, direct-type point light source backlight units equipped with color conversion films such as QD films are quite recent, and therefore, while fully recognizing the above-mentioned issues specific to such direct-type point light source backlight units, it appears that there is still no product that can meet the practical industrial challenges of reducing manufacturing costs by adopting simple manufacturing methods, reducing the number of parts and types of materials, and reducing manufacturing costs by utilizing common parts and materials, and further reducing the thickness of the parts themselves as well as the overall thickness by making each part multifunctional, and that can also achieve an excellent balance between the demand for uniform luminance (elimination of uneven front luminance on the light exit surface) and the demand for high front luminance (increasing the amount of light exiting from the light exit surface).

Now, in a direct-type point light source backlight unit, when trying to uniformize the brightness on the light exit surface while at the same time maintaining the amount of light from the light exit surface (preventing a decrease in light transmittance) or increasing the amount of emitted light (increasing the front brightness on the light exit surface), a common approach would be to increase the amount of light-diffusing fine particles contained within the light diffusion film to improve its light diffusion ability in order to uniformize the brightness on the light exit surface. However, doing so would result in a decrease in the amount of light emitted from the light exit surface of the direct-type point light source backlight unit.

Conversely, if you try to increase the amount of light emitted from the light exit surface of a direct-type point light source backlight unit, it is necessary to naturally reduce the amount of light-diffusing fine particles contained in the light diffusion film, but doing so will result in a strong eyeball-shaped lamp image (hot spot) appearing directly above the point light source, causing significant non-uniformity in the front brightness. As is clear from this simple thought simulation, uniforming the brightness at the light exit surface and maintaining the amount of light from the light exit surface (preventing a decrease in light transmittance) are mutually contradictory characteristics.

Possible solutions to this problem include increasing the number of point light sources used to increase the light source density, or using special point light sources with a large peak angle of light from the point light source, but doing so would result in the inconvenience of a dramatic increase in manufacturing costs. Another option would be to increase the distance between the light source and the light entrance surface of the opposing direct-type point light source backlight unit, but this would clearly create a problem that conflicts with the demand for a slimmer design.

In order to solve the above problems, the present inventors have made extensive research and efforts, and as a result, the above problems, particularly,
The present invention was made based on the discovery that 1) reduction in manufacturing costs through a manufacturing method suitable for mass production, 2) reduction in manufacturing costs by reducing the number of parts and types of materials and by utilizing common parts and materials, 3) reduction in overall thickness by making each part multifunctional, in addition to making the parts themselves thinner, and 4) an excellent balance can be achieved between the need to uniformly optimize the brightness at the light exit surface (reduce brightness unevenness) and the need to maintain the amount of light from the light exit surface (prevent a decrease in light transmittance) or increase the amount of light (increase in front brightness at the light exit surface).

That is, the first invention is a direct-type point light source backlight unit in which an optical film is mounted between a light source and a color conversion sheet, the light source being configured by a plurality of point light sources constituted by blue LED packages that do not have a fluorescent material in a sealing material of the LED packages and arranged in a lattice shape at a predetermined pitch, and
The optical film has an integral structure including an optical waveguide introduction layer on a light incident side, a light extraction layer on a light exit side, and an optical waveguide layer between the optical waveguide introduction layer and the light extraction layer,
the light extraction layer has a first exposed surface on a surface side where light from the point light source is emitted from the optical film, the first exposed surface configuring an uneven surface with a random pattern,
the optical waveguide layer includes light diffusing particles and constitutes a layer between the light extraction layer and the optical waveguide introduction layer;
the optical waveguide introduction layer constitutes a single-layer triangular prism layer, and has a second exposed surface consisting of triangular prism surfaces in a regular pattern, the second exposed surface being disposed on a surface side where light from the point light source enters the optical film;
The optical waveguide introduction layer is a direct type point light source backlight unit having a feature of changing one ray of light into two ray of light.

The second invention is characterized in that in the first invention, the triangular prism layer has a triangular cross-sectional shape, and the triangular shape in a cross section perpendicular to the longitudinal direction of the prism is an isosceles triangle, and the apex angle is 65 to 100 degrees.

The third invention is characterized in that in the first or second invention, the arrangement pitch of the prisms whose cross-sectional shape is an isosceles triangle is 20 to 100 μm.

The fourth invention is any one of the first to third inventions, characterized in that the optical film is mounted in such a manner that at least two of the optical films are stacked, each with the triangular prism layer facing the light source side, and the longitudinal directions of the prisms of the isosceles triangular cross section of the triangular prism layer are arranged so as not to coincide with each other.

The fifth invention is the fourth invention, characterized in that the arrangement is such that at least two of the optical films are stacked together and arranged so that the longitudinal directions of the prisms of the isosceles triangular cross section of the triangular prism layer are mutually perpendicular.

The sixth invention is a liquid crystal display device equipped with a direct-type point light source backlight unit according to one of the first to fifth inventions.

The seventh invention is the sixth invention, characterized in that the optical film is mounted by stacking at least two of the optical films, each of which is mounted with the triangular prism layer facing the light source side and arranged so that the longitudinal directions of the prisms of the isosceles triangular cross section of the triangular prism layer do not coincide with each other.

The eighth invention is a liquid crystal display device according to the seventh invention, characterized in that the arrangement is such that at least two of the optical films are stacked together and arranged so that the longitudinal directions of the prisms of the isosceles triangular cross section of the triangular prism layer are perpendicular to each other.

1) The optical film according to the present invention can be obtained by a simple method suitable for mass production, in which a large-area continuous film is first integrally molded by a roll-to-roll production method, and then the continuous film is cut into pieces of required dimensions. From this perspective, the manufacturing cost can be reduced.
2) Furthermore, a direct-type point light source backlight unit equipped with the optical film of the present invention makes it possible to achieve an excellent balance between the brightness and appearance (uniform brightness or elimination of brightness unevenness) at the light exit surface simply by stacking optical films of the same material and configuration so that the extension direction of the triangular prisms in the triangular prism layer is at a specified angle, which leads to a reduction in the number of parts and types of materials and a reduction in manufacturing costs by utilizing common parts and materials.
3) The optical film of the present invention has an integrated structure in which an optical waveguide introduction layer that deflects the light incident from an LED light source to convert a single beam of light into two beams of light, an optical waveguide layer that diffuses the light from this optical waveguide introduction layer, and a layer that extracts the diffused light from this optical waveguide layer are integrated into one structure, thereby providing multiple functions in one component and is expected to enable the overall film to be made thinner.
4) The direct-type point light source backlight unit of the present invention can be obtained simply by replacing the light diffusion sheet or film mounted in a conventional direct-type point light source backlight unit (e.g., see FIG. 1 ), in which light from a point light source is directly incident on a light diffusion sheet or film, with the laminated optical sheet (two-sheet laminate or multi-layer type) of the present invention, making it easy to design and simple in structure.
5) According to the direct type point light source backlight unit of the present invention, it is important to achieve an optimal balance between the two opposing characteristics of the variation in brightness at the light exit surface and the front brightness, but this optimal balance can be achieved.
6) The above-mentioned effects 1) to 5) can be obtained either alone or in combination, and these effects can also be said to be true for a liquid crystal display device equipped with a direct-type point light source backlight unit according to the present invention.

In addition, when evaluating the properties of the optical film according to the present invention, as described below, a blue LED that does not use a fluorescent material as an encapsulant for the LED is used as a point light source in a direct-type point light source backlight unit equipped with a color conversion sheet. When the light source is placed between the light source and the color conversion sheet, the unique property is obtained in that, on the light exit surface from the backlight unit, as the haze increases, the front brightness increases, i.e., the front brightness and the haze value on the light exit surface show a positive correlation.

FIG. 1 is a conceptual diagram showing a cross section of a direct-type point light source backlight unit according to the prior art that is equipped with a color conversion film such as a QD film. FIG. 2 is a conceptual diagram showing a schematic enlarged cross section (particularly enlarged in the thickness direction) of an optical film (one sheet) constituting the laminated optical sheet according to the present invention. FIG. 3 is a photograph of an example of a random pattern uneven surface on the surface (exposed surface) of the light extraction layer of the optical film according to the present invention, viewed from directly above (magnification is as shown in the figure). FIG. 4 is a conceptual diagram showing a cross-sectional configuration of a direct-type point light source backlight unit equipped with a laminated optical sheet according to the present invention. FIG. 5 is a diagram showing the size and positional relationship between one cell and an LED point light source when the measurement range is divided during luminance measurement in the examples and comparative examples. FIG. 6 shows a schematic diagram of the number of LED point light sources covered by one cell during measurement. FIG. 7 is a graph showing the measurement data in Table 2, with the standard deviation of luminance on the vertical axis and the average luminance normalized with the average luminance of Comparative Example 1 set to 100 on the horizontal axis.

The optical film according to the present invention has a three-layer structure consisting of a light extraction layer (top layer, light exit layer) as the first optical characteristic layer, a light guide layer (middle layer) as the second optical characteristic layer, and a light guide introduction layer (bottom layer, light entrance layer) as the third optical characteristic layer, and these layers each have the following characteristics and properties.

1. Light extraction layer (top layer)
(Manufacturing method and materials used)
The manufacturing method of the light extraction layer according to the present invention can be suitably adopted according to the method disclosed in Japanese Patent Application Laid-Open No. 6-59108 or Japanese Patent Application Laid-Open No. 2003-032074 filed by the same applicant. The outline of the manufacturing method is to fix the first fine particles for light diffusion to one side of the optical waveguide layer described in detail below with a light-transmitting organic adhesive, and more specifically, the kneaded product (mixture) of the first fine particles for light diffusion and the light-transmitting organic adhesive is applied almost uniformly to one side of a resin film containing the second fine particles for light diffusion described in detail below, and cured to fix the first fine particles for light diffusion to the surface, and the first fine particles for light diffusion fixed to the surface (however, each fine particle is covered with a cured coating of the light-transmitting organic adhesive) form a random pattern uneven surface. As the light-transmitting organic adhesive used here, at least one resin selected from polyester resin, acrylic resin, silicone acrylic resin, fluororesin, fluorine-acrylic resin, or a resin obtained by adding a crosslinkable resin having a curing function to these resins, or a curable resin such as a polyurethane resin or an epoxy resin, etc. It goes without saying that a curing method most suitable for the light-transmitting organic adhesive used can be appropriately adopted as a curing method for this kneaded product.

In this case, the mixing ratio (weight ratio or volume ratio) of the first light-diffusing fine particles to the light-transmitting organic adhesive can be preferably such that after the light-transmitting organic adhesive hardens, at least a portion of the light-diffusing fine particles form unevenness on the surface together with the hardened body (hardened film) of the light-transmitting organic adhesive. Specifically, the mixing ratio is such that Ssk (skewness: degree of bias), an index of the profile shape characteristics of the unevenness on the surface, is preferably 3.0 or less, more preferably 1.40 to 2.30, and the Haze value of only the light extraction layer is preferably within the range of 20% to 70%, more preferably 40% to 60%. However, these numerical ranges may be influenced by other variable factors such as the arrangement pitch of the LED light sources mounted in the direct-type point light source backlight unit and the Lambertian distribution characteristics of the light emitted from the light sources, so these numerical values are merely for reference. It goes without saying that any numerical range can be used as long as it provides the overall effect of the present invention.

By adopting the above-described simple method for forming the light extraction layer according to the present invention, it is possible to simplify the process and, ultimately, reduce manufacturing costs.

In addition to the above-mentioned coating method, other methods of forming the uneven surface with a random pattern include preparing a mold (master) with an inverted uneven shape having a desired predetermined Ssk value, pouring a thermosetting resin or ultraviolet-curing resin between this mold and one side of the light diffusion film that constitutes the light guide layer described below, and then curing the resin to transfer the shape of the master to one surface of the light diffusion film that constitutes the light guide layer, or transferring the shape of the master to one surface of the light diffusion film by heat pressing, or directly roughening the surface of the light diffusion film by a method such as sandblasting.

While not limiting the technical scope of the present invention in any way, the presence of the first light diffusing fine particles is not essential, and as long as the purpose and function of the light extraction layer constituting a part of the present invention can be obtained by providing a randomly uneven surface, a configuration that does not include the first light scattering fine particles can be suitably used. From a general optical viewpoint, when two substances with different refractive indices are in contact with each other at an interface, if light traveling from one substance A to the other substance B hits the interface, a part of the light is refracted at the interface and propagates into substance B, and the remaining part of the light is reflected at the interface and returns toward substance A. Therefore, if the light extraction layer does not exist and the light exit surface of the optical waveguide layer is exposed (in contact with the atmosphere), it can be said that the reflection (reflection into the optical waveguide layer) phenomenon occurs on the back side of the light exit surface (the inner surface of the optical waveguide layer) due to the difference between the refractive index of the atmosphere (1.0) and the refractive index of the resin used in the optical waveguide layer (1.58 when polycarbonate is used) (the greater the difference, the greater the loss of the amount of emitted light due to reflection into the optical waveguide layer). However, the fine structure of this light extraction layer is a complex structure, and some of the first fine particles for light diffusion protrude beyond the thickness of the resin binder that fixes them, and the protruding parts are covered with an extremely thin film of the fixing resin binder, so the propagation behavior of the light passing through them is extremely complex. Furthermore, coupled with the fact that there may be parts only of the fixing resin binder, a phenomenon that cannot be predicted by the general light refraction and reflection phenomenon at the boundary surface has appeared. As a result, as will be described in detail below, if the haze value of the light extraction layer is increased, and the optical film of the present invention having such a light extraction layer is placed directly above the light source in a direct-type point light source backlight unit equipped with a color conversion film, the front brightness at the light output surface of the backlight unit increases, resulting in a poor appearance (the standard deviation value from the average front brightness becomes large), so in some cases, it may be possible to obtain the favorable effect sought by the present invention by making the refractive index of the light extraction layer higher than the refractive index of the optical waveguide layer (increasing the amount of reflected light at the boundary surface).

As the first fine particles for light diffusion, fine particles made of at least one material selected from inorganic materials such as glass and silica, and organic materials such as urethane, polyamide, polycarbonate, polymethyl methacrylate, polyacrylonitrile, polyester, silicone, polyethylene, epoxy, melamine-formaldehyde condensate, benzoguanamine-formaldehyde condensate, or benzoguanamine-melamine-formaldehyde condensate can be preferably used, but fine particles such as urethane, polyamide, and polycarbonate, and more preferably fine particles of polymethyl methacrylate can be preferably used. However, in selecting the material, it goes without saying that any material can be used as long as the refractive index of the material is close to the refractive index of the resin so that the diffuse reflection due to the difference between the refractive index of the material and the refractive index of the resin constituting the light extraction layer does not increase in the environment in which the optical film of the present invention is used.

The particle size is preferably about 1 to 20 μm, more preferably about 5 to 15 μm. If it is less than 1 μm, the light extraction function of the light extraction layer in the present invention (the extraction function meaning the function of extracting the light diffused in the optical waveguide layer from the light exit surface described in detail below) is insufficient. Also, depending on the viscosity of the translucent organic adhesive used and the mixing ratio of the first fine particles for light diffusion, if the particle size is large, it becomes difficult to control the Ssk of the surface (exposed surface, light exit surface) after the translucent adhesive mentioned below hardens and the first fine particles for light diffusion are fixed, making coating more difficult. In addition, the layer thickness of the entire optical film becomes too thick. Therefore, the upper limit of the particle size range that can express the characteristics of the present invention is thought to be about 20 μm, but any particle size can be suitably adopted as long as it has the same effect as the optical film of the present invention.

Generally, when light enters one optical material at the boundary between two optical materials with different refractive indices, due to the difference in the refractive indices between the two materials, not all of the light is refracted at the boundary as it propagates into the other optical material. If the angle of incidence becomes large, a phenomenon occurs in which part of the light is reflected at the boundary and returned to one of the optical materials.

In short, the function of the light extraction layer is to refract light at the interface between the light extraction layer and the optical waveguide layer, and to transmit light that has propagated from one material toward the interface into the other material. However, the light extraction layer contains light diffusing particles, and the presence of these particles causes light to propagate in a much more complicated manner than if this layer were made of a single resin. Therefore, in order to optimize the amount of light that is reflected from the optical waveguide layer at the interface and returns to the optical waveguide layer as required by the present invention, it may be necessary to find the right combination of material configuration and refractive index by trial and error.

As the light-transmitting organic adhesive, at least one resin selected from urethane resin, polyether resin, polyester resin, acrylic resin, silicone acrylic resin, fluororesin or fluorine-acrylic resin, urethane-acrylic resin, etc., or a crosslinkable resin based on these resins, such as ultraviolet-curable, electron-beam-curable, heat-curable, isocyanate-curable, or amine-epoxy-curable resins, or a mixture thereof, can be suitably used. In some cases, thermoplastic resins can also be used. As described above, in view of the fact that the propagation of light at the interface between the light extraction layer and the optical waveguide layer exhibits complex behavior, the refractive index should be appropriately selected in relation to the refractive index of the resin used in the optical waveguide layer, and any value can be suitably used as long as the propagation behavior does not deviate from the purpose and intent of the present invention.

(Features and characteristics)
The mixing ratio of the light-diffusing fine particles to the light-transmitting organic adhesive is such that after the light-transmitting organic adhesive is cured, at least a part of the light-diffusing fine particles combine with the cured body (cured film) of the light-transmitting organic adhesive to form unevenness on the surface. The mixing ratio is such that Ssk (skewness: degree of bias), which is an index of the profile shape characteristics of the unevenness on the surface, is preferably 3.0 or less, more preferably 1.40 to 2.30. However, such a numerical range may be influenced by other variable factors such as the arrangement pitch of the LED light sources mounted in the direct-type point light source backlight unit and the characteristics of the Lambertian distribution of the light emitted from the light source, so such numerical values are merely for reference, and it goes without saying that any numerical range can be used as long as the effects of the present invention can be obtained overall.

The light extraction layer is characterized not only by the surface irregularity characteristics described above, but also by the composite refractive index of the final light extraction layer obtained by hardening the adhesive being in the range of 1.30 to 1.70, preferably in the range of 1.45 to 1.55, and the haze value of the entire light extraction layer (composite of resin and light diffusing particles) being in the range of 20% to 70%, preferably in the range of 40% to 60%. In view of the fact that the main function of the first light diffusing particles in the light extraction layer is light diffusion and light scattering as described above, the mixing amount (see FIG. 3) of the first light diffusing particles contained in the light extraction layer is preferably selected so that the first light diffusing particles form a single layer (a state in which the particles do not overlap each other) on the light exit surface of the optical waveguide layer, without any limitation to the scope of the rights of the present invention.

The main function of the light extraction layer included in the present invention is to improve the front luminance at the light exit surface (increase in luminance), and has the following three functions.
(1) Light is extracted from the optical waveguide layer, which will be described in detail below; that is, the light diffused in all directions by the optical waveguide layer is returned to the front, thereby improving the front brightness.
(2) Promoting the guiding of light (diffusion of light) by the optical waveguide layer, which will be described in detail below, increases the amount of light emitted from locations other than the point directly above the point light source.
(3) It has the effect of preventing a significant decrease in front luminance by suppressing light absorption and loss.

2. Optical waveguide layer (intermediate layer)
(Manufacturing method and materials used)
The optical waveguide layer is preferably made of a milky white translucent plastic film and can be manufactured by the same manufacturing method as that for manufacturing a general light diffusing film. Specifically, the optical waveguide layer is made of a transparent resin in the form of a film or sheet. Before molding, inorganic fine particles such as titanium oxide, barium sulfate, calcium carbonate, aluminum hydroxide, magnesium carbonate, or aluminum oxide, or organic fine particles such as urethane, polyamide, polycarbonate, or silicone ( The second light diffusing particles are added to a transparent resin, and the resulting pellets are formed into a film by a film extrusion method (T-die method). It is possible to use particles having the same chemical composition (type) as the first light diffusing particles (however, the powder characteristics such as particle size and particle size distribution may differ), but silicone particles are preferably used. Used.

From the viewpoint of the light guiding ability of the optical waveguide layer, the resin of the above-mentioned plastic film constituting the optical waveguide layer is preferably a transparent resin such as polycarbonate, polymethyl methacrylate, or styrene-methyl methacrylate copolymer, the refractive index of which is suitable for the present invention, but polycarbonate is more preferably used from the viewpoints of reliability and availability.

When selecting the resin for the above plastic film, it is necessary to consider the components of the backlight unit. When using white LEDs as point light sources, it is sufficient to select taking into consideration the light transmittance of only visible light. However, when using a QD film or a color conversion film that uses phosphors, light with wavelengths shorter than that of visible light is also converted into visible light, so it is necessary to select a resin that absorbs less short-wavelength light.

Regarding the thickness of the plastic film that constitutes the optical waveguide layer, it can be said that from the viewpoint of improving the optical waveguide effect, the thicker it is, the better; however, there are restrictions from the viewpoint of making the entire backlight unit thinner, and from the viewpoint of the roll-to-roll production method that is preferably adopted as the manufacturing method for the optical film of the present invention, it is considered that the maximum thickness is about 400 μm.

The content of the fine particles contained in the optical waveguide layer is not particularly limited as long as it is within the range in which the effects of the present invention can be obtained, but a content that causes the haze value of the optical waveguide layer alone to be preferably in the range of 25% to 85%, and more preferably in the range of 50% to 70%, is preferably adopted. Also, although there is no direct one-to-one correspondence with these numerical ranges, from the standpoint of weight percentage, empirically, it is preferable that the content be 5 weight percent or less, and more preferably 1 weight percent or less, relative to the weight of the plastic that constitutes the optical waveguide layer.

As for the type of such fine particles, basically any fine particles can be used as long as they have a light diffusing function and can obtain the effects of the present invention. However, when a color conversion film using a QD film or phosphor is used in the backlight unit, light with a shorter wavelength than the wavelength of visible light is also converted into visible light, so it is necessary to use fine particles that absorb less short-wavelength light. From this perspective, it is preferable not to select TiO2 particles because they absorb on the short-wavelength side.

(Features and characteristics)
The refractive index of the plastic constituting the optical waveguide layer is automatically determined when the type of plastic to be used is determined, but in the present invention, as described above, polycarbonate can be preferably used, in which case the nominal refractive index is 1.58. Note that in the optical waveguide layer according to the present invention, the above-mentioned second fine particles for light diffusion are dispersed in this polycarbonate.

The optical film according to the present invention has the advantage that the refractive index of the plastic constituting the optical waveguide layer and the refractive index of the plastic constituting the optical waveguide introduction layer, which will be described in detail below, can be controlled independently of each other.

The function of the optical waveguide layer included in the present invention is to provide light diffusion, and has the following three functions.
1) By guiding light in all horizontal directions along the optical waveguide layer, the amount of light in places other than directly above the LED point light source is increased.
2) By scattering and emitting the guided light, the amount of light coming from the front of the LED point light source is increased in places other than directly above the LED point light source.
3) By suppressing light absorption and loss, a significant decrease in front brightness is prevented.

3. Optical waveguide introduction layer (bottom layer)
(Manufacturing method and materials used)
The optical waveguide introduction layer included in the present invention can be formed from any resin as long as it is a transparent resin, and such resin is applied in an uncured state to one side of the plastic film constituting the optical waveguide layer. The mixture is applied onto a forming roller having a surface on which a pattern of fine prisms (prisms with a cross section of an approximately isosceles triangle) is formed in a single direction, forming a concave-convex shape on the roller surface. The resin to be used may be polycarbonate resin, urethane resin, polyether resin, polyester resin, acrylic resin, silicone acrylic resin, fluorine resin or fluorine-acrylic resin, urethane-acrylic resin, etc. Alternatively, at least one resin selected from among crosslinkable resins based on these resins, such as ultraviolet curing, electron beam curing, heat curing, isocyanate curing, and amine epoxy curing, or a mixture thereof, is preferably used. In some cases, it may be possible to use a simple thermoplastic resin. In addition, as a modified type of the manufacturing method, when a suitably selected resin is molded into a film and surface-bonded onto one side of the plastic film constituting the optical waveguide layer, the resin is juxtaposed along one direction on the surface. A forming roll having a fine prism pattern (a prism having an approximately isosceles triangular cross section) formed thereon is used, and the prism pattern having an approximately isosceles triangular cross section is transferred to the surface of a formed film that is surface-bonded thereto. can also be suitably used.

The optical film according to the present invention has the characteristic that the refractive index of this optical waveguide introduction layer (the refractive index of the resin that constitutes the optical waveguide introduction layer) and the refractive index of the optical waveguide layer that is in planar contact with it (the refractive index of the resin used in the optical waveguide layer) can be controlled independently of each other.

In this case, the apex angle of the triangle in the prism having an approximately isosceles triangular cross section is preferably 60° to 100°, more preferably 70° to 90°, and most preferably 75° to 85°, but the apex does not necessarily have to be the point where two straight lines intersect, and may be slightly rounded or have a slightly flat surface as long as the effects of the present invention are achieved. Also, the apex may have a small recess (concave stripe) formed therein, and a small protrusion (convex stripe) of approximately isosceles triangular cross section may be formed within this.

The arrangement pitch of the prisms with an approximately isosceles triangular cross section is not uniformly determined in relation to the arrangement pitch of the LED point light sources mounted in the direct-type point light source backlight unit, but in the case of a typical arrangement pitch of LED point light sources, it is preferably 20 to 100 μm. However, if a single ray of light incident on the triangular prism layer (light guide introduction layer) from the LED point light source is branched or deflected by the triangular prism layer, and the effect of light appearing to be emitted from two or more light sources is obtained on the light emission surface from the triangular prism layer, a pitch exceeding this range is also acceptable. Also, considering the principle of light refraction, even if a triangular prism having a similar cross-sectional shape with different cross-sectional sizes is formed in a random mixture, the effect of light appearing to be emitted from two or more light sources can be obtained, so such an embodiment can also be suitably applied.

(Characteristics)
The optical film according to the present invention has the characteristic that the refractive index of the optical waveguide introduction layer (the refractive index of the resin constituting the optical waveguide introduction layer) and the refractive index of the optical waveguide layer in planar contact therewith (the refractive index of the resin used in the optical waveguide layer) can be controlled independently of each other, but there may be a difference in the refractive index between these layers, or they may be the same.

Assuming that the present invention does not limit its technical scope in any way, it is believed that the remarkable effect is achieved when combined with a special point light source in which the width of the light beam from the blue LED point light source used in a direct-type point light source backlight unit equipped with a color conversion film such as a QD film is narrow and the directivity of the light is extremely high. The mechanism is assumed to be that the propagation direction of the light is divided and branched (deflected) into multiple parts by a geometric shape such as the triangular prism layer used in the light guide introduction layer, and the branched light is propagated through functional layers each having a different diffusion function to eliminate the unevenness of the final front brightness. Therefore, the branching (deflection) phenomenon of the optical axis due to the shape of the surface unevenness on the light incident surface side of the light guide introduction layer is a very important factor. Even with a similar shape, it is also possible to change the deflection angle by increasing the refractive index and further promote the guide of light. From this perspective, the optical waveguide introduction layer can be said to be suitable for use regardless of the shape of its exposed surface or the material it is made of, as long as it can split and branch (deflect) the propagation direction of light into multiple parts and allow the branched light to propagate through a functional layer that has a diffusion function.

The optical waveguide introduction layer included in the present invention is intentionally interposed between the LED point light source and the above-mentioned optical waveguide layer , and has the following three functions in a direct-type point light source backlight unit equipped with a color conversion film such as a QD film.
(1) The amount of light (amount of front light) in the vicinity of 0° directly above the LED point light source is reduced.
(2) By deflecting the highly directional light from an LED point light source to a desired angle, the waveguide of the light emitted from the optical waveguide introduction layer to the optical waveguide layer is promoted.
(3) It has the effect of preventing a significant decrease in front luminance by suppressing absorption and loss of light from the LED point light source.

The interface between the layers prepared by the above manufacturing method and having the above characteristics is not necessarily required to be flat, and it goes without saying that even if it has fine waves, it can be suitably used as long as it does not exceed the scope of the purpose and intent of the present invention.

4. Multi-layer optical sheet (two-layer type)
(Production method)
A laminated optical sheet is obtained by stacking two (or more) of the optical films described above in detail (optical films consisting of three layers, a light extraction layer, a light guide layer, and a light guide introduction layer, each having the same configuration) in such a manner that the prism layer with a cross section of approximately isosceles triangle that constitutes the light guide introduction layer faces the light source side. In this case, the method of stacking can be preferably adopted such that the longitudinal directions of the prisms with a cross section of approximately isosceles triangle of the triangular prism layer are mutually orthogonal. It is also preferably adopted to fix the stacked optical films in some way so that the relative positions do not change.

5. Direct-type point light source backlight unit and liquid crystal display device of the present invention (manufacturing method)
In a conventional direct-type point light source backlight unit equipped with a color conversion film such as a QD film, the light diffusion film or sheet that is mounted so as to directly receive light from the point light source (i.e., so that there is an air layer between it and the point light source) is simply replaced with the laminated optical sheet (at least two sheets laminated) of the present invention, that is, without increasing the number of parts, a direct-type point light source backlight unit and a liquid crystal display device that exhibit the remarkable effects of the present invention can be obtained.

The following is a detailed description of an embodiment in which the optical film according to the present invention is stacked to form a laminated optical sheet and mounted in a direct-type point light source backlight unit.

1. Configuration of a direct-type point light source backlight unit according to the present invention (cross-sectional conceptual diagram):
The cross-sectional structure of the direct type point light source backlight unit used in the evaluation of the examples is as shown in the conceptual diagram in Fig. 4. In the part corresponding to the bottom layer in the diagram, blue LED point light sources are arranged at a predetermined pitch (generally in a grid pattern), on top of which a laminated optical sheet in an embodiment in which two optical films according to the present invention are stacked is arranged parallel to a plane passing through all of the LED point light sources, on top of which a series of sheet-like bodies including a color conversion sheet are arranged in the same arrangement as in the conventional type, and on the top layer a diffuser is arranged to constitute a direct type point light source backlight unit according to the present invention.

2. LED point light source (470):
The LED used was manufactured by EPISTER (model number: ES-FFBCPE05C), and the driving current during use was 11.4 mA and the driving voltage was 30 V. The blue LED point light source (470) using the above LED elements was arranged in a lattice pattern with a pitch of 2.5 mm x 2.5 mm, 20 x 20 pieces. The light quantity characteristics at this time were as follows: When the measurements were taken, the following settings were made:
Maximum brightness: 16494.747 cd/cm2
Minimum brightness: 204.847 cd/cm2
Minimum brightness/maximum brightness: 1.24%
Average brightness: 821.365 cd/cm2
Standard deviation: 1.7980

3. Laminated optical sheet (laminated optical film 450 and optical film 450):
A laminated optical sheet obtained by stacking two optical films (450, 460) according to the present invention, as described above, so that the extension directions of the triangular prisms of the triangular prism layer are perpendicular to each other, was placed on the blue LED point light sources arranged as described above. The distance from the upper end of this blue LED point light source to the plane formed by the apexes of the triangular prisms of the lower optical film (in FIG. 4 ) constituting the laminated optical sheet according to the present invention was intentionally set to almost zero so that hot spots on the light emission surface of the direct-type point light source backlight unit equipped with the laminated optical sheet are most likely to appear (i.e., so that comparison and examination can be performed under the worst conditions), but during actual use, it can be appropriately set in consideration of the total thickness required for the direct-type point light source backlight unit.

The pitch of the triangular prisms in this triangular prism layer (reference number 230 in Figure 2) is 41 μm. The apex angle of the triangular prisms in the triangular prism layer is 80°. The resin that constitutes part of this triangular prism layer (reference number 210 in Figure 2) is an acrylic resin. Its refractive index after curing is 1.58.

The resin used in the optical waveguide layer that constitutes part of the two optical films according to the present invention that make up the laminated optical sheet is polycarbonate resin. The thickness of this optical waveguide layer is 200 μm.

The second microparticles for diffusing light that are dispersed and contained within the optical waveguide layer are silicone microparticles, which are dispersed microparticles with a refractive index of 1.42 and an average particle size of approximately 4 μm, are roughly spherical, and their content is such that the haze value of the optical waveguide layer alone is 58%.

The resin (reference number 210 in FIG. 2) that fixes the particles in the light extraction layer of the optical film is an acrylic urethane resin, which has a refractive index of 1.52 after curing, into which light diffusing particles (reference number 200 in FIG. 2) with a refractive index of 1.49 are fixed. The haze value of the light extraction layer alone at this time is 45%. Sa and Ssk, which are roughness indices of the uneven surface of the random pattern of the light extraction layer, are 1.74 and 1.92, respectively.

4. Color conversion sheet (reference number 440 in FIG. 4):
A color conversion film (440) is placed on the laminated optical sheet described above, with a predetermined gap between them, and is laminated with a predetermined functional film material. The light exit surface of the lower optical film, designated by reference number 480 in FIG. 4, is an uneven surface, and the lower surface of the upper optical film, designated by reference number 470, placed on top of it, has triangular prisms protruding from it, so that the protrusions are placed so as not to bite into the uneven surface of the lower optical film. The configuration and mutual spacing of the 3M BEFs (420, 430) placed above the light exit surface of this color conversion film (440) are the same as those in the conventional direct-type point light source backlight unit.

Comparative Example

Comparative Example 1:
A direct-type point light source backlight unit was constructed and evaluated using the same conditions as in the above examples, except that a conventional optical film was used in which only the triangular prism layer was removed from the optical film configuration used in the above examples.

Comparative Example 2:
Instead of the optical film used in the above-mentioned embodiment, a direct-type point light source backlight unit was constructed and evaluated using the same conditions as those in the above-mentioned embodiment, except that the light exit layer and the light input layer were made of a three-layer structure different from the material composition of the light extraction layer of the optical film used in the above-mentioned embodiment (see Table 1), and the optical waveguide layer was made of a transparent PET (polyethylene terephthalate) layer with a thickness of 188 μm (not including fine particles for light diffusion), so that the front brightness was as close as possible to the front brightness in the embodiment. Note that the configuration of Comparative Example 2 is conceptually a configuration that applies the configuration of the invention disclosed in JP-A-6-59108 by the same applicant.

Comparative Example 3:
The optical films used were the same as those used in the examples, but when two of them were stacked and mounted, they were inverted upside down and stacked so that the triangular prism layer of each optical film was on the light exit surface side to form a direct-type point light source backlight unit and evaluate it.

Detailed data on the configurations of the above-mentioned Examples and Comparative Examples are shown in the table below.

In the above configurations of the example and comparative example, the luminance at each pixel point on the light exit surface (light exit surface from the diffuser) was measured under the following conditions. The luminance measuring device used was a two-dimensional luminance and chromaticity measuring device, ACE3-2000, manufactured by Hyland. The measurement conditions were as follows:
Measurement range: Area equivalent to 8 x 8 LED point light sources arranged in the center of the backlight unit (rectangular area with each side measuring approximately 17.5 mm)
Number of divisions: 64 x 64 (total 4096 cells)
Compression: 0% (no compression)
Measurement size: 4 pixels (1 pixel is 1 pixel of the measurement camera)
Measurement area (measurement position): Center of 64 x 64 cells Shutter speed: Auto (high dynamic sampling mode)
The relative size relationship between the LED point light sources and the divided cells is as shown in FIGS.

The appearance evaluation (evaluation of the degree of elimination of unevenness in luminance) was performed by taking the average luminance value of the 4096 cells as the evaluation value of the front luminance and its standard deviation as the evaluation value of the appearance. The evaluation results are shown in the table below.

As is clear from the above comparison data, the average brightness of the Example is slightly higher than that of the Comparative Example (0.24% higher than the Comparative Example), and at the same time, the standard deviation value of brightness (i.e., variation (appearance evaluation factor)) is reduced from 202.318 to 178.901. In other words, the Example has improved appearance while maintaining brightness compared to the Comparative Example.

Figure 7 shows the measurement data in Table 2 above, plotted on a graph with the standard deviation of brightness on the vertical axis and the average brightness normalized to the average brightness of Comparative Example 1 set at 100 on the horizontal axis. It is clear from this figure that the example maintains roughly the same average brightness as Comparative Example 2, while improving the appearance indicated by the standard deviation value.

The present invention has been described in detail above based on a representative embodiment, but it goes without saying that various variations may be adopted as long as they do not go beyond the scope of the purpose and intent of the present invention.

The laminated optical sheet, backlight unit, and liquid crystal display device according to the present invention can be used for digital signage in the display devices of various digital devices.

1: Direct-type point light source backlight unit according to the prior art 100: Diffuser 110: 3M BEF (triangular prism faces upward and its extension direction is parallel to the paper surface)
120: 3M BEF (The triangular prism faces upward and its extension direction is perpendicular to the paper surface)
130: Color conversion film 140: BLT
150: Light diffusion sheet 160: Blue LED light source 2: Optical film of the present invention 200: Particles fixed to the light extraction layer 210: Resin of the light extraction layer (after curing)
220: Optical waveguide layer 230: Optical waveguide introduction layer (extension direction of triangular prism is perpendicular to the paper surface)
4: Cross-sectional conceptual diagram of a direct-type point light source backlight unit of the present invention 410: Diffuser 420: 3M BEF (the triangular prism faces upward and its extension direction is parallel to the paper surface)
430: 3M BEF (The triangular prism faces upward and its extension direction is perpendicular to the paper surface)
440: Color conversion film 450: BLT
460: The optical film of the present invention arranged on the upper side (see FIG. 2). As described in the specification, it has a three-layer structure, and is composed of a light extraction layer, a light guide layer, and a light guide introduction layer (the extension direction of the triangular prisms in the triangular prism layer is parallel to the paper surface) from top to bottom of the paper, but the three-layer structure is not shown in the drawing. In addition, the internal structures of the light extraction layer and the light guide layer are not shown.
470: Optical film of the present invention arranged on the lower side (see FIG. 2). As described in the specification, it has a three-layer structure, and is composed of a light extraction layer, a light guide layer, and a light guide introduction layer (the extension direction of the triangular prisms in the triangular prism layer is perpendicular to the paper surface) from top to bottom of the paper, but the three-layer structure is not shown in the drawing. In addition, the internal structure of the light extraction layer and the light guide layer is not shown.
480: Blue LED point light source 500: Range of one cell 510: Blue LED point light source 520: One measurement size (4 pixels)
600: Full measurement range (64 x 64 cells)
610: Close-up of the area covering the four LED point sources in the entire measurement range. 620: One cell (corresponding to reference number 500 in FIG. 5).
630: Corresponding to reference number 520 in FIG. 5 (one measurement size (4 pixels)) 640: Individual blue LED point light source (corresponding to reference number 510 in FIG. 5)

Claims (6)

A direct-type point light source backlight unit,
The present invention has a light source in which a plurality of point light sources are arranged in a grid at a predetermined pitch, the point light sources being constituted by a blue LED package that does not have a fluorescent material in the sealing material of the LED package, a color conversion sheet , two BEF films in which the extension directions of the respective triangular prisms are perpendicular to each other , and a diffuser,
An optical film is placed between the light source and the color conversion sheet,
The optical film has an integral structure including an optical waveguide introduction layer on a light incident side, a light extraction layer on a light exit side, and an optical waveguide layer between the optical waveguide introduction layer and the light extraction layer,
the light extraction layer has a first exposed surface on a surface side where light from the point light source is emitted from the optical film, the first exposed surface forming an uneven surface with a random pattern;
The optical waveguide layer (excluding the porous layer) contains fine particles for light diffusion and constitutes a layer between the light extraction layer and the optical waveguide introduction layer.
The optical waveguide introduction layer is a single triangular prism layer, and the apex side of the triangular prism of the triangular prism layer faces the light source, and the surface of the apex side of the triangular prism constitutes a second exposed surface.
It is characterized by:
The optical waveguide introduction layer has a feature of changing one beam of light into two beams of light,
The optical film is mounted in such a manner that at least two of the optical films are stacked and arranged such that the longitudinal directions of the prisms in the triangular prism layer are perpendicular to each other.
A direct-type point light source backlight unit characterized by the above-mentioned.
2. The direct-type point light source backlight unit according to claim 1, characterized in that the refractive index of the optical waveguide layer (excluding the porous layer) is not smaller than either the refractive index of the light extraction layer or the refractive index of the optical waveguide introduction layer.
The shape of the prisms included in the triangular prism layer in a cross section perpendicular to the longitudinal direction is an isosceles triangle, and the apex angle is 65 to 100 degrees.
3. The direct type point light source backlight unit according to claim 2.
The arrangement pitch of the prisms is 20 to 100 μm.
4. The direct type point light source backlight unit according to claim 2 or 3.
The optical film is mounted by stacking at least two of the optical films, with the triangular prism layer facing the light source side and the longitudinal directions of the prisms of the triangular prism layer being arranged so as to be perpendicular to each other.
5. The direct type point light source backlight unit according to claim 1, wherein the direct type point light source backlight unit is a backlight unit having a plurality of light sources.
A liquid crystal display device comprising the direct type point light source backlight unit according to any one of claims 1 to 5.

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