JP2007225853A - Optical sheet, backlight unit using the same and display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To further effectively utilize a backlight light source in improvement of an optical sheet used for controlling an illumination optical path in a backlight unit for a display using a liquid crystal display element. <P>SOLUTION: In the optical sheet which is used for controlling the illumination optical path in the backlight unit for the display and provided with a light reflection layer having a surface having high light reflectivity facing the backlight light source side and reflecting light made incident on the surface to the backlight light source side on a flat surface on the side opposite to a lens part of a lens sheet having the lens part having unit lenses arranged in parallel on one surface of the lens sheet and wherein the light reflecting layer has a stripe shape having aperture parts while being made to correspond to the unit lenses of the lens sheet by one to one, the unit lens is a half cylindrical convex cylindrical lens as a whole and its sectional shape has two or more curvatures different from each other in a center part and a peripheral part. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an improvement in an optical sheet used for illumination light path control in a display backlight unit mainly using a liquid crystal display element, and relates to a backlight unit and a display on which the sheet is mounted.

A display typified by a liquid crystal display device (LCD) is remarkably widespread in a type including a light source necessary for recognizing provided information. In a battery-powered device such as a laptop computer, the power consumed by the light source occupies a substantial portion of the power consumed by the entire battery-powered device.
Thus, reducing the total power required to provide a given brightness increases battery life, which is particularly desirable for battery powered devices.

  A brightness enhancement film (BEF) which is a registered trademark of 3M Corporation of the United States is widely used as an optical sheet for solving this problem.

BEF is a film in which unit prisms 72 having a triangular cross section are periodically arranged in one direction on a member 70 as shown in FIG.
The prism 72 has a size (pitch) larger than the wavelength of light.
BEF collects light from “off-axis” and redirects this light “on-axis” or “recycle” towards the viewer. To do.

  When using the display (when observing), BEF increases on-axis brightness by reducing off-axis brightness. Here, “on-axis” is a direction that coincides with the visual direction of the viewer, and is generally the normal direction (direction F shown in FIG. 1) side with respect to the display screen.

  When the repetitive array structure of the prisms 72 is arranged in only one direction, only the direction change or recycling in the parallel direction is possible, and in order to control the luminance of the display light in the horizontal and vertical directions, the prism groups are arranged in parallel. Two sheets are stacked and combined so that the directions are substantially orthogonal to each other.

The adoption of BEF allows display designers to achieve the desired on-axis brightness while reducing power consumption.
As patent documents disclosing that a brightness control member having a repetitive array structure of prisms 72 typified by BEF is adopted for a display, many are known as exemplified in Patent Documents 1 to 3. ing.
Japanese Patent Publication No. 1-378001 JP-A-6-102506 Japanese National Patent Publication No. 10-506500

In the optical sheet using the BEF as a brightness control member as described above, the light P from the light source 20 is finally emitted at a controlled angle φ by the refraction action x as shown in FIG. Thus, it is possible to control to increase the intensity of light in the visual direction F of the viewer.
However, at the same time, the light component due to the reflection / refraction action y is unnecessarily emitted in the lateral direction without proceeding in the visual direction F of the viewer.

  Therefore, the light intensity distribution emitted from the optical sheet using the BEF as shown in FIGS. 1 and 2 is the light in the viewer's visual direction F, that is, the angle at 0 ° with respect to the visual direction F as shown in FIG. Although the intensity can be maximized, there is a problem that the amount of light emitted unnecessarily from the lateral direction increases as shown by a small light intensity peak around ± 90 ° shown on the horizontal axis in the figure.

  In order to overcome such drawbacks, there is a backlight unit 40 using an optical film 38 having a repetitive array structure of unit lenses instead of a prism as shown in FIG. 4 (Patent Document 4).

A lens 44 for guiding the light traveling in the optical film 38 to the liquid crystal panel 42 is provided on the surface of the transparent base 39 of the optical film 38 on the liquid crystal panel 42 side. As shown in the perspective view of FIG. 5, the lens 44 has a plurality of unit lenses repeatedly forming an array structure.
Further, on the other surface, a reflector 48 having a stripe pattern having an opening 46 in the vicinity of the focal plane of the lens 44 is provided.

The reflecting material 48 is obtained by mixing a mixture of white titanium dioxide (TiO ₂ ) powder in a solution such as a transparent adhesive with a predetermined pattern (when the unit lens is a semi-cylindrical convex cylindrical lens group, each unit lens Is formed in a stripe shape having an opening corresponding to 1: 1).

FIG. 6 is an explanatory diagram showing optical path control characteristics of the backlight when the optical sheet of FIGS. 4 and 5 is applied to the backlight unit.
Of the light emitted from the diffusion film 32, only the light that has passed through the opening 46 enters the lens 44 and is emitted after being condensed in a certain direction by the lens 44.
Then, the light enters the polarizing plate 49 and only light having a predetermined polarization component is guided to the liquid crystal panel 42.

  On the other hand, the light that could not pass through the opening 46 is reflected by the reflecting material 48, returned to the diffusion plate 26 side, and guided to the reflecting plate 27. Then, the light is incident on the diffusion plate 26 again by being reflected by the reflection plate 27 and is diffused again on the diffusion plate 26. 6 and is emitted by the lens 44 after being narrowed down within a predetermined angle φ as shown in FIG.

In the backlight unit 40 using such an optical film 38, the size and the position of the opening 46 of the optical film 38 are adjusted to increase the light use efficiency, and the light is emitted from the lens 44 in the front direction S. Can be controlled to increase the proportion of light emitted.
JP 2000-284268 A

  In the case of a display having a large screen such as a liquid crystal TV or a personal computer monitor rather than a display having a relatively small screen such as a mobile phone or a mobile terminal, in order to make the luminance distribution in the screen uniform. It is preferable to employ a direct type backlight unit in which a light source is housed in a lamp house behind the screen, rather than a backlight unit using an edge light to a light guide plate.

  As shown in FIG. 4, the backlight unit in the liquid crystal display device is in the form of a laminated structure of each element, and the optical film 38 / the diffusion film 32 / the diffusion plate 26 includes the lamp house 21 in which the light source 23 is accommodated. Placed on top.

  As described above, as compared with the brightness enhancement film having a configuration in which unit prisms having a triangular cross section are arranged in parallel, semi-cylindrical convex cylindrical lenses are arranged in parallel, and each unit lens has a corresponding 1: 1 aperture. An optical sheet having a stripe pattern is advantageous in terms of utilization efficiency of a backlight light source, but depending on the design, there may be a loss of light amount according to the phenomenon described below.

FIG. 7 is an explanatory diagram showing an example of ray tracing (simulation) in one unit lens of the optical sheet.
In the figure, light incident from the backlight unit (light source) on the left side through the opening in the reflective layer having an aperture ratio of 40% is refracted (or reflected) by the lens unit and is reflected on the right side. The behavior of emitting light to the liquid crystal panel side (and the observer side) is shown.

  As shown in FIG. 7A, when the light that has entered the lens sheet through the opening hits the measuring unit (near the boundary with the adjacent unit lens) instead of the center (top) of the unit lens ( When the horizontal direction of the drawing, which is a perpendicular to the lens sheet incident surface, is 0 ° and the incident angle with respect to the lens sheet is large), there are many components that cause internal reflection at the lens measurement unit, and the vertical direction in FIG. Later, there is also a component of light returning to the light source side, and much light is not emitted directly to the right liquid crystal panel side.

  On the other hand, as shown in FIG. 7B, when the light incident on the lens sheet through the opening hits the vicinity of the center (top) of the unit lens, the light path is directly refracted by the lens. After being controlled (as substantially parallel light in the horizontal direction of the drawing), much light is emitted to the right liquid crystal panel side.

  The main object of the present invention is to eliminate the light loss caused by the phenomenon shown in FIG. 7A and to achieve a configuration that can realize more effective use of the backlight light source by the optical sheet. .

In order to achieve the above object, the optical sheet according to the present invention comprises:
The flat surface on the side opposite to the lens portion of the lens sheet having a lens portion in which unit lenses are arranged in parallel on one side has a highly light-reflective surface facing the backlight light source side. It has a light reflection layer that reflects incident light to the backlight source side,
In the optical sheet used for illumination light path control in a backlight unit for a display having a configuration in which the light reflecting layer has a stripe shape corresponding to each unit lens of the lens sheet in a 1: 1 ratio,
The unit lens is a semi-cylindrical convex cylindrical lens as a whole, and has a cross-sectional shape having two or more kinds of curvatures different in a central portion and a peripheral portion.

By improving the shape of the unit lens, the utilization efficiency of the backlight light can be further improved, and a display device that can visually display high-luminance display light is obtained.
In addition, as will be described later, while maintaining the front luminance of the display light, it is possible to have a distribution in which the luminance gently changes in the left and right (unit lens parallel direction), similar to a light diffusion sheet Thus, the lens sheet itself has both the optical function and the light diffusing sheet can be omitted, so that the number of members can be reduced and the cost of the backlight unit can be reduced accordingly.
Furthermore, it is more effective in eliminating moire caused by the relationship between the periodicity of the unit lens and the periodicity of the pixel arrangement in the liquid crystal panel.

Embodiments of the present invention will be described below.
FIG. 8 is an explanatory diagram showing an example of ray tracing (simulation) in one unit lens of the optical sheet according to the present invention.
In the figure, light incident from the backlight unit (light source) on the left side through the opening in the reflective layer having an aperture ratio of 40% is refracted (or reflected) by the lens unit and is reflected on the right side. The behavior of emitting light to the liquid crystal panel side (and the observer side) is shown.

As shown in the figure, the unit lens is a semi-cylindrical convex cylindrical lens as a whole, and the cross-sectional shape thereof has two or more kinds of curvatures different in the central part and the peripheral part.
That is, the curvature of the central part constituting the unit lens is small and protrudes to the right side of the figure, the curvatures on both sides (up and down in the figure) are large, and the peripheral part with the large curvature / the central part with the small curvature. / It consists of a peripheral part with a large curvature, and is vertically symmetrical in the figure.

7A is different from the prior art in FIG. 7A. As shown in FIG. 7A, the light incident on the lens sheet through the opening portion is measured by the measuring unit (adjacent unit lens) of the unit lens. In the vicinity of the boundary of the unit lens, there are many components that are incident around the vertical direction according to the curvature change of the measurement unit of the unit lens, so there is no component that causes total reflection (internal reflection at the lens measurement unit), and the lens That is, a large amount of light is emitted to the right liquid crystal panel side after being refracted by the portion.
Therefore, in the same figure, there is no component light returning to the light source side after going up and down, and light not directly emitted to the right liquid crystal panel side is reduced.

In FIG. 8B, since the light that has entered the lens sheet through the opening hits the vicinity of the center (top) of the unit lens, the refraction by the lens is directly performed as in the case of FIG. 7B. After receiving and controlling the optical path (as substantially parallel light in the horizontal direction of the drawing), much light is emitted to the right liquid crystal panel side.
In the case of FIG. 8B, since the curvature of the central portion of the unit lens is small and the curve is steep compared to FIG. 7B, the incident light causes total reflection at the “end in the central portion”. The possibility increases, but within the range of the incident angle of the light incident on the lens sheet through the opening, the above-mentioned possibility does not need to be considered, and the thickness of the lens sheet, the opening width, and the central portion. The curvature is designed.

FIG. 9 is a graph showing a comparison between the light distribution characteristic (solid line) of the display light by the optical sheet of the present embodiment and the light distribution characteristic (dotted line) of the display light by the optical sheet according to the prior art shown in FIG.
7 and 8, the unit lens arrangement pitch, lens height, aperture ratio, and the like are the same, and only the cross-sectional shape of the unit lens is different.

As is apparent from the graph, in FIG. 7 (dotted line of the conventional shape), the high luminance range is wide (about ± 20 °) centering on the front direction (0 °) of the screen, but the luminance is outside that range. The reduction is remarkable, and as a result, the viewing area is limited to the front direction.
On the other hand, in FIG. 8 (solid line in the present embodiment), the high brightness range is relatively narrow around the front direction (0 °) of the screen, but even when observed from an angle deviating from the front direction, The decrease in brightness becomes smooth, and as a result, the viewing area is not limited to the front direction.
The luminance distribution according to FIG. 8 is equivalent to the effect of uniforming the luminance distribution in the screen or widening the viewing area by employing a light diffusion sheet obtained by dispersing and mixing a light diffusing agent (filler). It can be said that the optical sheet according to the above also has a light diffusion function based on the lens design.

10 and 11 are plan views (photographs) of the optical sheets according to FIGS. 7 and 8 viewed from the lens unit side.
In FIG. 10 (conventional shape in FIG. 7), a stripe pattern is observed by the boundary line of the unit lens according to the parallel pitch of the unit lenses (semi-cylindrical cylindrical lens).
In FIG. 11 (the shape of the present embodiment in FIG. 8), the curvature does not continuously and smoothly change at the boundary between the peripheral portion (large curvature) and the central portion (small curvature) constituting the unit lens. A stripe due to the above is also observed, and the pitch of the stripe pattern is reduced (about 1/3 of FIG. 10).

In a liquid crystal panel (an image display element composed of a liquid crystal display element that defines a display image according to transmission / light-shielding in pixel units), the pixel pitch is remarkably miniaturized to realize high-resolution image display. The arrangement of about 500 μm pitch is also in the direction of further miniaturization.
Therefore, even in the lens sheet (or the prism sheet according to the prior art) as the optical path control element, the parallel pitch of the unit lenses (unit prisms) is also reduced in order to avoid moire caused by the relationship with the periodicity with the liquid crystal pixel pitch. Although it is necessary to reduce the size, in the present embodiment, the pitch of the unit lens can be kept constant and the stripe pitch can be reduced to about 1/3, which is effective in eliminating moire.
When the unit lens is composed of a plurality of parts (3 parts, 5 parts) having different curvatures, if the width of each part is constant, the pitch is accurately 1/3 (1/5), and the unit lens pitch is strong. Since it does not appear, it is effective in further reducing the moire contrast.

  The arrangement pitch of the unit lenses of the optical sheet needs to be selected within a range from 1: 1 or 1: 1.5 in consideration of moire caused by the overlapping of periodicity between the liquid crystal pixels and the unit lenses. There are many cases where it is designed around 0.3 mm (300 μm) due to the formability in the press and extrusion molding using a thermoplastic resin, but depending on the material (configuration) of the lens sheet, such as adopting the 2P method, it is 0. Miniaturization of around 1 mm is possible.

<Other embodiments>
FIG. 12 is a schematic diagram showing another embodiment of the present invention. The unit lens has a cross-sectional shape of peripheral part 2 / peripheral part 1 / central part / peripheral part 1 / peripheral part 2, and three types A lens having a curvature gradually decreases in the order of peripheral part 2, peripheral part 1, and central part, and has a symmetrical shape as a whole.
In the case of the figure, the curvature does not change smoothly and smoothly at the boundary between the peripheral part and the central part constituting the unit lens or at the boundary between the peripheral part 2 and the peripheral part 1 and the boundary line is clear. Thus, the pitch of the stripe pattern becomes 1/5.

<Other embodiment 2>
8 and 12 show the case where unit lens elements having different curvatures are stacked in order. As shown in FIG. 13, unit lens elements having different curvatures are discontinuously arranged on a plane. An improvement in which the whole constitutes one convex lens does not depart from the gist of the present invention.
In the case of FIG. 13, the inclination corresponding to the Fresnel surface becomes steep in steps as each unit lens element moves from the peripheral part to the central part as in a so-called linear Fresnel lens.
In the case of this embodiment, the optical sheet can be thinned.

The optical sheet according to the present invention is manufactured in the same manner as in the prior art except that the shape of a stamper (molding die) used to form the unit lens shape is changed.
In the formation of the light reflecting layer on the surface of the lens sheet opposite to the lens part, generally, various methods such as printing (coating), transfer, and photolithography are appropriately selected. In the case of a fine lens, alignment accuracy is required to form a stripe shape having an opening corresponding to each unit lens at a ratio of 1: 1. It is effective to employ a so-called “self-alignment technique” that defines the location where the reflective layer (opening) is formed by using.

  As an example of the self-alignment method, a photopolymer layer having a characteristic that the adhesiveness disappears due to light exposure is formed on the entire flat surface on the side opposite to the lens, and the light from each unit lens is condensed by exposure from the lens side. Of the condensing part (part where the photopolymer adhesive property disappears) / non-condensing part (the part where the photopolymer adhesive property remains) specified according to the action, white only at the part corresponding to the non-condensing part A technique of transferring and forming the transfer layer (or attaching a powdery light reflective material) is effective.

Explanatory drawing which shows "BEF" which is an optical sheet which concerns on a prior art. Explanatory drawing which shows the optical path control characteristic of the backlight by BEF. The graph which shows the optical path control characteristic of the backlight by BEF. Explanatory drawing which shows the optical sheet which concerns on another type of prior art from BEF. The perspective view which shows the optical sheet | seat of FIG. Explanatory drawing which shows the optical path control characteristic of the backlight by the optical sheet of FIG. Explanatory drawing which shows an example of the ray tracing (simulation) in one unit lens of the optical sheet which concerns on a prior art. Explanatory drawing which shows an example of ray tracing (simulation) in one unit lens of the optical sheet by this invention. The graph which compares and shows the light distribution characteristic (solid line) of the display light by the optical sheet of this embodiment, and the light distribution characteristic (dotted line) of the display light by the optical sheet by the prior art shown in FIG. The top view (photograph) which looked at the optical sheet of FIG. 7 from the lens part side. The top view (photograph) which looked at the optical sheet of FIG. 8 from the lens part side. Schematic which shows other embodiment of this invention. Schematic which shows other embodiment of this invention.

Explanation of symbols

23 Light source 26 Diffusing plate 27 Reflecting plate 38 Optical sheet 40 Backlight unit 42 Liquid crystal panel 44 Lens portion 46 Opening portion 48 Reflecting material 70 formed of stripe pattern 70 Member 72 Unit prism

Claims (7)

The flat surface on the side opposite to the lens portion of the lens sheet having a lens portion in which unit lenses are arranged in parallel on one side has a highly light-reflective surface facing the backlight light source side. It has a light reflection layer that reflects incident light to the backlight source side,
In the optical sheet used for illumination light path control in a backlight unit for a display having a configuration in which the light reflecting layer has a stripe shape corresponding to each unit lens of the lens sheet in a 1: 1 ratio,
The optical sheet, wherein the unit lens is a semi-cylindrical convex cylindrical lens as a whole, and the cross-sectional shape thereof has two or more kinds of curvatures different in a central part and a peripheral part.   The arrangement pitch of the unit lenses is 0.3 mm or less, the curvature of the central part constituting the unit lens is small, and the curvature of both sides (peripheral parts) is large. Optical sheet.   The unit lens is composed of five sections: a peripheral portion with a large curvature / a central portion with a small curvature / a peripheral portion with a large curvature, and the width of each portion is the same and symmetrical. The optical sheet according to claim 1 or 2, characterized in that:   The cross-sectional shape of the unit lens is composed of five parts of peripheral part 2 / peripheral part 1 / central part / peripheral part 1 / peripheral part 2, and the width of each part is the same and symmetrical. The optical sheet according to claim 1 or 2.   The curvature does not continuously and smoothly change at the boundary between the peripheral portion and the central portion constituting the unit lens, or the boundary between the peripheral portion 2 and the peripheral portion 1, and the boundary line is clear. The optical sheet according to claim 1. On the back of the image display element that defines the display image,
A backlight unit for a display, comprising at least a direct light source and the optical sheet according to claim 1. An image display element comprising a liquid crystal display element that defines a display image according to transmission / shading in pixel units;
A light source using a cold cathode ray tube or LED;
A display comprising the backlight unit according to claim 6.