JP4600077B2 - Illumination device and liquid crystal display device - Google Patents

Description

The present invention relates to an improvement of a backlight unit (illumination device) in a liquid crystal display device including a built-in light source (backlight) for generating display light.
More specifically, a so-called edge light type backlight having a configuration in which light from a light source disposed on a side surface with respect to a display screen is guided and light is emitted from a light emitting surface to a liquid crystal panel (LCD panel) side. -It is related with the improvement of the planar illuminating device provided with the light-guide plate and the light source in a unit.

Usually, a so-called backlight, which is an illumination light source used on the back surface of a transmissive LCD panel, uses a light guide plate made of a transparent resin in order to uniformly guide light from the light source to the LCD panel.
As shown in FIG. 9, in the illumination device 14 in which the light source 12 is disposed on this type of light guide plate 10, the light incident on the light guide plate 10 from the end surface 11 of the light guide plate 10 is transmitted through the flat portion of the light guide plate 10. The light guide plate 10 travels in the light guide plate 10 along the average light guide direction F, which is the average direction of light guided by the light guide plate 10, while being totally reflected.
Although FIG. 9 shows an example in which a linear light source is used as the light source 12, the shape of the light source 12 is not limited to a linear shape, and may be, for example, a dot shape.
Prisms 16 are provided in places on the plane portion of the light guide plate 10, and light hitting the prisms 16 is emitted upward from the light guide plate 10 as indicated by an arrow 56 in the drawing.

  As an example of an illuminating device that does not use a prism as means for taking out light that enters the light guide plate 10 from the end surface 11 of the light guide plate 10 and propagates through the light guide plate 10 from the light exit surface (upper side in the figure). Have proposed a method of diffusing and emitting light by printing scattering dots on the surface of the light guide plate 10 and a method of forming a diffraction grating on the back surface of the light guide plate 10 (Patent Document 1).

In the above light guide plate, it is also an important technical problem to make the light emission amount uniform in the light emission surface corresponding to the display screen of the liquid crystal panel (that is, to make the luminance distribution in the display screen uniform). Yes, as an improved method of forming a diffraction grating on the back of the light guide plate 10,
A light guide plate that diffracts light from a light source incident from at least one end surface of a transparent plate-like body toward the surface of the plate-like body by a diffraction grating provided on the back surface of the plate-like body,
That at least one of the cross-sectional shape of the diffraction grating or the ratio of the grating part width / non-grating part width in the unit width is changed so that the luminance on the surface of the light guide plate increases and becomes uniform. A proposal such as a characteristic light guide plate (Patent Document 2) is also known.
Japanese Patent Laid-Open No. 7-248496 JP-A-9-325218

However, in such an illuminating device, when a point light source such as an LED is installed on the end face of the light guide plate, it is difficult to make the emitted light intensity uniform on the exit surface without reducing the light utilization efficiency. is there.
In particular, the emitted light in a direction orthogonal to an average light guide direction (an average direction of light guided in the light guide plate) F, which is an average direction in which light travels from an end surface on the light source side to an end surface on the side far from the light source It is extremely difficult to increase both the uniformity of the distribution of light and the utilization efficiency of light. Of course, it is not easy to make the light intensity constant on the side closer to the light source and on the side farther from the light source.

In addition, it is difficult to increase the ratio of the light incident on the light guide plate from the light source to the desired emission light while making the light intensity in the emission surface uniform, and when used as a backlight of a transmissive LCD panel, etc. In addition, there is a problem that the light use efficiency (the ratio of the light incident on the light guide plate from the light source that is converted into the emitted light in the desired angle range as illumination light) is low.
In particular, it is desirable for a backlight of an LCD panel to emit strong light in the normal direction of the exit surface, but it is extremely difficult to achieve this with only a combination of a light source and a light guide plate.

On the other hand, in order to convert the light emitted from the light guide plate into an appropriate light distribution, a method of inserting various optical films between the light guide plate and the transmissive display element has been proposed. Another problem arises that the thickness increases and the manufacturing cost increases.
Also, when using a prism, the structure is relatively large, so it is difficult to hide the prism arrangement pattern during visual observation, the light guide plate is thickened by the prism, the emission angle range of emitted light, There is a problem that the intensity of emitted light cannot be freely controlled.

  The present invention has been made in view of such circumstances, and an object thereof is to uniformly emit light on the light exit surface of the light guide plate without using an expensive special optical film (such as a prism) other than the light guide plate. An object of the present invention is to provide an illuminating device having a simple configuration capable of obtaining intensity and increasing light utilization efficiency and obtaining emission light having sufficient light intensity in the normal direction of the light emission surface.

A lighting device according to claim 1 of the present invention is provided.
One or more point light sources;
In an illuminating device comprising a light guide plate having a substantially sheet-like shape that enters and guides light from the light source from an end surface, and exits from a light exit surface.
A diffraction grating is disposed on the light exit surface of the light guide plate and / or on the surface facing the light exit surface so that the diffraction efficiency increases as the angle from the normal direction of the end surface of the light guide plate that is the light entrance portion increases. It is characterized by.

  In the illuminating device according to the first aspect of the present invention, in the illuminating device including the planar light guide plate that enters and guides light from the point light source from the side surface and exits from the light exit surface, the light of the light guide plate A diffraction grating is disposed on the surface facing the light exit surface and / or the light exit surface so that the diffraction efficiency increases as the angle from the normal direction of the light guide plate side surface (hereinafter referred to as the light incident side surface), which is the light incident portion, increases. is doing.

Therefore, the light intensity varies depending on the direction in which light incident on the light guide plate from a point light source such as an LED travels in the light guide plate, and the light intensity decreases as the angle from the normal direction of the light incident side surface increases. This is compensated for by an increase in, so that a very uniform light intensity is obtained at the exit surface.
At this time, since the light is diffracted in a direction designed in advance by the function of the diffraction grating, a desired emission angle of the emitted light can be obtained and a light loss can be reduced, and an illumination device with high light utilization efficiency can be realized.

In the invention of claim 2, since the diffraction grating in a small region is a basic unit (hereinafter referred to as a diffraction grating cell), and the diffraction efficiency is changed depending on the arrangement density and size of the cell, a desirable diffraction efficiency distribution is obtained. It can be easily obtained and a uniform emission light distribution can be easily and reliably realized.
It is possible to construct a light plate.

  In the invention of claim 3, the angle of the grating vector of the diffraction grating is substantially equal to the angle from the normal direction of the light incident side surface, so that the light guide plate diverges from a point light source installed near the incident side surface. With respect to the light guided inside, the light can be diffracted in the direction of the exit surface or its opposite surface, and it is easy to obtain the desired exit light as illumination light.

  In the invention of claim 4, by setting the spatial frequency of the diffraction grating to 1000 to 4000 lines / mm, the visible light that is guided through the light guide plate is emitted in the direction of the exit surface or its opposite surface. Light can be diffracted in the normal direction of the surface, and the optimum light can be efficiently obtained as the direction of emission of illumination light.

  According to the fifth aspect of the invention, the optical element whose efficiency for emitting light from the light guide plate increases as the distance from the light incident side surface of the light guide plate increases, opposes the light exit surface and / or the light exit surface of the light guide plate. Because it is arranged on the surface, a simple light guide plate can be produced using conventional optical elements such as prisms, and the illumination device with the above-mentioned uniformity can be realized, and a uniform and large light guide plate is also relatively easy. Can be created.

  In the invention of claim 6, since the diffraction grating is used as the optical element, the light guided in the light guide plate along the average light guide direction is transmitted by the light emitting surface of the light guide plate and the diffraction grating disposed on the opposite surface. Diffracted at a significantly different angle from total reflected light, and can be emitted especially in the normal direction of the light exit surface of the light guide plate. Therefore, illumination with high light utilization efficiency under conditions suitable for illumination such as LCD panels The apparatus can be easily realized with a simple configuration.

According to the lighting device of the present invention, it is not necessary to use an expensive and special optical film or the like, and the lighting device can be realized with a small number of member configurations.
Furthermore, since the emission optical element is composed of a diffractive optical element, the structure is extremely minute and can be easily formed in an arbitrary region by a fine processing technique or the like. The light guide plate having a very uniform emission light distribution can be configured.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a perspective view illustrating a configuration example of a lighting device in the lighting device of the present invention.
FIG. 2 is a side view showing a cross section of the illumination device of the present invention.
FIG. 3 is a plan view showing an example of a cell arrangement and a diffraction grating constituting the cell in the illumination device of the present invention.

  In other words, the illumination device according to the present embodiment enters light from the light source into the light guide plate from the side surface, guides the light from the light guide plate while totally reflecting the light, and is disposed on the light emission surface or the surface facing the light emission surface. Diffracted light is generated by the diffraction grating thus formed, and light emitted from the light guide plate, that is, illumination light can be obtained.

  Here, light emitted from a point light source such as an LED almost always has an angular distribution of light intensity similar to Lambertian or similar. Therefore, the strongest light is emitted in the front direction of the light source (substantially coincident with the normal direction of the light incident side surface of the light guide plate), and the light intensity decreases as the angle increases thereafter.

When such light is extracted from the light guide plate with the intensity distribution as it is, the light intensity distribution is non-uniform on the exit surface of the light guide plate.
This non-uniformity is particularly remarkable in the direction perpendicular to the normal direction of the light incident end surface near the light incident portion.

  In the present invention, by disposing a diffraction grating whose diffraction efficiency increases as the angle from the normal direction of the light incident side surface increases, on the light exit surface of the light guide plate and / or the surface facing the light exit surface, Since the change in light intensity according to the angle of light incident on the light guide plate from the light source is compensated by the increase in diffraction efficiency, extremely uniform light intensity can be obtained on the exit surface.

  At this time, since the light is diffracted in the direction designed in advance by the function of the diffraction grating, a desired emission angle of the emitted light can be obtained and a light loss can be reduced, and an illumination device with high light utilization efficiency can be realized. At this time, the diffraction grating function diffracts at a significantly different angle from the totally reflected light, and in particular, the light can be emitted toward the normal direction of the light exit surface of the light guide plate. Under the conditions, a lighting device with high light utilization efficiency can be easily realized with a simple configuration.

As shown in FIGS. 1 and 2, by disposing the reflection sheet on the surface facing the light exit surface of the light guide plate, the main diffracted light component can be effectively used as illumination light.
An example in which a display device is configured in combination with the transmissive liquid crystal display panel 18 is shown in FIG.

FIG. 2 shows a state in which the diffraction grating 38 in the illumination device of FIG. 1 emits light in the light guide plate.
When the illumination device of the present invention is used as a backlight of a transmissive LCD panel, it is desirable that the diffraction grating 38 converts the light being guided into light that exits in a direction substantially perpendicular to the exit surface. This can be easily realized by using a diffraction grating as the diffraction grating 38.
Furthermore, it is possible to set the angle range of the emitted light as appropriate by the function of the diffraction grating, and to realize illumination light having an optimal angular distribution as a display without using any other optical film. Can do.

  Although the diffraction efficiency can be realized by changing the efficiency of the diffraction grating itself, the present invention includes a change in the area occupied by the diffraction efficiency per unit area as a macroscopic diffraction efficiency control method.

FIG. 3 shows an example in which the diffraction grating in a small region is a basic unit (hereinafter referred to as a cell), and the diffraction efficiency is changed depending on the arrangement density and size of the cells.
Therefore, a desirable diffraction efficiency distribution can be easily obtained, and a uniform emission light distribution can be easily and reliably realized.
FIG. 3 shows the case where one point light source is used. However, when three point light sources are used, for example, the configuration shown in FIG.

When a relief type diffraction grating is used as the diffraction grating, it is easy to design a pattern and to form it on a light guide plate.
At this time, since the height (depth) of the structure of the relief type diffraction grating is typically about 0.1 to 1 μm, it is possible to realize the light guide plate 10 that can be regarded as a substantially flat surface with no unnecessary protrusions. That is, the lighting device can be thinned.
Furthermore, it can be integrally formed with the light guide plate 10 and can be manufactured very simply and inexpensively.

Here, the function of the diffraction grating will be discussed in more detail.
Of the light guided through the light guide plate, the light incident on the diffraction grating produces diffracted light, but the normal main diffracted light is first-order diffracted light. In the diffraction grating which is the most basic structure of the diffractive optical element, the relationship between the grating pitch d of the diffraction grating and the emission angle (diffraction angle) θR of the first-order diffracted light is expressed by the following equation (1).
d = mλ / (sin θi−sin θR) (1)
Here, m is the diffraction order, λ is the wavelength of light in the light guide plate, and θi is the regular reflection angle (when the diffraction grating acts during reflection). An almost similar expression is established during transmission.

In order to convert the light traveling in the light guide plate into the emitted light, the diffraction grating that works most effectively is a case where it has a grating vector along the light guide direction.
That is, by making the grating vector direction and the light guide direction F substantially the same and appropriately setting the grating pitch d, the light traveling in the light guide direction F while being totally reflected is diffracted by the diffraction grating at an angle of θR and totally reflected. The light exits from the light exit surface 26 of the light guide plate 10 outside the conditions. In particular, when θR to 0 °, illumination light is emitted almost perpendicularly to the surface of the light guide plate, and is most preferable as illumination light for a transmissive display.

The diffraction grating not only emits the light guided through the light guide plate as diffracted light, but also can control how the diffracted light spreads (exit angle range).
Specifically, the linear diffraction grating pattern as shown in FIG. 5A has only a function of bending the light being guided, and the curved diffraction grating pattern as shown in FIG. 5B is emitted. The range of diffracted light can be arbitrarily designed according to the pattern.
Here, the diffraction grating pattern on the light guide plate can be configured to effectively act on light satisfying the total reflection condition when the light guide direction and the average grating vector direction are matched. That is, the direction of the emitted light can be greatly different from the light being guided, and the light can be reliably emitted from the light guide plate.

Furthermore, in the light guide direction, by giving a width to the spatial frequency of the diffraction grating, the first-order diffracted light emitted from the light guide plate can be emitted within an angle range corresponding to the width of the spatial frequency, and set in advance. Since light is intensively emitted in the angle range, light can be used efficiently.
Here, the outer shape of the cell of the emission optical element may be any of a rectangular shape, a circular shape, and an elliptical shape as shown in FIG.
Moreover, all the cells arranged on the same light guide plate 10 may have the same shape, or a rectangular shape, a circular shape, and an elliptical shape may be mixed.

As a design example, if the cell has a short shape in the average light guide direction F and a long shape in a direction perpendicular to the average light guide direction F, the light is emitted in the same direction as the average light guide direction F by the diffraction effect due to the shape of the cell. The angle distribution of the emitted light can be controlled independently of the design of the diffraction grating. The spread of the emitted light contributes to suppressing unnecessary coloring in the diffracted light.
Further, when the arrangement interval of the emission optical elements is 100 μm or less, the resolution is less than the resolution of the human eye under general observation conditions. Even when the light emission surface 26 of such a lighting device is visually observed, the size of the emission optical elements is reduced. Since the height is sufficiently small and a sufficient number of emission optical elements can be arranged per unit area, it can be observed as a surface that emits uniform emission light.

The diffraction grating has a very small structure, and can easily form a diffractive optical element having an arbitrary optical function in an arbitrary region by using a microfabrication technique. It is possible to construct a light guide plate having a distribution.
Normally, the light emitted from the light guide plate has a higher light emission intensity toward the end face closer to the light source (incident side), and the light emission intensity lower toward the end face farther from the light source.
Therefore, by increasing the diffraction efficiency of the diffraction grating as the distance from the light incident side of the light guide plate 10 increases, the ratio of the light emitted from the light guide plate 10 on the incident side where the light intensity is strong decreases, and as the distance from the incident side increases. The ratio can be increased, and light with uniform intensity can be emitted over the entire light exit surface 26 of the light guide plate 10.

As shown in FIG. 8, it is also possible to use a combination of the above-described diffraction grating and a conventional emission optical element.
For example, the light emitting surface of the light guide plate and / or the surface facing the light output surface so that the efficiency for emitting light from the light guide plate increases as the distance from the light incident side surface of the light guide plate increases. If it arrange | positions, a simple light-guide plate can be produced using the optical element which consists of the conventional prism etc. In addition, a large light guide plate that realizes uniform emission light can be produced relatively easily.

Here, if a diffraction grating is used as the emitting optical element, the light guided in the light guide plate along the average light guide direction is totally reflected by the light emitting surface of the light guide plate and the diffraction grating disposed on the opposite surface. Diffracted at an angle different from that of light, and in particular, can be emitted toward the normal direction of the light exit surface of the light guide plate. It can be easily realized with a simple configuration.
In addition, since a diffraction grating that functions effectively to make it uniform can be manufactured at one time, the manufacturing process can be simplified.

  As described above, the illuminating device of the present invention can obtain illumination light that is emitted substantially in the normal direction of the light emission surface without using a relatively expensive optical sheet such as a prism, and has an emission angle range. It is possible to provide an illumination device that is controlled, can achieve a uniform distribution in the exit surface, has high light utilization efficiency, and can obtain exit light with little color change due to an angle, with a simple configuration.

In addition, the light guide plate 10 may be configured not only to have a flat plate shape (a constant thickness) as shown in FIG. 1 but also to gradually reduce its thickness as it proceeds along the average light guide direction F.
In addition, the present invention does not prevent the use of a cheap and thin structure sheet such as a diffusion sheet, and a diffusion sheet is disposed on the light exit surface side of the light guide plate in order to ensure the uniformity of light within the exit surface. You may do it.

As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, this invention is not limited to this structure.
Within the scope of the invented technical idea of the scope of claims, a person skilled in the art can conceive of various changes and modifications. The technical scope of the present invention is also applicable to these changes and modifications. It is understood that it belongs to.

The perspective view which shows the structural example of the illuminating device of this invention. The side view which showed the cross section in the illuminating device of this invention. The top view which shows the example of the diffraction grating which comprises the cell arrangement | positioning and cell in the illuminating device of this invention. The top view which shows the example of cell arrangement | positioning in the illuminating device of this invention in the case of using three point light sources. The figure which shows the structural example of the diffraction grating in the illuminating device of this invention. The top view which shows the example of the cell of this invention. The perspective view which shows the display apparatus with which the illuminating device of this invention is applied. The top view which shows the example of cell arrangement | positioning in the illuminating device of this invention in the case of using three point light sources. The perspective view which shows the display apparatus with which the illuminating device by a prior art is applied.

Explanation of symbols

F ... Average light guide direction 10 ... Light guide plate 12 ... Light source 14 ... Illumination device 16 ... Prism 18 ... Transmission type LCD panel 24 ... Display device 26 ... Light exit surface 28 ... Opposite surface 32 ... Reflector 38 ... Diffraction grating 39 ... Exit Optical element 50 ... Light 51 being guided 51 ... Diffracted light 56 ... Illumination light 58 ... Display light

Claims (7)

One or more point light sources;
In an illuminating device comprising a light guide plate having a substantially sheet-like shape that enters and guides light from the light source from an end surface, and exits from a light exit surface.
A diffraction grating is disposed on the light exit surface of the light guide plate and / or on the surface facing the light exit surface so that the diffraction efficiency increases as the angle from the normal direction of the end surface of the light guide plate that is the light entrance portion increases. A lighting device characterized by the above.   2. The illumination device according to claim 1, wherein a diffraction grating formed in a cell-like small region is used as a basic unit, and the diffraction efficiency is changed depending on the arrangement density and size of the cells.   The illumination device according to claim 1 or 2, wherein an angle of a grating vector of the diffraction grating is substantially equal to an angle from a normal direction of the end face.   4. The illumination device according to claim 1, wherein a spatial frequency of the diffraction grating is set to 1000 to 4000 lines / mm.   The optical element that increases the efficiency of emitting light from the light guide plate according to the distance from the light incident end face of the light guide plate is disposed on the light exit surface of the light guide plate and / or the surface facing the light exit surface. The illuminating device according to claim 1, wherein:   6. The illumination device according to claim 5, wherein a diffraction grating is used as the optical element. A liquid crystal panel that defines a display image in accordance with translucency / light shielding in pixel units;
A liquid crystal display device comprising at least the illumination device according to claim 1.