Taking a lighting device for an image display devices an example, there is an edge light type in which light from a light source provided to a side edge of a light guide plate is guided toward the front direction and uniformized by a diffusion sheet and a direct type in which a light source is provided to the backside of an illuminated surface and light is uniformized by a light diffusion plate.
The direct type tends to be thicker as the light source is provided to the backside of a device, thus in the fields of requiring thinness such as cellular phone and mobile personal computer, the edge light type has been the mainstream, which is advantageous by having a light source provided to the side edge.
On the other hand, the request has been increasing for larger size and higher luminance of a display centering on the market of television and personal computer monitor etc. Especially along with the larger size of a display, in the abovementioned edge light type, as the rate of the length of a periphery part where a light source can be placed to a display area decreases and the amount of light runs short, sufficient luminance cannot be obtained.
Therefore, there is a method suggested to place a plurality of films for improving luminance over a surface light source to improve light utilization efficiency (see for example Patent Document 1).
However, as the luminance improvement film leads to an increase in cost and the number of films to use, it is not necessarily advantageous in terms of productivity and thinner device. Moreover, in the edge light type, there is also a problem that the weight of the light guide plate increases along with the larger size of a display. As described above, it has become difficult for the edge light type to meet the request in the market of recent years such as larger size and higher luminance of displays.
Thus, the direct type using a plurality of light sources is attracting attention. FIG. 15 shows an example of a lighting device of this style. In this example, a lighting device has a rectangle emission surface formed of X direction and Y direction, which is vertical to the X direction, includes a plurality of linear light sources 1, a light diffusion plate 5 and a reflective plate 4. The linear light sources 1 are placed in one virtual flat surface which is parallel to the X direction and the Y direction and the linear light sources in the longitudinal direction are placed in parallel to the Y direction and also placed at a regular interval along with the X direction. The light diffusion plate 5 is placed to the emission surface side of the arranged linear light sources 1 and the principal surface is parallel to the virtual flat surface where the linear light sources 1 are arranged. The reflective plate 4 is positioned to the opposite side of the light diffusion plate across the arranged linear light sources 1 and the principal surface of the reflective plate 4 is parallel to the virtual flat surface where the linear light sources are arranged. Moreover, usually the light diffusion plate 5 is uniformly dispersed with a light diffusion material and has uniform optical performance in the principal surface.
The rectangular emission surface is the most common in many usages of this lighting device such as an image display device and a lighting signboard. In this direct type, the light utilization efficiency of light emitted from a light source, which is the ratio of luminous flux emitted from a light emitting surface to luminous flux emitted from the light source, is high and also the number of light sources can be increased freely. That is, as the amount of light can be increased freely, requested high luminance is obtained easily and there is no decrease in luminance and luminance uniformity caused by larger size. Furthermore, since the light guide plate for turning light to the front becomes unnecessary, the weight can be reduced.
Moreover, as it is easy to eliminate luminance non-uniformity with linear light source than point light source and it is also easy as the wiring is short, the linear light source is the most common for a light source of these lighting devices. Cold cathode tube etc. is often used as a linear light source. Furthermore, although it is advantageous to use the same type of linear light sources for production and for uniformizing luminance, in this case, it is desirable to arrange the linear light sources in the direction parallel to the long side of the rectangle shape of the emission surface as the number of the linear light sources can be reduced. Moreover, by placing the linear light sources at a regular interval in the same flat surface, the problematic luminance non-uniformity becomes periodical along with the arrangement of the linear light sources and it becomes easy to eliminate luminance non-uniformity in the light diffusion plate having uniform optical performance in the principal surface. Although the reflective plate is not necessary, it is advantageous in terms of improving light utilization efficiency as it has a function to reflect light emitted from the linear light sources and the light diffusion plate in the opposite direction to the emission direction and use it again as emitting light.
Moreover, in this direct type, since the light utilization efficiency emitted from a light source, which is the ratio of luminous flux emitted from an emission surface to the luminous flux emitted from a light source, is high and also the number of light sources can be increased freely, the high luminance requested is obtained easily. Furthermore, since the light guide plate for turning light to the front is unnecessary, the weight can be reduced.
Moreover, in a lighting signboard as an example of other lighting devices, the direct type using a plurality of light sources is the mainstream because the configuration is simple and high luminance can be easily obtained without using the film or the like for improving luminance.
As a typical configuration example of such lighting device of direct type, there is a known configuration in which one direction is made to be the front direction, which is the main outgoing light direction, where the direction is parallel to Z direction vertical to XY flat surface, which is parallel to X direction and Y direction and the Y direction is vertical to the X direction. At least a plurality of light sources, a reflective plate and a light diffusion plate for transmitting light from the light sources and the reflective plate to the outgoing light side are included. The reflective plate, the light sources and the light diffusion plates are placed along the Z direction to the outgoing light side in order of the reflection plate, the light sources and the light diffusion plate. The light sources are arranged regularly in one virtual flat surface which is parallel to the XY flat surface and the periphery of the principal surfaces of the reflective plate and the light diffusion plate are rectangle shapes parallel to the XY flat surface. With this configuration, the diffusion plate has a function to eliminate luminance non-uniformity of the linear light sources and the reflective plate has a function to return light traveling to the opposite direction to the target outgoing light direction back to the reflective plate by the outgoing light side.
As the light sources for this lighting device, linear light sources are often used, which have less luminance non-uniformity, less light source points and are easy to wire as compared to point light sources such as LED. The linear light sources are arranged at a regular interval in parallel and also along the X direction or the Y direction.
However, in the direct type, unique problems such as elimination of a lamp image, thinner shape and energy saving must be solved. Especially for usages to observe an illuminated surface such as an image display device and a lighting signboard, the luminance uniformity in the surface is required, not only the elimination of a lamp image. Furthermore, for usages to observe an illuminated surface mainly from the front direction such as television and personal computer monitor etc., the uniformity of the front luminance in the surface is the most important. Since the lamp image appears as luminance non-uniformity far more remarkable than in the edge light type, it is difficult to eliminate it by the means such as a diffusion film which is coated with a light diffusion material to the film surface used in the edge light type in related arts. Furthermore, reduction in the number of light sources, higher luminance and control of view angle characteristics, etc. are requested and it has been an issue of how to effectively utilize the light of the light sources.
Therefore, a light diffusion plate dispersed with a light diffusion material is widely used to a base material resin and a light diffusion plate is provided to the front surface side of a light source having a reflective plate placed to the backside. An example of the direct style display device using a light diffusion plate is already explained with reference to FIG. 15. In order to achieve favorable diffusibility and light utilization efficiency, to the base material resin such as methacrylate type resin, polycarbonate type resin, styrene type resin and vinyl chloride type resin, there are various light diffusion materials such as inorganic particulates and bridged organic particulates under consideration (for example see Patent Document 2). However, the method to use the light diffusion material is not preferable from the aspect of energy saving due to absorption of light into the light diffusion material and diffusion of light in unnecessary direction. Moreover, although a lamp image can be reduced by placing many light sources closely, there is a problem of power consumption increase.
Therefore, the method of improving front luminance using the luminance improvement films is used as in the edge light type. These films collect outgoing light toward the front direction and increase the front luminance usually requested as a lighting device so as to increase the light utilization efficiency. The front luminance can be further improved by combining a plurality of luminance improvement films and light in each direction of the X direction and the Y direction for example can be collected by placing them with different angles. However as with the edge light type, it is not necessarily advantageous in terms of productivity and thinner shape as it leads to an increase in cost and the number of films to use.
On the other hand, there is a method suggested to give a unique reflective plate shape for eliminating a lamp image (for example see Patent Document 3). However, it is not preferable because it requires to align the shape of the reflective plate with light sources and it may prevent from reducing the thickness due to the shape of the reflective plate.
Furthermore, there are methods suggested such as the method to provide a reflective member opposite to light sources (for example see Patent Document 4) and the method to provide a light beam direction conversion device like a Fresnel lens, for example, to every light source (see for example Patent Document 5). However there is a problem generated such as deterioration in productivity as an accurate alignment between the member and the light sources is required.
Moreover, a light diffusion plate with concave-convex surface is suggested (for example see Patent Document 6). Since these light diffusion plates can achieve desired diffusibility while avoiding to use or reduce light diffusion material, the light utilization efficiency can be improved. However, as there is no detailed investigation for concave-convex shape, precise adjustment of luminance non-uniformity is difficult. Likewise, it is difficult to obtain uniformity of front luminance within the emission surface.
Furthermore, there is a method suggested to give an unique shape to a reflective plate which is parallel to linear light sources so as to eliminate a lamp image (see for example Patent Document 7). However, in this case, an accurate alignment between the shape of the reflective plate and the linear light source is required. This not only causes to reduce production efficiency due to the alignment but requires to change the shape of the reflective plate if the light source arrangement differs due to a design change and a difference in specification. Moreover, the method to increase productivity by cutting out a desired shape from a large-sized shaping sheet is also not preferable as it leads to limit cutting position and a decrease in yield.
Further, a prism sheet with a small loss in the amount of light is suggested (see for example Patent Document 8). A large number of convex portions whose cross-sections are triangular or ridge shapes and continuously extending in one direction are formed to the both sides of the sheet. However, since these prism sheets aim to reduce the loss in the amount of light by turning diffused light to the front, a lamp image generated in the direct type cannot be eliminated.
In a large-sized lighting device, the request for thinner shape is not severe as compared to cellular phones and mobile personal computers etc., thus it is possible deal with it by shortening the distance between light sources and a light diffusion plate and reducing the number of optical films or the like. Moreover, it is necessary to increase the light utilization efficiency in order to realize energy saving. Although in the direct type, it is easy to increase the number of linear light sources as described above to achieve high luminance, reduction in the light utilization efficiency by using a large amount of light diffusion material for eliminating a lamp image must be suppressed in terms of energy saving.
The light diffusion plate in related arts is a simple light diffusion style in which light diffusion material particulates are kneaded in a transparent base material resin by extrusion method or injection molding process, and for the optical performance, luminance non-uniformity (transmitted afterimage of the light source) is practical usage level but there is a problem that it is difficult to control view angle. Moreover, it is necessary to consider the formation condition in order to prevent dispersion failure of the light diffusion material particulates, thus it has been difficult to increase productivity consequently.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2-17
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 54-155244
[Patent Document 3] Japanese Patent No. 2852424
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2000-338895
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 2002-352611
[Patent Document 6] Japanese Unexamined Patent Application Publication No. 10-123307
[Patent Document 7] Japanese Unexamined Patent Application Publication No. 1-169482
[Patent Document 8] Japanese Patent No. 3455884