Illumination – Revolving
Reexamination Certificate
2001-04-16
2001-12-25
Sikder, Mohammad (Department: 2872)
Illumination
Revolving
C362S029000, C362S035000, C362S026000, C362S302000
Reexamination Certificate
active
06332691
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a surface light source device for a display apparatus such as a liquid crystal display device for use in a portable personal computer, a liquid crystal television or the like; a sign display apparatus such as a guide marking board, a large-size signboard or the like used in a station, public facilities or the like; or for a traffic sign display apparatus such as various types of guide signs, traffic signs or the like on a highway road or a general road; and the present invention relates more particularly to a surface light source element for emitting light which has high brightness and a uniform brightness distribution on a light emitting plane without performing any uniformity-enhancing processing such as treatment with a spot pattern or the like.
DESCRIPTION OF THE RELATED ART
Recently, a color liquid crystal display device has been widely used in various applications such as in portable personal computers, liquid crystal televisions, video built-in type liquid crystal televisions, etc. This liquid crystal display device comprises a back light portion and a liquid crystal display portion. An under-lighting system in which a light source is disposed just under the liquid crystal display portion and an edge-lighting system in which a light source is disposed on the side surface of a light conductor are used as a lighting system for the back light portion.
Recently, the edge-lighting system has been more frequently used because it is more suitable for reducing the size of the liquid crystal display device. In the edge-lighting system, the light source is disposed at a side surface portion of a planar light conductor so as to emit light from the entire surface of the light conductor, and thus the back light portion using this system is called a “surface light source device”.
According to such a surface light source device, a light conductor is formed of a planar transparent member such as an acrylic resin plate or the like, and light emitted from a light source which is disposed at the side surface of the light conductor is introduced through the side surface (light incident face) of the light conductor into the light conductor. The incident light totally (completely) reflects from the obverse surface of the light conductor (light emitting face) and the back surface of the light conductor and then passes through the light conductor. Further, a light emitting function or light emitting portion such as a light scattering portion is provided on the obverse surface or back surface of the light conductor to emit the light from the whole light emitting face. However, when the light emitting portion is uniformly formed on the obverse surface or the back surface of the light conductor, the brightness of the emitted light is more reduced as the light is farther away from the light source, so that the brightness distribution on the light emitting plane becomes disuniform and thus a high-quality display image cannot be obtained.
This result is more noticeable as the size of the liquid crystal display device increases, and thus the surface light source device cannot practically be used for a large-size liquid crystal display device of 10-inches or more in size. Despite this, large-size liquid crystal display devices have recently been in demand and, following this recent demand, liquid crystal display devices used for portable personal computers, liquid crystal televisions or the like have additionally been required to have a brightness distribution of very high uniformity on the screen thereof.
Furthermore, marking apparatuses such as a guide signboard or a large-size signboard, and traffic sign apparatuses such as a guide sign, a traffic signboard or the like, have used two illumination systems, i.e., an internal illumination system and an external illumination system, to enhance visual recognition and character recognition at night. According to the internal illumination system, characters, figures, photographs, etc., are formed on a semi-transparent plastic plate such as a methacrylate plate or the like by cut-out, print or the like to form a display plate. A light source is disposed at the inside of the display plate, and the display plate is illuminated by the light source. A rod (linear pipe shape) or annular type fluorescent lamp is generally used as the light source. According to the external illumination system, a light source is disposed at any of the upper and lower sides or right and left sides of a display plate on which an information display is formed, and the whole surface of the display plate is illuminated by the light source. A rod type fluorescent lamp is generally used as the light source.
In the conventional display devices as described above, the brightness distribution on the entire surface of the display plate is disuniform, that is, the ratio of the maximum value/minimum value of the brightness is very large. Therefore, it is very difficult to provide a display device having an uniform brightness distribution by using these illumination systems. In particular, this problem is more serious for the external illumination system. Further, the internal illumination system has another problem in that a fluorescent lamp or the like which is used as the light source can be unintentionally seen through the display plate (i.e., a see-through phenomenon occurs).
Therefore, attempts have been made to apply the edge lighting type back light system in which a light source is disposed at the side surface portion of a planar light conductor to emit light from the entire surface of the light conductor to the display devices as described above. However, these display devices need a large-size surface light source device, and thus they have the same problem as the liquid crystal display device in that sufficient uniformity in brightness cannot be obtained within the light emitting face.
In order to solve this problem of “disuniformity of brightness” of the surface light source device, various proposals have been made. For example, Japanese Laid-open Patent Application No. Hei-1-245220 proposes a surface light source device having a light emitting portion which is obtained by coating or sticking light scattering material to the back surface confronting the light emitting face of the light conductor so that the density of the light scattering material increases with increasing distance from the light incident face. Further, Japanese Laid-open Patent Application No. Hei-1-107406 proposes a light conductor comprising plural laminated transparent plates on which fine spots formed of light scattering material are formed in various patterns. In such a surface light source device, since white pigment such as titanium oxide, barium sulfate or the like is used as the light scattering material, optical loss occurs due to light absorption or the like when the light impinging against the light scattering material is scattered. Therefore, although uniformity of the brightness distribution can be achieved, the brightness of the emitted light is reduced.
Further, Japanese Laid-open Patent Application No. Hei-1-244490 and Japanese Laid-open Patent Application No. Hei-1-252933 propose a surface light source device in which an emitted light adjusting member or a light diffusion plate having a light reflection pattern which is matched to the reciprocal of a light emission distribution is disposed on the light emitting face of the light conductor. However, in such a surface light source device, since the light reflected from the emitted light adjusting member or the light diffusion plate can not be reused, the same optical loss also occurs. Therefore, the brightness of the emitted light in a desirable direction is reduced.
Still further, Japanese Laid-open Patent Application No. Hei-2-17 and Japanese Laid-open Patent Application No. Hei-2-84618 propose a surface light source device in which a satin-finished face is or many lens units are formed on at least one of the light emitting face or the back surface confronting the light emitting face of th
Chiba Issei
Hayashi Yasuko
Oda Masaharu
Mitsubishi Rayon Co. Ltd.
Morgan & Lewis & Bockius, LLP
Sikder Mohammad
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