Liquid crystal cells – elements and systems – Particular structure – Lens or prism separate from projection system
Patent
1997-09-30
1999-06-22
Nguyen, Tiep H.
Liquid crystal cells, elements and systems
Particular structure
Lens or prism separate from projection system
349 64, G02F 11335
Patent
active
059147598
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a surface light source device utilizing a light scattering guide plate (a plate-like light scattering guide), a liquid crystal display utilizing the same for back lighting, and an asymmetric prism sheet which can be advantageously used for them. In the present specification, the term "asymmetric prism sheet" means "a sheet-like optical element including a prism surface formed by a multiplicity of rows of asymmetric prism elements".
BACKGROUND OF THE INVENTION
Light source devices utilizing a light scattering guide plate and a prism sheet have been proposed and put in use for purposes such as back-lighting for liquid crystal displays. A conventional prism sheet is constituted by a plate-like member made of an optical material having a prism surface formed with repetitive rows of V-shaped irregularity. It is known that such a prism sheet has a function of modifying the directional propagation characteristics of a beam.
FIG. 1 shows a general arrangement of a conventional surface light source device utilizing a light scattering guide plate and a prism sheet. A light scattering guide plate 1 having a wedge-like section comprises a matrix made of polymethyl methacrylate (PMMA) and a material of a different refractive index mixed and dispersed uniformly in the matrix. The term "material of a different refractive index" means a material having a refractive index which is substantially different from the refractive index of the matrix.
One of the end faces that define the larger thickness of the light scattering guide plate 1 serves as an incidence surface 2, and a light source device (fluorescent lamp) L is arranged in the vicinity of the same.
A reflector 3 is arranged along one surface (rear surface 6) of the light scattering guide plate 1. The reflector 3 is constituted by a sheet of silver foil having regular reflectivity or a white sheet having diffuse reflectivity. Illuminating light is obtained at an exiting surface 5 on the other side of the light scattering guide plate 1. The prism sheet 4 is arranged outside the exiting surface 5.
For convenience in illustration, the interval between the light scattering guide plate 1 and the rows of the prism elements and the pitch and depth of the rows of prism elements are exaggerated. The inner surface of the prism sheet 4 is a prism surface formed by repetitive symmetric V-shaped slopes 4a and 4b. The outer surface of the prism sheet 4 constitutes a flat surface (luminous surface) 4e from which an illuminating beam 4f emits. A back-lighting configuration for a liquid crystal display can be arranged by disposing a well-known liquid crystal display panel outside the prism sheet 4.
Thickness of the light scattering guide plate 1 decreases as distance from the incidence surface 2 increases. Thus, repetitive reflection occurs in an effective manner within the light scattering guide plate 1. As a result, the surface light source device has excellent light utilization efficiency and uniformity of luminance.
Light introduced from the light source device L into the light scattering guide plate 1 is guided toward an end face 7 that defines the larger thickness thereof while being subjected to scattering and reflection in the light scattering guide plate 1. During this process, illuminating light is gradually emitted from the exiting surface 5.
The light emitted from the exiting surface 5 has a preferred propagating direction that depends on the particle diameter of particles of a different refractive index dispersed in the light scattering guide plate 1 (in general terms, correlation distance associated with the structure having a non-uniform refractive index). In other words, the exiting surface 5 emits a beam which has been strongly or weakly collimated.
The greater the diameter of the particles of a different refractive index (in general terms, the greater the correlation distance associated with structure of non-uniform refractive index), the stronger the light emitted from the exiting surface 5 is collimated. The pref
REFERENCES:
patent: 5598280 (1997-01-01), Nishio et al.
patent: 5600455 (1997-02-01), Ishikawa et al.
patent: 5694186 (1997-12-01), Yanagawa et al.
patent: 5838403 (1998-11-01), Jannson et al.
Higuchi Eizaburo
Koike Yasuhiro
Nguyen Tiep H.
Nitto Jushi Kogyo Kabushiki kaisha
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