Illumination – Revolving
Patent
1996-05-30
1999-08-31
O'Shea, Sandra
Illumination
Revolving
362551, F21V 704
Patent
active
059444053
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a flat light source that can be used in applications such as back-lighting for a light transmissive type of display device such as a liquid crystal display device, an illuminated advertisement, or a traffic sign. In particular, it relates to a flat light source that uses a light-diffusing sheet.
BACKGROUND OF THE INVENTION
Flat light sources that are used for back-lighting in devices such as liquid crystal displays (LCDs) are known in the art, as described below.
A first known type of flat light source is an edge-lit system that uses a flat optically transmissive plate as an optically conductive member. The flat light source used in this system causes light to be incident on one or both side edge surfaces of the optically conductive member which is formed of a transparent flat plate. Total reflection within the flat optically transmissive plate is utilized to propagate the light throughout the entire optically conductive plate. Part of the thus propagated light becomes diffused reflected light of less than the critical angle from a light-scattering reflective plate on the rear surface of the optically conductive plate, and thus diffused light is emitted from the outer surface of the optically conductive plate (refer to Japanese Utility Model Laid Open No. 55-162,201).
A second known type of flat light source has a lens sheet wherein one surface has projections and the other surface is smooth, which is placed with the projection side thereof on the outer surface of the optically conductive plate of the flat light source of the above first type. The light-focusing action of this lens is utilized to ensure that the diffused, reflected light is diffused uniformly and isotropically within a predetermined angular range (refer to Japanese Utility Model Laid Open No. 4-107,201).
The above described lens sheet could be used in combination with a frosted transparent diffusion plate (a frosted transparent sheet) formed by dispersing particles of a light-diffusing agent such as TiO.sub.2 within a transparent plastic. In such a case, the optical energy of the light source is distributed in a more concentrated manner within a predetermined limited angular range, than when a frosted transparent diffusion plate alone is placed over the optically conductive plate (refer to U.S. Pat. No. 4,729,067 and Japanese Patent Laid Open No. 61-55,684). Moreover, a uniform and highly isotropic diffused light can be obtained within this angular range.
However, both of the above prior-art techniques have problems. The first one simply places a light-scattering reflective plate on the rear surface of the optically conductive plate so that the emitted light has a comparatively sharp distribution that peaks at an angle of 60 degrees to the normal of the surface of the optically conductive plate. Therefore a phenomenon is observed in which the degree of luminance is insufficient in the normal direction (the forward direction) where brightness is most required, while optical energy is wasted in the lateral directions where it is completely unnecessary.
The second prior-art technique has a problem in that, when a lenticular sheet that comprises an array of a large number of individual triangular prismatic lenses is superimposed on the light-emitting surface of the optically conductive plate as the lens sheet, the ratio of optical energy emitted within angles between 30 and 60 degrees from the normal to the light-emitting surface is comparatively high, but even if the portion within 2 to 4 cm of the side edge portion of the optically conductive plate is very bright, the luminance drops gradually further away from this portion so that the edge at the opposite side from the light source is noticeably darker.
If a frosted transparent scattering diffusion plate is used, a further problem arises in that the particles of light-diffusing agent within the optically conductive plate absorb some of the light, so that the optical energy thereof is lost.
In addition, interference patterns such as Newton
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Ishii Taiji
Nishio Toshikazu
Takeuchi Michiko
Dai Nippon Printing Co. Ltd.
Honeyman Marshall
O'shea Sandra
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