Illumination – Revolving
Reexamination Certificate
2000-09-20
2002-07-30
Cariaso, Alan (Department: 2875)
Illumination
Revolving
C362S330000, C362S339000, C362S561000, C362S558000, C349S065000
Reexamination Certificate
active
06425673
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to a light guide pipe useful for example in a planar light source unit. The invention also relates to a planar light source unit generally, and to a liquid crystal display device.
2. Discussion of the Background
Liquid crystal display devices are broadly used in personal-computer monitors and flat-panel TV displays. A transmissive liquid crystal display device usually has a planar light source unit (backlight) arranged on a backside of the liquid crystal unit. The planar light source unit converts into planar light the light given from a linear light source such as a cold cathode fluorescent lamp or from a light source arranged with a plurality of point light source, such as an LED array.
For example, there are descriptions of planar light source units using a side-light scheme in Japanese Patent Laid-open No. 99187/1986 and Japanese Patent Laid-open No. 62104/1988 which disclose an increase in the density of a light extracting mechanism (light extractor) according to an increase in the distance from light source. The side-light scheme is a scheme having a linear light source provided at a side surface of a transparent light guide pipe of acrylic resin to convert the light from the linear light source into planar light.
Japanese Patent Laid-open No. 17/1990 and Japanese Patent Laid-open No. 84618/1990 describe a planar light source unit comprising a light guide pipe (first element) having a light incident surface, one light emitting surface orthogonal to the light incident surface and a reflecting surface opposed to the light emitting surface and formed with a light extracting mechanism (in this application another name for a light extracting mechanism is light extractor, neither of which are means-plus-function terms), light sources provided at opposite side ends of the light guide pipe, a light control sheet (second element) having a prism array provided in proximity to the light emitting surface of the light guide pipe and having triangular prisms and arranged such that the prisms have a top vertex directed toward the light emitting surface of the light guide pipe where a generating line of an arbitrary prism constituting the prisms is arranged nearly in parallel with the light source, and a reflecting sheet (reflecting surface) is provided in proximity to the light reflecting surface of the light guide pipe. The latter reference (JP Laid-open No. 84618/1990) also discloses a uniform surface-roughening treatment of the light emitting surface of the first element. The inventions described in both publications emit light in a particular direction. These, however, are unsatisfactory with respect to practical optical characteristics, e.g., satisfactory evenness is not achieved in the emission light on the light-emitting surface (see Japanese Patent Laid-open No. 18879/1998). In particular, no existing planar light source device adequately provides a broad view angle suited for a planar light source unit used as a liquid crystal display for monitors or thin panel televisions.
Japanese Patent Laid-open No. 18879/1994 describes a planar light source unit comprising a light guide pipe having a light incident surface, one light emitting surface orthogonal to the light incident surface and a light reflecting surface opposed to the light emitting surface and formed with a light extracting mechanism, the light guide pipe having a roughened surface having a directivity emission function or a plurality of lens units provided in one or both of the light guide pipe light emitting surface and reflecting surface, a smooth area formed in a surface having the roughened surface or the lens units so that the ratio of the smooth area increases as the light emitting surface is approached thereby having a control function to make the luminance value of the light emitted from the light emitting planar light sources provided at opposite side ends of the light guide pipe even, a light control sheet provided in proximity to the light emitting surface of the light guide pipe and having a prism array comprising triangular prisms so that the triangular prisms have a top vertex directed toward the light emitting surface of the light guide pipe and a generating line of an arbitrary prism constituting the prism array arranged nearly in parallel with the light source, a reflecting sheet provided in proximity to the reflecting surface of the light guide pipe. According to this invention, front brightness is nearly sufficiently achieved. However, the view angle characteristic is extremely narrow in a direction perpendicular to a major axis of the light source and hence not well suited in application to a liquid crystal display device or the like. Even with this reference, it is still difficult to provide brightness evenly throughout the entire light guide pipe surface in attempting to solve the problem of the existing dark region in a light guide pipe surface, as shown in FIG.
13
and
FIG. 14
herein. (The reason a dark region
22
exists is explained below based on
FIG. 14.
) A fluorescent lamp, such as a cold cathode fluorescent lamp, has a light emission characteristic similar to a light emission angular distribution as seen in so-called Lambert type scattering with high light emission without variation in each of the light emission angles. Consideration is made on the light emitted from a linear light source taking into account such a light distribution characteristic. In FIG.
14
the light emitted, e.g. from a major portion of each linear light source propagates up to a center region in a light emitting surface of a light guide pipe. That is, most of the linear light source contributes to light emission through the center region. On the other hand, in the dark region
22
of the light-emitting surface (FIG.
14
), there is no effective reach of the light from the part of the light sources positioned far from the dark region (e.g. emission light
34
c in the dark region
37
. Due to this, the amount of light reaching the part corresponding to the dark regions
31
,
32
of the light-reflecting surface is insufficient.
Meanwhile, the light rays propagating into the light guide pipe at a middle area of two light sources (
FIG. 14
) or its vicinity has directivity distributed nearly symmetric left and right as represented by optical vectors of emitting light as viewed from above the light emitting surface. However, the directivity of the light ray in respective areas near the light sources distributes asymmetric left and right as represented by optical vectors of emitting light.
A light guide pipe using a conventional light extracting mechanism (extractor), which means a print-type light guide pipe made by a normal screen printing process using light scattering ink, such as resins composed of TiO2 or SiO2 microparticles, has directivity varying so much because of the multi-scattering process with microparticles, thus, this asymmetric directivity caused no problems. However, a light guide pipe which has surface-roughened protrusions as the light extracting mechanism has directivity which does not vary so much when light diffuses-reflects through the light extracting mechanism. Asymmetric left and right directivity distribution as represented by the optical vectors of emitting light is maintained as it is, and light emits from the light guide pipe. Due to this, at the side area of the light guide pipe, in the vicinity of the light sources, the brightness of light is insufficient in the most-necessary forward direction.
A conventional planar light source unit is also set to have a maximum brightness in a normal line direction from the light emitting surface. A two-lamp type planar light source unit is set to provide a maximum brightness in a normal line direction of the light-emitting surface for light from each of the light sources. The view angle characteristic in this case is as shown in FIG.
23
. That is, in
FIG. 23
, the dotted lines represent a brightness distribution for each one light source, while solid line is
Miwa Masanobu
Suga Yoshinori
Cariaso Alan
Mitsubisshi Chemical Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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