Optical: systems and elements – Prism – With reflecting surface
Reexamination Certificate
2003-06-06
2004-10-26
Cherry, Euncha P. (Department: 2872)
Optical: systems and elements
Prism
With reflecting surface
C359S528000
Reexamination Certificate
active
06809892
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a light-conducting unit, which is used as a surface light source of a back-light for a liquid crystal display or an outdoor signboard, or as an interior or exterior lighting device. The light-conducting unit of the present invention can be used as a substitute for a conventional light-conducting plate, and can reduce the weight of a light-conducting unit and in turn a surface light source, since it has a hollow light-conducting space therein.
BACKGROUND OF THE INVENTION
As shown in
FIG. 1
, a conventional surface light source uses a solid light-conducting plate
10
to emit light, which is received from a spot light source or a linear light source, that is, a lamp
4
in FIG.
1
. The light-conducting plate is usually made of a polymer material such as an acrylic resin. The light from the light source enters in the light-conducting plate through the light-incident edge (light-incident side face), and is allowed to emit from one of the major surfaces of the light-conducting plate while the light propagates through the plate from the light-incident edge towards the remote edge of the plate opposing the light-incident edge. Thus, the surface light emission is achieved.
The propagation of the light through the light-conducting plate utilizes total internal reflection of light at the interface between air and the light-conducting plate due to the difference of the refractive index between the air and the light-conducting plate (polymer material). Such a light-conducting plate is an essential part of a back-lighting surface light source for a liquid crystal display. Such a light-conducting plate and a back-light comprising the same are disclosed, for example, in Japanese Laid Open Patent Application A-11-142845.
However, when a solid light-conducting plate is used, it is difficult to reduce the weight of the surface light source. For example, the area of the light-conducting plate (light-emitting area) and also the thickness of the surface light source are being increased with the enlargement of the liquid crystal display screen. Thus, the weight increase of the light-conducting plate makes it very difficult to reduce the weight of the surface light source and, in turn, the weight of the liquid crystal display. Even in the case of small-sized liquid crystal displays, it is highly desired to reduce the weight of the surface light sources in the case of portable terminals such as mobile PCs, cellular phones, PHS, etc
Accordingly, it has been proposed to use a hollow light-conducting unit instead of the solid light-conducting plate. For example, a hollow light-conducting unit is known, which comprises a light-emitting plate and a back plate which are arranged in parallel with each other to form a hollow light-conducting space, and two prismatic films which are placed on the respective surfaces of the light-emitting plate and the back plate facing inside of the light-conducting space.
The prismatic film is a well known optical element and means a light-transmitting film one major surface of which is flat, while the other major surface of which carries a plurality of prisms arranged in parallel with each other in the lengthwise direction (so-called parallel prisms). Such a light-transmitting film is usually placed between the light-conducting plate and an object to be illuminated (e.g. a liquid crystal display panel, etc.).
To allow the light to emit from the light-conducting unit, in general, a linear light source (e.g. a fluorescent tube, etc.) is placed along the opening which is formed at one end of the above light-conducting space, and the light is irradiated in the light-conducting space through the opening. Such an opening is formed by arranging the above pair of the prismatic films substantially in parallel with each other with leaving a gap between them. The pair of the prismatic films are usually arranged with allowing their flat surfaces to be in parallel with each other.
Such a light-conducting unit has a first opening formed at one end of the unit, and a second opening formed at the other end of the unit opposing the one end (the farthest end from the one end), and two light sources are placed at the respective openings to irradiate light into the light-conducting space. Since the pair of the prismatic films are arranged in parallel with each other, the height of the light-conducting space (in the thickness direction), that is, the size perpendicular to the light-emitting surface, does not substantially change from the first opening at the one end to the second opening at the farthest end from the one end.
One embodiment of such a hollow light-conducting unit, and a surface light source comprising the light-conducting unit and a light source is proposed by the present inventor in JP-A-6-180552. Prior to the invention of JP-A-6-180552, another embodiment of the light-conducting unit was known.
In the other embodiment of the light-conducting unit, (i) the arranging direction of the prisms of the prismatic film on the light-emitting plate is in parallel to the direction of light from the light source, and the prismatic surface carrying the parallel prisms is arranged to face the outside of the light-conducting space. Furthermore, (ii) the direction of the prisms of the prismatic film on the back plate is also in parallel with the direction of light from the light source, and the prismatic surface is arranged to face the outside of the light-conducting space. In the case of the surface light source comprising such a light-conducting unit, the illumination of light emitted from the light-emitting surface tends to decrease as the distance from the light source increases. Thus, an additional optical element is necessary to rectify such the decrease of the illumination. Such an additional optical is a so-called extractor, which has sawtooth-form reflective projections the distances of which increase as the distance from the light source increases. Such an extractor is usually arranged on the flat surface of the prismatic film on the back plate side.
The present inventor proposed, in JP-A-6-180552, an improved light-conducting unit to provide an inexpensive light-conducting unit and a surface light source which does not require any extractor and thus can be very easily designed and produced, and a surface light source.
That is, the proposed hollow light-conducting unit comprises a pair of prismatic films which are fixed in respective positions so that they are arranged in the specific three-dimensional configuration, wherein (I) the prismatic surface of one prismatic film, which functions as a light-emitting plane, is arranged, so that the direction of the prisms of this prismatic film is perpendicular to the direction of light from the light source, and the prismatic surface faces the inside of the light-conducting space. Furthermore, (II) the other prismatic film is arranged so that the direction of the prisms of the other prismatic film is in parallel with the direction of light from the light source, and the prismatic surface faces the inside of the light-conducting space. Thereby, the illumination on the light-emitting surface is made uniform without the use of an extractor.
With this above light-conducting unit, a part of the light emitted from the light source is reflected by the prismatic film, while the remaining part of the light passes through the prismatic film on the light-emitting side and is emitted from the light-emitting surface (leakage of light). The light, which is reflected by the prismatic film, is emitted from the light-emitting surface or propagated from one end near the light source to the farthest end from the light source by being repeatedly ed and transmitted by the prismatic films in the light-conducting space. That is, the balance between the leakage of light and the propagation of light is important. If the light-leaking effect is too high, the amount of light to be propagated decreases so that the uniformity of light emission tends to deteriorate. The increase of the light-leaking effect leads t
3M Innovative Properties Company
Buckingham Stephen W.
Cherry Euncha P.
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