Direct-type backlight device
The direct-type backlight device of the present invention includes a reflecting plate, a plurality of point light sources and a light diffusing plate in this order. A light emitting surface of the light diffusing plate is provided with a fine concavo-convex structure which is a split image forming member. By the fine concavo-convex structure, images A 1 , B 1 , C 1 and D 1 on the light emitting surface corresponding to four point light sources A, B, C and D which constitute a convex quadrilateral which has the smallest area and the shortest circumference length among the plurality of point light sources are observed in a specific region W in a region surrounded by positions A 0 , B 0 , C 0 and D 0 which are positions of the projected image of the point light sources A, B, C and D on the light diffusing plate.
1. A direct-type backlight device comprising:
a reflecting plate;
a plurality of point light sources; and
a light diffusing plate which receives direct light from said point light sources and reflected light from said reflecting plate on a light receiving surface thereof, and then diffuses and emits the light from a light emitting surface thereof,
said reflecting plate, said point light sources and said light diffusing plate being placed in this order,
wherein:
at least either one of said reflecting plate and said light diffusing plate is provided with an image forming position adjusting member which produces images A 1 , B 1 , C 1 and D 1 of the point light sources A, B, C and D, respectively, on said light emitting surface,
said four point light sources A, B, C and D being selected among said plurality of point light sources such that said four point light sources constitute a convex quadrilateral which has the smallest area and the shortest circumference length, and
said images A 1 , B 1 , C 1 and D 1 being observed within a specific region which satisfies the following relationships (1) to (4) in a region surrounded by positions A 0 , B 0 , C 0 and D 0 which are positions of the projected image of the point light sources A, B, C and D on said light diffusing plate:
0.8× L ( A 0 C 0 )> L ( A 0 A 1 )>0.2× L ( A 0 C 0 ) (1)
0.8× L ( A 0 C 0 )> L ( C 0 C 1 )>0.2× L ( A 0 C 0 ) (2)
0.8× L ( B 0 D 0 )> L ( B 0 B 1 )>0.2× L ( B 0 D 0 ) (3)
0.8× L ( B 0 D 0 )> L ( D 0 D 1 )>0.2× L ( B 0 D 0 ) (4);
wherein L(XY) indicates a length of a line segment XY.
2. The direct-type backlight device according to claim 1 wherein:
the shortest distance LP between two images selected from said images A 1 , B 1 , C 1 and D 1 satisfies the following relationships (5) and (6):
L ( A 0 C 0 )×0.5≧ LP≧L ( A 0 C 0 )×0.1 (5)
L ( B 0 D 0 )×0.5≧ LP≧L ( B 0 D 0 )×0.1 (6).
3. The direct-type backlight device according to claim 1 wherein:
said image forming position adjusting member is a fine concavo-convex structure formed on said light receiving surface and/or said light emitting surface, said fine concavo-convex structure being a structure of a plurality of pyramids having at least three planes which are aligned along a predetermined direction.
4. The direct-type backlight device according to claim 3 wherein:
said fine concavo-convex structure is formed on said light emitting surface.
5. The direct-type backlight device according to claim 3 wherein:
said light diffusing plate is in a shape approximate to a rectangle when viewed planarly,
said plurality of point light sources are arranged along a lengthwise direction and a crosswise direction of said light diffusing plate with a predetermined interval, and said plurality of pyramids are aligned along a predetermined direction intersecting with said lengthwise direction and said crosswise direction.
6. The direct-type backlight device according to claim 1 wherein:
a front luminance of the image corresponding to a point light source at a position corresponding to an outer edge of the point light source is 15 to 100% relative to the maximum value of the front luminance in said image, wherein said front luminance is a luminance observed from a normal line direction of said light diffusing plate.
7. The direct-type backlight device according to claim 1 wherein:
in said image, a luminance observed from a direction inclined by 15° with respect to a normal line direction of said light diffusing plate when measured at a position having a maximum front luminance is 20% or more and less than 100% relative to a front luminance which is the luminance observed from the normal line direction.
8. The direct-type backlight device according to claim 3 wherein:
a mean line average roughness Ra of planes which compose said concavo-convex structure is 3 μm≧Ra≧0.05 μm when a haze of said light diffusing plate is less than 20%, and is 2 μm≧Ra≧0.0001 μm when the haze of said light diffusing plate is 20% or more and 100% or less.
9. The direct-type backlight device according to claim 1 wherein:
said image forming position adjusting member is a concavo-convex structure formed on said reflecting plate.