Planar luminescent device and planar luminescent element

A planar luminescent device useful as a backlight for liquid crystal disp. The planar luminescent device comprises a synthetic resin sheet having at least one fold and a plurality of sheet sections foldable along this fold. A sheet holder is adapted to hold the sheet sections of the sheet, folded along the fold, in a predetermined accordion-like folded position. A reflective layer and a light emitting layer are respectively provided on the sheet sections defining opposite ends of the sheet. A light source emits light rays towards the reflective layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a planar luminescent device and a planar 
luminescent element. 
2. Description of Related Art 
A planar luminescent device has been used in various applications, 
particularly as a backlight device used to light a liquid crystal panel of 
a liquid crystal display from behind. As shown in FIGS. 8 and 9, a 
conventional backlight device 1 basically comprises a light guide plate 2 
having a wedge-shaped cross-section, a reflective sheet 3 laid on the rear 
side of the light guide plate 2, a diffusive plate 4 and a pair of prism 
sheets 5, 6 laid on the front side of the light guide plate 2, and a 
cylindrical light source (fluorescent lamp) 7 positioned within a thicker 
end of the light guide plate 2. The light guide plate 2 is usually made of 
transparent acrylic resin and is provided on its rear side with a 
diffusive pattern 2a selected so as to uniformalize light rays reflected 
on the reflective sheet 3. Behind the cylindrical light source 7, there is 
provided a cylindrical reflector 7a having a substantially semi-circular 
(or semi-elliptical) cross-section. To serve as condensing means the prism 
sheets 5 and 6 have 90.degree. angular sections. The cylindrical light 
source 7 may also be of a dual type, i.e., it may comprise a pair of 
halves placed laterally on opposite sides of the reflective sheet 3. 
With such a conventional backlight device 1, light rays emitted from the 
cylindrical light source 7 into the light guide plate 2 are repeatedly 
reflected on the front and rear sides of the light guide plate 2. The 
portion of the light rays passing beyond the critical angle passes (upward 
as viewed in FIG. 8) through the diffusive plate 4 and the prism sheets 5, 
6. Due to the condensing function of the prism sheets 5, 6 the central 
luminance of the planar luminescent body is increased. 
However, the conventional backlight device 1 has been disadvantageous in 
that the components thereof, particularly the light guide plate 2 as well 
as the prism sheets 5, 6 inevitably require unacceptably high unit costs 
and the cost required for assembly and for positioning these components is 
also relatively high, contrary to a desired cost reduction, necessary for 
liquid crystal displays to be mass-produced. Additionally, weight 
reduction is required when it is desired that the liquid crystal display 
should be used in a laptop computer. 
In view of the above-mentioned problems inherent to the conventional 
backlight, it is a primary object of the present invention to obtain a 
planar luminescent device and a planar luminescent element having a 
simplified construction thus leading to a drastic reduction in the costs 
of manufacture and assembly. It is another object of the present invention 
to obtain a lightweight planar luminescent device and a lightweight planar 
luminescent element. 
SUMMARY OF THE INVENTION 
The present invention is based on the fact that the basic element of the 
planar luminescent device (i.e., planar luminescent element) is 
constructed from a foldable synthetic resin sheet. 
More specifically, the planar luminescent element of the invention 
comprises a synthetic resin sheet having at least one fold and a plurality 
of sheet sections foldable along this fold, wherein the respective sheets 
are formed on their front and/or rear sides with optically active layers 
adapted to deflect light rays such as a reflective layer, a diffusive 
layer, a Fresnel lens layer and a prism sheet layer. Of these optically 
active layers, the reflective layer and a light emitting layer 
(luminescent layer) may be provided, for example, on the sheet sections 
defining opposite ends of the synthetic sheet folded along the fold, 
respectively. 
The fold serves as a guide or a positioning reference along which the 
synthetic resin sheet is folded and may be realized in the form of a 
thinned line, or a U-shaped or square U-shaped recess. 
The reflective layer may be formed by coating the sheet section with 
reflective film or laying a reflective sheet on the sheet section. 
The light distribution characteristic required for the planar luminescent 
body may be selected by an appropriate combination of characteristics 
presented by the optically active layers such as the reflective layer, the 
diffusive layer, the Fresnel lens layer and the prism sheet layer provided 
on the front and/rear sides of each sheet section. The respective 
optically active layers may be obtained in accurate configurations by 
stamping the synthetic resin sheet. 
More specifically, these optically active layers provided on the sheet 
sections may be arranged, for example, in the order of the reflective 
layer, the diffusive layer, a first prism sheet layer and a second prism 
sheet layer. 
Alternatively, these optically active layers may be arranged in the order 
of the reflective layer, the diffusive layer and the Fresnel lens layer. 
The planar luminescent device using the planar luminescent element 
mentioned above further comprises sheet holders adapted to hold the sheet 
sections of the sheet folded along the fold at a predetermined folded 
position; and a light source which emits light rays towards the reflective 
layer. 
The planar luminescent device may be assembled merely by folding the sheet 
along the fold, holding it in the sheet holders and placing the light 
source at a predetermined position. The foldable sheet as well as the 
sheet holders may be produced at a relatively low cost, thus leading to a 
reduction in the total manufacturing cost of the planar luminescent 
device. 
According to an aspect of the present invention there is provided a planar 
luminescent device which comprises a synthetic resin sheet having at least 
one fold and a plurality of sheet sections foldable along this fold. A 
sheet holder is adapted to hold the sheet sections of the sheet folded 
along the fold in a predetermined accordion-like folded position. A 
reflective layer and a light emitting layer are respectively provided on 
the sheet sections defining opposite ends of the sheet folded and held by 
the sheet holder. A light source emits light rays towards the reflective 
layer. 
According to another aspect of the present invention there is provided a 
planar luminescent element comprising a synthetic resin sheet having at 
least one fold and a plurality of sheet sections foldable along the fold. 
At least one optically active layer is provided on at least one of the 
front or rear sides of the sheet sections, to change the direction of 
light incident upon the same. 
According to another aspect of the present invention, there is provided a 
planar luminescent device which comprises a synthetic resin sheet having 
at least two folds and at least three sheet sections, foldable along the 
folds, having the same rectangular shape. A sheet holder is adapted to 
hold the sheet sections folded along the fold in a predetermined 
accordion-like folded position. A reflective layer and a light emitting 
layer are respectively provided on the sheet sections defining opposite 
ends of the sheet folded and held by the sheet holder. A light source is 
provided at one side of the accordion-like folded sheet, parallel with the 
fold to emit light toward the reflective layer from an open end of the 
folded sheet sections. 
The present disclosure relates to subject matter contained in Japanese 
Patent Application No. 7-42122 (filed on Mar. 1, 1995) which is expressly 
incorporated herein by reference in its entirety.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will be better understood from the following 
description of the embodiments made in reference to the accompanying 
drawings. 
FIGS. 1 and 2 show a first embodiment of the planar luminescent element and 
the planar luminescent device constructed according to an aspect of the 
present invention. A synthetic resin sheet 10 is foldable along a fold 11 
of a pair of rectangular sheet sections 12A and 12B. The sheet section 12A 
is provided on its outer side with a reflective film 14 and on its inner 
side with a diffusive pattern 15, for example, coated thereon by 
vaporization. It should be noted here that the term inner sides refers to 
those sides of the sheet sections 12A, 12B that have been folded along the 
fold 11 towards each other. It should also be understood that the 
reflective film 14 may be replaced by a separate reflective sheet 
adhesively bonded to the sheet section 12A. The fold 11 is formed, for 
example, by thinning or perforating the sheet 10 along this line so that 
the pair of sheet sections may be put one upon the other in a 
predetermined positional relationship, i.e., the fold 11 is used to 
position the sheet sections. 
The sheet section 12B is formed on its inner side with a diffusive layer 16 
and on its outer side with a Fresnel lens layer 17. 
The synthetic resin sheet 10 is folded along the fold 11, as has been 
described above, and is then held in this folded state by a pair of 
holders 20 provided laterally on both sides of the sheet 10 (note that 
only one of the holders is shown in the drawings). Each of these holders 
20 has holding grooves 21, 22 adapted to position the respective sheet 
sections 12A, 12B and a supporting hole 23 adapted to receive an 
associated end of a cylindrical light source 30. The cylindrical light 
source 30 is positioned between free ends of the respective sheet sections 
12A, 12B. The configuration and the angle defined between the holding 
grooves 21, 22 is previously selected so that a desired light distribution 
characteristic may be provided by the sheet 10. The sheet 10 is flexible 
so that, if desired, the holding groove 21 and/or the holding groove 22 
may be curved in shape. 
The above-mentioned device of the present invention is assembled by 
inserting both ends of the sheet sections 12A, 12B into the respective 
holding grooves 21, 22 and then inserting opposite ends of the cylindrical 
light source 30 into the respective receiving holes 23 of the sheet 
holders 20, as shown in FIG. 2. When the cylindrical light source 30 is 
switched on, light rays emitted from the cylindrical light source 30 are 
reflected by the reflective film 14 of the sheet section 12A before 
passing through a diffusive pattern 15, a diffusive layer 16 and a Fresnel 
lens layer 17 in this order. The Fresnel lens layer 17 serves as a 
luminescent layer. Depending on the light distribution characteristic 
desired, the pattern of the diffusive pattern 15, the diffusion degree of 
the diffusive layer 16 and the focal distance of the Fresnel lens layer 17 
can be decided. It is also possible, without departing from the scope of 
the present invention, to provide a dual type of cylindrical light source 
30 comprised of a pair of halves placed on either side of the foldable 
sheet 10. 
Instead of the reflective film 14, aluminium powder is appliable to the 
inner or outer surface of the sheet section 12A by, for example, coating 
or evaporation coating the same. In addition, it is possible to insert 
small irregular-reflection particles at the bottom 10' of the folded sheet 
sections 12A and 12B to ensure a good diffusion efficiency. 
FIG. 3 shows the case in which five sheet sections 12 are connected 
successively by the fold 11 one to another. The fold 11, along which the 
respective sheet sections are folded, may be selected from those having 
various cross-sections, such as, a square U-shaped cross-sectional fold 
11C, a semi-circular cross-sectional fold 11D and a V-shaped 
cross-sectional fold 11E, as shown in FIG. 3. Although in FIG. 3 these 
folds are shown together on the single foldable sheet 10, it is also 
possible for the foldable sheet 10 to have only one type of fold. 
In order to improve the utilization efficiency of the light rays, the inner 
sides of the respective sheet sections may be formed as reflective layers, 
if desired. The diffusive layer 16 may be formed by stamping a fine 
irregular pattern or, as shown in FIG. 4, by embedding fine members 16A 
having different refractive indices, such as fine glass fiber threads or 
beads, into the sheet section to a predetermined depth or to random 
depths. 
FIGS. 5 and 6 show another embodiment of the present invention, in which 
the foldable sheet 10 comprises three sheet sections 12A, 12B, 12C 
connected successively to each other by the folds 11. The sheet section 
12A is formed on its outer side with the diffusive pattern 15 which is, in 
turn, coated with the reflective film 14, and formed on its inner side 
with the diffusive layer 16. Each of the remaining sheet sections 12B, 12C 
has one side formed with a prism sheet layer 18 or 19. The prism sheet 
layers 18, 19 each comprise linear fine irregular angular profiles 
extending parallel to one another. However, the linear fine irregular 
angular profiles of the prism sheet layer 18 extend orthogonally to those 
of the prism sheet 19, as shown in FIG. 7. 
According to the arrangement described hereinabove, an effect similar to 
the effect obtained by the conventional backlight, mentioned in reference 
to FIG. 8, can be expected. The embodiment shown in FIGS. 5 and 6 may be 
modified in a variety of ways. For example, the number of sheet sections 
12 may be increased, or the diffusive layer 16 may be formed on only one 
of the sheet sections. The number of sheet sections 12 may be selectively 
increased depending on the light distribution characteristic desired. The 
prism sheet layers 18, 19 may be replaced by linear Fresnel layers formed 
by cylindrical lenses. In view of the well known fact that a cylindrical 
Fresnel lens gives a condensing effect in only one of two directions 
orthogonal to one another, a set of cylindrical Fresnel lenses, having an 
appropriate focal distance and condensing in mutually orthogonal 
directions, may be employed to increase a central luminance of the 
luminescent surface. The cylindrical Fresnel lens comprises fine 
undulations (irregularities) extending in parallel to one another and has 
a cross-section similar to those of the prism sheet layers 18, 19. 
The type of optically active layers formed on either side of the respective 
sheet sections 12, as well as the order in which these optically active 
layers are arranged, are not limited to those adopted in the specific 
embodiments as have been described above, but may be appropriately 
selected depending on the light distribution characteristic desired. 
As will be apparent from the foregoing description, according to the planar 
luminescent device and the planar luminescent element provided by the 
invention, the desired optically active layers may be formed on the 
respective sheet sections of the foldable synthetic resin sheet and merely 
folded along the respective folds to obtain the crucial part of the planar 
luminescent device. Such a feature advantageously contributes to a 
reduction in the manufacturing cost of the liquid crystal display's 
backlight. Furthermore, the basic element of the inventive planar 
luminescent device comprises the individual sections of the synthetic 
resin sheet which are foldable along the respective folds. This feature 
allows for a lightweight backlight, particularly suitable for a laptop 
computer.