Back light and liquid crystal display having protecting sheet with protrusions

A reflection sheet and a frame are arranged below a light guide plate having a light collecting effect. A tubular light source is arranged near the end surface (edge) of light guide plate. A reflector is arranged to surround tubular light source. A sheet having the light collecting effect and a sheet having a light diffusing effect are provided above light guide plate. Further, a liquid crystal panel is arranged thereabove. A sheet with protrusions is provided between a lens like grooved surface having the light collecting effect of light guide plate and reflection sheet. Thus, damage is not caused to lens like grooved surface, and lens like grooved surface is prevented from catching reflection sheet. Thus, a liquid crystal display or a back light which prevents a luminous dot phenomenon is provided.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to back lights and liquid crystal displays, 
and more particularly to a back light illuminating a liquid crystal panel 
with light from a back surface and a liquid crystal display provided with 
the same. 
2. Description of the Background Art 
Recently, the liquid crystal display has a wide range of applications 
because of its low power consumption, light weight and small thickness. 
The liquid crystal display is used, for example, as a display of a 
household video monitor, industrial machine monitor or personal digital 
assistant to be fit to use. In particular, the personal digital assistant 
is increasingly becoming low in power consumption, light in weight, and 
small in picture-frame and thickness. 
FIG. 13 is a cross sectional view showing a conventional structure of a 
back light illuminating a liquid crystal display with light from a back 
surface. As shown in FIG. 13, the back light includes a reflection sheet 3 
and a frame 8 below a light guide plate 22 uniformly transmitting light. A 
tubular light source 1 is arranged near the end surface (edge) of light 
guide plate 22. A reflector 2 is arranged to partially surround tubular 
light source 1. 
Provided above light guide plate 22 are a sheet having a light collecting 
effect, two sheets 6 having a light diffusing effect (each provided with 
grooves both in longitudinal and lateral directions), and another sheet 5 
having the light collecting effect. A liquid crystal panel 7 is arranged 
thereabove. Light guide plate 22 is formed of transparent acrylic resin, 
and its surfaces are all smooth and planar. 
It is noted that, the phrases "above light guide plate 22" and "below light 
guide plate 22" are respectively used to indicate directions toward liquid 
crystal panel 7 and frame 8 from light guide plate 22. 
As the back light has four sheets in total, the overall thickness of the 
liquid crystal display is large. Further, provision of two sheets 6 having 
the light diffusing effect makes the liquid crystal display more 
expensive. 
Then, a back light as shown in FIG. 14 has been developed to reduce cost as 
well as thickness and weight of the liquid crystal display. In the back 
light, two sheets are eliminated to reduce the overall thickness of the 
liquid crystal display, so that reduction in thickness and weight is 
achieved. 
FIG. 14 is a cross sectional view showing the structure of the conventional 
back light illuminating the liquid crystal display from a back surface. 
FIG. 15A is a cross sectional view showing a structure of frame 8 taken 
along the line 15A--15A in FIG. 15B. 
Referring to FIG. 14, the back light is provided with a reflection sheet 3 
and a frame 8 below a light guide plate 4 having a light collecting 
effect. A tubular light source 1 is arranged near the end surface (edge) 
of light guide plate 4. A reflector 2 is arranged to partially surround 
tubular light source 1. Provided above light guide plate 4 are a sheet 5 
having the light collecting effect and a sheet 6 having a light diffusing 
effect. A liquid crystal panel 7 is arranged thereabove. 
Instead of having two sheets, light guide plate 4 has at the bottom a lens 
like grooved surface 4a having the light collecting effect. Further, as 
shown in FIGS. 15A and 15B, an opening is formed in frame 8 supporting the 
light guide plate for reduction in weight and thickness and in 
consideration of a positional relationship with respect to an electric 
circuit or mechanical part. 
In the case of light guide plate 4 having lens like grooved surface 4a at 
the bottom as described above, lens like grooved surface 4a may be brought 
into contact with frame 8 or a part supporting light guide plate 4. In 
such case, lens like grooved surface 4a is damaged, thereby causing 
irregular reflection. In some cases, reflection sheet 3 may be caught by 
and fixed to lens like grooved surface 4a. As a result, display by the 
liquid crystal display becomes white or bright (the phenomenon is called a 
luminous dot phenomenon). 
Furthermore, as shown in FIG. 15A, sharp edge 16 of frame often causes 
damage to lens like grooved surface 4a, possibly resulting in the luminous 
dot phenomenon. In particular, if edge 16 is within a liquid crystal 
display area, sharp edge 16 would more often cause damage to lens like 
grooved surface 4a, possibly resulting in the luminous dot phenomenon. 
In addition, lens like grooved surface 4a of light guide plate 4 may be 
damaged by a protrusion 20 by a pin or a protrusion by a gate pin, which 
are caused during manufacture. 
SUMMARY OF THE INVENTION 
The present invention is made to solve the aforementioned problem. One 
object of the present invention is to provide a back light preventing 
damage to a lens like grooved surface of a light guide plate and 
preventing the lens like grooved surface from catching a member which is 
in contact with the lens like grooved surface for good display. Another 
object of the present invention is to provide a liquid crystal display 
provided with such back light. 
A back light according to one aspect of the present invention is provided 
with a light source, light guide plate, reflection sheet and frame. The 
light guide plate collects light from the light source and has grooves in 
one surface. The reflection sheet is arranged in vicinity of one surface 
of the light guide plate and emits the collected light from the other 
surface of the light guide plate. The frame is arranged on the side of one 
surface of the light guide plate and supports the light source, light 
guide plate and reflection sheet. Provided between one surface of the 
light guide plate with grooves and the reflection sheet is a sheet with 
protrusions. 
In the structure, the sheet with protrusions is provided in direct contact 
with the grooves of the light guide plate, so that damage to the surface 
with grooves (grooved surface) of the light guide plate is prevented. In 
addition, as the grooved surface does not catch the sheet with 
protrusions, the luminous dot phenomenon is prevented. 
A back light according to the second aspect of the present invention is 
provided with a light source, light guide plate, reflection sheet and 
frame. The light guide plate collects light from the light source and has 
grooves in one surface. The reflection sheet is arranged in vicinity of 
one surface of the light guide plate and emits the collected light from 
the other surface of the light guide plate. The frame is arranged on the 
side of one surface of the light guide plate and supports the light 
source, light guide plate and reflection sheet. The reflection sheet is 
provided with protrusions. 
In the structure, the reflection sheet with protrusions is in direct 
contact with the grooves of the light guide plate, so that damage to the 
grooved surface of the light guide plate is prevented. In addition, as the 
grooved surface does not catch the reflection sheet, luminous dot 
phenomenon is prevented. 
Preferably, the frame supports a portion of the light guide plate which is 
outside a prescribed light emitting region. More specifically, a portion 
of the frame positioned near a periphery of the prescribed light emitting 
region has a step or chamfer. 
In this case, even if the luminous dot phenomenon is caused at the portion 
supported by the frame, as the portion is positioned outside, the problem 
associated with display is avoided. 
A liquid crystal display according to the third aspect of the present 
invention is provided with a light source, light guide plate, reflection 
sheet, liquid crystal panel and frame. The light guide plate collects 
light from the light source and has grooves in one surface. The reflection 
sheet is arranged on the side of one surface of the light guide plate and 
emits the collected light from the other surface of the light guide plate. 
The liquid crystal panel is arranged on the side of the other surface of 
the light guide plate. The frame is arranged on the side of one surface of 
the light guide plate and supports the light source, light guide plate, 
reflection sheet and liquid crystal panel. A sheet with protrusions is 
arranged between one surface of the light guide plate with grooves and the 
reflection sheet. 
In the structure, the sheet with protrusions is in direct contact with the 
grooves of the light guide plate, so that damage to the grooved surface of 
the light guide plate is prevented. In addition, as the grooved surface 
does not catch the sheet, the luminous dot phenomenon is not caused. 
A liquid crystal display according to the fourth aspect of the present 
invention is provided with a light source, light guide plate, reflection 
sheet, liquid crystal panel and frame. The light guide plate collects 
light from the light source and has grooves in one surface. The reflection 
sheet is arranged on the side of one surface of the light guide plate and 
emits the collected light from the other surface of the light guide plate. 
The liquid crystal panel is arranged on the side of the other surface of 
the light guide plate. The frame is arranged on the side of one surface of 
the light guide plate and supports the light source, light guide plate, 
reflection sheet and liquid crystal panel. The reflection sheet is 
provided with protrusions. 
In the structure, the reflection sheet with protrusions is in direct 
contact with the grooves of the light guide plate, so that damage is not 
caused to the grooved surface of the light guide plate. In addition, as 
the grooved surface does not catch the reflection sheet, the luminous dot 
phenomenon is prevented. 
Preferably, the frame supports a portion outside a liquid crystal display 
region for display of the liquid crystal panel. More specifically, the 
portion of the frame positioned near a periphery of the liquid crystal 
display region has a step or chamfer. 
In this case, even if the luminous dot phenomenon is caused at the portion 
supported by the frame, as the portion is outside the liquid crystal 
display, the problem associated with display is avoided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment 
FIG. 1 is a cross sectional view showing a structure of a back light (or a 
liquid crystal display) according to the first embodiment. The back light 
includes a reflection sheet 3 and a frame 8 below a light guide plate 4 
having a light collecting effect. A tubular light source 1 is arranged 
near the end surface (edge) of light guide plate 4. A reflector 2 is 
arranged to surround tubular light source 1. A sheet 5 having a light 
collecting effect and a sheet 6 having a light diffusing effect are 
arranged above light guide plate 4. A liquid crystal on panel 7 is 
arranged thereabove. A lens like grooved surface 4a having the light 
collecting effect is provided at the bottom of light guide plate 4. 
Reflection sheet 3 has a thickness of about 150 .mu.m and its surface is 
white. 
The back light differs from the conventional back light in that a 
transparent sheet or plate like member with protrusions is provided 
between lens like grooved surface 4a of light guide plate 4 having the 
light collecting effect and reflection sheet 3. 
More specifically, embossed sheet 11 is used as the sheet with protrusions. 
Polyethylene terephthalate (hereinafter abbreviated as PET) is for example 
used as a material of sheet 11. As embossed sheet 11, for example, PC-ES 
produced by Kimoto Co., Ltd. is employed. The sheet is transparent and 100 
.mu.m in thickness. Any sheet other than that of PET may be used as long 
as the sheet is transparent and has protrusions. 
As shown in FIG. 2, a pitch of the embossed protrusions is about 0.2 mm and 
the protrusion is in the shape of a small column. A pitch of the groove of 
lens like grooved surface 4a of light guide plate is about 30 .mu.m and 
the groove is in a triangular shape. As the pitches of the protrusion and 
groove are different, the contact area of reflection sheet and lens like 
grooved surface 4a is reduced and lens like grooved surface 4a of light 
guide plate 4 and reflection sheet 3 are not in close contact with each 
other. 
Provision of embossed transparent sheet 11 between lens like grooved 
surface 4a of light guide plate 4 and reflection sheet 3 prevents damage 
to lens like grooved surface 4a and prevents reflection sheet 3 from being 
caught by lens like grooved surface 4a, so that the luminous dot 
phenomenon associated with display is prevented. 
It is noted that the cross section of lens like grooved surface 4a is not 
particularly limited to the triangular or wave shape. In addition, 
characteristics of lens like grooved surface 4a, including a vertical 
angle, interval, height or the like, as well as a positional relation with 
respect to the light source and liquid crystal display, are not 
particularly limited. Further, the protrusion is not limited to a column, 
and may be a cone or hemisphere. 
The grooves of lens like grooved surface 4a refer to grooves between lenses 
in stripe pattern provided in the light guide plate and those between 
lenses in dotted pattern provided in the light guide plate. In particular, 
provision of sheet 11 with respect to the grooves between the lenses in 
the stripe shape effectively prevents the above described problem 
associated with catching of the sheet by the grooves or luminous dot 
phenomenon. 
Second Embodiment 
FIG. 3 is a cross sectional view showing a structure of a back light (or a 
liquid crystal display) according to the second embodiment. The second 
embodiment is different from the back light of the first embodiment in 
that a transparent sheet having a certain degree of stiffness is provided 
between lens like grooved surface 4a having the light collecting effect 
and reflection sheet; 3. 
PET is for example used as a material of transparent sheet 9 having a 
certain degree of stiffness. Any material other than PET may be used as 
long as it has the above mentioned function. More specifically, a sheet 
which has a stiffness higher than a reflection sheet RW 125 (having a 
thickness of 125 .mu.m) produced by Kimoto Co. may be used. 
Provision of transparent sheet 9 having a certain degree of stiffness 
between lens like grooved surface 4a of light guide plate 4 and reflection 
sheet 3 prevents damage to lens like grooved surface 4a and prevents lens 
like grooved surface 4a from catching reflecting sheet 3. Thus, the 
luminous dot phenomenon associated with display is prevented. 
Third Embodiment 
FIG. 4 is a cross sectional view showing a structure of a back light (or a 
liquid crystal display) according to the third embodiment. The back light 
of the third embodiment is different from that of the first embodiment in 
that a transparent sheet 10 having a light diffusing effect is provided 
between lens like grooved surface 4a having a light collecting effect and 
reflection sheet 3. 
A sheet 100TL produced by Kimoto Co. is for example used as transparent 
sheet 10 having the light diffusing effect. The sheet is transparent, has 
a thickness of 100 .mu.m and includes PET with bead coatings on either 
side. The bead coating is preferably of a particle type for diffusion of 
light and may include material. 
Use of sheet 10 effectively reduces the contact area with lens like grooved 
surface 4a. The sheet is not limited to sheet 100TL, and any material may 
be used as long as it has the above mentioned function. 
Provision of transparent sheet 10 having the light diffusing effect between 
lens like grooved surface 4a of light guide plate 4 and reflection sheet 3 
prevents damage to lens like grooved surface 4a and prevents lens like 
grooved surface 4a from catching reflection sheet 3. Thus, the luminous 
dot phenomenon associated with display is prevented. 
Fourth Embodiment 
FIG. 5 is a cross sectional view showing a structure of a back light (or a 
liquid crystal display) of the fourth embodiment. In the back light, a 
sheet 12, which is one of the sheets described in the first to third 
embodiments, adheres to reflection sheet 3. A material with high 
transmittance is selected for adhesion. In addition to the effects 
described in the first to third embodiments, the back light of the present 
embodiment produces an effect that efficiency in assembly operation is 
increased as the number of sheets does not increase. 
Fifth Embodiment 
FIG. 6 is a cross sectional view showing a structure of a back light (or a 
liquid crystal display) according to the fifth embodiment. The back light 
of the fifth embodiment is different from that of the first embodiment in 
that a reflection sheet 13 provided with protrusions and a reflecting 
function is arranged on the side of lens like grooved surface 4a. The 
shape of the protrusion is the same as that of sheet 11 of the back light 
according to the first embodiment. 
The protrusions are formed by an emboss processing. Preferably, a sheet 
based on embossed reflection sheet RW 188 or RW 125, produced by Kimoto 
Co., is used. Sheets RW 188 and RW 125 respectively have thicknesses of 
188 .mu.m and 125 .mu.m, both having white surfaces. Any sheet having the 
above described function may be employed. 
Provision of the protrusions on the side of lens like grooved surface 4a of 
light guide plate 4 and sheet 13 having the reflecting function makes 
pitches of the protrusions of sheet 13 and lens like grooved surface 4a 
different, so that the contact area of lens like grooved surface 4a 
decreases. 
Accordingly, damage is not caused to lens like grooved surface 4a and lens 
like grooved surface 4a is prevented from catching reflection sheet 13. 
Thus, the luminous dot phenomenon associated with display is prevented. In 
addition, efficiency in the assembly operation increases as the number of 
sheets is not increased. Furthermore, reduction in the number of sheets 
reduces the overall thickness of the liquid crystal display. 
Sixth Embodiment 
FIG. 7 is a cross sectional view showing a structure of a back light (or a 
liquid crystal display) of the sixth embodiment. The back light of the 
sixth embodiment is different from that of the first embodiment in that a 
transparent coating 14 is applied to the surface of reflection sheet 3 on 
the side of lens like grooved surface 4a. 
Preferably, sheet BS-01 or BS-03 produced by Keiwa Shoukou Co. with a 
coating is used. It is noted that the coating is not limited to this as 
long as it has the above mentioned function. 
Transparent coating 14 on reflection sheet 3 below lens like grooved 
surface 4a of light guide plate 4 prevents damage to lens like grooved 
surface 4a and prevents lens like grooved surface 4a from catching 
reflection sheet 3. Thus, the luminous dot phenomenon associated with 
display is prevented. Further, efficiency in the assembly operation 
increases as the number of sheets is not increased. 
Seventh Embodiment 
FIG. 8 is a cross sectional view showing a back light (or a liquid crystal 
display) of the eighth embodiment. The back light of the eighth embodiment 
is different from that of the sixth embodiment in that a transparent 
coating 15 is applied to the surface of lens like grooved surface 4a of 
light guide plate 4. For example, although not limited, SiO.sub.2 is used 
for the coating. In addition, coating 15 is not limited to a transparent 
coating. 
Application of transparent coating 15 on the surface of lens like grooved 
surface 4a of light guide plate 4 prevents damage to lens like grooved 
surface 4a and prevents lens like grooved surface 4a from catching 
reflection sheet 3. Thus, the luminous dot phenomenon associated with 
display is prevented. Further, efficiency in assembly operation increases 
as the number of sheets is not increased. 
Eighth Embodiment 
FIGS. 9A and 9B are cross sectional views showing structures of a back 
light (or a liquid crystal display) according to the eighth embodiment. 
FIG. 9A shows a cross section taken along the line 9A--9A of frame 8, and 
FIG. 9B shows a top view of frame 8. 
As in the case of the conventional back light, frame 8 supporting light 
guide plate 4 is provided with an opening. A sheet or coating 21, which 
has been described in the first to seventh embodiments, is applied on the 
side of lens like grooved surface 4a of light guide plate 4. If the 
portion in the vicinity of a boundary of the liquid crystal area or a 
portion of frame 8 is within a liquid crystal display area, a chamfer 17 
in an arc shape is formed inside frame 8 on the side of lens like grooved 
surface 4a. 
Unlike a chamfer in the arc shape (R0.2 mm to R0.3 mm) generally formed by 
molding, a chamfer in an arc shape, which is as large as possible in terms 
of design, is intentionally formed. For example, in frame 8, the chamfer 
is R1 mm at a portion having a thickness of 1.1 mm. The chamfer is not 
limited to the arc shape, and any shape without a corner may be employed. 
Provision of chamfer 17 in the arc shape in frame 8 prevents damage to lens 
like grooved surface 4a in a display area and prevents lens like grooved 
surface 4a from catching reflection sheet 3. Thus, the luminous dot 
phenomenon associated with display is prevented. 
Ninth Embodiment 
FIG. 10A is a cross section view showing a structure of a back light (or a 
liquid crystal display) of the ninth embodiment of the present invention. 
For frame 8 supporting light guide plate 4, when a boundary between a 
light shielding film 23 and a liquid crystal area is viewed at a viewing 
angle of 45.degree., a step 18 is provided inside frame 8 and outside the 
viewing angle 45.degree.. 
As shown in FIG. 10B, an edge 16 may be positioned outside the viewing 
angle of 45.degree.. Step 18 is in a shape defining an opening with an 
upper portion larger than a lower portion. The direction of the viewing 
angle of 45.degree. corresponds to the direction of the largest viewing 
angle when viewing the liquid crystal display. The shape of frame 8 is 
shown in FIG. 10C. If a portion of frame 8 is within the liquid crystal 
display area, the shapes shown in FIGS. 9 and 12 may be employed. 
Step 18 of frame prevents damage to lens like grooved surface 4a and 
prevents lens like grooved surface 4a from catching reflection sheet 3, so 
that the luminous dot phenomenon associated with display is prevented. 
Further, even if the luminous dot phenomenon is caused at edge 16 on the 
side of lens like grooved surface 4a of step 18 of frame 8, the luminous 
dot phenomenon does not affect display as edge 16 is outside the liquid 
crystal display area. Therefore, the problem associated with display is 
not caused. 
Tenth Embodiment 
FIG. 11 is a cross sectional view showing a structure of a back light (or a 
liquid crystal display) of the tenth embodiment. A recess is provided 
around a protrusion 19 by a pin of frame 8 or a protrusion by a gate pin 
caused during manufacture, so that the protrusion is not brought into 
contact with lens like grooved surface 4a of light guide plate 4. 
Protrusion 19 by a pin or the protrusion by the gate pin is not limited to 
this as long as it is not brought into contact with lens like grooved 
surface 4a. With such a structure, damage to lens like grooved surface 4a 
is prevented. 
Eleventh Embodiment 
FIG. 12 is a cross sectional view showing a structure of a back light (or a 
liquid crystal display) of the eleventh embodiment. The back light is a 
combination of the structures of frame 8 described in the eighth to tenth 
embodiments. 
The combination of structures of frame 8 also produces an effect similar to 
those described in the eighth to tenth embodiments. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.