U.S. patent application number 09/572052 was filed with the patent office on 2001-11-29 for light guide device enhancing a polarized component and liquid crystal display device.
Invention is credited to Kawada, Koji, Oki, Yoji, Suzuki, Masaru.
Application Number | 20010046006 09/572052 |
Document ID | / |
Family ID | 17462221 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010046006 |
Kind Code |
A1 |
Oki, Yoji ; et al. |
November 29, 2001 |
Light guide device enhancing a polarized component and liquid
crystal display device
Abstract
A polarized component is obtained with a high conversion
efficiency in a light guide which produces one of the polarized
components by having it transmitted. The light from a light source
is incident to a light guide which comprises a plurality of light
guide layers and reflected by the end surface to an interface
between the light guide layers. The polarized component
transmitting through the end surface is rotated in its polarization
plane by a wave length plate and reflected by a reflecting plate
for reentrance to the light guide at the end surface of the light
guide toward the interface. The reentering light mostly transmits
through the interface because the polarization plane is rotated. A
reflected light polarized component is returned to the wave length
plate and the reflecting plate, and directed back to the interface
again. The polarized component transmitting through the interface
is similarly transmitted and reflected in the next interface. The
number of interfaces can be reduced by increasing the reflection of
the polarized component reflected in the interface. For this
purpose, the index of refraction in the direction along the axis of
the reflected polarized component is increased by making the index
of refraction of the light guide layer anisotropic.
Inventors: |
Oki, Yoji; (Yokohama,
JP) ; Kawada, Koji; (Hadano, JP) ; Suzuki,
Masaru; (Hodogaya, JP) |
Correspondence
Address: |
HOLLAND & HART LLP
PO BOX 8749
555 17TH STREET, STE 3200
DENVER
CO
80201
US
|
Family ID: |
17462221 |
Appl. No.: |
09/572052 |
Filed: |
May 17, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09572052 |
May 17, 2000 |
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09399124 |
Sep 20, 1999 |
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6118503 |
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Current U.S.
Class: |
349/65 |
Current CPC
Class: |
G02B 6/005 20130101;
G02B 6/003 20130101; G02B 6/0056 20130101; G02B 6/0046 20130101;
G02F 1/133615 20130101; G02B 6/0038 20130101; G02B 6/0055 20130101;
G02B 6/0053 20130101; G02F 1/13362 20130101 |
Class at
Publication: |
349/65 |
International
Class: |
G02F 001/1335 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 22, 1998 |
JP |
10-268704 |
Claims
What is claimed is:
1. A light guide device comprising: a light guide unit consisting
of a lamination of a plurality of light guide layers in which one
end thereof is a light incidence surface and the other end is cut
obliquely with respect to the direction of the lamination; a
reflecting plate disposed adjacent to said other end surface; and
means disposed between said other end surface and said reflecting
plate for changing the polarization direction of light, said light
guide layers having an anisotropic index of refraction in which the
index of refraction in the axis of reflecting one of the polarized
components of the light from said incident surface is larger than
the index of refraction in the axis of transmitting the other one
of said polarized components.
2. A light guide device of claim 1 in which said other end surface
is cut obliquely in an angle such that the light reflected by said
other end surface and incident to an interface between said light
guide layers is incident in the Brewster angle.
3. A light guide device of claim 1 in which said other end surface
has a plurality of sloped surfaces in the direction perpendicular
to the direction of the lamination of the light guide layers and
the direction of incidence of the light.
4. A light guide device of claim 3 in which said sloped surface
comprises a combination of a surface having an angle of inclination
smaller than the angle of inclination of said other end surface and
a surface having a larger angle of inclination which is an angle
causing the light reflected from the latter surface and incident to
the interface between the light guide layers to be incident in the
Brewster angle.
5. A light guide device of claim 4 in which the angle .phi. of the
surface having a larger angle is given by the following expression:
.phi.=cos.sup.-1 {(n.sub.2/n.sub.1)
[n.sub.1.sup.2/(n.sub.1.sup.2+n.sub.2- .sup.2)].sup.1/2}/2 (rad)
where n.sub.1 is an index of refraction of the light guide and
n.sub.2 is an index of refraction of a material other than the
light guide.
6. A light guide device of claim 5 in which a plurality of light
guide films are further laminated on the top layer of said
plurality of light guide layers of said light guide unit.
7. A light guide device of claim 26 in which said light guide film
is thinner than said light guide layer.
8. A light guide device of claim 6 in which a phase film for
changing the direction of polarization is further disposed on said
plurality of light guide films.
9. A liquid crystal display device comprising a liquid crystal cell
and a light guide device disposed in the back of said liquid
crystal cell, said light guide device comprising: a light guide
unit consisting of a lamination of a plurality of light guide
layers in which one end thereof is a light incidence surface and
the other end is cut obliquely with respect to the direction of the
lamination; a reflecting plate disposed adjacent to said other end
surface; means disposed between said other end surface and said
reflecting plate for changing the polarization direction of a
light; and a prism sheet disposed on the top light guide layer of
said light guide unit and having an apex part only on one side
thereof, the side where said apex is formed facing said light guide
unit, said light guide layers having an anisotropic index of
refraction in which the index of refraction in the axis of
reflecting one of the polarized components of the light from said
incident surface is larger than the index of refraction in the axis
of transmitting the other one of said polarized components.
10. A liquid crystal display device comprising a liquid crystal
cell and a light guide device disposed in the back of said liquid
crystal cell, said light guide device comprising: a light guide
unit consisting of a lamination of a plurality of light guide
layers in which one end thereof is a light incidence surface and
the other end is cut obliquely with respect to the direction of the
lamination; a reflecting plate disposed adjacent to said other end
surface; means disposed between said other end surface and said
reflecting plate for changing the polarization direction of a
light, and a prism sheet disposed on the top light guide layer of
said light guide unit and having an apex part only on one side
thereof, the side where said apex is formed facing said light guide
unit; the pitch of a plurality of prisms of said prism sheet is the
same as the pitch of the array of the liquid crystal cells.
11. A liquid crystal display device of claim 10 in which a
reflecting surface of each prism of said prism sheet is aligned
with an opening part of the liquid crystal cell.
12. A liquid crystal display device of claim 11 in which said
liquid crystal cell is adhered to said prism sheet.
13. A liquid crystal display device of claims 10 in which said
prism sheet is integrally formed with a substrate of said liquid
crystal cell.
14. A liquid crystal display device comprising: a matrix array of
liquid crystal cells arranged on a transparent substrate; and a
column of a plurality of prisms formed on the back of said
substrate with respect to said liquid crystal cells with a
reflecting surface of each prism being aligned with an opening part
of said liquid crystal cell.
15. A liquid crystal display device of claim 14 in which said
substrate is formed of a glass.
16. A liquid crystal display device of claim 14 in which said
substrate is formed of a plastic material.
17. A liquid crystal display device of claim 11 in which said array
of liquid crystal cells is formed directly on said prism sheet.
18. A liquid crystal display device of claim 10 in which a
polarizer plate is further disposed on said prism sheet.
19. A liquid crystal display device of claim 10 in which said prism
sheet is formed of a polarizing material.
20. A liquid crystal display device of claim 10 in which said light
guide layers has an anisotropic index of refraction in which the
index of refraction in the axis of reflecting one of polarized
components of the light from said incident surface is larger than
the index of refraction in the axis of transmitting the other one
of said polarized components.
21. A liquid crystal display device of claim 17 in which a
polarizer plate is further disposed on said prism sheet.
22. A liquid crystal display device of claims 17 in which said
prism sheet is formed of a polarizing material.
23. A liquid crystal display device of claim 19 in which said light
guide layers has an anisotropic index of refraction in which the
index of refraction in the axis of reflecting one of the polarized
components of the light from said incident surface is larger than
the index of refraction in the axis of transmitting the other one
of said polarized components.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a light guide unit for use
in a liquid crystal display device in which a polarized component
of light is enhanced and a liquid crystal display device which is
provided with such light guide unit. Particularly, this invention
relates to a light guide unit for efficiently converting the light
from a light source to a polarized light and a liquid crystal
display device having means for efficiently directing-the polarized
light emitted from such light guide unit to a liquid crystal
cell.
[0003] 2. Description of the Related Art
[0004] A liquid crystal display device is conventionally observed
by directing polarized light to a liquid crystal cell to cause the
polarization plane to be rotated depending on the condition of the
cell for passage through a polarizer plate. A light source of the
polarized light is placed in the back of the liquid crystal plate
and thus is called a "back light". For obtaining such polarized
light wave, a non-polarized light was conventionally incident to a
polarizer plate and either one of the polarized components; i.e., S
component and P component, was absorbed.
[0005] Assuming that a plane defined by a light incident to a point
of incidence on a surface is an incident plane, a polarized
component parallel to the incident plane is called a P component
while a component perpendicular to the incident plane is called an
S component. Therefore, more than 50-percent of an incident light
was not effectively utilized in principle and an actual measurement
shows that about 58-percent of the incident light is absorbed.
[0006] Further, a light dispersing sheet having printed dots was
typically used in addition to a polarization device for obtaining
polarized light by absorbing a polarized component in a
conventional Liquid Crystal Display (LCD) device, and this makes an
additional 20-percent of the light unavailable.
[0007] In FIG. 1, a LCD module 100 of a conventional LCD device is
shown. The light emanating from a light source 101 transmits
through a light guide plate 102 having 96% transmittance, a
dispersion sheet 103 having 80% transmittance, a lower polarizer
plate 104 having 42% transmittance, a glass substrate 105 having a
numerical aperture of 40%, a color filter 106 having 30%
transmittance, and an upper polarizer plate 107 having 90%
transmittance, resulting in an actually available light intensity
which is 3.5% of the light generated in the light source 101. This
greatly prevents the energy from being utilized efficiently.
[0008] A back light system of a high intensity for use in a low
power consumption LCD device is especially desired because it is an
important objective in a portable personal computer to assure a
longer usable time with a given capacity of a battery and the power
consumption of a back light 108 is a major percentage of total
power consumption.
[0009] Also, the light energy absorbed in the lower polarizer plate
104, etc., is converted to heat energy which contributes to
degradation of parts of the LCD device. Particularly for a liquid
crystal material of STN (Super Twisted Nematic) type in which the
display quality is degraded by heat, it is an important objective
to reduce such heat generation. As seen from FIG. 1, 66.4% of the
light energy is converted to heat energy by the light absorption in
the lower polarizer plate 104 and the dispersion sheet 103 (this is
69% of heat generation by the light energy).
[0010] In order to solve such problems, the applicant of this
application filed Japanese patent application no. 9-249139 relating
to a method of improving the efficiency of light utilization in
obtaining a polarized light by making available for use at least a
part of a polarized component which had not been utilized. The
principle of this method is shown in FIG. 3.
[0011] Light from a fluorescent lump CFL which is a light source is
incident to the end surface of a laminated light guide plate unit
via a reflecting mirror and a collimator. It propagates through the
layers of the light guide plates, and arrives at the other end
surface which is cut in an angle. The incident light is partly
reflected at the other end surface with the rest being transmitted
therethrough. The polarization plane of the light transmitting
through the end surface is rotated by a quarter wave length plate
placed thereunder and reflected by a reflecting plate placed under
the quarter wave length plate for reentrance to layers of the light
guide plate again through the quarter wave length plate as a P
component.
[0012] The P component reentering the light guide plates is
incident to the interface with an adjacent light guide plate layer.
The angle of incidence of the light on the interface is the
Brewster angle (to be described later in detail). Therefore, all
the P component and a part of the S component of the light incident
to the interface transmit through the interface with the rest of
the S component reflected back to the quarter wave length plate and
the reflecting plate. The light reflected again by the reflecting
plate is again directed to the interface after being converted to a
P component by the quarter wave length plate where all the P
component and a part of the S component, if any, transmit with the
rest being reflected.
[0013] The light reflected here is reflected repeatedly in a
similar manner and a light converted to a P component for each
reflection transmits through the interface. As such, the light
guide unit ultimately emits a large portion of the light from the
light source as a P component. The polarized light is emitted in
the direction largely deviated from the normal to the front. A
prism sheet for redirecting the light to the front toward the
liquid crystal cell is used. The polarization can be further
improved by placing a further polarization plate on the prism
sheet.
[0014] Because the reflectance and transmission characteristics are
different between the S component and the P component, the light
transmitting through the interface and the light reflected by the
interface have different polarization components. To explain the
principle of operation of this invention, a change of polarization
components of the light in transmitting through or reflecting from
the interface between materials of different indices of refraction
is described with reference to FIGS. 4, 5 and 6.
[0015] In FIG. 4, when light 204 reaches an interface 203 between
two materials 201 and 202 having different indices of refraction
n.sub.1 and n.sub.2, respectively, a part of the light 205 is
reflected when the angle of incidence .phi..sub.1 is less than a
critical angle while a part of the light 206 transmits through the
interface. Assuming that a plane defined by a light incident to a
point of incidence on a surface is an incident plane, the incident
light 204 is divided into a P component parallel to the incident
plane and an S component perpendicular to the incident plane.
[0016] Modifying Maxwell equation for a dielectric material, the
transmittance of the polarized components P and S are given by;
Tp=sin (2.phi..sub.1).times.sin
(2.phi..sub.2)/(sin.sup.2(.phi..sub.130
.phi..sub.2).times.cos.sup.2(.phi..sub.1-.phi..sub.2))
Ts=sin (2.phi..sub.1).times.sin (2.phi..sub.2)/sin.sup.2
(.phi..sub.1+.phi..sub.2)
n.sub.1.times.in (.phi..sub.1)=n.sub.2.times.sin (.phi..sub.2)
[0017] where
[0018] Tp: transmittance of P component (1-reflectance Rp)
[0019] Ts: transmittance of S component (1-reflectance Rs)
[0020] .phi..sub.1: incident angle of light
[0021] .phi..sub.2: exit angle of light
[0022] n.sub.1: index of refraction of material 201
[0023] n.sub.2: index of refraction of material 202
[0024] or it is known that;
Rp=((n.sub.1/cos .phi..sub.1-n.sub.2/cos .phi..sub.2)/(n.sub.1/cos
.phi..sub.1+n.sub.2/cos.phi..sub.2)).sup.2
Rs=((n.sub.1.times.cos .phi..sub.1-n.sub.2.times.cos
.phi..sub.2)/(n.sub.1.times.cos .phi..sub.1+n.sub.2.times.cos
.phi..sub.2)).sup.2
[0025] where
[0026] Rp: reflectance of P component (1-transmittance Tp)
[0027] Ts: reflectance of S component (1-transmittance Ts)
[0028] The reflectance of the P polarized component and S polarized
component vary depending on the incident angle .phi..sub.1 and the
exit angle .phi..sub.2 as shown in FIG. 5 and FIG. 6, and differ
from each other even in a same incident angle .phi..sub.1
(reflectance/transmittanc- e characteristics are different between
S and P polarized components).
[0029] For example, when the light proceeds from an acrylic
material having an index of refraction of 1.49 to air which has an
index of refraction of 1.00 (FIG. 6), the critical angle in which a
total reflection takes place is 42.1-degrees. If the light is
incident at 40-degrees which is less than the critical angle, the
exit angle .phi..sub.2 will be 77.8-degrees according to Snell's
law. Substituting the above equation of Rs and Rp with this, the
reflectance for the S component is 35.69% while the reflectance for
the P component is 7.98%.
[0030] It should be clearly understood from the above description
referring to FIGS. 4 to 6 how the polarized components of the light
are transmitted and reflected in the interface in this
invention.
[0031] It is understood from the above-described principle that it
is important for the layers of the light guide to be laminated in
multiple layers to cause the unnecessary S component to be
reflected back each time the light reaches the interface between
the layers and to be returned as a P component for transmitting
through the interface thereby improving the efficiency of
converting the light emitting from the unit eventually to a P
component.
[0032] However, it is disadvantageous to laminate too many layers
from the view point of the efficiency of utilizing the energy of
the light source because each layer invites some loss of light. In
addition, the increased number of laminated layers would result in
the increase of the thickness of the entire unit even if a thin
layer is used. The increase of the thickness would also invite an
increase of the weight. It is the most important objective for a
portable information processing device, such as a notebook
computer, to decrease the power consumption of its battery as well
as the thickness and the weight of the entire unit as much as
possible.
SUMMARY OF THE INVENTION
[0033] This invention relates to an improvement of a light guide
unit of the above-described type, and it is an object of this
invention to provide a light guide unit having an unchanged
performance with a decreased thickness of the entire unit.
[0034] It is another object of this invention to improve the
brightness of a liquid crystal display device without resulting in
an increase of power consumption by efficiently combining the
polarized light from such light guide unit of a high efficiency to
a liquid crystal cell.
[0035] The basic configuration of this invention lies in a
structure in which the light from a light source incident to an end
surface of a unit of laminated light guide plates propagates
through each layer of the light guide plates and is partly
reflected by the other end surface which is obliquely cut. The rest
of the light transmitting therethrough causes the polarization
plane of the transmitting light to be rotated by a wave length
plate lying thereunder and reflected by a reflecting plate lying
under the wave length plate for reentrance to the light guide plate
again through the wave length plate as a P component.
[0036] The P component reentering the light guide plate is incident
to an interface between neighboring light guide plates. The
incident angle of the light incident to the interface is adapted to
be the Brewster angle. Therefore, all P component light incident to
the interface and a part of the S component light transmit the
interface while the rest of the S component light is reflected back
to the wave length plate and the reflecting plate. The light
reflected again by the reflecting plate is directed back to the
interface after being converted to a P component by the wave length
plate, and all P components and a part of S component, if any,
transmit through the interface while the rest is reflected.
[0037] The light reflected here is subject to the same process
repeatedly, and a light converted to a P component in every
repetition transmits through the interface. As such, the light
guide unit eventually emits a large portion of the light from the
light source as a P component. Because the polarized light is
emitted in the direction largely deviated from the normal to the
front, a prism sheet for redirecting the light to the front toward
the liquid crystal cell is used.
[0038] In this invention, it is important in the principle of this
invention that the S component is reflected in the interface of the
light guide layers. The number of the interfaces; i.e., the number
of the light guide layers can be reduced by causing as much S
component as possible to be reflected to reduce the S component
transmitting through the interface.
[0039] This invention provides a conversion efficiency comparable
to light guide layers using an isotropic material with a lesser
number of light guide layers by using a material of an anisotropic
index of refraction as the light guide layers to improve the
reflectance of the S component in the interface. The axes of two
indices of refraction of the anisotropic material coincide with the
planes of P and S components, respectively. While the index of
refraction in the direction of the axis lying in the plane of the P
component may be the same as a conventional one, the index of
refraction in the plane of the S component is higher than the
conventional one. The higher, the better. It is seen from the
expression of the reflectance Rs described above that the
reflectance of the S component becomes larger when the index of
refraction in the axis of the plane of the S component in the
interface is larger.
[0040] The light guide unit comprising laminated light guide layers
of such anisotropic index of refraction receives an incident light
from a light source at the end surface thereof which is a
cross-section of the laminated layers to cause a part of the
incident light to be reflected at the opposite end surface which is
obliquely cut and the rest of the light to be transmitted
therethrough. A quarter wave length plate is attached to the
obliquely cut end surface and a reflecting plate is provided under
the wave length plate.
[0041] The light transmitting through the end surface is reflected
by the reflecting plate after being rotated by the wave length
plate and is incident to the end surface after being rotated by the
wave length plate again. The light incident to the end surface is
incident to the interface where it is transmitted and reflected as
described herein. However, the majority of the S component is
reflected in the interface with the rest transmitting through the
interface in this invention. Therefore, the light from the light
source can be converted to the P component with a lesser number of
layers.
[0042] In this invention, it is preferred that the light incident
to the first interface of the light guide is incident in the
Brewster angle. It is readily seen by drawing a geometrical drawing
that the angle of incidence of the light to the obliquely cut end
surface of the light guide unit decides the angle of incidence at
the interface. In this invention, the angle of incidence of the
light to the obliquely cut end surface of the light guide unit is
so adjusted that the light incident to the first interface of the
light guide is incident in the Brewster angle.
[0043] The light guide unit is so inclined with respect to the wave
length plate and the reflecting plate as to provide an incident
angle decided in this manner. In order to reduce the inclination, a
plurality of slopes making such incident angle may be formed in the
obliquely cut end surface. This allows a necessary incident angle
to be provided without inclining the entire light guide unit in
this angle. This allows the thickness of the entire unit to be
further reduced.
[0044] In this invention, the light guide unit may be formed into a
shape of a triangular wedge consisting of the top layer of the
laminated layers, the obliquely cut end surface and the surface to
which the light from the light source is incident. This allows a
wedge-shaped space to be provided under the unit for receiving
various components. This is advantageous for a portable data
processing device in which a thin and light weight type is
especially desired.
[0045] In another aspect of this invention, the prism means for
directing the polarized light to the front has a plurality of
prisms disposed in a same pitch as columns of the liquid crystal
cells. Each prism has an incident surface and a reflecting surface.
Because the light is emitted from the reflecting surface, a portion
corresponding to the incident surface is dark. In this invention,
the dark incident surface portion is so disposed as not to
contribute illuminating the liquid crystal cell by having the
portion corresponding to the reflecting surface align the columns
of the liquid crystal cell. All the polarized light emitted from
the light guide is thus directed to the liquid crystal cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 is a schematic diagram showing a conventional LCD
device.
[0047] FIG. 2 is a diagram showing the structure of a conventional
LCD polarizer plate unit.
[0048] FIG. 3 is a diagram showing the structure of a conventional
LCD polarizer plate unit.
[0049] FIG. 4 is a diagram showing refraction of light between
different materials.
[0050] FIG. 5 shows a characteristic plot of reflectance when the
light is incident from a material having an index of refraction of
1.0 to a material having an index of refraction of 1.49.
[0051] FIG. 6 shows a characteristic plot of reflectance when the
light is incident from a material having an index of refraction of
1.49 to a material having an index of refraction of 1.0.
[0052] FIG. 7 is a diagram showing deflection of the light by a
prism sheet.
[0053] FIG. 8 is a diagram showing deflection of the light by a
prism sheet.
[0054] FIG. 9 is a schematic diagram showing a concept of another
embodiment of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0055] The basic structure of this invention is shown in FIG. 2.
The laminated light guide unit is made of thin light guide layers
laminated as shown and a light source is attached to an end surface
thereof. The light source comprises a fluorescent lamp and a
reflecting sheet. The lamination is cut so as to assume an oblique
end surface to which a combination of a quarter wave length plate
and a reflecting plate is affixed.
[0056] The end surface makes an angle .phi. with respect to the
quarter wave length plate and the reflecting plate. Therefore, the
light is incident to the end surface at an angle of .pi./2-2.phi..
It is readily found from a geometrical drawing that an incident
angle to the interface between layers is .pi./2-2.phi..
[0057] It is preferable that this incident angle is the Brewster
angle. When the light is incident to the interface at the Brewster
angle, all polarization component lying in the incident plane (P)
transmits the interface while all polarization component lying in a
plane orthogonal to the incident plane (S) is reflected. Any S
component which may transmit through the interface will be
reflected by the next interface. The S component reflected from the
interface is returned back to the light guide layers by the wave
length plate and the reflecting plate as a P component and incident
again to the interface. The S component is reflected each time the
above process is repeated at a plurality of interfaces transmits
through the interface as a P component so that a large portion of
the light from the light source is emitted from the light guide
unit as a P component.
[0058] A plurality of thin light guide films may be further
laminated on the top light guide layer of the light guide unit as
shown in FIG. 3. This further adds interfaces of transmission and
reflection.
[0059] The light guide member and the plurality of light guide
layers are preferably of a material which assumes a low internal
absorption of the light, such as an acrylic sheet and preferably
transparent materials including acrylic resin, PMMA
(polymethylmethacrylate), polycarbonate, polyethylene, Se, and
AgCl. The shape of the light guide member may be in a shape
suitable for use such as a bar and a curved surface without being
limited to a plate and a sheet.
[0060] The light guide member may be of a single piece or a
lamination of a plurality of sheets. These light guides are not
limited to a same size or a same material and a member requiring
stiffness may be designed thick while a member which does not
require stiffness may be designed thin. Also, materials of
differing indices of refraction may be deposited in multiple layers
on a stiff light guide to increase the number of laminated layers
while maintaining a stiffness.
[0061] In using an acrylic sheet in the light guide member, the
thickness of the sheet is preferably 0.1 to 4.0 mm from the
consideration of the stiffness and the efficiency of light
utilization. The lamination as used in this invention is not
limited to insertion of air between the light guides and water
vapor may be introduced between the guides for preventing
degradation of the light guide unit, water or an adhesive may be
inserted between the guides for preventing the guides from being
peeled off, or a material having an index of refraction differing
from the light guide may be inserted. Higher reflectance of the
reflecting plate is preferable in this invention and the reflecting
plate may be made of an aluminum deposited sheet, a silver
deposited sheet and a metal foil, etc.
[0062] In this invention, the light guide layer is made of a
material having a high index of refraction in the axis lying in the
plane of the S component. For example, while the isotropic index of
refraction of an acrylic material is normally 1.49, the index can
be increased up to about 1.69 in the direction of the axis lying in
the plane of the S component. By doing so, an increased portion of
the S component is reflected in the interface (lesser amount of the
S component transmit the interface) so that an unchanged effect can
be resulted with a lesser number of layers than those required for
an isotropic material.
[0063] For example, when an acrylic material having an index of
refraction 1.49 is used as an isotropic material, the reflectance
of the S component is 28% while the transmittance is 72% per layer.
With ten layers laminated, the overall transmittance will be
0.72.sup.10=0.04.
[0064] On the other hand, if the index of refraction in the
direction of the S component is 1.66, the reflectance is 40% while
the transmittance is 60% and the same effect is obtained with 6
layers (0.6.sup.6=0.04). A sheet having such anisotropic index of
refraction is easily available in the market.
[0065] While the thickness of the light guide film is not important
and it is preferable that the number of the interfaces is as large
as possible, the light guiding layer is preferably as thin as
possible from the view point of reducing the weight of the light
guide unit. An extra space is created by making the thickness of
the light guiding layer in this portion extremely thin and the
layers of substantially same size may be used in lamination without
requiring the layers to be progressively in different sizes
resulting in a stepped structure as shown in FIG. 2.
[0066] As such, the light is not lost by re-entering from the edge
of the layers and dark and bright stripes are eliminated. Even if
the steps remain in the layers as shown in FIG. 2, there is little
possibility of the light re-entering and recognizable stripes are
not generated because the layer is thin and the size of the edge is
very small.
[0067] By employing the above structure, this invention allows the
cross-sectional shape of the light guide unit to be of a triangular
shape as shown in FIG. 3, in contrast to the conventional unit
which had a rectangular cross-section as shown in FIG. 2. By this
structure, the weight and the volume of the unit can be about half
the conventional unit. Also, this invention can implement a mode
which is similar to the mode in which a conventional back light
(not generating a polarized light) uses a light guide of a wedge
shaped cross section to provide an effective use of a space and
allows a conventional back light to be replaced with the present
polarized back light in a form compatible with the conventional
type.
[0068] While the light guide layer is acrylic material and the
surrounding material is air in the example so far described, any
material of the layer and any surrounding material may be used so
long as the indices of refraction of the materials allow the
incident light to satisfy the Brewster angle or an angle which is
near the Brewster angle.
[0069] The following condition is required for the incident angle
.pi./2-2.phi. to be the Brewster angle .theta..sub.B. In the
expression, n.sub.1 is an index of refraction of the light guide,
n.sub.2 is an index of refraction of a material other than the
light guide (air in FIG. 5), and .phi. is the angle of the groove
(the slope of the larger angle of inclination). The relationship
between Brewster angle .theta..sub.B and n.sub.1, n.sub.2 is given
by;
.theta..sub.B=sin.sup.-1
[n.sub.1.sup.2/(n.sub.1.sup.2+n.sub.2.sup.2)].sup- .1/2 (rad)
[0070] The angle of incidence to the upper surface of the light
guide is given by a geometric analysis using .phi.;
.pi./2-2.phi. (rad)
[0071] Snell's law is expressed on the upper surface of the light
guide as;
sin .theta..sub.B/sin (.pi./2-2.phi.)=n.sub.1/n.sub.2
[0072] solving this expression for .phi. gives the following
general solution;
.phi.=cos.sup.-1 {(n.sub.2/n.sub.1)
[n.sub.1.sup.2/(n.sub.1.sup.2+n.sub.2.- sup.2)].sup.1/2}/2
(rad)
[0073] Any medium satisfies the condition of this invention so long
as it satisfies the above general expression.
[0074] While the entire light guide unit is inclined with respect
to the wave length plate and the reflecting plate so as to provide
an incident angle which is equal to the Brewster angle, many sloped
surfaces which provide such incident angle can be formed in the
obliquely cut end surface. As shown in the enlarged view in FIG. 3,
many sloped surfaces running perpendicularly to the face of the
drawing are formed in the obliquely cut end surface and are so
disposed as to provide a desired angle to the incident light in the
light guide. An incident angle satisfying the Brewster angle is
thus provided though the entire light guide is not inclined in this
angle. A necessary incident angle can be thus provided while the
light guide unit is not entirely inclined in this angle thereby
reducing the thickness of the entire unit.
[0075] This invention is contemplated for use as a back light of a
liquid crystal display device. The liquid crystal display device
comprises a light source and glass substrates sandwiching a liquid
crystal to which a polarized light emitted from the light guide
unit of this invention is incident.
[0076] The light emitted from the light guide is largely inclined
in 70-degrees from the front thereof in this invention. Two methods
are available for deflecting the light to the right angle to the
front surface. The first method is to have the light refract twice
to deflect it to the front, in which a prism sheet is used with the
apex thereof oriented upward as shown in FIG. 8. When the index of
refraction n of the material of the prism is 1.58, a prism sheet
having an angle of apex of 32-degrees is required to deflect the
light to the front.
[0077] A second method is to have the light refract once and
totally reflect once to deflect to the front, in which the prism
sheet is used with the apex thereof oriented downward as shown in
FIG. 9. In this case, a prism having an angle of apex of
65.4-degrees is required. As seen in the above, a same effect is
resulted whether the prism is oriented upward or downward. From the
view point of fabrication, it is more advantageous in the view
point of yield and cost to use the prism with the apex oriented
downward because a smaller apex angle of a prism is more difficult
to fabricate (a larger apex angle can be used when the apex is
oriented downward). The prism sheet is made of a glass or plastic
material.
[0078] In FIGS. 7 and 8, it is seen that the sloped surface of each
prism which is not the light reflecting surface does not emit the
light to the front. In other words, the light emitted from the
prism sheet is in a stripe pattern. This may possibly induce an
interference pattern with a gate line or a data line of the liquid
crystal cell. In order to prevent the stripe pattern from being
generated, the pitch of the prisms of the prism sheet (50 microns,
for example) can be made smaller than the pitch of the liquid
crystal cell (200 microns, for example) to mismatch the pitches. By
doing so, the prism sheet is observed as if it emits the light
uniformly from the front and the interference pattern can be
prevented from being generated because the pitch of the prism sheet
is very small.
[0079] However, the light incident to a portion of the liquid
crystal cell array which has no opening is absorbed there and
wasted in this case. Another aspect of this invention provides a
structure in which such waste is avoided.
[0080] According to this structure of this invention, the pitch of
the prisms of the prism sheet is made the same as the pitch of the
liquid crystal cell array so that the opening part of the liquid
crystal cell coincides with a portion of the prism corresponding to
the reflecting surface which emits the light. The portion
corresponding to the slope of the prism which is not the reflecting
surface coincides with the part having no opening.
[0081] FIG. 9 is a schematic diagram showing a concept of the
inventive structure. As shown in FIG. 9, the pitch of the prisms of
the prism sheet is made the same as the pitch of the liquid crystal
cell array. The light reflected by the reflecting surface of the
prism is directed to the opening part of the liquid crystal cell.
The part having no opening does not receive the light because it
faces to the surface which is not a reflecting surface. All the
light emitted from the light guide unit is thus directed to the
opening part, and there is no light which is absorbed without being
utilized. It is easy to manufacture the prism because the prism has
a larger pitch than those shown in FIGS. 7 and 8. The apex angle
and the ratio of reflecting/transmitting surfaces may be suitably
decided in a specific design work.
[0082] As shown in FIG. 9, the liquid crystal cell array may be
formed directly on the prism sheet. In this case, the prism sheet
also plays a role of a glass substrate of the liquid crystal cell.
The number of interfaces between media is decreased by 2 when
compared to a case where an independent prism sheet is disposed
between the liquid crystal cell and the light guide film, resulting
in a corresponding improvement of efficiency.
[0083] The thickness and the weight of the light guide are reduced
because the light is converted to the P polarized component with
the number of layers less than those of a conventional light guide
according to this invention. There is no light which is absorbed
without being utilized in another aspect of this invention because
all the light from the light guide is directed to the opening part
of the liquid crystal cell.
[0084] The following is a brief description of the reference
numbers as used in the drawings:
[0085] 100: Conventional LCD device
[0086] 101: Light source
[0087] 102: Light guide plate
[0088] 103: Diffusion sheet
[0089] 104: Lower polarizer plate
[0090] 105: Glass substrate
[0091] 106: Color filter
[0092] 107: Upper polarizer plate
[0093] 108: Back light
[0094] 201: Material 1
[0095] 202: Material 2
[0096] 203: Interface between the materials
[0097] 204: Incident light
[0098] 205: Reflected light
[0099] 206: Transmitted light
[0100] While the exemplary preferred embodiments of the present
invention are described herein with particularity, those having
normal skill in the art will recognise various changes,
modifications, additions and applications other than those
specifically mentioned herein without departing from the spirit of
this invention.
* * * * *