Liquid crystal display having backscattering polarizer

A liquid crystal display includes: a scattering-polarizer provided between a liquid crystal and a back light for transmitting a polarization component in one direction of light from back light, but scattering and reflecting a polarization component in a direction which is orthogonal thereto: and a scattered light reflector reflecting the reflected and scattered light toward scattering-polarizer along with the polarization component in above mentioned one direction. Thus, a liquid crystal display which is inexpensive and has high display quality with reduced power consumption is obtained.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to liquid crystal displays, and more
 particularly to a liquid crystal display of a direct view type.
 2. Description of the Background Art
 Recently, a liquid crystal display is actively used for a personal OA
 (Office Automation) instrument such as a word processor, notebook personal
 computer or desktop personal computer as well as an image display unit
 such as a television because of its small thickness, lightweight and small
 power consumption. Particularly, a liquid crystal display of an active
 matrix type is actively developed as a portable display because it enables
 a high resolution display in addition to the advantage of small thickness,
 lightweight and small power consumption. (Hisao Ishii, Denshi-Gijutsu
 (Electrotechnology), July 1997, p.7)
 A liquid crystal itself does not emit light. It functions as a display by
 controlling light which is transmitted therethrough. There are generally
 two ways of directing light through the liquid crystal. One is related to
 a liquid crystal of a direct view or transmission type in which a back
 light is provided behind the liquid crystal, and the other is related to a
 liquid crystal of a reflective type which allows light in the environment
 surrounding a viewer to be transmitted through the liquid crystal from
 front, reflected by a reflector plate behind the liquid crystal and again
 transmitted through the liquid crystal from back.
 The liquid crystal display of the direct view type is always provided with
 the back light of a light source behind the liquid crystal panel. As shown
 in FIGS. 7A and 7B, there are two types of back lights: edge light type
 and vertical type. The edge light type shown in FIG. 7B includes: a bar
 like light 51 (which is either a cold cathode tube or hot cathode tube) at
 an end of a light conducting plate 54; a reflector plate 53 behind light
 conducting plate 54; and a scattering or diffusion plate 52 in front of
 light conducting plate 54 which has a lens sheet (that is, an optical
 sheet such as a prism sheet and wave sheet) and is provided with a
 protection function. The vertical type shown in FIG. 7A has a structure
 having light 51 immediately below diffusion plate 52, and it does not
 require the light conducting plate used in the edge light type (see for
 example, Keiichi Nakajima, Denshi-Gijutsu (Electrotechnology), July 1997,
 p.11).
 The above described liquid crystal display does not have a large volume nor
 weight unlike a CRT(Cathode-Ray Tube). In addition, it needs not be
 operated at a high voltage, so that power consumption thereof is small. In
 other words, the liquid crystal display is characterized by its small
 thickness, lightweight and small power consumption unlike the CRT.
 However, transmittance of a polarizer for a liquid crystal panel is about
 40% to 45%, and half of the light from the back light which consumes the
 largest amount of power in the liquid crystal display is not effectively
 utilized. This is because only polarization component in one direction is
 transmitted of all light which is directed to the polarizer from the back
 light, and that in a direction which is orthogonal to the above mentioned
 direction is absorbed by the polarizer.
 To solve this problem, a method of inserting a so called
 reflecting-polarizer or a highly transmissive polarizer between the liquid
 crystal panel and the back light has been proposed, and a product
 manufactured in accordance with the method has been obtained. Such
 products include DBEF (Double Brightness Enhancement Film) of 3M
 Corporation and Transmax of Merck & Co., Inc. Such reflecting-polarizer is
 obtained by forming a multilayer film having an optical function on a
 polymer film by an evaporation method, or by specifically aligning
 molecules of a cholesteric liquid crystal for application on a polymer
 film. As shown in FIG. 8, such reflecting-polarizer allows transmission of
 only one of two polarization components from the back light (that is, two
 linearly polarized light which are orthogonal to each other or two
 circularly polarized light which rotate in opposite directions) and
 reflects the other. As the reflection is mirror reflection, the
 reflecting-polarizer looks like a mirror.
 The polarized light which has been reflected by the reflecting-polarizer is
 returned to and reflected by the back light, so that the completely
 polarized state is partially cancelled. In other words, light having one
 polarization component is converted to light having two polarization
 components and again directed to the reflecting-polarizer. Again, one of
 the polarization components is transmitted through the
 reflecting-polarizer and the other reflected thereby. The process is
 repeated. Therefore, a larger amount of light is directed from the back
 light through the reflecting-polarizer, and therefore light is effectively
 utilized. As a result, even when brightness of the back light is reduced,
 high brightness is obtained for the liquid crystal display. In addition,
 as a back light which consumes a small amount of power can be applied,
 overall power consumption of the liquid crystal display is reduced.
 However, the function of the reflecting-polarizer which allows transmission
 of only one polarization component and reflection of the other is
 insufficient. Further, as light is absorbed by various optical components
 during travel, utilizing efficiency of the light would not be so high as
 compared with the conventional case.
 Besides the above mentioned reflecting-polarizer, referring to FIG. 9, a
 method of using an anisotropic scatterer which allows transmission of
 polarization component in one direction and forward scattering of that in
 a direction which is orthogonal thereto has been proposed (Japanese Patent
 Laying-Open No. 8-76114). The scattering by the anisotropic scatterer is
 mainly forward scattering. It is thus different from a
 scattering-polarizer which is used in the present invention and will later
 be described. The principle of forward scattering by the anisotropic
 scatterer is still unclear in many respects.
 In a conventional liquid crystal display in which the reflecting-polarizer
 is held between a back light and a liquid crystal panel, light from the
 back light is effectively utilized. Thus, brightness of the back light is
 reduced as compared with the conventional display, whereby power
 consumption of the liquid crystal display can be reduced. However,
 disadvantageously, the reflecting-polarizer is expensive as it is
 manufactured with a complicated manufacturing process and requires high
 technology for aligning molecules of a liquid crystal. In addition, a
 material of the reflecting-polarizer may readily allow deformation due to
 heat and coloring, thereby reducing quality of display. Further, the
 anisotropic scatterer which allows transmission of light component in one
 direction and forward scattering of that in the direction orthogonal
 thereto had a problem which is similar to that of the
 reflecting-polarizer.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide a liquid crystal display
 which is inexpensive and provides enhanced display quality with reduced
 power consumption by providing a scattering-polarizer, which is different
 from the above mentioned reflecting-polarizer or anisotropic scatterer,
 between a back light and a liquid crystal.
 The most basic liquid crystal display according to the present invention is
 a liquid crystal display which allows display of a liquid crystal portion
 to be viewed from a front side of a liquid crystal by transmitting light
 through the liquid crystal from a back light provided behind the liquid
 crystal. The liquid crystal display includes: a scattering-polarizer
 provided between the back light and the liquid crystal for transmitting a
 polarization component of the light in one direction from the back light,
 but reflecting with scattering a polarization component in a direction
 which is orthogonal thereto toward a direction which is opposite to the
 direction of light; and a scattered light reflector reflecting the above
 mentioned reflected and scattered light toward the scattering-polarizer,
 including the above mentioned polarization component in one direction.
 The above described structure makes it possible to utilize part of the
 polarization component in the direction which is orthogonal to one
 direction, which has not conventionally contributed to display, so that
 utilizing efficiency of the light from the back light is increased. As a
 result, a liquid crystal display which is inexpensive and provides
 enhanced display quality with reduced power consumption can be achieved.
 In addition, coloring or reduction in the display quality is prevented
 unlike the case where the reflecting-polarizer is used.
 In the above described liquid crystal display, desirably, light scattered
 from the scattering-polarizer is reflected and scattered by the back light
 and again emitted as scattered light as described below. In other words,
 the back light includes a fluorescent tube emitting light and a reflector
 reflecting light and directing the reflected light toward the
 scattering-polarizer. The reflector desirably functions as the scattered
 light reflector.
 Utilizing efficiency of light from the back light is increased as light of
 the polarization component orthogonal to one direction which is scattered
 by the scattering-polarizer is reflected, and the polarized state only in
 this direction is partially cancelled, so that the light includes the
 polarization component transmitted through the scattering-polarizer. This
 increase in the utilizing efficiency makes it possible to ensure high
 display quality even when brightness of the back light and the power
 consumption are reduced.
 If a lens sheet having a function of a lens is used with the above
 described structure, brightness is further increased. Desirably, the above
 described liquid crystal display further includes a lens sheet provided
 between the scattering-polarizer and the scattered light reflector for
 refracting and transmitting light which has been reflected with scattering
 by the scattering-polarizer, and collecting it onto the scattered light
 reflector.
 By employing the lens sheet which refracts the light reflected with
 scattering by the scattering-polarizer and collects it onto the scattered
 light reflector, the scattered light can be effectively utilized. As a
 result, high display quality of the display is ensured even if the
 brightness of the back light and power consumption are reduced.
 By employing a scattering-controlling-film in addition to the above
 mentioned sheet, utilizing efficiency of the light from the back light is
 further increased. In this case, it is desirable to have a
 scattering-controlling-film between the scattering-polarizer and the
 scattered light reflector for transmitting light which has been reflected
 with scattering by the scattering-polarizer by changing a scattering
 degree in accordance with an angle of incidence.
 By employing the above mentioned scattering-controlling-film for
 transmitting the light which has been reflected with scattering by the
 scattering-polarizer by changing the scattering degree in accordance with
 the angle of incidence, the light which has not conventionally been
 utilized is directed into the back light, so that utilizing efficiency of
 the light is increased.
 More specifically, the above mentioned scattering-controlling-film is
 provided between the scattering-polarizer and the lens sheet for
 transmitting light which has a relatively small angle of incidence of the
 light reflected with scattering by the scattering-polarizer as it is and
 transmitting with scattering light which has a relatively large angle of
 incidence. Desirably, the lens sheet refracts and transmits the diagonally
 travelling scattered light from the scattering-controlling-film for
 collecting it onto the scattered light reflector.
 The scattering-controlling-film can increase an amount of light reaching
 the scattered light reflector. As a result, utilizing efficiency of light
 from the back light is increased and high display quality can be ensured
 without use of a diffusion plate having a protection function.
 The above described scattering-controlling-film can be inserted between the
 lens sheet and the scattered light reflector. In this case, the display
 has a structure having the scattering-controlling-film between the lens
 sheet and the scattered light reflector for transmitting and scattering
 light having a relatively small angle of incidence of the light which has
 been reflected with scattering by the scattered light reflector and
 transmitted through the lens sheet, and for transmitting light having a
 relatively large angle of incidence as it is.
 More specifically, it is desirable that the transmission through the
 scattering-polarizer of the polarization component in one direction is
 desirably performed with a little scattering.
 As described above, by transmitting light through the scattering-polarizer
 while making it slightly extend radially, scattered light which is
 necessary for display is obtained without use of the scattering or
 diffusion plate having the protection function of the back light. Thus, an
 inexpensive liquid crystal display is obtained in which coloring and
 reduction in display quality are prevented.
 Arrangement of an optical component having the above described function
 enables display with high visibility and quality.
 The foregoing and other objects, features, aspects and advantages of the
 present invention will become more apparent from the following detailed
 description of the present invention when taken in conjunction with the
 accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments of the present invention will now be described in detail with
 reference to the drawings.
 First Embodiment
 Referring to FIG. 1, light emitted from a back light 2 is directed to a
 scattering-polarizer 3. A polarization component 11 (S-polarization) in
 one direction of two polarization components which are orthogonal to each
 other is forwardly transmitted while being slightly scattered by
 scattering-polarizer 3, and a polarization component 12 (P-polarization)
 in a direction which is orthogonal to the above mentioned one direction is
 backwardly scattered by scattering-polarizer 3. The polarized state of one
 component of polarization component 14 which has been backwardly scattered
 is partially cancelled while it is reflected inside back light 2,
 ultimately reflected by a scattered light reflector 6 and emitted from
 back light 2 including not only P-polarization 16 but also S-polarization
 15. Thus, ratio of S-polarization of the light emitted from back light 2
 is not negligible. As a result, an amount of light directed to a liquid
 crystal 1 increases, thereby enabling bright display. In other words,
 ratio of the light which is effectively utilized of the light emitted from
 the back light increases.
 Scattered light reflector 6 corresponds to a reflector 53 in the back light
 shown in FIGS. 7A and 7B. However, as scattered light reflector 6
 generally refers to a component including a fluorescent tube surface and a
 surface or a corner of light conducting plate, if it has a reflecting
 function. Therefore, it is not limited to the reflector.
 When the above mentioned structure is used, brightness of the liquid
 crystal display is about 1.5 times that in the case where
 scattering-polarizer 3 is not held between back light 2 and liquid crystal
 panel 1. Even when scattering-polarizer 3 is used, coloring of image which
 often occurs during use of the reflecting-polarizer and reduction in
 display quality are prevented. Further, image with high visibility is
 ensured without use of a scattering or diffusion plate which has a
 protection function and is generally provided at an outermost portion of
 back light 2. This is because scattering-polarizer 3 has a function which
 is similar to that of the scattering or diffusion plate, that is, a
 function of slightly and forwardly scattering light rather than
 transmitting it completely in a straight direction.
 The above described scattering-polarizer may be anything as long as it
 scatters and reflects one of two polarization components which are
 orthogonal to each other in a direction which is opposite to a travelling
 direction of light rather than directly reflecting it, and forwardly
 transmits and slightly scatters the other radially. A desirable structure
 of the scattering-polarizer includes a macromolecule film in which a
 transparent material having anisotropy as to refractive index, which is
 different from a binder of a main material, is uniformly scattered. Such
 scattering-polarizer can readily be manufactured without any complicated
 step of evaporation or high technology for aligning molecules of a liquid
 crystal. In addition, as the material therefore is relatively easily
 acquired, it is inexpensive. Such scattering-polarizer may also be
 combined with a conventional polarizer to serve as a scattering-polarizer.
 When a scattering-polarizer having such function is used, a polarizer on
 the side of a back light of two polarizers generally provided on a liquid
 crystal panel would be unnecessary. As in the above described embodiment,
 scattering or diffusion plate 52 having a protection function needs not be
 provided for the back light as the scattering-polarizer is provided
 between the back light and the liquid crystal. Thus, the liquid crystal
 display can be obtained which is inexpensive and has high display quality
 with reduced power consumption while preventing coloring.
 Second Embodiment
 Referring to FIG. 2, a liquid crystal display according to the second
 embodiment of the present invention has a structure in which a lens sheet
 4 is provided between a back light 2 and a scattering-polarizer 3 (light
 which is again emitted from the back light is not shown). In this case,
 light 14 scattered by scattering-polarizer 3 is transmitted through lens
 sheet 4, directed to and reflected by back light 2, reflected by a
 scattered light reflector 6 and again emitted including S-polarization 16
 and P-polarization 15, as shown in FIG. 3. As a result, the use of the
 above mentioned scattering-polarizer 3 and lens sheet 4 has increased
 front brightness to 1.6 times that in the case where they are not used. In
 addition, light distribution, that is, an amount of light in accordance
 with directions of light emitted from the back light, can now be
 controlled. In this second embodiment, a prism sheet (BEF: Brightness
 Enhancement Film, manufactured by 3M Corporation) is used. However, this
 is not limited to the use of a particular prism sheet, and a wave sheet
 providing similar performance may be used.
 The scattering-polarizer generally performs backward scattering and cannot
 control light distribution of the back light by itself. However, use of
 the above mentioned lens sheet or wave sheet for refraction has allowed
 the light distribution to be controlled. It is noted that the light
 distribution can be controlled even if the lens sheet is provided closer
 to the liquid crystal than the scattering-polarizer, that is, between the
 liquid crystal and the scattering-polarizer. This arrangement, however,
 only allowed increase in front brightness of 1.1 times, and this is not
 very desirable.
 The structure of scattering-polarizer 3 and lens sheet 4 shown in FIG. 2
 makes it possible to eliminate scattering (diffusion) plate 52 having the
 protection function of back light 2 and to control the light distribution
 of the back light. Thus, a liquid crystal display which is inexpensive and
 has high display quality with reduced power consumption while preventing
 coloring is provided.
 Third Embodiment
 Referring to FIG. 4, a liquid crystal display according to a third
 embodiment of the present invention has a structure in which a lens sheet
 4 and a scattering-controlling-film 5 are provided between back light 2
 and liquid crystal 1 in addition to scattering-polarizer 3. In this case,
 scattering-polarizer 3 is provided in a position which is the closest to
 the liquid crystal, scattering-controlling-film 5 between
 scattering-polarizer 3 and lens sheet 4, and lens sheet 4 in a position
 which is the closest to the back light. FIGS. 5A and 5B show
 representative patterns of transmission of light-ray of
 scattering-controlling-film 5. FIG. 5A shows a scattering-controlling-film
 having a non-transparent front and allows significant scattering when
 light is directed to the front, that is, when an angle of incidence is
 small. FIG. 5B shows a scattering-controlling-film having a transparent
 front and allows significant scattering when light is directed to a
 periphery, that is, when an angle of incidence is large.
 As shown in FIG. 6, P-polarization 14 which is backwardly scattered by
 scattering-polarizer 3 is transmitted through scattering-controlling-film
 5 and lens sheet 4, reflected by a scattering reflector 6 and again
 emitted from back light 2.
 By arranging scattering-polarizer 3, lens sheet 4 and
 scattering-controlling-film in this way, front brightness is increased to
 1.8 times that in the case where these components are not arranged, and
 light distribution of the back light can be controlled. In the third
 embodiment, a prism sheet (BEF: Brightness Enhancement Film, manufactured
 by 3M Corporation) is used as the lens sheet. As in the second embodiment,
 the lens sheet is not particularly limited to the prism sheet, and
 anything which has similar performance may be used.
 Scattering-controlling-film 5 having a function described in connection
 with FIGS. 5A and 5B need only to have an optical characteristic of again
 scattering light having a specific angle of emission of light which has
 been backwardly scattered from scattering-polarizer 3, and is not limited
 to a particular component. In the present embodiment, a field control film
 (Lumisty: MFZ-2555) of Sumitomo Chemical Co., LTD. is used as the
 scattering-controlling-film (Kazumitsu Kawamura, New Materials, November,
 1993, p.71). Use of such scattering-controlling-film 5 increases front
 brightness as an amount of light which returns to lens sheet 4 of light
 which is emitted from back light 2 and backwardly scattered by
 scattering-polarizer 3 can be increased. Further, provision of only
 scattering-polarizer 3 did not enable control of light distribution
 characteristic of the back light, that is, control of the amount of light
 in accordance with directions of light emission from the back light.
 However, lens sheet 4 (a prism sheet or wave sheet) and a
 scattering-controlling-film or field control film such as Lumisty enables
 control of the light distribution characteristic.
 In the above described example, the scattering-controlling-film (in the
 case of FIG. 5B) having the transparent front and the periphery which
 allows scattering is used. When a film which scatters only light directed
 to the front and transmits the diagonally directed light without
 scattering (in the case of FIG. 5A: for example, the field control film
 Lumisty MFX-1515, manufactured by Sumitomo chemical, Co., LTD.) is used,
 the scattering-controlling-film can be arranged in a position which is the
 closest to the back light.
 In this case, the scattering-polarizer can be arranged in a position which
 is the closest to the liquid crystal, and the prism sheet is desirably
 used as lens sheet 4. Then, the front brightness can be increased to about
 1.1 times that in the conventional case where scattering-polarizer 3 or
 the like is not used. However, control of light distribution of back light
 2 is difficult. The above described structure eliminates the need for a
 scattering (diffusion) plate having a protection function of back light 2
 as scattering-polarizer 3, lens sheet 4 and scattering-controlling-film 5
 are held between back light 2 and the liquid crystal.
 Thus, the need for the scattering plate is eliminated and the light
 distribution characteristic of back light 2 is controlled. As a result, a
 liquid crystal display which is inexpensive and has high display quality
 with reduced power consumption while preventing coloring is provided.
 Fourth Embodiment
 In the above described third embodiment, the liquid crystal display is
 manufactured by adhering scattering-controlling-film 5 to
 scattering-polarizer 3. As a result, the front brightness has been
 increased to 1.8 times that in the case without such structure, and light
 distribution of the back light can be controlled. In addition, it has been
 proved that degradation of display quality does not readily occur.
 Further, the use of such optical components which are adhered together
 facilitates assembly of the liquid crystal display.
 Fifth Embodiment
 In the above described third embodiment, the liquid crystal display has
 been manufactured by adhering scattering-controlling-film 5 to lens sheet
 4. As a result, the front brightness has been increased to about 1.1 times
 that in the case without such structure, and light distribution of back
 light 2 can be controlled. Further, it has been proved that degradation of
 display quality does not readily occur. Moreover, the assembly of the
 liquid crystal display is facilitated.
 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.