OPTICAL SHEET AND DISPLAY DEVICE

An optical sheet (40) is used in a display device for switchably displaying a two-dimensional image and a naked-eye visible three-dimensional image. The optical sheet (40) includes: a first layer (51) including a thermoplastic resin; and a second layer (52) laminated to the first layer. The first layer is an optically anisotropic An optical interface (55) for changing the traveling direction of light of one polarization component is formed between the first layer (51) and the second layer (52).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described with reference to the drawings. In the drawings attached to the present specification, for the sake of illustration and easier understanding, scales, horizontal to vertical dimensional ratios, etc. are exaggeratingly modified from those of the real things.

FIGS. 1 through 4are diagrams illustrating an embodiment of the present invention. Of these,FIG. 1is a perspective view showing a display device.FIGS. 2 and 3are diagrams illustrating the actions of the display device when it displays a three-dimensional image and a two-dimensional image, respectively.FIG. 4is a perspective view showing the refractive index ellipsoids of the first layer and the second layer of an optical sheet.

The display device10of this embodiment can switchably display a two-dimensional image and a naked-eye visible three-dimensional image. As shown inFIG. 1, the display device10includes an image display unit15and an optical sheet40which is disposed so as to face the image display unit15. The image display unit15is configured to emit light of one linear polarization component for forming a three-dimensional image and light of the other linear polarization component for forming a two-dimensional image. The optical sheet40controls the traveling direction of light depending on the polarization state of the light. More specifically, the optical sheet40controls the traveling direction of light of the one linear polarization component for forming a three-dimensional image while maintaining the traveling direction of light of the other linear polarization component that vibrates in a direction perpendicular to the direction of vibration of the one linear polarization component.

The “two-dimensional image” herein refers to an image which is viewed two-dimensionally on a display surface10a, while the “three-dimensional image” refers to an image having a sense of depth, which can be viewed also at a position at a distance from the display surface10a.The display device10of this embodiment is configured to be capable of displaying a three-dimensional image by utilizing binocular parallax and motion parallax. As shown inFIG. 2, when displaying a three-dimensional image, the pixels21of the image forming device20of the image display unit15are assigned to those positions where the left eye or the right eye of a viewer is supposed to be located. Those pixels21which are assigned to the same position form an image to be viewed from the assigned position. The optical sheet10, on the other hand, controls the path of light so that light, emitted from each pixel21, travels toward the position to which the pixel21is assigned of those positions at which the left eye or the right eye of a viewer is supposed to be located. Accordingly, the viewer's left and right eyes view different images, and the viewer perceives a three-dimensional image. When the viewer changes the viewing direction, the viewer can view a different three-dimensional image according to the viewing position.

The components of the display device10will now be described in greater detail. In the following description, one linear polarization component that forms a three-dimensional image will be referred to as “first polarization component” that vibrates in the x-axis direction (seeFIG. 1) parallel to the sheet plane of the optical sheet40. The other linear polarization component that forms a two-dimensional image will be referred to as “second polarization component” that vibrates in the y-axis direction (seeFIG. 1) perpendicular to the x-axis direction and parallel to the sheet plane of the optical sheet40.

The terms “sheet”, “film” and “plate” are not used herein to strictly distinguish them from one another. Thus, the term “film” includes a member which can also be called a sheet or plate. An “optical sheet” is not strictly distinguished from a member called “optical film” or “optical plate”.

The term “sheet plane (film plane, plate plane, panel plane)” herein refers to a plane which coincides with the planar direction of an objective sheet-like (film-like, plate-like, panel-like) member when taking a wide and global view of the sheet-like (film-like, plate-like, panel-like) member. In this embodiment the image forming surface20aof the image forming device20, the panel plane of the liquid crystal display panel25, the panel plane of the polarization control device30, the sheet plane of the optical sheet40and the display surface10aof the display device10are parallel to each other. The term “front direction” herein refers to the normal direction of the sheet plane of the optical sheet40.

The terms used herein to specify shapes or geometric conditions, such as “parallel”, “perpendicular”, etc., should not be bound to their strict sense, and should be construed to include equivalents or resemblances from which the same optical function or effect can be expected.

The image display unit15includes the image forming device20and the polarization control device30which transmits light from the image forming device20. The polarization control device30is disposed between the image forming device20and the optical sheet40. The image forming device20has a large number of pixels21arranged in a plane parallel to the image forming surface20a.In the illustrated embodiment, the pixels21are arranged in a stripe arrangement. The following description illustrates an exemplary case where the image forming device20forms an image by using light of the first polarization component. In this case, the polarization control device30maintains the first polarization state of light, emitted from the image forming device20, when a three-dimensional image is to be displayed, or converts the polarization state of the light into the second polarization state when a two-dimensional image is to be displayed. However, it is also possible for the image forming device20to emit light of the second polarization component, and for the polarization control device30to convert the polarization state of light, emitted from the image forming device20, into the first polarization state when a three-dimensional image is to be displayed, or maintains the second polarization state of the light when a two-dimensional image is to be displayed.

In the illustrated embodiment, the image forming device20is constructed as a liquid crystal display device. Thus, the image forming device20includes a liquid crystal display panel25and a backlight24disposed at the rear of the liquid crystal display panel25. The backlight24may have any known construction, including that of the edge-light type or the direct-light type.

The liquid crystal display panel25includes a pair of polarizing plates26,28and a liquid crystal cell27disposed between the polarizing plates26,28. The polarizing plates26,28include polarizers which function to resolve incident light into two orthogonal polarization components, and transmit one polarization component and absorbs the other polarization component perpendicular to the one polarization component. In this embodiment the lower polarizing plate26, disposed on the backlight24side, transmits light of the second polarization component, while the upper polarizing plate28, disposed on the polarization control device30side, transmits light of the first polarization component.

The liquid crystal cell27includes a pair of support plates and liquid crystal molecules (liquid crystal material) disposed between the support plates. An electric field can be applied to each pixel area of the liquid crystal cell27. When an electric filed is applied to a pixel area, the orientation of the liquid crystal of the liquid crystal cell27changes in the pixel area. For example, light of the second polarization component, which has passed through the lower polarizing plate26, turns its vibration direction by 90 degrees when it passes through those pixel areas of the liquid crystal cell27to which an electric field is being applied, whereas light of the second polarization component maintains its polarization state when it passes through those pixel areas of the liquid crystal cell27to which no electric field is being applied. Thus, transmission through or absorption and blocking by the upper polarizing plate28, disposed on the light exit side of the lower polarizing plate26, of light of the second polarization component, which has passed through the lower polarizing plate26, can be controlled by application or no application of an electric field to each pixel area of the liquid crystal cell27. Light of the first polarization component, which has thus selectively passed through the upper polarizing plate28and has been emitted from pixels21, will form an image.

The polarization control device30will now be described. The polarization control device30basically comprises a pair of a first electrode34and a second electrode36, and a medium layer35disposed between the first electrode34and the second electrode36. The medium layer35generates refractive index anisotropy when a voltage is applied between the pair of the electrodes34,36. In the illustrated embodiment, the first electrode34, the medium layer35and the second electrode36are disposed between a pair of a first support film33and a second support film37. The first electrode34, the medium layer35and the second electrode36are supported and protected by the pair of the support films33,37. The following description illustrates a case where the medium layer is constructed as a liquid crystal layer35.

The pair of electrodes34,36and the liquid crystal layer35have a size that expands the entire area of the image forming surface20aof the image forming device20. As shown inFIGS. 2 and 3, the liquid crystal layer35contains liquid crystal molecules31. The electrodes34,36are electrically connected to a not-shown voltage application means. The electrodes34,36are kept at a predetermined distance from each other e.g. by the use of a spacer (not shown).

When the liquid crystal molecules31contained in the liquid crystal layer35are typical liquid crystal molecules of the TN type, the liquid crystal molecules31are aligned when a voltage is applied between the pair of electrodes34,36, as shown inFIG. 2. The first polarization state of light (first polarization component) from the image forming device20is maintained upon passage of the light through the liquid crystal layer35to which a voltage is being applied. On the other hand, when no voltage is applied between the pair of electrodes34,36, the liquid crystal molecules31are in a 90 degree-twisted or turned state as shown inFIG. 3. When light from the image forming device20passes through the liquid crystal layer35to which no voltage is being applied, the vibration direction of the light is converted from the x-axis direction to the y-axis direction, i.e. the light is converted from the first polarization component to the second polarization component.

The above description of the image display unit15, the image forming device20and the polarization control device30is merely exemplary. Thus, for example, it is also possible to generate light of the first polarization component by turning the vibration direction of light from the image forming device20by 45 degrees, and to generate light of the second polarization component by turning the vibration direction of light from the image forming device 20 by −45 degrees.

The optical sheet40will now be described. As shown inFIGS. 1 through 3, the optical sheet40includes a first layer51and a second layer52provided adjacent to the first layer51. In the illustrated embodiment, the optical sheet40further includes a film layer43provided on the second layer52.

The film layer43may be formed as a single layer or as a stack of multiple layers. The film layer43is expected to exert a particular function, and forms the outermost light exit-side surface of the display device10, i.e. the display surface10aof the display device10. The film layer43may comprise at least one of an antireflective layer (AR layer) having an antireflective function, an anti-glare layer (AG layer) having an anti-glare function, an abrasive-resistant hard coat layer (HC layer), an antistatic layer (AS layer) having an antistatic function, etc.

The interface between the first layer51and the second layer52is formed as a surface having a three-dimensional (corrugated) pattern. The interface serves as an optical interface55which changes the traveling direction of light of at least the first polarization component. In the illustrated embodiment, the optical interface55between the first layer51and the second layer52is constructed as a surface consisting of a plurality of unit optical interfaces55a.As shown inFIG. 1, the unit optical interfaces55aare arranged in an arrangement direction. Each unit optical interface55aextends in a direction not parallel to the arrangement direction. Particularly in the illustrated embodiment, the unit optical interfaces55aare arranged in the x-axis direction without any space between adjacent unit interfaces, and each unit optical interface55aextends linearly in the y-axis direction. Each unit optical interface55ahas the same shape at varying positions along the y-axis direction. The unit optical interfaces55aall have the same construction.

As described above, the unit optical interfaces55aare designed so that light, emitted from each pixel21, is directed to a predetermined position. In the illustrated embodiment, in a cross-direction parallel to both the front direction and the arrangement direction of the unit optical interfaces55a,each unit optical interface55ahas a convex lens-like contour and focuses a divergent light flux LF1(seeFIG. 2) from each pixel on a preset position. The optical interface55as an assembly of the unit optical interfaces55aforms a lenticular lens.

The unit optical interfaces55aand the optical interface55, shown in the Figures, are merely examples and are capable of various changes and modifications. For example, the cross-sectional contour of each unit optical interface55amay be arbitrarily changed. Further, the unit optical interfaces55amay have different shapes. For example, the optical interface55may form a Fresnel lens. Though the illustrated unit optical interfaces55aare composed of elongated elements arranged one-dimensionally, the unit optical interfaces55amay be arranged two-dimensionally.

The refractive indices of the first layer51and the second layer52will now be described. The first layer51is optically anisotropic, and is birefringent at least in a plane. Thus, the refractive index n1xof the first layer51in the x-axis direction differs from the refractive index n1yof the first layer51in the y-axis direction. In addition, in the optical sheet40of this embodiment, the refractive index n1xof the first layer51in the x-axis direction, the refractive index n2xof the second layer52in the x-axis direction, the refractive index n1yof the first layer51in the y-axis direction and the refractive index n2yof the second layer52in the y-axis direction satisfy the following relation:

Accordingly, the optical sheet40exerts different optical effects on light of the first polarization component that vibrates in the x-axis direction and light of the second polarization component that vibrates in the y-axis direction. In particular, light of the first polarization component and light of the second polarization component, both traveling in the same direction, come to travel in different directions after passing through the optical interface55of the optical sheet40.

Particularly in this embodiment the following relation is satisfied:

In this case, the optical interface55of the optical sheet40no more functions as an effective optical interface, having a refractive index difference, on light of the second polarization component that vibrates in the y-axis direction. Thus, while the optical interface55of the optical sheet40exerts an optical effect (e.g. lens effect) on light of the first polarization component, light of the second polarization component does not change its traveling direction when it passes through the optical interface55of the optical sheet40. A refractive index value is herein expressed as a value rounded off to two decimal places.

In application of the optical sheet40in a display device which switchably displays a two-dimensional image and a naked-eye visible three-dimensional image, it is not practically essential for the refractive indices n1y, n2yto satisfy the relation: |n1y−n2y|=0, and it is sufficient if the following relation is satisfied:

In this case, light of the second polarization component will not change its traveling direction at the optical interface55of the optical sheet40to such an extent as to cause problems, such as ghost and crosstalk.

In application of the optical sheet40in a display device which switchably displays a two-dimensional image and a naked-eye visible three-dimensional image, the level of the optical effect, exerted on light of the second polarization component, is affected not only by the absolute value of |n1y−n2y| but also by other factors, including the shape of the optical interface55of the optical sheet40, as will be described in detail below. From the above viewpoint, the optical sheet40may be designed so that light of the polarization component (second polarization component) that vibrates in the y-axis direction, traveling in a direction perpendicular to the sheet plane of the optical sheet40, i.e. in the front direction, before entering the optical sheet40, comes to travel in a direction at an angle of not more than 2 degrees with respect to the front direction after passing through the optical sheet40. This can effectively prevent an optical effect which could cause image degradation e.g. upon display of a two-dimensional image, due to the occurrence of a problem such as ghost, from being exerted on light of the second polarization component, passing though the optical sheet40.

FIG. 4shows exemplary refractive index ellipsoids that indicate refractive index distributions in the first layer51and the second layer52in varying directions. In the illustrated embodiment the following relation is satisfied:

The refractive index n1xof the first layer51in the x-axis direction is higher than the refractive index n1yof the first layer in the y-axis direction. Further, in the embodiment illustrated inFIG. 3, the second layer52is formed as an optically isotropic layer. Thus, the refractive index n2xof the second layer52in the x-axis direction is equal to the refractive index n2yof the second layer52in the y-axis direction. Therefore, the refractive index n1xof the first layer51in the x-axis direction is higher than the refractive index n2xof the second layer52in the x-axis direction. Accordingly, the optical interface55shown inFIG. 1can exert the same lens effect as a convex lens.

In the embodiment illustrated inFIG. 1, the direction of the slow axis, in which the refractive index is maximum, coincides with the x-axis direction in a plane in the first layer51, while the direction of the fast axis, in which the refractive index is minimum, coincides with the y-axis direction in a plane in the first layer51. In addition, the refractive index n1yin the y-axis direction (direction of the fast axis) in a plane in the first layer51is made equal to the refractive index n2yin the y-axis direction in a plane in the second layer52. Therefore, the difference in the x-axis direction refractive index between the first layer51and the second layer52can be set to be large while setting the difference in the y-axis direction refractive index between the first layer51and the second layer52to zero. In application of the optical sheet40in a home display device, on the condition that the optical interface55is manufactured in a shape easy to manufacture, the birefringent index Δn (=n1x−n1y) of the first layer51is preferably not less than 0.13. On the other hand, when the optical anisotropy of the first layer51is provided by stretching as described below, the birefringent index Δn of the first layer51is preferably not more than 0.22 e.g. in view of the in-plane uniformity in a stretching process.

The refractive indices of the first layer51and the second layer52can be measured, for example, by using “KOBRA-WR” manufactured by Oji Scientific Instruments, “Ellipsometer M150” manufactured by JASCO Corporation, or an Abbe refractometer (NAR-4, manufactured by Atago Co., Ltd.).

Such an optical sheet40can be produced in the following manner: First, as shown inFIG. 5, a resin film71is produced by using a thermoplastic resin. Thereafter, the resin film71is subjected to stretching to produce a first layer51composed of the stretched resin film71. Thereafter, a second layer52is formed on the first layer51to obtain an optical sheet40.

The resin film71can be produced by molding of a resin material comprising a thermoplastic resin as a main component, or consisting only of a thermoplastic resin. The molding of the resin material may be performed by injection molding or melt extrusion. Such a molding method can produce the resin film71having a three-dimensional (corrugated) pattern that forms the optical interface55. As shown inFIG. 5, the resin film71has raised portions71aarranged in a direction not parallel to the longitudinal direction of each raised portion71a.

A mold, having a mold surface made of metal or plastic, can be used for the molding of the resin film71. Compared to the use of a mold having a metal mold surface, the use of a mold having a plastic mold surface can prevent rapid absorption of heat from a heated thermoplastic resin into the mold surface upon application of the thermoplastic resin onto the mold surface. This enables the heated thermoplastic resin to fully spread over the mold surface, making it possible to enhance the rate of shaping. Further, the resin film71produced can be easily released from the mold surface. This can prevent the formation of a defect in the resin film71upon its release from the mold surface. A long film-like mold can be used as a mold having a plastic mold surface.

Stretching of the resin film71is performed in order to impart optical anisotropy to the resin film71and, insofar as this object is achieved, may be performed by any of uniaxial stretching, sequential biaxial stretching and simultaneous biaxial stretching. When the resin film71comprises a polyester resin, the stretching direction (stretching axis) coincides with the slow axis. For example, when it is intended to make the longitudinal direction of the unit optical interfaces55aparallel to the slow axis of the first layer51, the resin film is stretched in a direction parallel to the longitudinal direction of the raised portions of the resin film71which are to form the unit optical interfaces55aof the optical interface55, as shown inFIG. 5.

Stretching of the resin film71is carried out while heating the resin film71at a temperature above the glass transition temperature of the thermoplastic resin of the resin film71. In the case where the resin film71is produced by melt extrusion, the high-temperature resin film71immediately after extrusion may be subjected to stretching. Thus, there is no need to separately provide a heating process for stretching of the resin film71. As shown inFIG. 5, the shape of the resin film71is changed by stretching to form the first layer51. Therefore, in the molding of the resin film71, the resin film71is produced in a shape that takes into account the deformation of the resin film71by stretching.

Next, the second layer52is formed on the first layer51by applying a resin onto the first layer51and curing the resin on the first layer51. The second layer52, thus formed on the first layer51, has a three-dimensional pattern, corresponding to or complementary to the three-dimensional (corrugated) pattern of the first layer51, in the surface facing the first layer51. Alternatively, a second layer52, which has been produced separately, may be laminated to the first layer51. A resin material for the second layer52may be a thermoplastic, thermosetting or ionizing radiation-curable resin which is non-birefringent, i.e. having an isotropic refractive index (n2x=n2y). The optical sheet40can be produced in the above manner. Such a non-birefringent resin for the second layer52is usually solidified in the unstretched state.

The optical sheet40can also be produced by a production method as illustrated inFIG. 6.

In the production method illustrated inFIG. 6, a resin film71having the above-described three-dimensional (corrugated) pattern (seeFIG. 5) and a second rein film72having a three-dimensional pattern corresponding to, or complementary to the three-dimensional (corrugated) pattern of the resin film71are prepared first. Next, the resin film71and the second resin film72are laminated to each other e.g. with an adhesive or glue in such a manner that the respective three-dimensional patterns engage each other. Thereafter, the laminate of the resin film71and the second resin film72are stretched e.g. in the longitudinal direction of each raised portion71aof the resin film71to obtain an optical sheet40consisting of the first layer51composed of the resin film71and the second layer52composed of the second resin film72.

Also in the production method illustrated inFIG. 6, the resin film71and the second resin film72can be produced by molding using a thermoplastic resin as in the above-described production method illustrated inFIG. 5. Also in the production method illustrated inFIG. 6, in-plane birefringence is imparted to the resin film71by stretching of the resin film71. Though the second resin film72is also stretched together with the resin film71, it is not necessary to intentionally impart optical anisotropy to the second resin film72. Therefore, in order to prevent in-plane birefringence from being produced in the second resin film72, the electric dipole moment of a molecule in the second resin film72is preferably low. In particular, the electric dipole moment of a molecule in the second resin film72is preferably at least lower than the electric dipole moment of a molecule in the resin film71. The measurement of the electric dipole moment of a film can be performed by first measuring the dielectric constant of the film with a test fixture HP 16451B electrode of precision LCR meter, manufactured by Yokogawa-Hewlett-Packard Ltd., and then determining the electric dipole moment using the measured dielectric constant.

The level of birefringence (refractive index anisotropy) produced in a film depends on the electric dipole moment of the constituent molecule of the film. Accordingly, by using the resin film71and the second resin film72which satisfy the above relation in the electric dipole moments of the respective constituent molecules, the following relation is satisfied even when the resin film71and the second resin film72are stretched to the same extent to cause the same degree of molecular alignment in the films:

birefringent index Δn1of the resin film71>birefringent index Δn2of the second resin film72or, when expressed with the refractive indices of the films in the x- and y-axis directions, n1x−n1y>n2x−n2y(ideally→0).

The optical sheet40consisting of the optically anisotropic first layer51comprising a thermoplastic resin, and the second layer52which is laminated to the first layer51and which forms, between it and the first layer52, the optical interface55for changing the traveling direction of light of the first polarization component, can thus be produced.

A polycarbonate resin, a cycloolefin polymer resin, an acrylic resin, a polyester resin, etc. can be used as the thermoplastic resin of the first layer51. Of these, a polyester resin is advantageous in terms of cost and mechanical strength. Specific examples of the polyester resin include polyethylene naphthalate, polyethylene terephtha late, polyethylene isophtha late, polybutylene terephtha late, poly(1, 4-cyclohexylene dimethylene terephthalate), and polyethylene-2, 6-naphthalate. The polyester resin, forming the first layer50, may be a copolymer of such a polyester resin or a resin blend of a major amount (e.g. not less than 80 mol %) of such a polyester resin and a minor amount (e.g. not more than 20 mol %) of other resin(s). Of the above polyester resins, polyethylene naphthalate is preferred because it can ensure a high birefringent index. Of the above polyester resins, polyethylene terephthalate or polyethylene-2, 6-naphthalate is preferred because of good balance between mechanical properties and optical properties. From the viewpoint of stability of the optical sheet40, the glass transition temperature of the material of the first layer51is preferably not less than 100° C.

The display device10, which includes such an optical sheet40, can display a two-dimensional image and a naked-eye visible three-dimensional image in the following manner. The case of displaying a two-dimensional image will be described first mainly with reference toFIG. 3.

The backlight24illuminates an area of the liquid crystal display panel25from the back. The liquid crystal display panel25transmits light from the backlight24selectively for each pixel21. Two-dimensional image lights L31to L36thus formed, exiting the image forming surface20aof the image forming device20, are of the first polarization component that can pass through the upper polarizing plate28of the image forming device20. The two-dimensional image lights L31to L36then enter the polarization control device30. When displaying a two-dimensional image, no voltage is applied between the pair of electrodes34,36of the polarization control device30. The liquid crystal molecules31are therefore in a 90 degree-turned state as shown inFIG. 3. Accordingly, the two-dimensional image lights L31to L36passing through the polarization control device30change their polarization state, and have turned into the second polarization component when exiting the image display unit15.

The two-dimensional image lights L31to L36that have exited the image display unit15enter the optical sheet40. The optical sheet40has the optical interface55which is formed as a corrugated surface. The optical interface55is formed between the optically anisotropic first layer51and the second layer52. The refractive index n1yof the first layer51in the y-axis direction, i.e. in the vibration direction of the two-dimensional image lights L31to L36of the second polarization component, is set equal to the refractive index n2yof the second layer52in the y-axis direction. The two-dimensional image lights L31to L36therefore travel in the optical sheet40without changing their travelling directions at the optical interface55. The two-dimensional image lights L31to L36then exit the display surface10aof the display device10, whereby a viewer can view a two-dimensional image.

Light from the backlight24, illuminating the liquid crystal display panel25, has a light axis in the front direction (i.e. has the peak of brightness in the front direction), while the light travels in a direction with a certain angular range around the front direction. Therefore, light that has passed through each pixel21travels and exits the display surface10aof the display device10as divergent light in a certain angular range. Accordingly, as shown inFIG. 3, a viewer can view the same two-dimensional image, formed on the display surface10a,in a certain angular range.

The case of displaying a three-dimensional image that can be viewed with the naked eye will now be described with reference toFIG. 2. As with the case of displaying a two-dimensional image, three-dimensional image lights Ll1to Ll6, Lr1to Lr6exit the image forming device20. The three-dimensional image lights Ll1to Ll6, Lr1to Lr6then enters the polarization control device30. When displaying a three-dimensional image, each pixel21of the image forming device20of the image display unit15is assigned to one of those positions where the left eye or the right eye of a viewer is supposed to be located. The image display unit15controls transmission and blocking of light for each pixel21so that an image is formed by lights from those pixels21which are assigned to the same position.

As shown inFIG. 2, when displaying a three-dimensional image, a voltage is applied between the electrodes34,36of the polarization control device30. Accordingly, the three-dimensional image lights Ll1to Ll6, Lr1to Lr6pass through the polarization control device30while maintaining their first polarization state.

The three-dimensional image lights Ll1to Ll6, Lr1to Lr6that have exited the image display unit15enter the optical sheet40. The refractive index n1xof the first layer51in the x-axis direction, i.e. in the vibration direction of the three-dimensional image lights Ll1to Ll6, Lr1to Lr6of the first polarization component, is made higher than the refractive index n2xof the second layer52in the x-axis direction. The optical interface55of the optical sheet40thus controls the traveling direction of the three-dimensional image lights Ll1to Ll6, Lr1to Lr6from the pixels21.

As described above, a divergent light flux from each pixel21enters the optical sheet40. The unit optical interfaces55aof the optical interface55each exert a lens effect and focus a divergent light flux from each pixel21on a position corresponding to the focal point of each optical interface55athat functions as a lens. In particular, each unit optical interface55afocuses a divergent light flux (e.g. divergent light flux LF1shown inFIG. 2), emitted from a pixel21located opposite to the unit optical interface55a,on a position to which the pixel21is assigned, i.e. one of those positions where the left eye or the right eye of a viewer is supposed to be located. The three-dimensional image lights Ll1to Ll6, Lr1to Lr6from the pixels21thus travel toward their respective scheduled positions.

When a viewer views the display device10from a supposed position, an image to be viewed from the position of the right eye of the viewer can be viewed by the right eye, while an image to be viewed from the position of the left eye of the viewer can be viewed by the left eye. The viewer can therefore view a three-dimensional image with the naked eye by binocular parallax. When a viewer views the display device10from another supposed position as shown inFIG. 2, an image to be viewed from that position can be viewed three-dimensionally with the naked eye. Thus, when the viewer changes the viewing direction, the viewer can view different images with the naked eye according to the viewing directions. Thus, the viewer can view an image with a higher stereoscopic effect by motion parallax.

In a conventional display device for switchably displaying a two-dimensional image and a naked-eye visible three-dimensional image, a birefringent lens having an optically anisotropic layer containing liquid crystal (liquid crystal molecules, liquid crystal material) is used to control the traveling direction of light depending on the polarization state of the light. The optically anisotropic layer is typically produced by curing an ultraviolet curable resin containing liquid crystal.

The optically anisotropic layer of the conventional birefringent lens contains a high proportion of liquid crystal and has a large thickness of e.g. more than 5 μm in order to ensure a sufficiently high birefringent index. Because of the high content of liquid crystal, the conventional birefringent lens lacks stability, especially thermal stability. This imposes restrictions on the environment in which the birefringent lens and a display device having the birefringent lens are installed.

According to this embodiment, on the other hand, the first layer51of the optical sheet40, having an in-plane birefringent index, contains no liquid crystal (liquid crystal molecules, liquid crystal material). Optical anisotropy is imparted to the first layer51by stretching of the first layer51composed of a thermoplastic resin. Accordingly, it is quite possible for the first layer51to have a glass transition temperature of not less than 100° C. The optical sheet40of this embodiment and the display device10incorporating the optical sheet40therefore exhibit excellent thermal stability. For example, compared to the conventional birefringent lens containing liquid crystal, the optical sheet40of this embodiment can dramatically improve dimensional stability as measured according to JIS C2151 using the heating conditions of 150° C., 30 minutes. Specifically, the dimensional stability value of the optical sheet40of this embodiment, measured according to JIS C2151 using the heating conditions of 150° C., 30 minutes, can be made as low as not more than 2%. The optical sheet40of this embodiment can therefore be used, without significant restriction on it, in a common environment where a home television receiver, for example, is used, and the optical sheet40can exert the expected optical effect.

Various changes and modifications may be made to the above-described embodiment. Some variations will now be described. In the following description, the same reference numerals are used for the same members or elements as used in the above-described embodiment, and a duplicate description thereof will be omitted.

The optical sheet40is merely an example and can be arbitrarily changed: The film layer43is not essential and may be omitted from the optical sheet40. An additional film layer, which is expected to perform a certain function, may be provided at a position nearer to the polarization control device30than the first layer51and the second layer52. As described above, the construction of the optical interface55and the unit optical interfaces55acan be arbitrarily changed depending on a desired optical effect. Further, the optically anisotropic first layer51may be disposed nearer to the viewer than the second layer52.

The above-described relation between the refractive index n1xof the first layer51in the x-axis direction, the refractive index n1yof the first layer51in the y-axis direction, the refractive index n2xof the second layer52in the x-axis direction and the refractive index n2yof the second layer52in the y-axis direction is merely exemplary, and is not intended to limit the scope of the present invention.

In the above-described embodiment the refractive index difference between the first layer51and the second layer52is made zero in either one of the x-axis direction and the y-axis direction. However, it is possible to make the refractive index difference between the first layer51and the second layer52not zero in both of the x-axis direction and the y-axis direction. Also in this case, the same effect as described above can be obtained by appropriately designing the construction of the optical interface55and the unit optical interfaces55a.

Though in the above-described embodiment the main axes (the slow axis and the fast axis) in a plane of the first layer51coincide with the directions of vibration of light that forms a three-dimensional image and light that forms a two-dimensional image, it is possible not to make the main axes coincide with the vibration directions. Also in this case, the same effect as described above can be obtained by appropriately adjusting the refractive indices n1x, n1y, n2xand n2y.

The modifications described above can of course be made in an appropriate combination to the above-described embodiment.