Optical film

To provide a thin optical sheet having improved efficiency for light utilization, an optical sheet (5) of one mode of the present invention includes, in sequence from a light entry side to a light emission side, a plurality of first prisms (13), a ¼ wavelength plate (11), and a polarized-light separating element (12), the plurality of first prisms (13) each having (i) a first surface (13a) through which light enters the first prism and (ii) a second surface (13b) that reflects the light, having entered the first prism, toward the light emission side, the optical film further including, between the plurality of first prisms in an in-plane direction of the optical film, a second prism (14) that reflects light.

TECHNICAL FIELD

The present invention relates to an optical film, a backlight, and a liquid crystal display device.

BACKGROUND ART

In recent years, a typical liquid crystal display device includes a backlight and a liquid crystal panel, the liquid crystal panel including two polarizers and a liquid crystal layer sandwiched between the two polarizers. A typical liquid crystal display device displays an image by controlling the alignment of liquid crystal molecules by means of voltage. The backlight emits light, which enters the liquid crystal panel in an unpolarized state. Thus, half of the light is absorbed by the polarizer through which the light first passes.

Patent Literature 1 discloses a technique for reducing a light loss caused by a polarizer. The technique of Patent Literature 1 provides a polarized-light selection reflecting surface on a light entry side of a prism array, and also provides a ¼ wavelength plate and a reflecting mirror on a side toward which light reflected by the polarized-light selection reflecting surface travels. The light source emits light toward the polarized-light selection reflecting surface, the light including polarized light that is incapable of passing through the polarized-light selection reflecting surface. Such polarized light is reflected by the polarized-light selection reflecting surface to subsequently strike the ¼ wavelength plate and the reflecting mirror, which are provided ahead of the reflected light. For improved efficiency for light utilization, the polarized light reflected by the polarized-light selection reflecting surface needs to be changed into polarized light that is capable of passing through the polarized-light selection reflecting surface and to be then emitted to the polarized-light selection reflecting surface again. To achieve such an arrangement, the technique of Patent Literature 1 disposes the ¼ wavelength plate and the reflecting mirror at such positions that the ¼ wavelength plate and the reflecting mirror are located apart from the prism array and the polarized-light selection reflecting surface provided on the side toward which light reflected by the polarized-light selection reflecting surface travels. With this arrangement, (i) P polarized light, for example, included in the light emitted by the light source passes through the polarized-light selection reflecting surface, whereas S polarized light included in the light is reflected by the polarized-light selection reflecting surface, (ii) the S polarized light reflected by the polarized-light selection reflecting surface passes through the ¼ wavelength plate and is then reflected by the reflecting mirror to become P polarized light to be emitted again to the polarized-light selection reflecting surface, and (iii) the P polarized light thus emitted passes through the polarized-light selection reflecting surface. The above arrangement consequently allows reuse of reflected polarized light.

CITATION LIST

Patent Literature 1

SUMMARY OF INVENTION

Technical Problem

According to the technique of Patent Literature 1, both the polarized-light selection reflecting surface and the ¼ wavelength plate are in contact with air, which has low refractive index. The polarized-light selection reflecting surface and the ¼ wavelength plate thus reflect light unnecessarily at their respective interfaces, with the result of decreased efficiency for reuse of polarized light. Further, according to this technique, the prism array and the polarized-light selection reflecting surface are apart from the polarized light converting elements (namely, the ¼ wavelength plate and the reflecting mirror). This problematically results in a backlight having large thickness and requires fixing means for each member.

The present invention has been accomplished in view of the above problems. One mode of the present invention is a thin optical film having improved efficiency for utilization of light of a light source.

Solution to Problem

An optical film of the present invention is an optical film including, in sequence from a light entry side of the optical film to a light emission side of the optical film: a plurality of first prisms; a first phase-difference plate; and a polarized-light separating element, the plurality of first prisms each having (i) a first surface through which light enters the first prism and (ii) a second surface that reflects the light, having entered the first prism through the first surface, toward the light emission side, the optical film further including, between the plurality of first prisms in an in-plane direction of the optical film, a light reflecting section that reflects light, having been reflected by the polarized-light separating element toward the light entry side, back toward the light emission side.

Advantageous Effects of Invention

An optical film of the present invention is an optical film including, in sequence from a light entry side of the optical film to a light emission side of the optical film: a plurality of first prisms; a ¼ wavelength plate; and a polarized-light separating element, the plurality of first prisms each having (i) a first surface through which light enters the first prism and (ii) a second surface that reflects the light, having entered the first prism through the first surface, toward the light emission side, the optical film further including, between the plurality of first prisms in an in-plane direction of the optical film, a light reflecting section that reflects light, having been reflected by the polarized-light separating element toward the light entry side, back toward the light emission side.

With the above arrangement, the optical film allows light having entered the optical film in an unpolarized state to be converted efficiently into polarized light that is capable of passing through the polarized-light separating element, and allows such polarized light to be emitted. Further, the polarized-light separating element and the ¼ wavelength plate are included in the optical film. This can reduce the difference in refractive index at the respective boundary surfaces of the polarized-light separating element and the ¼ wavelength plate, and can thus reduce Fresnel reflectance at the respective boundary surfaces of the polarized-light separating element and the ¼ wavelength plate, with the result of reduction in a light loss caused by Fresnel reflection. In addition, the polarized-light separating element, the ¼ wavelength plate, and the light reflecting section are included in the optical film. This allows production of a thin optical film that allows light in an unpolarized state to be converted into predetermined polarized light and that allows such polarized light to be emitted. Furthermore, since the polarized-light separating element, the ¼ wavelength plate, and the light reflecting section are included in the optical film, there is no need for, for example, alignment of the individual optical members. Also, the polarized-light separating element, the first phase-difference plate, and the light reflecting section, which are included in the optical film, may be stacked up on top of each other for integral production. This facilitates assembly of the optical film to an optical product such as a liquid crystal display device, and can consequently reduce the cost of producing an optical product including the optical film.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described below in detail with reference to the drawings.

(Arrangement of Liquid Crystal Display Device)

FIG. 1is a cross-sectional view illustrating a configuration of a liquid crystal display device1of the present embodiment.FIG. 1shows arrows to indicate an example optical path of light from a light source6. The liquid crystal display device1includes a backlight2and a liquid crystal panel3provided on a front surface side of the backlight2(that is, in the direction of a user). The description below uses (i) the term “x direction” to refer to a direction that extends from one end of the liquid crystal display device1at which end the light source6is provided to the other end, (ii) the term “z direction” to refer to a direction that extends from a back surface of the liquid crystal display device1to a front surface thereof, and (iii) the term “y direction” to refer to a direction perpendicular to the x direction and the z direction.

The backlight2includes a light-emitting section4and an optical sheet (optical film)5provided on a front surface side of the light-emitting section4. The light-emitting section4includes a light source6, a reflector7, and a light guide plate8.

The light source6is a cold cathode fluorescent tube in the present embodiment. The light source6is, however, not limited to that, and may be, for example, a white LED or a plurality of LEDs that individually emit light of the colors of R, G, and B. The white LED may be, for example, an LED including a blue LED chip and a plurality of fluorescent substances applied on the blue LED chip and having respective wavelength peaks for such colors as red and green. The light source6may alternatively be, instead of an LED, an organic EL device provided at an entry end of the light guide plate8.

The description below uses the symbol “λ” to indicate the wavelength of a representative portion of light emitted by the light source6. The representative wavelength λ may be, for example, a middle wavelength (for example, 550 nm for green) within a wavelength range (visible light range) of light emitted by the light source6. The representative wavelength λ may be a green wavelength because the sight of the human being is sensitive to luminance of green light.

The reflector7reflects light from the light source6to collect it and cause it to converge at the entry end of the light guide plate8. Light emitted by the light source6enters the light guide plate8through its entry end, which corresponds to a side surface of the light guide plate8which side surface faces the light source6.

The light guide plate8has a cross section in a shape tapering from the entry end to the other end thereof, the thickness of the light guide plate8being larger at the entry end and smaller at the other end. The light guide plate8has a wedge shape in the present embodiment, but is not limited to such a shape. The light guide plate8may have asperities for light extraction on a surface opposite to an emission surface thereof. Light travels inside the light guide plate8from its entry end toward the other end while being reflected by surfaces of the light guide plate8, and is emitted from the emission surface, which faces the optical sheet5, toward the optical sheet5. The light emitted from the light guide plate8is so greatly angled as to travel in a direction away from the light source6with respect to a direction perpendicular to the emission surface. The light emitted from the light guide plate8, in other words, travels at an angle substantially parallel to the emission surface of the light guide plate8.

The optical sheet5receives unpolarized light emitted from the light guide plate8and incident on a back surface of the optical sheet5, changes the unpolarized light into linearly polarized light, and emits the linearly polarized light from a front surface (emission surface) thereof toward the liquid crystal panel3. The optical sheet5includes a prism array9, a phase-difference plate (wave plate, retardation plate)10, a ¼ wavelength plate11, and a polarized-light separating element12in that order from a back surface side of the optical sheet5to a front surface side thereof (that is, from a light entry side to a light emission side).

The prism array9, provided on the back surface side of the optical sheet5, includes a plurality of first prisms13and a plurality of second prisms (light reflecting sections)14provided in an in-plane direction of the optical sheet5. The first prisms13each have a vertex angle (that is, the angle of its vertex) smaller than that of each second prism14. The first prisms13and the second prisms14are provided alternately along the direction (x direction) extending away from the light source6. A later description will deal in detail with how the optical sheet5is arranged.

The liquid crystal panel3controls the alignment state of liquid crystal molecules to determine whether to allow transmission of light emitted from the optical sheet5and incident on a back surface of the liquid crystal panel3, and thus displays an image. The liquid crystal panel3includes a back surface polarizer15, an active matrix substrate16including a plurality of pixel electrodes, a liquid crystal layer17, a color filter substrate18including a common electrode and color filters for individual pixels, and a front surface polarizer19in that order from a back surface side of the liquid crystal panel3to a front surface side thereof (that is, from a light entry side to a light emission side). The back surface polarizer15has a polarized-light transmission axis identical in direction to that of the polarized-light separating element12. This prevents the back surface polarizer15from absorbing polarized light emitted from the optical sheet5and incident on the back surface of the liquid crystal panel3, with the result of high efficiency for light utilization. Since light emitted from the polarized-light separating element12is linearly polarized in the direction of the polarized-light transmission axis of the polarized-light separating element12, the back surface polarizer15may be omitted. The liquid crystal panel3may alternatively be in contact with the optical sheet5.

(Arrangement of Optical Sheet)

FIG. 2is an enlarged cross-sectional view of a portion of the optical sheet5.FIG. 2further illustrates respective optical axes of the individual layers.FIG. 2shows arrows to indicate an example optical path of light emitted from the light guide plate8and incident on the optical sheet5.

The optical sheet5includes (i) a prism array9on its back surface side, (ii) a phase-difference plate (second phase-difference plate)10on a front surface side of the prism array9, (iii) a ¼ wavelength plate (first phase-difference plate)11on a front surface side of the phase-difference plate10, and (iv) a polarized-light separating element12on a front surface side of the ¼ wavelength plate11.

The prism array9includes a plurality of first prisms13and a plurality of second prisms14. The first prisms13and the second prisms14are provided alternately along the direction (x direction) extending away from the light source6. The prism array9has a shape that is uniform along the direction (y direction) in which the first prisms13extend.

The first prisms13each have a first surface13ainclined on the side of the light source6and a second surface13binclined on the opposite side in symmetry to the first surface13a. The first surface13aand the second surface13bhave an equal angle to the y-z plane, and are symmetrical to each other with respect to the y-z plane. The first prisms13thus each have a cross section in the shape of an isosceles triangle. The first prisms13are taller than the second prisms14, and protrude toward the light guide plate8farther than the second prisms14.

The second prisms14each have a third surface14ainclined on the side of the light source6and a fourth surface14binclined on the opposite side in symmetry to the third surface14a. The third surface14aand the fourth surface14bhave an equal angle to the y-z plane, and are symmetrical to each other with respect to the y-z plane. The second prisms14thus each have a cross section in the shape of an isosceles triangle. The second prisms14each have a vertex angle larger than that of each first prism13. The second prisms14each have a vertex angle larger than that of each first prism13so that the two surfaces of each second prism14(that is, the third surface14aand the fourth surface14b) totally reflect light reflected by the polarized-light separating element12.

The phase-difference plate10imparts a predetermined phase difference to light having a wavelength λ and passing through the phase-difference plate10in its thickness direction. A later description will deal in detail with the size of the phase difference that the phase-difference plate10imparts. The phase-difference plate10has a slow axis along the y direction.

The ¼ wavelength plate11imparts a phase difference of a ¼ wavelength (λ/4) to light having the wavelength λ and passing through the ¼ wavelength plate11in its thickness direction. The ¼ wavelength plate11has a slow axis at an angle of 45 degrees with respect to the x direction.

Light passes through the phase-difference plate10and the ¼ wavelength plate11at an angle that is not exactly parallel to the thickness direction but that is inclined several degrees (for example, smaller than 10 degrees) with respect to the thickness direction. The light, however, travels in a direction that is substantially parallel to the thickness direction. The phase difference imparted to the passing light can thus be regarded as substantially equal to that which would be imparted to light passing through the phase-difference plate10and the ¼ wavelength plate11in a direction exactly parallel to the thickness direction (that is, perpendicular to the layers).

The polarized-light separating element12transmits only light polarized in a certain direction, and reflects light polarized in a direction perpendicular to that certain direction. The polarized-light separating element12may be, for example, a DBEF (registered trademark). The polarized-light separating element12has a polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12transmits) angled at 90 degrees with respect to the x direction.

The prism array9, the phase-difference plate10, the ¼ wavelength plate11, and the polarized-light separating element12are integrated with each other to form the optical sheet5. The phase-difference plate10, the ¼ wavelength plate11, and the polarized-light separating element12may be in contact with each other or separated from each other. In a case where the phase-difference plate10, the ¼ wavelength plate11, and the polarized-light separating element12are separated from each other, the phase-difference plate10, the ¼ wavelength plate11, and the polarized-light separating element12are preferably separated from each other by a space filled with either a material of the prism array9or a material (adhesive) having a refractive index close to those of materials of the individual layers. This arrangement can reduce reflectance at each interface between the individual layers (namely, the phase-difference plate10, the ¼ wavelength plate11, and the polarized-light separating element12), with the result of increased efficiency for light utilization. The above arrangement can reduce reflectance at the interface of the polarized-light separating element12for only light polarized in a direction parallel to the polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12transmits): The polarized-light separating element12has high reflectance at the interface for light polarized in a direction perpendicular to the polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12does not transmit).

With reference toFIGS. 1 and 2, the description below deals with how light having entered the optical sheet5behaves.FIG. 2illustrates polarization states at different positions. The description below uses the term “right-handed rotation” for a rotation direction of circularly polarized light to refer to a clockwise direction with respect to the direction in which the light travels.

Light is emitted from the emission surface of the light guide plate8in an unpolarized state at an angle close to the x direction. The light emitted from the emission surface of the light guide plate8thus strikes the first surface13aof each first prism13, which protrudes toward the light guide plate8.

The light incident on the first surface13aof each first prism13is refracted by the first surface13aand is then reflected (total reflection) by the second surface13b. This arrangement causes light having entered the optical sheet5to change its direction to a direction close to the direction (z direction) perpendicular to the emission surface of the optical sheet5. The present embodiment adjusts, for example, the direction in which light is emitted from the light guide plate8and the angle of each first prism13so that light reflected by the second surface13btravels in a direction that is not exactly parallel to the z direction but is slightly inclined from the z direction toward the x direction. If light reflected by the second surface13btravels in a direction exactly parallel to the z direction, polarized light reflected by the polarized-light separating element12will unfortunately travel back along the same optical path to be emitted from the first surface13a.

The light reflected by the second surface13bpasses through the phase-difference plate10and the ¼ wavelength plate11. The light reflected by the second surface13bis in an unpolarized state. The polarization state of that light is thus not changed by the phase-difference plate10or the ¼ wavelength plate11, with the result that the light remains unpolarized even after passing through the ¼ wavelength plate11.

The light having passed through the ¼ wavelength plate11and then reached the polarized-light separating element12is separated into two portions: polarized light passing through the polarized-light separating element12and polarized light reflected by the polarized-light separating element12. In the present embodiment, the polarized-light transmission axis extends along the y direction. The polarized-light separating element12thus (i) transmits light (S polarized light) polarized in the direction (y direction) perpendicular to the x-z plane and (ii) reflects light (P polarized light) polarized in the direction parallel to the x-z plane (plane of incidence). The light reflected by the polarized-light separating element12is thus linearly polarized light of P polarized light. The polarized-light separating element12reflects P polarized light so that the P polarized light travels back toward the third surface14aof each second prism14. The S polarized light having passed through the polarized-light separating element12is emitted from the emission surface of the optical sheet25toward the liquid crystal panel3.

The light reflected by the polarized-light separating element12enters the ¼ wavelength plate11at an angle substantially perpendicular to the ¼ wavelength plate11(that is, an angle close to a −z direction). The linearly polarized light (P polarized light) passes through the ¼ wavelength plate11, which has a slow axis angled at 45 degrees with respect to the y direction, to change into circularly polarized light.

Next, circularly polarized light having passed through the ¼ wavelength plate11passes through the phase-difference plate10. The circularly polarized light is then totally reflected by the third surface14aof each second prism14, and is thereafter totally reflected by the fourth surface14bto subsequently pass through the phase-difference plate10again. This means that circularly polarized light having passed through the ¼ wavelength plate11is subjected to (i) two phase difference changes caused by the phase-difference plate10and (ii) two other phase difference changes caused by total reflection, before returning to the ¼ wavelength plate11again. In the present embodiment, the phase-difference plate10is arranged to impart a phase difference that causes circularly polarized light to have circular polarization in an opposite rotation direction (left-handed or right-handed rotation) when returning to the ¼ wavelength plate11again. In other words, the present embodiment is arranged such that the phase-difference plate10and the second prisms14impart a phase difference of λ/2 (that is, a phase shift amount π) in total to the circularly polarized light, having passed through the ¼ wavelength plate11toward the second prisms14, before the circularly polarized light returns to the ¼ wavelength plate11again.

The description below deals with how polarized light is changed by total reflection. Assuming that light travels through a medium having a high refractive index (in the present embodiment, the second prisms14having a refractive index n1) toward a boundary between that medium and a medium having a low refractive index (in the present embodiment, air having a refractive index n2), the light is not refracted but all reflected in a case where the light is incident on the boundary at the critical angle θc or larger. This phenomenon is called total reflection.
sin θc=n2/n1<1

Strictly speaking, however, a boundary surface at which total reflection occurs lets an evanescent wave leak out to the medium having a low refractive index. Such an evanescent wave, after passing through the boundary surface, attenuates extremely sharply. The evanescent wave causes, at the boundary surface, a phase shift (phase lead) between light on the path of incident light and that on the path of reflected light. This phase shift is called Goos-Hanchen shift.

This phase lead differs between P polarized light and S polarized light. Total reflection causes (i) P polarized light to have a phase lead δp and (ii) S polarized light to have a phase lead δs. The phase leads δp and δs may be calculated with the following expressions:

where n1represents the refractive index of a medium having a high refractive index, n2represents the refractive index of a medium having a low refractive index, and θ1 represents the angle of incidence (that is, the angle between the optical path of light incident on a boundary surface and the direction perpendicular to the boundary surface).

FIG. 3is a graph illustrating example phase leads caused by total reflection.FIG. 3shows a phase lead δp caused by total reflection in P polarized light, a phase lead δs caused by total reflection in S polarized light, and a phase difference δp−δs between the P polarized light and the S polarized light. The present embodiment assumes that n1=1.511 and n2=1, which means that the critical angle θc=41.438 degrees. While the angle of incidence is not smaller than the critical angle θc (at and over which total reflection occurs), the phase difference δp−δs between the P polarized light and the S polarized light has a value within the range of 0 to 45 degrees (0 to π/4). In other words, a single instance of total reflection causes a phase difference within the range of 0 to π/4 between P polarized light and S polarized light. The two instances of total reflection at the third surface14aand fourth surface14bof each second prism14thus cause a phase difference within the range of 0 to π/2 between P polarized light and S polarized light. Since δp>δs, the phase of P polarized light leads that of S polarized light, meaning that the phase of S polarized light is delayed relatively with respect to that of P polarized light. InFIG. 2, the slow axis for total reflection corresponds to the y direction (that is, the direction in which the prisms extend).

Imparting a phase difference to light having a polarized state means changing the polarization state of that light. The present embodiment, to correct a phase shift caused by two instances of total reflection, includes a phase-difference plate10on a back surface side of the ¼ wavelength plate11. The phase-difference plate10, however, does not have a phase difference that cancels phase leads by total reflection: The phase-difference plate10has a phase difference such that undergoing two instances of total reflection and two instances of passing through the phase-difference plate10results in a total phase difference (delay of S polarized light) of λ/2 (phase delay π). For instance, in a case where a single instance of total reflection causes a phase difference (delay of S polarized light) of λ/12 (phase delay of 30 degrees), the phase-difference plate10is preferably arranged to impart a phase difference of λ/6 (phase delay of 60 degrees) to light each time the light passes through the phase-difference plate10. The phase-difference plate10is arranged to have a slow axis parallel to the direction in which the second prisms14of the prism array9extend.

In a case where second prisms each have a refractive index of 1.511 as in the present embodiment, the phase difference δp−δs resulting from total reflection has a value within the range of 0 to λ/8 (0 to 45 degrees) as illustrated inFIG. 3. In view of such a phase difference lead ofFIG. 3caused by total reflection, the phase-difference plate10has a phase difference of not less than λ/4 and not greater than λ/8 (that is, from 90 degrees to 45 degrees).

Further, in a case where, for instance, the phase difference resulting from total reflection is λ/8 (45 degrees), that is, the angle of incidence inFIG. 3is 50 degrees or in the vicinity thereof, the phase-difference plate10may alternatively have a phase difference of not greater than λ/6 (60 degrees) and not less than λ/12 (30 degrees). In a case where the phase-difference plate10has a phase difference of λ/6 (60 degrees), two instances of passing through the phase-difference plate10(phase difference of 60 degrees) and two instances of total reflection (phase difference of 45 degrees) result in a total phase difference of 210 degrees. This means that in this case, there is a difference of 30 degrees from a phase difference of 180 degrees, which is necessary for conversion of circularly polarized light into circularly polarized light having an opposite rotation. In a case where the phase-difference plate10has a phase difference of λ/12 (30 degrees), two instances of passing through the phase-difference plate10(phase difference of 30 degrees) and two instances of total reflection (phase difference of 45 degrees) result in a total phase difference of 150 degrees. This means that in this case also, there is a difference of 30 degrees from a phase difference of 180 degrees, which is necessary for conversion of circularly polarized light into circularly polarized light having an opposite rotation. Thus, regardless of whether the phase-difference plate10has a phase difference of λ/6 (60 degrees) or λ/12 (30 degrees), the same proportion of light passes through the polarized-light separating element12again. With a difference of 30 degrees or in the vicinity thereof from the ideal phase difference of 180 degrees, a large proportion of light passes through the phase-difference plate10again (that is, a large proportion of light is reusable).

As the above discussion indicates, the phase-difference plate10has a phase difference of not greater than λ/4 and not less than λ/12. The phase-difference plate10may alternatively have a phase difference of not greater than λ/6 and not less than λ/12.

It is needless to say that two instances of passing through the phase-difference plate10and two instances of total reflection may result in a total phase difference of λ/2+mλ (where m is an integer) instead of exactly λ/2 (180 degrees) and that the ¼ wavelength plate11may have a phase difference of λ/4+mλ (where m is an integer) instead of exactly λ/4 (90 degrees).

Circularly polarized light having passed through the ¼ wavelength plate11toward the second prisms14passes through the phase-difference plate10and is then totally reflected once by the third surface14aof each second prism14, at which time point the circularly polarized light becomes subjected to a phase shift of π/2. Specifically, the light is totally reflected once by the third surface14ato become linearly polarized light having a polarization direction inclined at an angle of 45 degrees with respect to the polarization direction of P polarized light (that is, linearly polarized light having a P polarized light component and an S polarized light component that are identical to each other or linearly polarized light having a polarization direction inclined at an angle of 45 degrees with respect to the x-z plane). Next, the light is totally reflected once again by the fourth surface14bof each second prism14, and then passes through the phase-difference plate10again to be subjected to another phase shift of π/2. The light, after passing through the phase-difference plate10toward the polarized-light separating element12, includes a P polarized light component having a phase lead of π. The light, which has passed through the phase-difference plate10toward the polarized-light separating element12, is circularly polarized light having a rotation direction opposite to that of the original circularly polarized light (at the time point at which the light passed through the phase-difference plate10toward the second prisms14).

The light is elliptically polarized between the phase-difference plate10and the second prisms14.

The circularly polarized light having passed through the phase-difference plate10toward the polarized-light separating element12passes through the ¼ wavelength plate11for conversion into linearly polarized light. Since the circularly polarized light had a reversed rotation direction, the linearly polarized light exiting the ¼ wavelength plate11has a polarization direction perpendicular to that of the linearly polarized light (P polarized light) at the time of the reflection by the polarized-light separating element12. The linearly polarized light exiting the ¼ wavelength plate11is, in other words, linearly polarized light of S polarized light.

Since the light having been reflected by the second prisms14and then passed through the ¼ wavelength plate11is S polarized light, it can pass through the polarized-light separating element12. The S polarized light having passed through the polarized-light separating element12is emitted from the emission surface of the optical sheet5to strike the liquid crystal panel3.

As the result of the process described above, only S polarized light is emitted from the emission surface of the optical sheet5(that is, the surface facing the liquid crystal panel3). Further, P polarized light having been reflected by the polarized-light separating element12to travel back toward the second prisms14is converted into S polarized light by the polarized light converting elements (namely, the ¼ wavelength plate11, the phase-difference plate10, and the second prisms14) to be emitted from the optical sheet5. The optical sheet5consequently makes it possible to (i) highly efficiently utilize light that is incident from the light-emitting section4on the optical sheet5and to (ii) emit only S polarized light at an angle close to 90 degrees with respect to the emission surface.

The present embodiment stacks up all of the prism array9, the polarized-light separating element12, and the polarized light converting elements (namely, the ¼ wavelength plate11and the phase-difference plate10) to integrally form an optical sheet5. This arrangement eliminates the need for alignment of the individual optical members, and facilitates assembly of the liquid crystal display device1, thereby reducing the cost of producing the liquid crystal display device1. Further, the above arrangement, which allows the individual optical members to integrally form an optical sheet5, allows production of an optical sheet5that is large-sized for use in a large screen yet reduced in thickness. This in turn allows production of a thin liquid crystal display device1.

Further, the present embodiment is arranged such that the individual optical members (namely, the polarized-light separating element12, the ¼ wavelength plate11, and the phase-difference plate10) are in contact with each other or that the individual optical members are separated from each other by a material having a refractive index equivalent or close to those of the individual optical members. This arrangement allows the optical sheet5to cause only a small refractive index change inside itself as compared at least to a conventional technique involving different optical members separated from each other by air or the like. With the above arrangement, polarized light reflected by the polarized-light separating element12(P polarized light) is converted through the optical sheet5, which causes only a small refractive index change, into light that is polarized so as to be able to pass through the polarized-light separating element12. The above arrangement can thus almost completely eliminate a light loss caused by Fresnel reflection at the interface between the individual optical members, thereby improving efficiency for light utilization.

The efficiency for light utilization was calculated of the optical sheet5of the present embodiment with use of an optical simulator.FIG. 4is a diagram illustrating a cross section of the optical sheet5and light intensities at different positions.

The simulation was run under the following conditions: Light was emitted from the outside (that is, from the light-emitting section4) to strike the first prisms13of the optical sheet5at an angle that was parallel to the x-z plane and that was 8 degrees with respect to the emission surface of the optical sheet5(horizontal direction). The light emitted had a wavelength of 550 nm. The first prisms13each had a cross section in the shape of an isosceles triangle of which the vertex angle was 80 degrees. The second prisms14each had a cross section in the shape of an isosceles triangle of which the vertex angle was 100 degrees. The polarized-light separating element12, the ¼ wavelength plate11, and the phase-difference plate10each had its optical axis extending in the direction illustrated inFIG. 2. Specifically, the polarized-light separating element12had a polarized-light transmission axis extending in a direction perpendicular to the x direction. The ¼ wavelength plate11had a slow axis extending in a direction angled at 45 degrees with respect to the x direction. The phase-difference plate10had a slow axis extending in a direction perpendicular to the x direction. The phase-difference plate10had a phase difference (retardation) of ⅛ wavelength along its thickness direction. The optical sheet5(specifically, the prism array9) had a refractive index of 1.5, whereas outside air had a refractive index of 1. The light used in the simulation had a single wavelength. The simulation was run under an ideal condition: The polarized-light separating element12would cause an absorption loss of 0%. The polarized-light separating element12had a transmittance of 100% for light polarized in a direction parallel to the polarized-light transmission axis, and had a reflectance of 100% for light polarized in a direction perpendicular to the polarized-light transmission axis.

The description below first deals with a preferable phase difference for the phase-difference plate10. Light enters each first prism13of the optical sheet5at an angle of 32 degrees with respect to a direction perpendicular to the first surface13a. The light having entered the first prism13travels at an angle of 20.68 degrees with respect to the direction perpendicular to the first surface13aby Snell's law. Next, the light is reflected (total reflection) by the second surface13bof each first prism13to strike the polarized-light separating element12. The light strikes the polarized-light separating element12at an angle of 9.3 degrees with respect to a direction perpendicular to the boundary surface of the polarized-light separating element12. Polarized light reflected by the polarized-light separating element12travels in a direction angled at 9.3 degrees toward the x direction from the direction (z direction) perpendicular to the boundary surface of the polarized-light separating element12, and reaches the third surface14aof each second prism14. The light strikes the third surface14aat an angle of 49.3 degrees with respect to a direction perpendicular to the third surface14aof the second prism14, and is thus totally reflected by the third surface14a. This instance of total reflection causes a phase difference change of approximately 44.55 degrees as calculated with reference toFIG. 3. This indicates that in the case illustrated inFIG. 4, using a phase-difference plate10having a phase difference of approximately λ/8 (45 degrees) along its thickness direction allows P polarized light reflected by the polarized-light separating element12to be mostly converted into S polarized light.

The description below now deals with the intensity of light emitted from the optical sheet5under the above conditions. The description below assumes that light reaching the optical sheet5from the outside has an intensity of 1. First, light striking each first prism13from the outside is partly reflected by a surface of the first prism13(that is, the first surface13a) as a result of Fresnel reflection. The light having been refracted at the surface and entered the first prism13has an intensity of 0.958. The light then passes through the optical sheet5to reach the polarized-light separating element12. Then, half of the light (with an intensity of 0.479) passes through the polarized-light separating element12in the form of S polarized light to be emitted from the emission surface of the optical sheet5. The remaining half of the light (with an intensity of 0.479) is P polarized light, and is thus reflected by the polarized-light separating element12to travel back toward the ¼ wavelength plate11.

The P polarized light reflected by the polarized-light separating element12passes through the ¼ wavelength plate11and the phase-difference plate10, and is then totally reflected (by the second prisms14) to change its polarization state. Most of the P polarized light is converted into S polarized light. The light reaching the polarized-light separating element12again includes S polarized light having an intensity of 0.4788 and P polarized light having an intensity of 0.0002. The light having the intensity of 0.4788 (S polarized light) passes through the polarized-light separating element12, whereas the remaining light having the intensity of 0.0002 (P polarized light) is reflected by the polarized-light separating element12again to travel back toward the ¼ wavelength plate11.

The component exiting the optical sheet5when the light first reaches the polarized-light separating element12has an intensity of 0.479, whereas the component exiting the optical sheet5when the light subsequently reaches the polarized-light separating element12has an intensity of 0.4788, the sum of the two intensities being 0.9578. This is a value with no consideration into Fresnel reflection of light from the polarized-light separating element12toward the outside (air). The light emitted from the optical sheet5thus has an actual intensity of 0.9158. The optical sheet5, therefore, makes it possible to utilize approximately 91.6% of emitted light in the form of linearly polarized light that is polarized in a predetermined direction.

Light emitted from the optical sheet5of the present embodiment is already linearly polarized. This ideally causes no absorption loss at the back surface polarizer15of the liquid crystal panel3.

In contrast, in a case where a conventional backlight including no polarized-light separating element emits light onto a liquid crystal panel, since such light from a conventional backlight is in an unpolarized state, half of the light is absorbed by a back surface polarizer of the liquid crystal panel, and only the remaining half is utilized for display. In other words, of light from a conventional backlight, only a component having an intensity of 0.5 is utilized.

The optical sheet5of the present embodiment makes it possible to utilize a light component having an intensity of 0.9158 for display. This means that the liquid crystal display device1of the present embodiment has an improved light utilization efficiency that is 1.83 times that of a liquid crystal display device including a conventional backlight. The liquid crystal display device1of the present embodiment, in other words, has a reduced backlight power consumption that is 0.546 times that of a liquid crystal display device including a conventional backlight.

Another embodiment of the present invention is described below. For convenience of explanation, members and arrangements of the present embodiment that are identical in function to those described for Embodiment 1 with reference to drawings are each assigned a common reference numeral, and are not described here in detail. The present embodiment differs from Embodiment 1 in how the optical sheet is arranged.

FIG. 5is a cross-sectional view illustrating a configuration of a liquid crystal display device20of the present embodiment.FIG. 5shows arrows to indicate an example optical path of light from a light source6. The liquid crystal display device20includes a backlight22and a liquid crystal panel3provided on a front surface side of the backlight22(that is, in the direction of a user). The description below uses (i) the term “x direction” to refer to a direction that extends from one end of the liquid crystal display device20at which end the light source6is provided to the other end, (ii) the term “z direction” to refer to a direction that extends from a back surface of the liquid crystal display device20to a front surface thereof, and (iii) the term “y direction” to refer to a direction perpendicular to the x direction and the z direction. The liquid crystal panel3of the present embodiment is identical in arrangement to that of Embodiment 1.

The backlight22includes a light-emitting section4and an optical sheet (optical film)25provided on a front surface side of the light-emitting section4. The light-emitting section4includes a light source6, a reflector7, and a light guide plate8.

(Arrangement of Optical Sheet)

The optical sheet25receives unpolarized light emitted from the light guide plate8and incident on a back surface of the optical sheet25, changes the unpolarized light into linearly polarized light, and emits the linearly polarized light from a front surface (emission surface) thereof toward the liquid crystal panel3. The optical sheet25includes a prism array26, a ¼ wavelength plate11, and a polarized-light separating element12in that order from a back surface side of the optical sheet25to a front surface side thereof (that is, from a light entry side to a light emission side).

The prism array26is provided on the back surface side of the optical sheet25, and includes a plurality of first prisms13and a plurality of light reflecting sections24. The first prisms13and the light reflecting sections24are provided alternately along the direction (x direction) extending away from the light source6.

FIG. 6is an enlarged cross-sectional view of a portion of the optical sheet25.FIG. 6further illustrates respective optical axes of the individual layers.FIG. 6shows arrows to indicate an example optical path of light emitted from the light guide plate8and incident on the optical sheet25.

The optical sheet25includes (i) a prism array26on its back surface side, (ii) a ¼ wavelength plate11on a front surface side of the prism array26, and (iii) a polarized-light separating element12on a front surface side of the ¼ wavelength plate11. The ¼ wavelength plate11and the polarized-light separating element12are similar in arrangement to those of Embodiment 1. The ¼ wavelength plate11has a slow axis at an angle of 45 degrees with respect to the x direction. The polarized-light separating element12has a polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12transmits) angled at 90 degrees with respect to the x direction.

The prism array26includes a plurality of first prisms13and a plurality of light reflecting sections24. The first prisms13and the light reflecting sections24are provided alternately along the direction (x direction) extending away from the light source6. The prism array26has a shape that is uniform along the direction (y direction) in which the first prisms13extend.

The first prisms13each have a first surface13ainclined on the side of the light source6and a second surface13binclined on the opposite side in symmetry to the first surface13a. The first surface13aand the second surface13bhave an equal angle, and are symmetrical to each other. The first prisms13thus each have a cross section in the shape of an isosceles triangle.

The light reflecting sections24each serve as a reflecting surface for reflecting light from the polarized-light separating element12so that the light travels back toward the polarized-light separating element12. The reflecting surface is parallel to the emission surface of the optical sheet25. The light reflecting sections24each include a metal film deposited at a surface thereof (that is, a surface on the back surface side of the optical sheet25). The metal film can be made of a material such as silver and aluminum. In a case where the metal film is made of aluminum in particular, the light reflecting sections24may each further include a reflective coating between the metal film and a surface of the material of the prism to increase reflectance.

The first prisms13may also each have a second surface13bprovided with a metal film. In this case, the second surface13bof each first prism13will have low reflectance as compared to a case where the second surface13bcauses total reflection, but the backlight22will have higher light utilization efficiency than a conventional backlight. Further, in the above case, there is no need to form metal films only on flat surfaces for light reflecting sections, with the result of facilitated production of the optical sheet25.

The prism array26, the ¼ wavelength plate11, and the polarized-light separating element12are integrated with each other to form the optical sheet25. The ¼ wavelength plate11and the polarized-light separating element12may be in contact with each other or separated from each other. In a case where the ¼ wavelength plate11and the polarized-light separating element12are separated from each other, the ¼ wavelength plate11and the polarized-light separating element12are preferably separated from each other by a space filled with either a material of the prism array26or a material (adhesive) having a refractive index close to those of materials of the individual layers. This arrangement can reduce reflectance at the interface between the individual layers (namely, the ¼ wavelength plate11and the polarized-light separating element12), with the result of increased efficiency for light utilization. The above arrangement can reduce reflectance at the interface of the polarized-light separating element12for only light polarized in a direction parallel to the polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12transmits): The polarized-light separating element12has high reflectance at the interface for light polarized in a direction perpendicular to the polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12does not transmit).

With reference toFIGS. 5 and 6, the description below deals with how light having entered the optical sheet25behaves.FIG. 6illustrates polarization states at different positions. The description below uses the term “right-handed rotation” for a rotation direction of circularly polarized light to refer to a clockwise direction with respect to the direction in which the light travels.

Light is emitted from the emission surface of the light guide plate8in an unpolarized state at an angle close to the x direction. The light emitted from the emission surface of the light guide plate8thus strikes the first surface13aof each first prism13, which protrudes toward the light guide plate8.

The light incident on the first surface13aof each first prism13is refracted by the first surface13aand is then reflected (total reflection) by the second surface13b. This arrangement causes light having entered the optical sheet25to change its direction to a direction close to the direction (z direction) perpendicular to the emission surface of the optical sheet25. The present embodiment adjusts, for example, the direction in which light is emitted from the light guide plate8and the angle of each first prism13so that light reflected by the second surface13btravels in a direction that is not exactly parallel to the z direction but is slightly inclined from the z direction toward the x direction. If light reflected by the second surface13btravels in a direction exactly parallel to the z direction, polarized light reflected by the polarized-light separating element12will unfortunately travel back along the same optical path to be emitted from the first surface13a.

The light reflected by the second surface13bpasses through the ¼ wavelength plate11. The light reflected by the second surface13bis in an unpolarized state. The polarization state of that light is thus not changed by the ¼ wavelength plate11, with the result that the light remains unpolarized even after passing through the ¼ wavelength plate11.

The light having passed through the ¼ wavelength plate11and then reached the polarized-light separating element12is separated into two portions: polarized light passing through the polarized-light separating element12and polarized light reflected by the polarized-light separating element12. In the present embodiment, the polarized-light transmission axis extends along the y direction. The polarized-light separating element12thus (i) transmits light (S polarized light) polarized in the direction (y direction) perpendicular to the x-z plane and (ii) reflects light (P polarized light) polarized in the direction parallel to the x-z plane (plane of incidence). The light reflected by the polarized-light separating element12is thus linearly polarized light of P polarized light. The polarized-light separating element12reflects P polarized light so that the P polarized light travels back toward the light reflecting sections24. The S polarized light having passed through the polarized-light separating element12is emitted from the emission surface of the optical sheet25toward the liquid crystal panel3.

The light reflected by the polarized-light separating element12enters the ¼ wavelength plate11at an angle substantially perpendicular to the ¼ wavelength plate11(that is, an angle close to a −z direction). The linearly polarized light (P polarized light) passes through the ¼ wavelength plate11, which has a slow axis angled at 45 degrees with respect to the y direction, to change into circularly polarized light.

Next, circularly polarized light having passed through the ¼ wavelength plate11is reflected by the light reflecting sections24. The light is reflected so that one of S polarized light and P polarized light both included in the light has a phase shifted by π. The light as reflected by the reflecting surfaces (that is, the light reflecting sections24) is consequently circularly polarized light having a rotation direction opposite to that of the circularly polarized light at the time of the entry into the optical sheet25.

The circularly polarized light reflected by the light reflecting sections24passes through the ¼ wavelength plate for conversion into linearly polarized light. Since the circularly polarized light had a reversed rotation direction, the linearly polarized light exiting the ¼ wavelength plate11has a polarization direction perpendicular to that of the linearly polarized light (P polarized light) at the time of the reflection by the polarized-light separating element12. The linearly polarized light exiting the ¼ wavelength plate11is, in other words, linearly polarized light of S polarized light.

Since the light having been reflected by the light reflecting sections24and then passed through the ¼ wavelength plate11is S polarized light, it can pass through the polarized-light separating element12. The S polarized light having passed through the polarized-light separating element12is emitted from the emission surface of the optical sheet25to strike the liquid crystal panel3.

As the result of the process described above, only S polarized light is emitted from the emission surface of the optical sheet25(that is, the surface facing the liquid crystal panel3). Further, P polarized light having been reflected by the polarized-light separating element12to travel back toward the light reflecting sections24is converted into S polarized light by the polarized light converting elements (namely, the ¼ wavelength plate11and the light reflecting sections24) to be emitted from the optical sheet25. The optical sheet25consequently makes it possible to (i) highly efficiently utilize light that is incident from the light-emitting section4on the optical sheet25and to (ii) emit only S polarized light at an angle close to 90 degrees with respect to the emission surface.

The present embodiment stacks up all of the prism array26, the polarized-light separating element12, and the polarized light converting elements (namely, the ¼ wavelength plate11and the light reflecting sections24) to integrally form an optical sheet25. This arrangement eliminates the need for alignment of the individual optical members, and facilitates assembly of the liquid crystal display device20, thereby reducing the cost of producing the liquid crystal display device20. Further, the above arrangement, which allows the individual optical members to integrally form an optical sheet25, allows production of an optical sheet25that is large-sized for use in a large screen yet reduced in thickness. This in turn allows production of a thin liquid crystal display device20.

Further, the present embodiment is arranged such that the individual optical members (namely, the polarized-light separating element12and the ¼ wavelength plate11) are in contact with each other or that the individual optical members are separated from each other by a material having a refractive index equivalent or close to those of the individual optical members. This arrangement allows the optical sheet25to cause only a small refractive index change inside itself as compared at least to a conventional technique involving different optical members separated from each other by air or the like. With the above arrangement, polarized light reflected by the polarized-light separating element12(P polarized light) is converted through the optical sheet25, which causes only a small refractive index change, into light that is polarized so as to be able to pass through the polarized-light separating element12. The above arrangement can thus almost completely eliminate a light loss caused by Fresnel reflection at the interface between the individual optical members, thereby improving efficiency for light utilization.

The efficiency for light utilization was calculated of the optical sheet25of the present embodiment with use of an optical simulator.FIG. 7is a diagram illustrating a cross section of the optical sheet25and light intensities at different positions.

The simulation was run under the following conditions: Light was emitted from the outside (that is, from the light-emitting section4) to strike the first prisms13of the optical sheet25at an angle that was parallel to the x-z plane and that was 8 degrees with respect to the emission surface of the optical sheet25(horizontal direction). The light emitted had a wavelength of 550 nm. The first prisms13each had a cross section in the shape of an isosceles triangle of which the vertex angle was 80 degrees. The light reflecting sections24each had a reflecting surface parallel to the emission surface of the optical sheet25. The polarized-light separating element12and the ¼ wavelength plate11each had its optical axis extending in the direction illustrated inFIG. 6. Specifically, the polarized-light separating element12had a polarized-light transmission axis extending in a direction perpendicular to the x direction. The ¼ wavelength plate11had a slow axis extending in a direction angled at 45 degrees with respect to the x direction. The optical sheet25(specifically, the prism array26) had a refractive index of 1.5, whereas outside air had a refractive index of 1. The light used in the simulation had a single wavelength. The simulation was run under an ideal condition: The light reflecting sections24each had a reflectance of 100%. The polarized-light separating element12would cause an absorption loss of 0%. The polarized-light separating element12had a transmittance of 100% for light polarized in a direction parallel to the polarized-light transmission axis, and had a reflectance of 100% for light polarized in a direction perpendicular to the polarized-light transmission axis.

The description below assumes that light reaching the optical sheet25from the outside has an intensity of 1. First, light striking each first prism13from the outside is partly reflected by a surface of the first prism13(that is, the first surface13a) as a result of Fresnel reflection. The light having been refracted at the surface and entered the first prism13has an intensity of 0.958. The light then passes through the optical sheet25to reach the polarized-light separating element12. Then, half of the light (with an intensity of 0.479) passes through the polarized-light separating element12in the form of S polarized light to be emitted from the emission surface of the optical sheet25. The remaining half of the light (with an intensity of 0.479) is P polarized light, and is thus reflected by the polarized-light separating element12to travel back toward the ¼ wavelength plate11.

The P polarized light reflected by the polarized-light separating element12passes through the ¼ wavelength plate11, and is then reflected (by the light reflecting sections24) to change its polarization state. Most of the P polarized light is converted into S polarized light. The light reaching the polarized-light separating element12again includes S polarized light having an intensity of 0.4789 and P polarized light having an intensity of 0.0001. The light having the intensity of 0.4789 (S polarized light) passes through the polarized-light separating element12.

The component exiting the optical sheet25when the light first reaches the polarized-light separating element12has an intensity of 0.479, whereas the component exiting the optical sheet25when the light subsequently reaches the polarized-light separating element12has an intensity of 0.4789, the sum of the two intensities being 0.9579. This is a value with no consideration into Fresnel reflection of light from the polarized-light separating element12toward the outside (air). The light emitted from the optical sheet25thus has an actual intensity of 0.9163. The optical sheet25, therefore, makes it possible to utilize approximately 91.6% of emitted light in the form of linearly polarized light that is polarized in a predetermined direction.

The optical sheet25of the present embodiment makes it possible to utilize a light component having an intensity of 0.916 for display. This means that the liquid crystal display device20of the present embodiment has an improved light utilization efficiency that is 1.83 times that of a liquid crystal display device including a conventional backlight (efficiency of 0.5). The liquid crystal display device20of the present embodiment, in other words, has a reduced backlight power consumption that is 0.546 times that of a liquid crystal display device including a conventional backlight.

Variations of Embodiments 1 and 2 of the present invention are described below. For convenience of explanation, members and arrangements of these variations that are identical in function to those described for Embodiment 1 with reference to drawings are each assigned a common reference numeral, and are not described here in detail.

FIG. 8is an enlarged cross-sectional view of a portion of an optical sheet27.FIG. 8further illustrates respective optical axes of the individual layers.FIG. 8shows arrows to indicate an example optical path of incident light.

The optical sheet27differs from the optical sheet5of Embodiment 1 in that (i) its polarized-light separating element12has a polarized-light transmission axis which is different in direction from that of the polarized-light separating element12of the optical sheet5and that (ii) its ¼ wavelength plate11has a slow axis which is different in direction from that of the ¼ wavelength plate11of the optical sheet5. Except for these points, the optical sheet27is identical in arrangement to the optical sheet5of Embodiment 1.

The optical sheet27is arranged such that (i) the polarized-light separating element12has a polarized-light transmission axis extending in the x direction, that (ii) the ¼ wavelength plate11has a slow axis extending in a direction angled at 45 degrees with respect to the x direction, that (iii) the phase-difference plate10has a slow axis extending in the y direction, and that (iv) the individual prisms extend in the y direction, that is, the prism array9has a shape that is uniform along the y direction.

The optical sheet27is arranged such that the polarized-light separating element12transmits P polarized light. It is thus P polarized light that is emitted from an emission surface of the optical sheet27. S polarized light reflected by the polarized-light separating element12passes through the ¼ wavelength plate11to become circularly polarized light. The circularly polarized light then passes through the phase-difference plate10and is totally reflected by the second prisms14to become circularly polarized light having an opposite rotation. This circularly polarized light passes through the ¼ wavelength plate11again to be converted into P polarized light. The P polarized light exiting the ¼ wavelength plate11then passes through the polarized-light separating element12to be emitted from the emission surface of the optical sheet27.

FIG. 9is an enlarged cross-sectional view of a portion of an optical sheet28.FIG. 9further illustrates respective optical axes of the individual layers.FIG. 9shows arrows to indicate an example optical path of incident light.

The optical sheet28differs from the optical sheet5of Embodiment 1 in that (i) its polarized-light separating element12has a polarized-light transmission axis which is different in direction from that of the polarized-light separating element12of the optical sheet5and that (ii) its ¼ wavelength plate11has a slow axis which is different in direction from that of the ¼ wavelength plate11of the optical sheet5. Except for these points, the optical sheet28is identical in arrangement to the optical sheet5of Embodiment 1.

The optical sheet28is arranged such that (i) the polarized-light separating element12has a polarized-light transmission axis extending in a direction angled at 45 degrees with respect to the x direction, that (ii) the ¼ wavelength plate11has a slow axis extending in a direction angled at 90 degrees with respect to the x direction, that (iii) the phase-difference plate10has a slow axis extending in the y direction, and that (iv) the individual prisms extend in the y direction, that is, the prism array9has a shape that is uniform along the y direction.

The optical sheet28is arranged such that the polarized-light separating element12transmits linearly polarized light (herein referred to as “first linearly polarized light”) having a polarization direction angled at 45 degrees in a left-handed rotation from the x direction as viewed from a front surface side (user side) of the optical sheet28. It is thus first linearly polarized light that is emitted from an emission surface of the optical sheet28. Linearly polarized light (herein referred to as “second linearly polarized light”) having a polarization direction perpendicular to that of the first linearly polarized light is reflected by the polarized-light separating element12. There is an angle of 45 degrees between the polarization direction of the second linearly polarized light and the slow axis of the ¼ wavelength plate11. The second linearly polarized light reflected by the polarized-light separating element12thus passes through the ¼ wavelength plate11to become circularly polarized light. The circularly polarized light then passes through the phase-difference plate10and is totally reflected by the second prisms14to become circularly polarized light having an opposite rotation. This circularly polarized light passes through the ¼ wavelength plate11again to be converted into first linearly polarized light. The first linearly polarized light exiting the ¼ wavelength plate11passes through the polarized-light separating element12to be emitted from the emission surface of the optical sheet28.

FIG. 10is an enlarged cross-sectional view of a portion of an optical sheet29.FIG. 10further illustrates respective optical axes of the individual layers.FIG. 10shows arrows to indicate an example optical path of incident light.

The optical sheet29differs from the optical sheet5of Embodiment 1 in that (i) its polarized-light separating element12has a polarized-light transmission axis which is different in direction from that of the polarized-light separating element12of the optical sheet5and that (ii) its ¼ wavelength plate11has a slow axis which is different in direction from that of the ¼ wavelength plate11of the optical sheet5. Except for these points, the optical sheet29is identical in arrangement to the optical sheet5of Embodiment 1.

The optical sheet29is arranged such that (i) the polarized-light separating element12has a polarized-light transmission axis angled at 10 degrees in a right-handed rotation from the x direction as viewed from a front surface side of the optical sheet29, that (ii) the ¼ wavelength plate11has a slow axis angled at 35 degrees in a left-handed rotation from the x direction as viewed from the front surface side of the optical sheet29, that (iii) there is an angle of 45 degrees between the polarized-light transmission axis of the polarized-light separating element12and the slow axis of the ¼ wavelength plate11, that (iv) the slow axis of the phase-difference plate10extends along the y direction, and that (v) the individual prisms extend in the y direction, that is, the prism array9has a shape that is uniform along the y direction.

The optical sheet29is arranged such that the polarized-light separating element12transmits linearly polarized light (herein referred to as “third linearly polarized light”) having a polarization direction angled at 10 degrees in a right-handed rotation from the x direction as viewed from the front surface side of the optical sheet29. It is thus third linearly polarized light that is emitted from an emission surface of the optical sheet29. Linearly polarized light (herein referred to as “fourth linearly polarized light”) having a polarization direction perpendicular to that of the third linearly polarized light is reflected by the polarized-light separating element12. There is an angle of 45 degrees between the polarization direction of the fourth linearly polarized light and the slow axis of the ¼ wavelength plate11. The second linearly polarized light reflected by the polarized-light separating element12thus passes through the ¼ wavelength plate11to become circularly polarized light. The circularly polarized light then passes through the phase-difference plate10and is totally reflected by the second prisms14to become circularly polarized light having an opposite rotation. This circularly polarized light passes through the ¼ wavelength plate11again to be converted into third linearly polarized light. The third linearly polarized light exiting the ¼ wavelength plate11passes through the polarized-light separating element12to be emitted from the emission surface of the optical sheet29.

FIG. 11shows diagrams illustrating the angle of an optical axis of each optical member of the optical sheets5,27,28, and29as viewed from the front surface side thereof. Specifically, (a) ofFIG. 11is a diagram illustrating the angle of an optical axis of each optical member of the optical sheet5illustrated inFIG. 2. (b) ofFIG. 11is a diagram illustrating the angle of an optical axis of each optical member of the optical sheet27illustrated inFIG. 8. (c) ofFIG. 11is a diagram illustrating the angle of an optical axis of each optical member of the optical sheet28illustrated inFIG. 9. (d) ofFIG. 11is a diagram illustrating the angle of an optical axis of each optical member of the optical sheet29illustrated inFIG. 10. The diagrams ofFIG. 11each indicate (i) the direction in which the prisms have a uniform shape (that is, the direction in which the prisms extend) and (ii) the direction in which light enters the optical sheet.

The optical sheets5,27,28, and29are each arranged such that (1) there is an angle of 45 degrees between the polarized-light transmission axis of the polarized-light separating element12and the slow axis of the ¼ wavelength plate11and that (2) the direction in which the prisms have a uniform shape is parallel to the slow axis of the phase-difference plate10. Satisfying these two conditions increases efficiency for light utilization.

Specifically, in a case where there is an angle of 45 degrees between the polarized-light transmission axis of the polarized-light separating element12and the slow axis of the ¼ wavelength plate11, polarized light reflected by the polarized-light separating element12is converted by the ¼ wavelength plate11into circularly polarized light. In a case where there is no angle between the direction in which the prisms have a uniform shape and the slow axis of the phase-difference plate10(that is, the direction and the slow axis are parallel to each other), circularly polarized light incident on the phase-difference plate10passes through the phase-difference plate10and then undergoes total reflection to be converted into circularly polarized light having an opposite rotation and return to the phase-difference plate10. The circularly polarized light having an opposite rotation passes through the ¼ wavelength plate11to be converted into linearly polarized light having a polarization direction identical to the polarized-light transmission axis of the polarized-light separating element12. Through this process, polarized light reflected by the polarized-light separating element12is converted into polarized light having a polarization direction perpendicular to that of the polarized light before the conversion, and is incident on the polarized-light separating element12again. Almost all of the returning light passes through the polarized-light separating element12.

In brief, very high efficiency for light utilization is achievable by arranging the optical sheet such that (i) the polarized-light transmission axis of the polarized-light separating element12is parallel to the transmission axis of the back surface polarizer of the liquid crystal panel and that (ii) the ¼ wavelength plate11and the phase-difference plate10are provided to satisfy the conditions (1) and (2) above.

In a case where the second prisms are replaced by light reflecting sections24as in the optical sheet25of Embodiment 2, very high efficiency for light utilization is achievable by arranging the optical sheet such that (i) the polarized-light transmission axis of the polarized-light separating element12is parallel to the transmission axis of the back surface polarizer of the liquid crystal panel and that (ii) the ¼ wavelength plate11is provided to satisfy the condition (1) above. The condition (2) above intends to produce the same effect as that produced by the light reflecting sections24reflecting light to impart a phase difference of λ/2 to the light.

(Angle-Dependent Characteristic of Rate of Reuse of Polarized Light)

Embodiment 1 uses the phase-difference plate10to correct a phase shift caused by total reflection. However, in a case where light does not travel in a direction perpendicular to the direction in which the prisms have a uniform shape (that is, the direction in which the prisms extend), in other words, in a case where light does not travel in a direction parallel to the x-z plane, the light will unfortunately be incident on the second prisms14at an unintended angle, and the plane of incidence (that is, the plane on which the path of incident light and the path of reflected light are present) will not coincide with the x-z plane. As illustrated inFIG. 3, a change in the angle of incidence on the second prisms14will unfortunately change the phase difference that the second prisms14cause. Further, a shift of the plane of incidence from the x-z plane will also change the phase difference to be caused between (i) polarized light having a polarization direction along the y direction and (ii) polarized light having a polarization direction perpendicular to the y direction.

FIG. 12is a graph showing a rate of reuse of polarized light with respect to different directions in which light travels through the optical sheet5illustrated inFIG. 4. The “DIRECTION ANGLE” refers to the angle of a light traveling direction with respect to the x direction as viewed from the front surface side of the optical sheet5(that is, on the x-y plane). The rate of reuse of polarized light refers to that proportion of polarized light reflected by the polarized-light separating element12which is converted into polarized light that the polarized-light separating element12transmits and which returns to the polarized-light separating element12.FIG. 12indicates that a larger direction angle results in a lower rate of reuse of polarized light.

Thus, in Embodiment 1, which uses total reflection, light emitted from the light-emitting section4toward the optical sheet5preferably travels in a direction having small angular distribution (that is, at a small angle with respect to the x-z plane). Embodiment 1 preferably includes a light source having low direction angle distribution (that is, having high directivity on the x-y plane).

Embodiment 2, on the other hand, includes the light reflecting sections24(flat surfaces) for reflection of circularly polarized light to reverse its rotation direction. Since the light reflecting sections24each include a metal film for reflection, even in a case where a change in direction angle has changed the angle of incidence, light does not undergo a large phase shift when reflected by the light reflecting sections24. Thus, in Embodiment 2, even a large direction angle does not result in a large decrease in the rate of reuse of polarized light.

Therefore, the optical sheet25of Embodiment 2 may simply be used in a case where the liquid crystal display device includes a light-emitting section that emits light having high direction angle distribution, whereas the optical sheet5of Embodiment 1 may simply be used in a case where the liquid crystal display device includes a light-emitting section that emits light having low direction angle distribution.

FIG. 13is a cross-sectional view illustrating a configuration of a backlight30as a variation of the backlight2of Embodiment 1. The backlight30includes the optical sheet5and a light-emitting section31. The light-emitting section31includes a light guide plate32, two light sources6provided respectively at opposite ends of the light guide plate32, and two reflectors7provided respectively at the opposite ends. The light guide plate32receives light emitted by the two light sources at the opposite ends, which causes the light guide plate32to emit light in two opposite directions (left and right).

As illustrated inFIG. 13, the first prisms13are each in the shape of an isosceles triangle. The optical sheet5thus has a similar optical characteristic for light incident on either of the two inclined surfaces (that is, the first surface13aand the second surface13b) of each first prism13. The backlight30is, therefore, capable of (i) converting light of both the light sources6into linearly polarized light efficiently and (ii) emitting the linearly polarized light toward a front surface side of the backlight30.

FIG. 14is a cross-sectional view illustrating a configuration of a backlight33as a variation of the backlight22of Embodiment 2. The backlight33includes the optical sheet25and a light-emitting section31.

The optical sheet25, as well as the optical sheet5, has a similar optical characteristic for light incident on either of the two inclined surfaces (that is, the first surface13aand the second surface13b) of each first prism13. The backlight33is, therefore, capable of (i) converting light of both the light sources6into linearly polarized light efficiently and (ii) emitting the linearly polarized light toward a front surface side of the backlight33.

Another embodiment of the present invention is described below. For convenience of explanation, members and arrangements of the present embodiment that are identical in function to those described for Embodiment 1 with reference to drawings are each assigned a common reference numeral, and are not described here in detail. The present embodiment includes a backlight that is particularly suitably used in a compact portable terminal such as a smart phone and a mobile telephone, each of which includes a compact liquid crystal display device.

FIG. 15is a perspective view illustrating a configuration of a backlight40of the present embodiment.FIG. 16shows plan views indicative of optical axes and the like of individual optical members placed on top of each other as viewed from a front surface side of the optical members.

The backlight40includes a light-emitting section41and an optical sheet42provided on a front surface side of the light-emitting section41. The light-emitting section41includes a light source43and a light guide plate44.

The light source43is a point light source provided at a corner of the substantially rectangular light guide plate44. The light source43may be, for example, an LED or a compact organic EL device. The light source43emits light, which enters the light guide plate44and travels therethrough while spreading radially.

The light guide plate44has a cross section in a shape tapering from (i) the corner at which the light source43is provided to (ii) the opposite end thereof, the thickness of the light guide plate44being larger at the entry end and smaller at the opposite end. The light emitted from the light guide plate44travels at an angle substantially parallel to the emission surface of the light guide plate44.

The optical sheet42receives unpolarized light emitted from the light guide plate44and incident on a back surface of the optical sheet42, changes the unpolarized light into linearly polarized light, and emits the linearly polarized light from a front surface (emission surface) thereof. The optical sheet42includes a prism array45, a phase-difference plate46, a ¼ wavelength plate47, and a polarized-light separating element48in that order from a back surface side of the optical sheet42to a front surface side thereof (that is, from a light entry side to a light emission side).

The prism array45is provided on the back surface side of the optical sheet42, and includes a plurality of first prisms13and a plurality of second prisms (light reflecting sections)14. The first prisms13each have a vertex angle (that is, the angle of its vertex) smaller than that of each second prism14. The first prisms13and the second prisms14have their respective vertices (edge lines) arranged concentrically with the position of the light source43as the center. Light having entered the optical sheet42travels in directions spreading radially from the light source43. Thus, with each prism in a concentric shape, light strikes each first prism13invariably in a direction orthogonal to the first surface13aof the first prism13(as viewed from a front surface side of the first prism13). The first prisms13and the second prisms14are provided alternately along the radial direction.

As illustrated inFIG. 16, the phase-difference plate46has a slow axis extending concentrically along the circumferential direction with the position of the light source as the center. This is because the slow axis of the phase-difference plate46is preferably parallel to the direction in which the second prisms14extend (that is, the circumferential direction). Such a phase-difference plate having a concentrically extending slow axis may be produced by, for example, combining a plurality of sectors of phase-difference plates arranged in the circumferential direction.

FIG. 17is a plan view of an example phase-difference plate49having a slow axis extending concentrically in a simulated manner. This phase-difference plate49includes a combination of three phase-difference plate parts49ato49ceach in a sector shape (triangle shape). The phase-difference plate parts49ato49ceach have a slow axis extending in a single direction.FIG. 17illustrates an example including three phase difference parts to form a phase-difference plate49having a concentrically extending slow axis. Using more phase difference parts makes it possible to form a phase-difference plate46having a slow axis extending more concentrically.

The ¼ wavelength plate47has a slow axis angled at 45 degrees with respect to the polarized-light transmission axis of the polarized-light separating element48.

The prism array45, the phase-difference plate46, the ¼ wavelength plate47, and the polarized-light separating element48are integrated with each other to form the optical sheet42.

The second prisms14of the prism array45of the present embodiment each have a reflecting surface perpendicular to the plane of incidence of light emitted by the point light source (that is, the plane on which the path of incident light and the path of reflected light are present). With this arrangement, the optical sheet42causes linearly polarized light reflected by the polarized-light separating element48to be efficiently converted with the use of the ¼ wavelength plate47, the phase-difference plate46, and the second prisms14into linearly polarized light having a polarization direction orthogonal to that of the linearly polarized light at the time of the reflection by the polarized-light separating element48. This means that the optical sheet42allows efficient reuse of polarized light.

Further, the backlight40, which includes a point light source (that is, the light source43), can be downsized easily.

The first prisms13and the light reflecting section24of the prism array26of Embodiment 2 may alternatively be arranged to extend concentrically as in the present embodiment to form an optical sheet.

Another embodiment of the present invention is described below. For convenience of explanation, members and arrangements of the present embodiment that are identical in function to those described for Embodiment 1 with reference to drawings are each assigned a common reference numeral, and are not described here in detail. The present embodiment differs from Embodiment 1 in how the optical sheet is arranged.

FIG. 18is a cross-sectional view illustrating a configuration of a liquid crystal display device50of the present embodiment.FIG. 18shows arrows to indicate an example optical path of light from a light source6. The liquid crystal display device50includes a backlight52and a liquid crystal panel3provided on a front surface side of the backlight52(that is, in the direction of a user). The description below uses (i) the term “x direction” to refer to a direction that extends from one end of the liquid crystal display device50at which end the light source6is provided to the other end, (ii) the term “z direction” to refer to a direction that extends from a back surface of the liquid crystal display device50to a front surface thereof, and (iii) the term “y direction” to refer to a direction perpendicular to the x direction and the z direction.

The back surface polarizer15of the liquid crystal panel3has a polarized-light transmission axis that coincides with the polarized-light transmission axis of the polarized-light separating element12. In the present embodiment, the polarized-light transmission axis of the liquid crystal panel3of the back surface polarizer15is angled at 45 degrees with respect to x direction.

The backlight52includes a light-emitting section4and an optical sheet (optical film)55provided on a front surface side of the light-emitting section4. The light-emitting section4includes a light source6, a reflector7, and a light guide plate8.

(Arrangement of Optical Sheet)

The optical sheet55receives unpolarized light emitted from the light guide plate8and incident on a back surface of the optical sheet55, changes the unpolarized light into linearly polarized light, and emits the linearly polarized light from a front surface (emission surface) thereof toward the liquid crystal panel3. The optical sheet55includes a prism array9, a phase-difference plate56, and a polarized-light separating element12in that order from a back surface side of the optical sheet55to a front surface side thereof (that is, from a light entry side to a light emission side).

FIG. 19is an enlarged cross-sectional view of a portion of the optical sheet55.FIG. 19further illustrates respective optical axes of the individual layers.FIG. 19shows arrows to indicate an example optical path of light emitted from the light guide plate8and incident on the optical sheet55.FIG. 20is a diagram illustrating the angle of an optical axis of each optical member of the optical sheet55as viewed from the front surface side thereof.

The optical sheet55includes (i) a prism array9on its back surface side, (ii) a phase-difference plate (first phase-difference plate)56on a front surface side of the prism array9, and (iii) a polarized-light separating element12on a front surface side of the phase-difference plate56. The polarized-light separating element12is similar in arrangement to that of Embodiment 1. The polarized-light separating element12of the present embodiment has a polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12transmits) angled at 45 degrees with respect to the x direction.

The prism array9includes a plurality of first prisms13and a plurality of second prisms14. The first prisms13and the second prisms14are provided alternately along the direction (x direction) extending away from the light source6. The prism array9has a shape that is uniform along the direction (y direction) in which the first prisms13extend. The first prisms13and the second prisms14are similar in arrangement to those of Embodiment 1.

The phase-difference plate56imparts a predetermined phase difference to light having a wavelength λ and passing through the phase-difference plate56in its thickness direction. A later description will deal in detail with the size of the phase difference that the phase-difference plate56imparts. The phase-difference plate56has a slow axis along the y direction.

The polarized-light separating element12transmits only light polarized in a certain direction, and reflects light polarized in a direction perpendicular to that certain direction. The polarized-light separating element12has a polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12transmits) angled at 45 degrees with respect to the x direction.

The prism array9, the phase-difference plate56, and the polarized-light separating element12are integrated with each other to form the optical sheet5. The phase-difference plate56and the polarized-light separating element12may be in contact with each other or separated from each other. In a case where the phase-difference plate56and the polarized-light separating element12are separated from each other, the phase-difference plate56and the polarized-light separating element12are preferably separated from each other by a space filled with either a material of the prism array9or a material (adhesive) having a refractive index close to those of materials of the individual layers. This arrangement can reduce reflectance at the interface between the individual layers (namely, the phase-difference plate56and the polarized-light separating element12), with the result of increased efficiency for light utilization. The above arrangement can reduce reflectance at the interface of the polarized-light separating element12for only light polarized in a direction parallel to the polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12transmits): The polarized-light separating element12has high reflectance at the interface for light polarized in a direction perpendicular to the polarized-light transmission axis (that is, the direction of polarization of light that the polarized-light separating element12does not transmit).

With reference toFIGS. 18 and 19, the description below deals with how light having entered the optical sheet55behaves.FIG. 19illustrates polarization states at different positions.

Light is emitted from the emission surface of the light guide plate8in an unpolarized state at an angle close to the x direction. The light emitted from the emission surface of the light guide plate8thus strikes the first surface13aof each first prism13, which protrudes toward the light guide plate8.

The light incident on the first surface13aof each first prism13is refracted by the first surface13aand is then reflected (total reflection) by the second surface13b. This arrangement causes light having entered the optical sheet55to change its direction to a direction close to the direction (z direction) perpendicular to the emission surface of the optical sheet55. The present embodiment adjusts, for example, the direction in which light is emitted from the light guide plate8and the angle of each first prism13so that light reflected by the second surface13btravels in a direction that is not exactly parallel to the z direction but is slightly inclined from the z direction toward the x direction. If light reflected by the second surface13btravels in a direction exactly parallel to the z direction, polarized light reflected by the polarized-light separating element12will unfortunately travel back along the same optical path to be emitted from the first surface13a.

The light reflected by the second surface13bpasses through the phase-difference plate56. The light reflected by the second surface13bis in an unpolarized state. The polarization state of that light is thus not changed by the phase-difference plate56, with the result that the light remains unpolarized even after passing through the phase-difference plate56.

The light having passed through the phase-difference plate56and then reached the polarized-light separating element12is separated into two portions: polarized light passing through the polarized-light separating element12and polarized light reflected by the polarized-light separating element12. The polarized-light separating element12has a polarized-light transmission axis angled at 45 degrees with respect to the x direction. The present embodiment uses (i) the term “u direction” to refer to the direction in which the polarized-light transmission axis of the polarized-light separating element12extends and (ii) the term “v direction” to refer to the direction perpendicular to the u direction on the x-y plane. The u direction is angled at 45 degrees with respect to the x direction, whereas the v direction is angled at 45 degrees with respect to the x direction toward an opposite direction. The polarized-light separating element12transmits light polarized in a direction parallel to the u direction, and reflects light polarized in a direction perpendicular to the u direction (that is, parallel to the v direction). The light reflected by the polarized-light separating element12is thus light polarized in a direction parallel to the v direction (hereinafter referred to as “v-direction polarized light”). The polarized-light separating element12reflects v-direction polarized light so that the v-direction polarized light travels back toward the third surface14aof each second prism14. The polarized light having passed through the polarized-light separating element12and having a polarization direction parallel to the u direction is emitted from the emission surface of the optical sheet55toward the liquid crystal panel3.

Next, the light reflected by the polarized-light separating element12(v-direction polarized light) passes through the phase-difference plate56. The light is then totally reflected by the third surface14aof each second prism14, and is thereafter totally reflected by the fourth surface14bto subsequently pass through the phase-difference plate56again. This means that light reflected by the polarized-light separating element12is subjected to (i) two phase difference changes caused by the phase-difference plate56and (ii) two other phase difference changes caused by total reflection, before returning to the polarized-light separating element12. The phase-difference plate56is arranged to impart a phase difference such that the linearly polarized light (v-direction polarized light) reflected by the polarized-light separating element12returns to the polarized-light separating element12as linearly polarized light having a polarization direction parallel to the u direction, which is perpendicular to the v direction.

With the above arrangement, the light reflected by the polarized-light separating element12is polarized in the v direction with respect to the direction in which the light travels from the polarized-light separating element12to the phase-difference plate56. The light is totally reflected by the second prisms14to travel back toward the polarized-light separating element12in a reversed direction. This means that in order to become linearly polarized light having a polarization direction parallel to the u direction, a phase difference of mλ (where m is an integer) between an x component and a y component needs to be imparted to the light returning to the polarized-light separating element12. The phase-difference plate56and the second prisms14thus impart, to the v-direction polarized light reflected by the polarized-light separating element12, a total phase difference of λ between the x component and the y component (that is, a phase shift amount of 2π or a phase shift amount of 0) before the v-direction polarized light returns to the polarized-light separating element12. The present embodiment includes a phase-difference plate56having a phase difference and slow axis that cause a phase delay of 2π in the y component relative to the x component. The phase-difference plate56may, as described above, simply have a phase difference so selected in consideration of a phase shift by total reflection that when the v-direction polarized light has been divided into P polarized light and S polarized light, the S polarized light has a phase delay of 2π relative to the P polarized light.

In a case where, for instance, a single instance of total reflection causes a phase difference of approximately 44.55 degrees (between P polarized light and S polarized light) as in Embodiment 1, using a phase-difference plate56having a phase difference of approximately 3λ/8 (135 degrees) allows most of the v-direction polarized light reflected by the polarized-light separating element12to be converted into linearly polarized light having a polarization direction parallel to the u direction.

With the above arrangement, the light is totally reflected twice by the second prisms14and then passes through the phase-difference plate56toward the polarized-light separating element12to be converted into linearly polarized light having a polarization direction parallel to the u direction.

Since the light having been reflected by the second prisms14and then passed through the phase-difference plate56is linearly polarized light having a polarization direction parallel to the u direction, the light can pass through the polarized-light separating element12. The linearly polarized light having passed through the polarized-light separating element12and having a polarization direction parallel to the u direction is emitted from the emission surface of the optical sheet55to strike the liquid crystal panel3.

As the result of the process described above, only linearly polarized light having a polarization direction parallel to the u direction is emitted from the emission surface of the optical sheet55(that is, the surface facing the liquid crystal panel3). Further, v-direction polarized light having been reflected by the polarized-light separating element12to travel back toward the second prisms14is converted into linearly polarized light having a polarization direction parallel to the u direction by the polarized light converting elements (namely, the phase-difference plate56and the second prisms14) to be emitted from the optical sheet55. The optical sheet55consequently makes it possible to (i) highly efficiently utilize light that is incident from the light-emitting section4on the optical sheet55and to (ii) emit only linearly polarized light (that is, linearly polarized light having a polarization direction parallel to the u direction) at an angle close to 90 degrees with respect to the emission surface.

The present embodiment stacks up all of the prism array9, the polarized-light separating element12, and the polarized light converting element (namely, the phase-difference plate56) to integrally form an optical sheet55. This arrangement eliminates the need for alignment of the individual optical members, and facilitates assembly of the liquid crystal display device50, thereby reducing the cost of producing the liquid crystal display device50. Further, the above arrangement, which allows the individual optical members to integrally form an optical sheet55, allows production of an optical sheet55that is large-sized for use in a large screen yet reduced in thickness. This in turn allows production of a thin liquid crystal display device50.

Further, the present embodiment is arranged such that the individual optical members (namely, the polarized-light separating element12and the phase-difference plate56) are in contact with each other or that the individual optical members are separated from each other by a material having a refractive index equivalent or close to those of the individual optical members. This arrangement allows the optical sheet55to cause only a small refractive index change inside itself as compared at least to a conventional technique involving different optical members separated from each other by air or the like. With the above arrangement, polarized light reflected by the polarized-light separating element12(v-direction polarized light) is converted through the optical sheet55, which causes only a small refractive index change, into light that is polarized so as to be able to pass through the polarized-light separating element12. The above arrangement can thus almost completely eliminate a light loss caused by Fresnel reflection at the interface between the individual optical members. Further, the present embodiment, as compared to Embodiment 1, does not need to include a ¼ wavelength plate. This can further reduce a loss caused by Fresnel reflection inside the optical sheet55, thereby improving efficiency for light utilization. The above arrangement, in addition, further facilitates production of the optical sheet55.

In a case where the polarized-light separating element has a transmission axis that is not angled at 45 degrees with respect to the direction in which the second prisms extend, the optical sheet55may include a ½ wavelength plate between the polarized-light separating element and the phase-difference plate so that light (linearly polarized light) having been reflected by the polarized-light separating element and passed through the ½ wavelength plate has a polarization direction angled at 45 degrees with respect to the direction in which the second prisms extend.

An optical film of one mode of the present invention is an optical film including, in sequence from a light entry side of the optical film to a light emission side of the optical film: a plurality of first prisms; a first phase-difference plate; and a polarized-light separating element, the plurality of first prisms each having (i) a first surface through which light enters the first prism and (ii) a second surface that reflects the light, having entered the first prism through the first surface, toward the light emission side, the optical film further including, between the plurality of first prisms in an in-plane direction of the optical film, a light reflecting section that reflects light, having been reflected by the polarized-light separating element toward the light entry side, back toward the light emission side.

The above arrangement causes polarized light reflected by the polarized-light separating element to be converted by the first phase-difference plate and the light reflecting section, both included in the optical film, into polarized light that is capable of passing through the polarized-light separating element. With the above arrangement, the optical film allows light having entered the optical film in an unpolarized state to be converted efficiently into polarized light that is capable of passing through the polarized-light separating element, and allows such polarized light to be emitted. Further, the polarized-light separating element and the first phase-difference plate are included in the optical film. This can reduce the difference in refractive index at the respective boundary surfaces of the polarized-light separating element and the first phase-difference plate, and can thus reduce Fresnel reflectance at the respective boundary surfaces of the polarized-light separating element and the first phase-difference plate, with the result of reduction in a light loss caused by Fresnel reflection. In addition, the polarized-light separating element, the first phase-difference plate, and the light reflecting section are included in the optical film. This allows production of a thin optical film that allows light in an unpolarized state to be converted into predetermined polarized light and that allows such polarized light to be emitted. Furthermore, since the polarized-light separating element, the first phase-difference plate, and the light reflecting section are included in the optical film, there is no need for, for example, alignment of the individual optical members. Also, the polarized-light separating element, the first phase-difference plate, and the light reflecting section, which are included in the optical film, may be stacked up on top of each other for integral production. This facilitates assembly of the optical film to an optical product such as a liquid crystal display device, and can consequently reduce the cost of producing an optical product including the optical film.

The optical film may be arranged such that the first phase-difference plate is a ¼ wavelength plate.

The optical film may be arranged such that the light reflecting section is a second prism having a third surface and a fourth surface each of which reflects light; an angle between the third surface and the fourth surface is larger than an angle between the first surface and the second surface; and the optical film further includes a second phase-difference plate between the ¼ wavelength plate and the second prism.

The optical film may be arranged such that the second phase-difference plate imparts a phase difference of not greater than λ/4 and not less than λ/12 to light having a wavelength λ, where the wavelength λ is a representative wavelength of incident light. The optical film may be arranged such that the second phase-difference plate imparts a phase difference of not greater than λ/6 and not less than λ/12 to light having a wavelength λ, where the wavelength λ is a representative wavelength of incident light.

The optical film may be arranged such that the second phase-difference plate has a slow axis that is parallel to a direction in which the second prism extends.

The optical film may be arranged such that two passages through the second phase-difference plate, total reflection at the third surface, and total reflection at the fourth surface cause a phase shift in a total amount of π.

The optical film may be arranged such that the light reflecting section totally reflects the light, having been reflected by the polarized-light separating element toward the light entry side, at each of the third surface and the fourth surface toward the light emission side.

The optical film may be arranged such that the light reflecting section is a second prism having a third surface and a fourth surface each of which reflects light; and two passages through the first phase-difference plate, total reflection at the third surface, and total reflection at the fourth surface cause a phase shift in a total amount of 2π.

The optical film may be arranged such that the first phase-difference plate has a slow axis that is parallel to a direction in which the second prism extends; and the polarized-light separating element has a polarized-light transmission axis that is angled at 45 degrees with respect to the direction in which the second prism extends.

The optical film may be arranged such that the plurality of first prisms each have a cross section in a shape of an isosceles triangle along a direction in which the plurality of first prisms and the light reflecting section are arranged next to each other.

The optical film may be arranged such that the first surface and the second surface of each of the plurality of first prisms are symmetrical to each other with respect to a direction perpendicular to a surface of the polarized-light separating element.

The optical film may be arranged such that the light reflecting section has a reflecting surface that is parallel to the polarized-light separating element.

The optical film may be arranged such that the light reflecting section includes a metal film that reflects light.

The optical film may be arranged such that the plurality of first prisms and the light reflecting section are provided concentrically.

The optical film may further include a ½ wavelength plate between the first phase-difference plate and the polarized-light separating element, wherein: the light reflecting section is a second prism having a third surface and a fourth surface each of which reflects light; two passages through the first phase-difference plate, total reflection at the third surface, and total reflection at the fourth surface cause a phase shift in a total amount of 2π; and the polarized-light separating element and the ½ wavelength plate are disposed so that light having been reflected by the polarized-light separating element and passed through the ½ wavelength plate has a polarization direction angled at 45 degrees with respect to a direction in which the second prism extends.

A backlight of one mode of the present invention includes: the optical film; and a light-emitting section disposed on the light entry side of the optical film, the light-emitting section emitting light toward the first surface of each of the plurality of first prisms included in the optical film.

The backlight may be arranged such that the light-emitting section includes a light source and a light guide plate.

A liquid crystal display device of one mode of the present invention includes: the backlight; and a liquid crystal panel disposed on a light emission side of the backlight.

The present invention is not limited to the description of the embodiments above, but may be altered in various ways by a skilled person within the scope of the claims. Any embodiment based on a proper combination of technical means disclosed in different embodiments is also encompassed in the technical scope of the present invention.

INDUSTRIAL APPLICABILITY

The present invention is applicable to an optical sheet for use in a liquid crystal display device and to a backlight.

REFERENCE SIGNS LIST

1,20,50liquid crystal display device

3liquid crystal panel

8,32,44light guide plate

17liquid crystal layer

24light reflecting section