Backlight unit and liquid crystal display device

A backlight unit includes a prism sheet (an optical path changing portion and a light gathering portion) having lenses and being disposed to face an output surface of a light guide member, and a plurality of second prism inclined surfaces that are inclined at a predetermined inclination angle with respect to the output surface. Further, a lens surface that gathers light from the second prism inclined surfaces toward a normal line direction of the output surface according to the second prism inclined surfaces is provided, and a central position of the lens surface in a transmitting direction of the light guide member is matched with a central position of its corresponding second prism inclined surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backlight unit and a liquid crystal display device using the same.

2. Description of the Related Art

Liquid crystal display devices change optical anisotropy of a liquid crystal layer according to a voltage applied to the liquid crystal layer so as to change light transmittances thereof, and thereby displaying information such as character and image. Such liquid crystal display devices are mainly classified into three types: a transmission type; a reflection type; and a semi-transmission type, according to incident light for the display into the liquid crystal layer.

That is, in the liquid crystal display device of the transmission type, a backlight unit is disposed on a rear surface (non-display surface) side of a liquid crystal display element that is provided with the liquid crystal layer, and light from the backlight unit passes through the liquid crystal display element, whereby a user can visually recognize displayed information. Moreover, in the liquid crystal display device of the reflection type, incident light from a front surface is reflected by the liquid crystal display element, whereby a user can visually recognize displayed information.

Further, the liquid crystal display device of the semi-transmission type is designed to function similarly to the liquid crystal display device of the transmission type or the reflection type depending on an environment in which it is used. More specifically, the liquid crystal display device of the semi-transmission type displays by reflecting light from the outside in its environment in which the incident light from the outside is strong, similarly to the liquid crystal display device of the reflection type. Whereas, in an environment in which the incident light from the outside is weak, the backlight unit that is provided on the rear surface side of the liquid crystal display element is turned ON, so that the liquid crystal display device of the semi-transmission type display by using the light from the backlight unit, similarly to the liquid crystal display device of the transmission type. Further, some of the liquid crystal display devices of the semi-transmission type display in two modes including the transmission-type mode and the reflection-type mode at the same time, regardless of intensities of the incident light from the outside.

Moreover, a conventional liquid crystal display device, in which a backlight unit is provided with a prism sheet having a prism surface that is formed to have a sawtooth configuration, has been suggested (see, for example, JP 11(1999)-224058 A).

Here, the backlight unit provided in a first conventional example that is described in the above-described cited reference JP 11(1999)-224058 A will be explained specifically with reference toFIGS. 11A-11C.

As shown inFIG. 11A, a backlight unit100is provided with a planar light source device101that emits light with a flat shape (hereinafter, called “planar light”), and a prism sheet104and a reflecting plate105that are provided on an upper side and a lower side with respect to the planar light source device101in the figure, respectively, so that the planar light is incident into the liquid crystal display element (not illustrated) via the prism sheet104.

The planar light source device101is provided with a light source102that emits light, a reflecting member102athat is arranged so as to surround the light source102, and a light guide member103having a wedge-shaped cross section that allows the light emitted by the light source102to be input therein and leads the input light toward a predetermined transmitting direction (illustrated as the arrow S1inFIG. 11A). The reflecting member102areflects the light that is emitted by the light source102toward the light guide member103, thereby allowing the light from the light source102to be incident into the light guide member103efficiently.

By also referring toFIG. 11B, the light guide member103is faces an output surface103athat outputs the planar light toward the prism sheet104side, an inclined surface103bthat faces the reflecting plate105and is inclined by a predetermined inclination angle K1with respect to the output surface103a, and an input surface103cfrom which the light from the light source102is input. In this light guide member103, the light from the light source102that is input from the input surface103ctoward an inner side is repeatedly reflected by the output surface103aand the inclined surface103bor the reflecting plate105so as to be led toward the transmitting direction S1, and is output from the output surface103atoward the prism sheet104appropriately.

In the prism sheet104, a prism surface is arranged to face the output surface103aof the light guide member103and is formed to have a sawtooth configuration. This prism surface is provided with a first prism inclined surface104aand a second prism inclined surface104bthat are disposed alternately, and a ridge line104cis formed on a boundary between these prism inclined surfaces104aand104b(see alsoFIG. 11C). Moreover, in the prism sheet104, a vertical angle between the first and second prism inclined surfaces104aand104bis set as K2, as illustrated inFIG. 11C.

In the backlight unit100having the structure as described above, the light from the light source102is transmitted toward the transmitting direction S1inside the light guide member103. More specifically, when the light from the light source102that is input into the inside of the light guide member103is incident upon an interface between the output surface103aand air at an incident angle that is smaller than a predetermined incident angle (total reflection angle), a portion of the light is refracted by the above-described interface and is output toward the prism sheet104side, and the remaining light is reflected by the interface toward the inner side of the light guide member103, as shown by the arrows with the solid lines and the dotted lines inFIG. 11A. Moreover, the entire light incident upon the interface at an incident angle that is the total reflection angle or larger is reflected by the interface toward the inner side of the light guide member103.

Moreover, the light that is output from the interface toward the prism sheet104side is incident upon the prism surface of the prism sheet104, and is reflected by the second prism inclined surface104btoward an upper side so as to be output from the prism sheet104as incident light into the liquid crystal display element, as shown by the arrows with the solid lines and the dotted lines inFIG. 11C.

Whereas, the light reflected by the interface between the output surface103aand the air toward the inner side of the light guide member103travels toward the inclined surface103binside the light guide member103, as shown by the arrow with the dotted line inFIG. 11A. Then, the light refracted by the interface between the inclined surface103band the air is output toward an outside of the light guide member103, and is subsequently reflected by the reflecting plate105toward the light guide member103side. Further, this light passes through the inclined surface103bso as to be incident into the light guide member103again, and is subsequently output from the output surface103atoward the prism sheet104side so as to be output from the prism sheet104as the incident light, as shown by the arrows of the solid lines and the dotted lines inFIG. 11C.

As described above, in the backlight unit100in the first conventional example, the light that is output once from the inclined surface103bof the light guide member103to the outside is reflected by the reflecting plate105toward the light guide member103side, so that the light output to the outside can be used for the display, thereby increasing an efficiency of utilizing the light from the light source102. Moreover, in the backlight unit100, by interposing the prism sheet104, the light having high directivity with respect to the liquid crystal display element can be output.

Moreover, another conventional backlight unit, which is provided with a lens for gathering output light toward the liquid crystal display element at a position facing each of a plurality of prisms that are provided on a prism surface with a sawtooth configuration, is suggested (see, for example, JP 10(1998)-12024 A).

Here, the backlight unit of a second conventional example described in the above-described JP 10(1998)-12024 A will be described specifically with reference toFIGS. 12A-12C.

As shown inFIG. 12A, a backlight unit200is provided with a planar light source device201that emits planar light, and a prism sheet204that includes lenses for gathering light from the planar light source device201and outputting the light toward the outside, so that the backlight unit200allows the planar light to be incident into the liquid crystal display element, which is not illustrated, via the prism sheet204.

The planar light source device201is provided with a light source202for emitting light, a reflecting member202athat is arranged so as to surround the light source202, and a light guide member203having a rectangular cross-section that allows the light emitted by the light source202to be input and leads the input light toward a predetermined transmitting direction (illustrated as the arrow S2inFIG. 12A). The reflecting member202areflects the light emitted by the light source202toward the light guide member203, thereby allowing the light from the light source202to be incident into the light guide member203efficiently.

By also referring toFIG. 12B, the light guide member203is provided with an output surface203athat outputs the light from the light source202toward the prism sheet204side, a non-output surface203bthat is formed in parallel with this output surface203a, and an input surface203cfrom which the light from the light source202is input. On the output surface203a, a prism surface having a sawtooth configuration is formed. That is, in the output surface203a, a first inclined surface203dthat is formed to have a predetermined inclination angle K3with respect to the output surface203a, and a second inclined surface203ethat constitutes a prism having a isosceles triangular cross section with the first inclined surface203dare provided, and a plurality of the prisms are provided along the transmitting direction S2. Then, in this light guide member203, the light from the light source202that is input from the input surface203cto the inside thereof is reflected repeatedly by the output surface203aand the non-output surface203bso as to be led toward the transmitting direction S2, and is output from the output surface203atoward the prism sheet204appropriately.

The prism sheet204is disposed to face the output surface203aof the light guide member203, and is provided with a prism surface that is constituted of the plurality of the prisms so as to have a sawtooth configuration, and a plurality of lenses that are formed on an opposite side of the light guide member203of this prism surface and are disposed facing the liquid crystal display element. By also referring toFIG. 12C, each of the prisms of the prism surface is provided with a first prism inclined surface204aand a second prism inclined surface204bthat are disposed alternately, and a ridge line204cis formed on a boundary between these prism inclined surfaces204aand204b. Moreover, as shown inFIG. 12C, a vertical angle between the first and second prism inclined surfaces204aand204bis set as K5in the prism sheet204.

Moreover, in the prism sheet204, a lens surface204dhaving a semicircular cross section that protrudes toward the outer side is formed at a position facing the prism having the isosceles triangular cross section that is constituted of the prism inclined surfaces204aand204b. One end side and the other end side of each lens surface204dare formed to be continuous to upper end sides of the prism inclined surfaces204aand204b, respectively, and a dimension of each lens surface204din the transmitting direction S2is equal to a dimension of the prism in the transmitting direction S2. That is, in the prism sheet204, the lens surface204dthat is included in each of the plurality of the lenses is provided for each of the plurality of the prisms that are arranged along the transmitting direction S2, thereby gathering the light reflected by the prism so as to output the light toward the liquid crystal display element side.

In the backlight unit200having the structure as described above, the light from the light source202is transmitted toward the transmitting direction S2inside the light guide member203. More specifically, when the light from the light source202that is input into the inside of the light guide member203is incident upon an interface between the output surface203aand the air at an incident angle that is the total reflection angle or larger, which is shown as K4inFIG. 12B, the light is reflected totally toward the inner side of the light guide member203as shown by the arrow with the solid line inFIG. 12Bso as to travel in the transmitting direction S2.

Whereas, when the light is incident upon the second inclined surface203ethat is provided on the output surface203a, the light is refracted by the inclined surface203eso as to be output to the outside of the light guide member203, as shown by the arrows with the dotted lines inFIG. 12B. Thereafter, the light output to the outside of the light guide member203is incident upon the prism sheet204, is reflected by the second prism inclined surface204btoward an upper side, and is subsequently refracted by the lens surface204din a substantially perpendicular direction so as to be incident into the liquid crystal display element, as shown by the arrows with the solid lines inFIG. 12A.

As described above, in the backlight unit200of the second conventional example, each of the plurality of the lens surfaces204dprovided on the prism sheet204gathers light, thereby enabling an increase in the directivity of the output light toward the liquid crystal display element.

However, the conventional backlight units as described above cannot increase the directivity of the output light, and may hardly increase brightness of the liquid crystal display device.

More specifically, in the backlight unit100of the first conventional example, it is necessary to increase the inclination angle of the inclined surface103bshown as K1inFIG. 11Bso as to allow the light from the light source102that is incident into the light guide member103to travel upward along a normal line direction of the output surface103aand to output the light from the output surface103aefficiently. When the inclination angle K1is increased as described above, an angle range of the light output from the output surface103ato the outside (an angle difference between the arrow with the solid line and the arrow with the dotted line shown inFIG. 11B) is also increased. That is, an angle range of the incident angle of the light that is incident from the light guide member103upon the prism sheet104is also increased, an amount of the light that is output from the second prism inclined surface104btoward the liquid crystal display element side at an angle inclined with respect to the normal line direction is increased, thereby decreasing the directivity of the output light from the prism sheet104(the backlight unit100). As a result, an amount of the light that is incident into the liquid crystal display element in the perpendicular direction is decreased, and a half value of a brightness angle of the output light toward the liquid crystal display element is increased, thereby degrading the brightness (from a front view) of the liquid crystal display device.

Whereas, in the case of decreasing the inclination angle K1of the inclined surface103bin order to increase the directivity in the prism sheet104, the light amount of the planar light that is output from the output surface103ais decreased, that is, a plane output efficiency of the light on the output surface103ais decreased. As a result, the efficiency of utilizing the light from the light source102is degraded, which consequently leads to the degradation of the brightness of the liquid crystal display device.

Moreover, in the backlight unit200of the second conventional example, the lens surface204d, and the prism that is constituted of the first prism inclined surface204aand the second prism inclined surface204bare structured so as to have the same dimensions in the above-described transmitting direction S2. Thus, the second prism inclined surface204bthat mainly allows the light to travel from the prism side to the lens surface204dis made smaller than the lens surface204d, so that it is difficult to improve the light gathering efficiency on the lens surface204d. Accordingly, it is also difficult to increase the directivity of the output light from the prism sheet204(backlight unit200), so that it is not easy to increase the brightness of the liquid crystal display device. In particular, in a part near from the light source202of the prism sheet204b, the light is likely to be incident upon the second prism inclined surface204at a large angle range similarly to the case of the backlight unit100of the first conventional example, and the lens surface204dcannot gather the light in the normal line direction of the output surface203a, so that the directivity of the output light from the backlight unit200may be decreased significantly.

In particular, in the case of applying the conventional backlight unit described above to the liquid crystal display device of the semi-transmission type, it is sometimes significantly difficult to increase the brightness of the liquid crystal display device. In detail, in the liquid crystal display device of the semi-transmission type, a plurality of reflecting electrodes are generally provided at a predetermined interval in a transmitting direction of light in a light guide member on an incident surface side of a liquid crystal display element. And, when utilizing the output light from the backlight unit, the liquid crystal display device of the semi-transmission type is necessary to allow the output light to pass through a transmission opening that is formed between the reflecting electrodes that are adjacent to each other in the transmitting direction, similarly to the liquid crystal display device of the transmission type. Thus, the liquid crystal display device of the semi-transmission type is required to allow the output light with high directivity to be incident from the backlight unit into the liquid crystal display element. However, in the above-described conventional backlight unit, the directivity of the output light is low, and an amount of the light passing through the transmission opening cannot be increased, so that it is very difficult to increase the brightness of the liquid crystal display device of the semi-transmission type.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodiments of the present invention provide a backlight unit that can increase directivity of output light and can increase brightness easily, and a liquid crystal display device including such a novel backlight unit.

A backlight unit according to a preferred embodiment of the present invention includes: a light source; a light guide member which has an input surface from which light from the light source is input and an output surface that outputs the light input from the input surface, leads the light that is input from the input surface toward a predetermined transmitting direction, and outputs the light from the output surface; an optical path changing portion which has a plurality of inclined surfaces that are inclined at a predetermined inclination angle with respect to the output surface, and changes an optical path of the light according to each of the plurality of the inclined surfaces such that the light output from the output surface travels substantially along a normal line direction of the output surface; and a light gathering portion having a plurality of light gathering surfaces each of which is disposed for each of the plurality of the inclined surfaces, and gathers light from the corresponding inclined surface toward the normal line direction of the output surface, wherein each of central positions of the plurality of the light gathering surfaces in the transmitting direction is matched substantially with a central position of its corresponding inclined surface in the transmitting direction.

In the backlight unit having the unique structure as described above, each of the plurality of the inclined surfaces that are provided in the optical path changing portion changes the optical path of the light from the light source that is output from the output surface of the light guide member such that the light travels substantially along the normal line direction of the output surface. Moreover, in the light gathering portion, a plurality of the light gathering surfaces are provided according to the plurality of the inclined surfaces, respectively, and each of the light gathering surfaces gathers the light from its corresponding inclined surface toward the above-described normal line direction. Further, since each of the central positions of the plurality of the light gathering surfaces in the transmitting direction of the light guide member is matched substantially with the central position of its corresponding inclined surface in the transmitting direction, each of the light gathering surfaces can output and allow the light to travel together in the normal line direction reliably, unlike the second conventional example described above. As a result, the directivity of the output light can be increased, thereby enabling to increase the brightness easily.

Also, in the above-described backlight unit, the light source may be structured by using a point light source, and the plurality of the inclined surfaces included in the optical path changing portion and the plurality of the light gathering surfaces included in the light gathering portion may be arranged along arcs having centers that are positioned at the point light source.

In this case, since the plurality of the inclined surfaces and the plurality of the light gathering surfaces are arranged along the arcs whose centers are positioned at the point light source, a size of the backlight unit can be decreased and the efficiency of utilizing the light from the light source can be improved.

Moreover, in the backlight unit, the optical path changing portion is preferably constituted of a prism sheet that is provided with a plurality of prisms having respective surfaces are used for the inclined surface.

In this case, the optical path changing portion provided with the plurality of the inclined surfaces that are formed integrally can be obtained easily, and an operation for incorporating them into the backlight unit can be simplified.

Also, in the backlight unit, each of the plurality of the light gathering surfaces may be constituted of a lens surface having a semicircular cross section that protrudes toward an outside.

In this case, compared with the case of structuring the light gathering surface by using a prism surface of a light gathering prism or the like, the structure of the light gathering portion can be simplified. Further, because of using the lens surface having the semicircular cross section, degradation of an efficiency of utilizing the light from the light source can be prevented, and the directivity of the output light can be increased easily.

Moreover, in the backlight unit, the optical path changing portion and the light gathering portion are preferably formed integrally.

In this case, the optical path changing portion and the light gathering portion can be incorporated into the backlight unit at the same time, the operation for assembling the backlight unit can be simplified easily. Moreover, since the optical path changing portion and the light gathering portion are formed integrally, an operation for coinciding the central position of the inclined surface with the central position of the light gathering surface can be omitted, the integrated optical path changing portion and light gathering portion can be attached in the backlight unit easily, without adjusting the positions of the inclined surface and the light gathering surface.

Moreover, the liquid crystal display device according to a preferred embodiment of the present invention is a liquid crystal display device including a liquid crystal display element, wherein the light from any one of the backlight units described above is input into the liquid crystal display element.

In the liquid crystal display device having the unique structure as described above, since the light with the increased directivity from the backlight unit is incident into the liquid crystal display element, the brightness of the liquid crystal display device can be increased easily.

Moreover, in the liquid crystal display device, a polarizing plate is preferably disposed between the light guide member and the liquid crystal display element such that a transmission axis is matched substantially with the transmitting direction of the light guide member.

In this case, it is possible to suppress the occurrence of optical absorption (loss) in the polarizing plate to a minimum, so that the liquid crystal display device with the high brightness can be structured easily.

Thus, preferred embodiments of the present invention provide a backlight unit that can increase directivity of output light and can increase brightness easily, and a liquid crystal display device including such a backlight unit.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the backlight unit and the liquid crystal display device of the present invention will be described below with reference to the drawings. It should be noted that, in the following description, a case of applying the present invention to a liquid crystal display device of a semi-transmission type will be exemplified for the explanation.

First Preferred Embodiment

FIG. 1is a view for explaining a structure of main portions of a backlight unit and a liquid crystal display device according to a first preferred embodiment of the present invention. In the figure, the liquid crystal display device1of the present preferred embodiment is provided with a backlight unit2according to a preferred embodiment of the present invention, a pair of polarizing plates3and4, a microlens array5and a liquid crystal display element (liquid crystal panel)6, and is structured such that output light from the backlight unit2is incident into the liquid crystal display element6as illumination light. Whereas, the polarizing plate3is disposed between the backlight unit2and the microlens array5that is provided on an upper side of the backlight unit2and on a lower side of the liquid crystal display element6. Moreover, the other polarizing plate4is provided on an upper side (display surface side) of the liquid crystal display element6.

The backlight unit2is provided with a planar light source device7that emits light in a flat shape (hereinafter, called as “planar light”), and a prism sheet10having lenses and a reflecting plate11that are respectively provided on an upper side and a lower side with respect to the planar light source device7in the figure, so that the planar light is allowed to be incident into the liquid crystal display element6via the prism sheet10and the polarizing plate3. Moreover, in the polarizing plate3, a (polarizing) transmission axis is arranged so as to be matched with a transmitting direction P described below, so that brightness of a display surface of the liquid crystal display element6can be increased more easily (detail will be described below).

The planar light source device7is provided with a light source8that emits and outputs light, a reflecting member8athat is arranged so as to surround the light source8, and a light guide member9that allows the light output from the light source8to be input and leads the input light to a predetermined transmitting direction (a horizontal direction ofFIG. 1shown as the arrow P therein). The light source8utilizes a cold cathode ray tube that is disposed, for example, in a direction perpendicular to the sheet ofFIG. 1, which is constituted of a linear light source. A reflecting member8areflects the light output from the light source8toward the light guide member9, thereby allowing the light from the light source8to be incident into the light guide member9efficiently.

The light guide member9is provided with a parallel output surface9athat outputs the planar light toward the prism sheet10side, an inclined surface9bthat is arranged to face the reflecting plate11and is provided with a plurality of inclined portions which are inclined with respect to the output surface9a, and an input surface9cthat is arranged to face the light source8and allows the light from the light source8to be input. In the output surface9a, when the light from the light source8that is input from the input surface9cis incident upon an interface between the output surface9aand air at an incident angle that is a predetermined total reflection angle or larger, which is shown as R inFIG. 2, the light is totally reflected toward an inner side of the light guide member9and travels in a transmitting direction P, as shown by the arrows with the solid lines inFIG. 2. Whereas, as shown by the arrows with the dotted lines inFIG. 2, when the light is incident upon the interface at the incident angle that is smaller than the total reflection angle R, a portion of the light is reflected toward the inner side of the light guide member9, and the remaining light is output from the output surface9atoward the outer side.

As shown inFIG. 3, the inclined surface9bis provided with a plurality of the inclined portions having an inclination angle with respect to the output surface9athat is set to be α in the transmitting direction P. This inclination angle α is preferably set to be larger than 0° and smaller than 90°. More specifically, the inclination angle α of the inclined surface9bis set to be, for example, about 12°, in order to improve an output efficiency of the planar light from the output surface9aof the light guide member9. Moreover, by setting the inclination angle α to be about 12° as described above, an angle of the output light that is output from the output surface9ashown as α′ inFIG. 3has a range around 12°. Then, in this light guide member9, the light from the light source8that is input into the inside thereof from the input surface9cis repeatedly reflected by the output surface9aand the inclined surface9bor the reflecting plate11so as to be lead toward the transmitting direction P, and is output from the output surface9atoward the prism sheet10appropriately.

The prism sheet10having lenses includes an optical path changing portion and a light gathering portion that are formed integrally, where the optical path changing portion changes an optical path of the light from the light source8output from the output surface9asuch that the light travels substantially along a normal line direction of the output surface9a, and the light gathering portion gathers the light from the light source8whose optical path is changed by this optical path changing portion toward the normal line direction.

More specifically, as shown also inFIGS. 4A-4C, the prism sheet10preferably includes a prism surface as the optical path changing portion, which is disposed facing the output surface9aof the light guide member9and is constituted of a plurality of (for example, nine) prisms in a sawtooth configuration; and a plurality of (for example, eight) lenses as the light gathering portion, which are provided on an opposite side of the light guide member9of this prism surface and are disposed facing the liquid crystal display element6. Each of the prisms of the prism surface preferably has, for example, an isosceles triangular cross section, and is provided with a first prism inclined surface10aand a second prism inclined surface10bthat are disposed alternately.

Moreover, on the prism sheet10, a vertical angle between the first and second prism inclined surfaces10aand10bis set to be θ (for example, about 62°) (see alsoFIG. 5). Further, on the prism sheet10, among the first and second prism inclined surfaces10aand10b, the second prism inclined surface10bconstitutes each of the plurality of the inclined surfaces that are formed in the optical path changing portion, and can change the optical path substantially by reflecting the light from the light source8as described below. Moreover, an inclination angle of this second prism inclined surface10bwith respect to the output surface9aof the light guide member9is set to be, for example, about 59° (=(180°−62°)/2), so that the prism inclined surface10bis structured so as to function as the inclined surface that contributes to change the optical path of the light appropriately and sufficiently.

Moreover, in the prism sheet10, a ridge line10cis disposed on a boundary between the first and second prism inclined surfaces10aand10b. This ridge line10cis structured so as to cross the transmitting direction P, preferably perpendicularly, as shown by the dotted lines inFIG. 4A, and a dimension of an interval between the two adjacent ridge lines10c(that is, a pitch dimension of the prisms) is preferably set to be about 30 μm, for example.

Moreover, on a surface of the prism sheet10on the liquid crystal display element6side facing the prism surface, a lens surface10dhaving a semicircular or substantially semicircular cross section that protrudes to the outside is formed according to the second prism inclined surface10b. That is, the prism sheet10is integrally made of a synthetic resin which is entirely transparent (for example, acrylic resin, polycarbonate resin, epoxy resin or the like), and has a base portion10ehaving a rectangular or substantially rectangular cross section. Moreover, on the prism sheet10, the prism having an isosceles triangular cross section that is constituted of the first and second prism inclined surfaces10aand10bis provided on a lower side of the base portion10e. Whereas, on an upper side of the base portion10e, the lens surface10das the light gathering surface included in the light gathering portion is provided at a pitch (for example, about 30 μm) that is equal to an arrangement pitch of the ridge lines10cof the prism.

Moreover, in the prism sheet10, a central position C2of each of the lens surfaces10din the transmitting direction P is matched with a central position C1of its corresponding second prism inclined surface10bin the transmitting direction P, as shown inFIG. 5. That is, in the prism sheet10, the central positions C2and C1of the lens surface10dand the second prism inclined surface10bare matched in the normal line direction of the light guide member9, so that the light gathering efficiency of the lens surfaces10dis enhanced. Thereby, in the prism sheet10, each of the lens surfaces10dgathers the light reflected by its corresponding second prism inclined surface10bto be parallel light that is parallel with the normal line direction reliably, and can output the parallel light toward the liquid crystal display element6side via the polarizing plate3, as shown by the arrows with the solid lines and the dotted lines inFIG. 5. Moreover, in the polarizing plate3, since the transmission axis thereof is arranged so as to be matched with the transmitting direction P of the light guide member9, the parallel light form the prism sheet10is output toward the liquid crystal display element6side without being mostly absorbed by the polarizing plate3. Thereby, it is possible to suppress the occurrence of optical absorption (loss) by the polarizing plate3to a minimum, so that the brightness of the display surface of the liquid crystal display element6can be increased more easily.

Incidentally, the above description explained the case where the number of the prisms disposed is larger by one than the number of the lenses disposed in the prism sheet10, as shown inFIG. 1, but the disposal of the prism on a right end inFIG. 1may be omitted. That is, light, which is reflected by the second prism inclined surface10bof this prism on the right end toward the liquid crystal display element6side, is not incident into an effective display region of the liquid crystal display element6, so that the light does not contribute to the display of the liquid crystal display element6.

Moreover, a radius of a curvature of each of the lens surfaces10dis set to be, for example, about 25 μm, and a thickness dimension f of the base portion10eis determined to be, for example, about 30 μm. Moreover, this thickness dimension f of the base portion10eis set appropriately within a range, for example, from about 20 μm to about 40 μm, according to the inclination angle α of the inclined surface9bof the light guide member9, the vertical angle θ of the above-described prism (the inclination angle of the second prism inclined surface10bwith respect to the output surface9a), the radius of the curvature of the lens surface10dand the like. Moreover, by providing the base portion10ehaving the thickness dimension f in such a range, the lens surface10dcan output the light that is reflected by the second prism inclined surface10bat various angles so as to allow the light to travel together with the above-described parallel light easily.

By referring toFIG. 1again, on the surface side of the reflecting plate11facing the inclined surface9bof the light guide member9, a reflecting film that is made of a dielectric substance or a metal is arranged so as to reflect the light, which is output from the inclined surface9bto the outside of the light guide member9, toward the light guide member9side. Thereby, the light that is output once to the outside of the light guide member9can be incident into the inside of the light guide member9again, thereby improving the efficiency of utilizing the light from the light source8.

The liquid crystal display element6is provided with an active matrix substrate12, a facing substrate13that is disposed facing the active matrix substrate12, and a liquid crystal layer14that is provided between the active matrix substrate12and the facing substrate13. This liquid crystal layer14includes a liquid crystal molecule that has, for example, positive dielectric anisotropy, and is sealed between the active matrix substrate12and the facing substrate13.

Moreover, a screen size of the liquid crystal display element6is, for example, about 2.4 inches in diagonal diameter (length: about 49.0 mm, width: about 36.7 mm), and pixels (R, G, B) that constitute a smallest display unit of the liquid crystal display element6are arranged in stripe with, for example, a horizontal pixel number 240×a vertical pixel number 320. Moreover, pitches of the above-described pixels are set to be, for example, about 0.153 mm in the vertical direction, and about 0.051 mm in the horizontal direction, and the liquid crystal display element6is structured such that it can drive the liquid crystal layer14by each pixel unit.

The active matrix substrate12preferably includes a transparent substrate12athat is made of, for example, a glass material, a plurality of reflecting electrodes12bthat are disposed at a predetermined interval on a surface of the transparent substrate12aon the liquid crystal layer14side (illustrated as a hatched portion inFIG. 1); and a transparent electrode12cthat is provided on the transparent substrate12aon the liquid crystal layer14side. Moreover, in the active matrix substrate12, a transmission opening12dis formed in a portion where the reflecting electrodes12bare not provided, that is, between the two adjacent reflecting electrodes12b.

To the transparent electrode12c, a driving circuit (not illustrated) for applying a voltage in order to change a state of an orientation of the liquid crystal molecule is connected, so that the driving circuit drives the transparent electrode12cso as to control the orientation of the liquid crystal molecule, thereby controlling an intensity of the light passing through the liquid crystal layer14.

Moreover, in the active matrix substrate12, a plurality of thin film transistors, which are not illustrated, are arranged in matrix, and each of the thin film transistors controls electric charges of the reflecting electrode12band the transparent electrode12c. Further, on a surface of the transparent substrate12aon the backlight unit2side, the microlens array5is provided integrally.

The microlens array5includes a plurality of spherical lenses with a curvature of, for example, about 80 μm, and the spherical lens is preferably formed by applying a transparent acrylic or epoxy resin with a refractive index of, for example, about 1.51 onto the transparent substrate12aand patterning the resin. More specifically, in the microlens array5, the spherical lenses are arranged substantially in a delta arrangement as illustrated inFIG. 6. Moreover, these spherical lenses are provided according to the transmission opening12dof the active matrix substrate12. More specifically, a shape of the transmission opening12dthat is determined by the reflecting electrode12band the transparent electrode12cis a circle with a diameter of, for example, about 0.042 mm, and the transmission opening12dand the spherical lens of the microlens array5are disposed so that central positions thereof are matched with each other.

Moreover, in the microlens array5, the pitch of the spherical lenses shown as P1inFIG. 6in a direction parallel with the above-described transmitting direction P in the light guide member9is preferably set to be about 76.5 μm, for example. Further, the pitch of the spherical lenses shown as P2inFIG. 6in a direction perpendicular to the transmitting direction P is set to be about 51 μm, for example.

Moreover, in the microlens array5, a thickness of the active matrix substrate12and a height of a protrusion of the spherical lens are adjusted such that a distance from the transmission opening12d(the surface of the transparent substrate12aon the liquid crystal layer14side) to a vertex of the spherical lens may be a predetermined dimension (for example, about 220 μm).

By referring toFIG. 1again, the facing substrate13is provided with a transparent substrate13aand a transparent electrode13bthat is provided on a surface of the transparent substrate13aon the liquid crystal layer14side. The transparent substrate13ais preferably made of, for example, a glass material that is the same as the material of the above-described transparent substrate12a, and the polarizing plate4is provided on the other surface of the facing substrate13on an opposite side of the liquid crystal layer14.

Operations of the liquid crystal display device1structured as described above will be explained specifically below, with reference toFIGS. 1 to 5. It should be noted that the following description will mainly provide a case where the liquid crystal display device1operates similarly to that of the transmission type.

Firstly, in the backlight unit2, when the light from the light source8is incident into the inside of the light guide member9through the input surface9c, the incident light travels in the transmitting direction P toward an end surface that is formed in parallel with the input surface9c. Then, as shown inFIG. 2, when the light that travels inside the light guide member9is incident upon the interface between the output surface9aand the air at the incident angle smaller than the predetermined total reflection angle R, a portion of the light is refracted by the interface so as to be output to the outside of the light guide member9, and a portion of the light is reflected by the interface toward the inner side of the light guide member9. Whereas, when the light is incident upon the interface at the incident angle that is the total reflection angle R or larger, the light is totally reflected toward the inner side of the light guide member9.

Moreover, as shown inFIG. 1, light L1that is output from the output surface9ato the outside has factors in the transmitting direction P and a direction from the light guide member9toward the prism sheet10, and travels toward a right oblique upper direction inFIG. 1. Then, this light L1is incident into the prism sheet10, and is reflected by the second prism inclined surface10bsubstantially in the normal line direction of the output surface9a. Thereafter, the light L1is refracted by the lens surface10dso as to travel together in the normal line direction, thereby being output from the prism sheet10perpendicularly with respect to the liquid crystal display element6.

Whereas, the light that is reflected by the interface between the output surface9aand the air travels toward the inclined surface9b, and reaches an interface between the inclined surface9band the air. Then, the light refracted by this interface is output from the inclined surface9bto the outside of the light guide member9. The light L2that is output from this inclined surface9bto the outside has factors in the transmitting direction P and a direction from the light guide member9toward the reflecting plate11, and travels toward a right oblique lower direction inFIG. 1. Then, when this light L2is regularly reflected by the reflecting plate11, the light L2is incident into the inside of the light guide member9again, and is output from the output surface9atoward the prism sheet10. Thereafter, similarly to the light L1, the light L2is reflected by the second prism inclined surface10b, and is refracted by the lens surface10dso as to travel together in the normal line direction, thereby being output from the prism sheet10perpendicularly with respect to the liquid crystal display element6.

Moreover, even if the light is totally reflected by the interface between the output surface9aand the air at the incident angle larger than the total reflection angle R, when the light is reflected by the inclined surface9bhaving the inclination angle α toward the inner side of the light guide member9after the total reflection, the incident angle of the light that is incident upon the interface again is smaller than the total reflection angle R. Thus, a portion of the light is output from the output surface9atoward the liquid crystal display element6side, and the remaining portion of the light is reflected toward the inclined surface9bside so as to be lead toward the transmitting direction P inside the light guide member9. Thereby, the light guide member9can utilize the light from the light source8efficiently, and can make uniform the brightness of the planar light from the output surface9b.

Incidentally, in the case of operating the liquid crystal display device1similarly to that of the reflection type, the external light that is incident from an upper side of the polarizing plate4is reflected by the reflecting electrode12bso as to be utilized for the display on the liquid crystal display element6, so that the operation for displaying information such as character images and the like can be performed without switching ON the backlight unit2.

In the present preferred embodiment having the unique structure as described above, the prism sheet (the optical path changing portion and the light gathering portion)10is provided with a plurality of the second prism inclined surfaces (the inclined surfaces)10b, and a plurality of the lens surfaces (the light gathering surfaces)10daccording to the second prism inclined surfaces10b. Moreover, the central position C2of each of the lens surfaces10din the transmitting direction P of the light guide member9is matched with the central position C1of its corresponding second prism inclined surface10bin the transmitting direction P. Thereby, unlike the above-described second conventional example, in the backlight unit2, even when the light is incident upon the second prism inclined surface10bat an angle in a wide range, each of the lens surfaces10dcan gather the light reliably in the normal line direction of the output surface9aso as to make the light into parallel light, thereby outputting the light toward the liquid crystal display element6side. As a result, the directivity of the output light that is output from the backlight unit2can be increased.

Moreover, as described above, the backlight unit2of the present preferred embodiment can output the output light with the high directivity toward the liquid crystal display element6, and thus can increase the brightness of the liquid crystal display device1of the semi-transmission type easily. Thus, it is possible to easily structure the liquid crystal display device1of the semi-transmission type that has an excellent efficiency of utilizing the light from the light source8, and can suppress power consumption. Further, because of using the backlight unit2with the high directivity, the reflecting electrode12bcan be large, so that the efficiency of utilizing the external light in the case of operating the liquid crystal display device1similarly to that of the reflection type can be increased, thereby increasing the brightness.

Here, a verification test carried out by the present inventors will be described specifically.

In this verification test, the liquid crystal display device1of the present preferred embodiment shown inFIG. 1and a liquid crystal display device51(hereinafter, called as a comparative liquid crystal display device) provided with a corresponding conventional backlight unit52shown inFIG. 7were prepared so as to carry out a comparative test as described below, thereby verifying that the directivity and the brightness of the output light of the liquid crystal display device1of the present preferred embodiment are increased. It should be noted that, in the comparative liquid crystal display device51, a prism sheet53that was equivalent to the prism sheet104shown inFIGS. 10A and 10Bwas used instead of the prism sheet10provided with the lenses of the present preferred embodiment. That is, as the prism sheet53, a prism sheet provided with a prism having a first prism inclined surface53a, a second prism inclined surface53band a ridge line53cthat are disposed only on a surface side of the light guide member9facing the output surface9awas used. Also, optical properties and brightness properties of the liquid crystal display device1of the present preferred embodiment and the comparative liquid crystal display device51were obtained when turning ON the same light source8.

That is, it was verified that, as shown as the solid line60inFIG. 8, the brightness (a.u.) of the output light that is output from the backlight unit2in the liquid crystal display device1of the present preferred embodiment had a transmission light flux at and around a visibility angle of 0° that was about twice or more the brightness of the comparative liquid crystal display device51, which is shown as the dotted line61in the figure, thereby increasing the directivity.

Moreover, it was verified that the brightness (nt) on the display surface of the liquid crystal display device1of the present preferred embodiment, which is shown as the solid line70inFIG. 9, had the transmission light flux at and around the visibility angle of 0° which was about 1.5 times or more that of the comparative liquid crystal display device51, which is shown as the dotted line71in the figure, thereby increasing the brightness.

Further, as shown in Table 1 below, it was verified that, in the liquid crystal display device1of the present preferred embodiment, the minimum value of the brightness and a variable value of the transmission light flux within a range of the visibility angle of about 0°±5° were increased by about 1.5 times and about 1.3 times the minimum value of a brightness and a variable value of the transmission light flux of the conventional liquid crystal display device, respectively.

TABLE 1BrightnessTransmission(nt)light flux (a.u)Liquid crystal display2002.2device of present embodimentComparative liquid crystal1301.7display device

As described above, it was verified that, in the liquid crystal display device of the present preferred embodiment, even when the light from the light source8was incident from the output surface9aupon the second prism inclined surface10bat an angle in a range shown by α′ inFIG. 3, and reflected light with low directivity was reflected toward the lens surface10dside, the lens surface10dgathered the reflected light with the low directivity toward the normal line direction of the output surface9a, thereby increasing the directivity of the output light from the backlight unit2. Moreover, it was also verified that, according to this increase of the directivity of the output light of the backlight unit2, the brightness of the liquid crystal display device1could be increased.

Second Preferred Embodiment

FIG. 10Ais a view for explaining a structure of main portions of a backlight unit and a liquid crystal display device according to a second preferred embodiment of the present invention, andFIG. 10Bis a plan view of the backlight unit shown inFIG. 10A. In the figures, a main distinctive point of the present preferred embodiment from the above-described first preferred embodiment lies in using of a light emitting diode instead of the cold cathode ray tube as the light source, and providing a plurality of the second prism inclined surfaces and a plurality of the lens surfaces along arcs whose centers are positioned at this light emitting diode. Incidentally, elements provided in common with the first preferred embodiment described above are given the same reference numerals, and the redundant description thereof will be omitted here.

That is, as shown inFIGS. 10A and 10B, in the present preferred embodiment, a light source18using a point light source, for example, a light emitting diode is disposed facing the light guide member9, so that light from this light source18can be efficiently incident upon the input surface9cof the light guide member9via a reflecting member18athat is provided so as to surround the light source18, and is transmitted radially.

Moreover, as shown inFIG. 10B, the light source18is disposed facing the central portion of the input surface9cof the light guide member9. Further, in the prism sheet10, as shown by the dotted lines inFIG. 10B, a plurality of the ridge lines10care provided along arcs (concentrically) having centers are positioned at the light source18, and each of the ridge lines10ccrosses the transmitting directions P and P′, preferably perpendicularly. That is, in the prism sheet10, the plurality of the first and second prism inclined surfaces10aand10bare also arranged along arcs whose centers are positioned at the light source18, respectively. Further, on the polarizing plate3side of the prism sheet10, similarly to the plurality of the first and second prism inclined surfaces10aand10b, the plurality of the lens surfaces10dare arranged along arcs whose centers are positioned at the light source18. And, for example, in the above-described transmitting direction P or P′ that connects respective vertexes of the plurality of the lens surfaces10d, the central position C1of the second prism inclined surface10band the central position C2of the lens surface10d, which correspond to each other in the vertical direction inFIG. 5, are matched with each other in the normal line direction of the light guide member9as shown inFIG. 5. Thereby, in the prism sheet10, a light gathering effect of the lens surface10dis increased, similarly to that of first preferred embodiment.

Moreover, in the present preferred embodiment, the plurality of the inclined surfaces9bof the light guide member9are arranged along arcs whose centers are positioned at the light source18according to the shape of the prism sheet10, similarly to the prism sheet10. Thereby, the light guide member9is structured such that the light from the light source18which is transmitted radially inside the light guide member9can be output toward the prism sheet10side more appropriately.

Moreover, in the present preferred embodiment, the output light (planar light) from the prism sheet10is incident into the liquid crystal display element6via the polarizing plate3whose transmission axis is arranged so as to be matched substantially with the transmitting direction P or P′, whereby it is possible to suppress the occurrence of optical absorption (loss) by the polarizing plate3to a minimum similarly to the first preferred embodiment, so that the brightness of the display surface of the liquid crystal display element6can be increased more easily.

According to the above-described structure, the present preferred embodiment can achieve effects similar to those of the first preferred embodiment. Moreover, in the present preferred embodiment, the plurality of the second prism inclined surfaces (the inclined surfaces)10band the plurality of the lens surfaces (the light gathering surfaces)10dare respectively arranged along the arcs whose centers are positioned at the (point) light source18. Thereby, in the present preferred embodiment, the size of the backlight unit2can be decreased more easily than that in the case of using a linear light source such as a cold cathode ray tube. Further, it is possible to dispose an effective light emitting region of the light source18so as to face the light guide member9easily without increasing the dimension of the light guide member9more than necessary, unlike that of the above-described cold cathode ray tube in which non-light emitting regions such as electrode portions are formed at both end portions, thereby increasing the efficiency of utilizing the light from the light source more easily than that in the case of using the linear light source.

Incidentally, in the above description, an example of applying the backlight unit according to a preferred embodiment of the present invention to the liquid crystal display device of the semi-transmission type was provided, but the backlight unit of preferred embodiments of the present invention is not limited to this, and may be applied also to a liquid crystal display device of a transmission type in which a reflecting electrode is not provided in a liquid crystal display element and a liquid crystal display device of a semi-transmission type in which a microlens array is not provided.

Moreover, the above description explained an example where the central positions of the inclined surface and the light gathering surface are preferably matched with each other in the transmitting direction of the light from the light guide member, for example, as shown by C1and C2inFIG. 5, but the present invention is not limited to this, and each of the central positions of the plurality of the light gathering surfaces in the transmitting direction may be matched substantially with the central position of its corresponding inclined surface in the transmitting direction.

More specifically, in the verification test carried out by the present inventors, the inclination angle α and the vertical angle θ were respectively set to be, for example, about 59° and about 62° so as to measure the transmission light flux within a range of the visibility angle of about 0°±5°. Then, it was judged that, while displacing the central position C1and the central position C2in the transmitting direction gradually, the directivity of the output light is increased, when a measurement result of the transmission light flux within the above-described range was about 1.2 times that of the comparative liquid crystal display device shown inFIG. 7. As a result, it was verified that a displacement amount between the central position C1and the central position C2that can increase the directivity was in a range from about −6.5 μm to about +2.5 μm in the transmitting direction.

However, it is preferable to adopt the case where the central positions of the inclined surface and the light gathering surface are matched with each other as described above, because the light can be output so as to travel together in the normal line direction of the output surface of the light guide member more reliably, so that the directivity of the output light can be increased more easily, and the brightness can be increased more easily.

Moreover, the above description explain an example of using the prism sheet having the lens in which the optical path changing portion and the light gathering portion are integral with each other, but the optical path changing portion and the light gathering portion that are structured separately can also be used. However, it is preferable to use the optical path changing portion and the light gathering portion that are integral with each other such as the above-described prism sheet having the lens, because it is possible to easily simplify an operation for incorporating into the backlight unit and an operation for assembling the backlight unit. Also, it is preferable in the point of enabling an operation for the attachment into the backlight unit easily, without carrying out any position adjusting operation for allowing the central positions of the inclined surface of the optical path changing portion and its corresponding light gathering surface of the light gathering portion to be matched with each other.

Moreover, other than the above description, for example, a contact surface of two different optical members that have different refractive indices can also be used as the inclined surface of the optical path changing portion, instead of using the prism sheet. However, the above-described case of using the prism sheet in which the inclined surface is constituted of respective single surfaces of the plurality of the prisms is more preferable, in the point of easily manufacturing the optical path changing portion with excellent handling capabilities, and simplifying an operation for incorporating into the backlight unit. Moreover, the above description also described an example of using a prism that preferably has the isosceles triangular cross section, but the shape of the prism is not limited to this at all, and for example, the prism having a right triangular cross section or other suitable configuration can also be used.

Moreover, the above description disclosed an example of structuring each of the light gathering surfaces of the light gathering portion by the lens surface having the semicircular cross section, but the light gathering portion of the present invention may be provided with the light gathering surface on each of the inclined surfaces of the optical path changing portion, in which the central positions of the pair of the inclined surface and the light gathering surface are matched substantially with each other in the transmitting direction in the light guide member. As the light gathering surface, those with other shapes or structures, for example, a light gathering surface with a convex shape and a prism surface of a light gathering prism can be used. However, the case of using the lens surface having the semicircular cross-section as the light gathering surface is preferable in the point of simplifying the structure of the light gathering portion. Also, it is preferable that, since a portion for outputting the light to the outside has an arc-shaped cross section, the light from the inclined surface is allowed to travel along the normal line direction of the output surface of the light guide member easily, so that the efficiency of utilizing the light from the light source is prevented from being decreased, and the directivity of the output light can be increased easily.

Moreover, the above description explained an example of using the inclined surface that is provided with the plurality of the inclined portions with the inclination angle of α on the surface of the light guide member facing the output surface, but the light guide member of the present invention is not limited to this, and a light guide member with a wedge-shaped cross section having an inclined surface that is inclined at the certain inclination angle of α, and a light guide member with a substantially rectangular-shaped cross section having a parallel surface that is parallel with the output surface can also be used. However, the case of using the inclined surface that is provided with the plurality of the inclined portions at the inclination angle of α is preferable in the point of outputting the efficiency of utilizing the light on the output surface and obtaining the uniform planar light easily.

Moreover, the above descriptions of the first and second preferred embodiments explained examples of using the cold cathode ray tube and the LED (light emitting diode) as the light source, respectively, but the light source of the present invention is not limited to these, and a hot cathode ray tube, other linear light source such as a fluorescent tube, and a point light source such as an EL (electroluminescence) element can also be used.

Moreover, the above description explained an example of using the reflecting plate that is disposed on the lower side of the light guide member, but the reflecting plate of the present invention is not limited to this, and, for example, by applying a paint with high reflectivity onto the reflecting surface that is provided on an inner surface of a box of the liquid crystal display device for storing the liquid crystal display element or the inner surface of the box, the inner surface can also be used as the reflecting plate.

Moreover, other than the above description, an optical member such as a diffusion sheet for adjusting the visibility angle of the liquid crystal display element can also be layered appropriately on the upper side (display surface side) of the liquid crystal display element, for example.

Since the backlight unit according to preferred embodiments of the present invention and the liquid crystal display device including the same can output light so that the light can travel together reliably along the normal line direction of the output surface of the light guide member, the backlight unit that can increase the directivity of the output light and can increase the brightness easily, and the liquid crystal display device including the same can be provided.