Surface emission device, optical element and liquid crystal display device

Using a distance L between the centers of light sources 12, 12; a refractive index n of an optical element 15; a thickness d of the optical element; a distance W from the center of the light source to the optical element; a refractive index n0 of air; an angle of incidence θ1 of light emitted from the light sources and coming into the optical element, relative to the direction of optical axes; an angle of refraction θ2 of light, incident on the optical element, in the optical element; a diameter of each light source as D; and a maximum tangential angle a formed between a tangential line in contact with an outer surface of a luminance distribution generating layer 18 and a plane orthogonal to the optical axes, there is included a maximum tangential angle a satisfying x>L/2−D/2 when calculating a travel range x of a split image of the light sources in a direction normal to the optical axes using (1) n0 sin(a)=n sin(a−θ2), (2) n0 sin θ1=n sin θ2 and (3) x=W tan θ1+d tan θ2. This facilitates uniformalizing of front luminance distribution.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent Document Nos. P2006-206008 filed on Jul. 28, 2006, and P2007-171078 filed on Jun. 28, 2007 the disclosures of which are herein incorporated by reference.

BACKGROUND

1. Technical Field

The present invention relates to technical fields of surface emission device, optical element and liquid crystal display device. For more detail, the present invention relates to technical fields of suppressing non-uniformity in luminance by allowing split images of light sources to overlap between the light sources.

2. Background Art

Liquid crystal display devices provided with backlight (surface emission device) have conventionally been used as display devices for word processors, laptop personal computers and so forth. As the surface emission device for this type of liquid crystal display devices, an edge-light-type backlight, having linear light sources just like fluorescent lamps disposed laterally on a transparent plate (light guide plate), in response to demands for weight reduction and thinning, has been in the main stream.

The edge-light-type backlight has, however, often resulted in insufficient luminance with recent expansion in size of the liquid crystal display device represented by those used for television sets, so that a direct-type backlight, having linear light sources arranged straightly under the liquid crystal display panel has more widely been adopted.

FIG. 32is a perspective view showing a schematic configuration of a conventional direct-type backlight unit1. The backlight unit1has light sources (linear light sources)2,2, . . . such as fluorescent lamps, a reflective plate3, and a diffuser plate4.

As the light sources (linear light sources)2,2, . . . , cold cathode fluorescent lamps (CCFL) or the like are used, which are formed into columns extended in a predetermined direction.

The reflective plate3is disposed so as to make use, in a recycled manner, of light reflected on the diffuser plate4, etc., or light emitted from the light sources2,2, . . . , but not reached the diffuser plate4.

The diffuser plate4is an optical element of at least 1 mm thick or more improved in diffusing and scattering performances, by virtue of having a transparent base and a resin component different from the transparent base in the refractive index randomly contained therein, and is used as an optical element for suppressing variation in front luminance distribution.

In the backlight unit1, the reflective plate3and the diffuser plate4are disposed respectively on both sides of the light sources2,2, . . . .

In thus-configured backlight unit1, light emitted from the light sources2,2, . . . is extracted from the diffuser plate4, wherein luminance of illumination flux of the backlight unit1may be high straightly above the light sources2,2, . . . and may be low between the light sources2,2, . . . as shown inFIG. 33, when the distance between the light sources2,2, . . . and the diffuser plate4becomes small, or the distance between the individual light sources2,2, . . . becomes large, and this may degrade uniformity in the front luminance distribution and may cause variation in luminance.

In order to suppress such variation in luminance, as shown inFIG. 34, there has been known a technique of disposing an optical sheet (optical element)5such as prism sheet or lenticular lens sheet between the light sources2,2, . . . and the diffuser plate4, or disposing an optical sheet (optical element)5such as prism sheet or lenticular lens sheet in place of the diffuser plate4(see Japanese Patent Application Publication (KOKAI) Nos. H5-333333, H6-250178, H10-283818, and 2004-6256).FIG. 34shows an exemplary case where the optical element (prism sheet)5is disposed in place of the diffuser plate4shown inFIG. 33.

The optical element (prism sheet)5has, on the front surface or the back surface thereof, a plurality of linear projections (prisms) consecutively provided at regular pitches, typically having a triangle profile, and is an optical element generally adopted as a sheet for improving luminance. These linear projections function as a luminance distribution generating layer5awhich suppresses variation in luminance in the direction of optical axes of light emitted from the light sources2,2, . . . .

The optical element5is disposed so that the direction of ridge of the linear projections which function as a luminance distribution generating layer5aagrees with the longitudinal direction of the light sources2,2, . . . . By using the optical element5, as shown inFIG. 34, extracted illumination flux is split into a plurality of fluxes to give split images2A,2A, . . . of each light source, and thereby the variation in the front luminance distribution may be suppressed.FIG. 34shows an exemplary case where the number of split images2A,2A, . . . of each light source was doubled from the number of the light sources2,2, . . . by the optical element5.

The above-described conventional surface emission device1has, however, been suffering from a problem in that a large non-uniformity in luminance is likely to occur, when the distance between the light sources2,2, . . . and the optical element5varies. Variation in the distance may be ascribable to accuracy in processing or assembling of the individual components, or to deformation of the optical element due to environmental changes such as changes in temperature.

For example, as shown inFIG. 35, in a surface emission device designed to ensure uniform front luminance distribution with respect to each of the split images2A,2A, . . . of the light sources2,2, . . . , when the distance between the centers of the light sources2,2, . . . and the optical element5is given as H, a change in the designed distance H of the optical element5to as much as ΔH may be highly causative of non-uniformity in luminance as shown inFIG. 36.

The surface emission device1is designed so as avoid overlapping of one split image2A of the light source2with the adjacent split image2A of the light source2, so far as the designed distance H is maintained, so that such non-uniformity in luminance may occur as a result of a sharp change in the front luminance distribution when the distance H varies. More specifically, a change in the distance H to as much as ΔH may cause overlapping of the split images2A,2A, . . . of the individual light sources2,2, . . . , and may raise a sharp change in the front luminance distribution, making non-uniformity in luminance more likely to occur.

The distance between the light sources2,2, . . . and the optical element5is therefore designed so that a uniform front luminance distribution, as shown inFIG. 35, may be obtainable, but only with a small degree of freedom in the design.

On the other hand, with recent trends in expansion in size of the liquid crystal display devices, also the surface emission devices (backlight unit) have been expanded in size. As a consequence, also the optical elements such as prism sheet, lenticular sheet and so forth are to be expanded in size, for the purpose of making the front luminance distribution uniform.

SUMMARY

Expansion in size of these optical elements, however, makes them more likely to cause sagging or warping due to their self weight, and makes it difficult to stably and uniformly maintain the distance between the optical element and the light sources over the entire surface of the optical element. As a consequence, variation in the distance may be caused between the optical element and the light sources as shown inFIG. 36, the front luminance distribution may be prevented from being made uniform, and non-uniformity in luminance may be more likely to occur.

It is therefore a subject of a surface emission device, an optical element and a liquid crystal display device of the present invention to overcome the above-described problems, and to suppress non-uniformity in luminance by ensuring uniformity in the front luminance distribution, even when the distance between the light sources and the optical element should vary.

Aiming at solving the above-described problems, a surface emission device, an optical element and a liquid crystal display device of the present invention is configured as containing a maximum tangential angle a which satisfies x>L/2−D/2, when travel range x of a split image of the light sources in the direction orthogonally crossing the optical axes is calculated using (1) n0sin(a)=n sin(a−θ2), (2) n0sin θ1=n sin θ2, and (3) x=W tan θ1+d tan θ2, assuming distance between the centers of every adjacent light sources as L; refractive index of the optical element as n; thickness of the optical element as d; distance from the center of the light sources to the optical element in the direction of optical axes as W; refractive index of air in the air layer as n0; angle of incidence of light emitted from the light sources and coming into the optical element, relative to the direction of optical axes, as θ1; angle of refraction of light, incident on the optical element, in the optical element as θ2; diameter of the light source as D; angles formed between a tangential line in contact with the outer surface of the luminance distribution generating layer and a plane orthogonal to the optical axes, as viewed in a sectional profile orthogonal to the longitudinal direction of the structural portions of the luminance distribution generating layer, as tangential angles ψ; and a tangential angle largest of all tangential angles ψ as maximum tangential angle a.

Accordingly, in the surface emission device, the optical element and the liquid crystal display device, at least part of the split images of the individual light sources positioned adjacent to each other may be overlapped.

The surface emission device of the present invention is a surface emission device having a plurality of light sources respectively shaped into a columnar form extending in a predetermined direction and disposed on the same plane as being extended in the same direction; an optical element having transparency and having, as formed therein, a luminance distribution generating layer suppressing variation, in the direction of optical axes, in luminance of light emitted from the plurality light sources; and a reflective surface positioned as being opposed to the optical element across the plurality of light sources, while keeping an air layer between the optical element and itself, and reflecting light emitted from the light sources, wherein the luminance distribution generating layer of the optical element being composed of a plurality of structural portions extending in the longitudinal direction of the light sources and projecting in the direction of optical axes. The surface emission device is characterized by, assuming distance between the centers of every adjacent light sources as L; refractive index of the optical element as n; thickness of the optical element as d; distance from the center of the light sources to the optical element in the direction of optical axes as W; refractive index of air in the air layer as n0; angle of incidence of light emitted from the light sources and coming into the optical element, relative to the direction of optical axes, as θ1; angle of refraction of light, incident on the optical element, in the optical element as θ2; diameter of each light source as D; angles formed between a tangential line in contact with the outer surface of the luminance distribution generating layer and a plane orthogonal to the optical axes, as viewed in a sectional profile orthogonal to the longitudinal direction of the structural portions of the luminance distribution generating layer, as tangential angles ψ; and a tangential angle largest of all tangential angles ψ as maximum tangential angle a; having the optical element containing the maximum tangential angle a which satisfies x>L/2−D/2, when travel range x of a split image of the light sources in the direction normal to the optical axes is calculated using the conditional equation (1) to conditional equation (3) below.
n0sin(a)=nsin(a−θ2)  (1)
n0sin θ1=n sin θ2(2)
x=Wtan θ1+dtan θ2(3)

The optical element of the present invention is an optical element being configured as having formed therein a luminance distribution generating layer suppressing variation, in the direction of optical axes, in luminance of light emitted from the plurality light sources respectively shaped into a columnar form extending in a predetermined direction and disposed on the same plane as being extended in the same direction, wherein the luminance distribution generating layer is composed of a plurality of structural portions extending in the longitudinal direction of the light sources and projecting in the direction of optical axes. The optical element is characterized by being, assuming distance between the centers of every adjacent light sources as L; refractive index of the optical element as n; thickness of the optical element as d; distance from the center of the light sources to the optical element in the direction of optical axes as W; refractive index of air in the air layer as n0; angle of incidence of light emitted from the light sources and coming into the optical element, relative to the direction of optical axes, as θ1; angle of refraction of light, incident on the optical element, in the optical element as θ2; diameter of each light source as D; angles formed between a tangential line in contact with the outer surface of the luminance distribution generating layer and a plane orthogonal to the optical axes, as viewed in a sectional profile orthogonal to the longitudinal direction of the structural portions of the luminance distribution generating layer, as tangential angles ψ; and a tangential angle largest of all tangential angles ψ as maximum tangential angle a; configured so as to contain the maximum tangential angle a which satisfies x>L/2−D/2, when travel range x of a split image of the light sources in the direction normal to the optical axes is calculated using the conditional equation (1) to conditional equation (3) below.
n0sin(a)=nsin(a−θ2)  (1)
n0sin θ1=n sin θ2(2)
x=Wtan θ1+dtan θ2(3)

The liquid crystal display device is a liquid crystal display device having a plurality of light sources respectively shaped into a columnar form extending in a predetermined direction and disposed on the same plane as being extended in the same direction; an optical element having transparency and having, as formed therein, a luminance distribution generating layer suppressing variation, in the direction of optical axes, in luminance of light emitted from the plurality light sources, the luminance distribution generating layer being composed of a plurality of structural portions extending in the longitudinal direction of the light sources and projecting in the direction of optical axes; a reflective surface positioned as being opposed to the optical element across the plurality of light sources, while keeping an air layer between the optical element and itself, and reflecting light emitted from the light sources; and a liquid crystal panel allowing thereon image display and irradiated with light emitted from the plurality of light sources. The liquid crystal display device is characterized by, assuming distance between the centers of every adjacent light sources as L; refractive index of the optical element as n; thickness of the optical element as d; distance from the center of the light sources to the optical element in the direction of optical axes as W; refractive index of air in the air layer as n0; angle of incidence of light emitted from the light sources and coming into the optical element, relative to the direction of optical axes, as θ1; angle of refraction of light, incident on the optical element, in the optical element as θ2; diameter of each light source as D; angles formed between a tangential line in contact with the outer surface of the luminance distribution generating layer and a plane orthogonal to the optical axes, as viewed in a sectional profile orthogonal to the longitudinal direction of the structural portions of the luminance distribution generating layer, as tangential angles ψ; and a tangential angle largest of all tangential angles ψ as maximum tangential angle a; having the optical element containing the maximum tangential angle a which satisfies x>L/2−D/2, when travel range x of a split image of the light sources in the direction normal to the optical axes is calculated using the conditional equation (1) to conditional equation (3) below.
n0sin(a)=nsin(a−θ2)  (1)
n0sin θ1=n sin θ2(2)
x=Wtan θ1+dtan θ2(3)

Accordingly, in the surface emission device, the optical element and the liquid crystal display device of the present invention, at least part of the split images of the individual light sources positioned adjacent to each other may be overlapped, so that the front luminance distribution is ensured with a desirable level of uniformity, and thereby the non-uniformity in luminance may be suppressed.

DETAILED DESCRIPTION

Paragraphs below will explain best modes for carrying out the surface emission device, the optical element and the liquid crystal display device of the present invention, referring to the attached drawings.

A surface emission device10is used as a direct-type backlight unit adapted to a liquid crystal display device50(seeFIG. 1).

The surface emission device10has the individual components disposed in a housing11, and is provided with a plurality of light sources (linear light sources)12,12, . . . , a reflective plate13, a diffuser plate14, an optical element (optical plate)15and an optical element component16.

As the light sources12,12, . . . , fluorescent lamps such as cold cathode fluorescent lamps or hot cathode fluorescent lamps are adopted. The light sources12,12, . . . are respectively shaped into a columnar form, and are disposed on the reflective plate13as being extended in the Y-direction shown inFIG. 1. The light sources12,12, . . . are disposed at regular intervals in the X-direction shown inFIG. 1, between the reflective plate13and the optical element15.

In the surface emission device10, since a plurality of light sources12,12, . . . are disposed at regular intervals on the reflective surface13aso as to ensure uniformity in the state of arrangement as shown in the above, so that light emitted from the light sources12,12, . . . is less likely to cause partial non-uniformity in luminance depending on the state of arrangement of the light sources12,12, . . . when it reaches a liquid crystal display panel described later.

The surface of the reflective plate13opposed to the light sources12,12, . . . is formed as the reflective surface13a. Part of light emitted from the light sources12,12, . . . is reflected on the reflective surface13aback to the optical element15. The reflective plate13may be anything so far as it has a property of reflecting light, allowing use of a variety of those composed of aluminum, PET (polyethylene terephthalate), polycarbonate and so forth.

The diffuser plate14is disposed as being opposed to the light sources12,12, . . . across the optical element15. The diffuser plate14has a function of diffusing light transmitted through the optical element15and uniformalizing a luminance distribution of the illumination flux emitted in the front direction, that is, front luminance distribution. Alternatively, in the surface emission device10, also a thin diffuser sheet may be adoptable in place of the diffuser plate14.

As the diffuser plate14, those composed of polystyrene, cycloolefin polymer, acryl and polycarbonate may typically be used, whereas as the diffuser sheet, those capable of assisting diffusion of light, such as having filler particles coated on a PET base, may be used. Note that, although it is good enough to use at least either one of the diffuser plate14and diffuser sheet, they may be used in a stacked manner.

On the light extraction surface side of the diffuser plate14, a liquid crystal display panel is disposed although not shown.

The optical element15is disposed between the light sources12,12, . . . and the diffuser plate14. The optical element15is typically a prism sheet or a lenticular lens sheet having transmissivity of light, and is configured with a luminance distribution generating layer18formed on the light extraction surface side of a base17as being integrated therewith.

The base17is formed using a plate material made of a transparent synthetic resin such as acrylic resin, polyethylene terephthalate, polyethylene naphthalate, polycarbonate, styrene-base resin, styrene-methyl methacrylate copolymer resin and so forth. Note that, although the base17may alternatively be configured in a form of sheet or film, formation using a highly rigid plate material may be more preferable, because such base17may be less likely to sag, warp or deform by heat when incorporated into the housing11, and less likely to vary the distance between the light source12and itself in the Z-direction. Thickness of the base17is not specifically limited, and even the thickness as small as that of sheet or film may be allowable so far as a predetermined rigidity may be ensured.

The luminance distribution generating layer18functions as suppressing variation in luminance in the front direction (Z-direction) of light emitted from the light sources12. The luminance distribution generating layer18is configured with a plurality of structural portions18a,18a, . . . , having the direction of ridges thereof agreed with the Y-direction shown inFIG. 1, and the structural portions18a,18a, . . . are consecutively disposed in the X-direction at predetermined pitches. Each structural portion18ais made project toward the Z-direction shown inFIG. 1, that is, the direction of optical axis of light emitted from the light source12, and has an outer surface thereof typically formed into a curved profile or polygonal profile. The structural portion18aformed into the curved profile may have an aspherical profile, for example.

Pitch of arrangement of the structural portions18a,18a, . . . is irrespective of the pitch of arrangement of the light sources12,12, . . . , wherein the structural portions18a,18a, . . . are arranged at micro pitches.

The luminance distribution generating layer18may be formed as being integrated with the base17, but may be formed by transferring the luminance distribution generating layer18formed using a UV-curable resin onto the base17, or may be formed by bonding the luminance distribution generating layer18to the base17by press forming.

The optical element component16is composed of one of, or a plurality of various optical elements such as diffuser sheet, prism sheet and reflective polarizer. For a case where the optical element component16is composed of a plurality of optical elements, these plurality of optical elements are disposed as being stacked. The optical element component16is disposed as being opposed to the optical element15across the diffuser plate14.

In thus-configured surface emission device10, a space between the reflective plate13and the optical element15is formed as an air layer19.

In the surface emission device10, once the light is emitted from the light sources12,12, . . . , the emitted light sequentially transmits the optical element15, the diffuser plate14and the optical element component16, and is irradiated onto the liquid crystal display panel. Part of the emitted light herein is reflected on the reflective surface13aof the reflective plate13, and is directed to the optical element15.

The light incident on the optical element15is refracted on the plane of incidence of the optical element15, and is again refracted also when it is emitted out from the optical element15, and is then directed to the diffuser plate14. The light incident on the diffuser plate14is extracted therefrom after being diffused, transmits through the optical element component16, and reaches the liquid crystal display panel.

FIG. 2shows positional relations of route of light emitted from the light sources12,12, . . . and the individual components.

InFIG. 2, distance between the centers of every adjacent light sources12,12is given as L, refractive index of the optical element15is given as n, thickness of the optical element15is given as d, distance from the center of the light source12to the optical element15in the direction of optical axes P is given as W, refractive index of air in the air layer19is given as n0, angle of incidence of light emitted from the light source12and coming into the optical element15, relative to the direction of optical axes P, is given as θ1, angle of refraction of light, incident on the optical element15, in the optical element15is give as θ2, and diameter of the light source12is given as D.

It is noted thatFIG. 2shows the size of the structural portions18a,18a, . . . of the luminance distribution generating layer18as being emphasized with respect to the base17of the optical element15, and that actual size of the structural portions18a,18a, is extremely smaller than that of the base17.

In addition, as shown inFIG. 3, in a sectional profile orthogonal to the longitudinal direction of the structural portions18a,18a, . . . of the luminance distribution generating layer18, an angle formed between a tangential line S in contact with the outer surface of the structural portion18aand a plane Q orthogonal to the optical axis P is given as tangential angle ψ. In this case, as shown inFIG. 2, a tangential angle largest of all tangential angles ψ is given as maximum tangential angle a, and travel range of a split image12A of the light source12in the direction normal to the optical axis P from the light source12is given as x. The travel range x represents a distance from the end face of the light source12.

With these equation (1) to equation (3), substituting an arbitrary tangential angle ψ in the equation (1) yields the angle of refraction θ2, substituting the calculated θ2in the equation (2) yields the angle of incidence θ1, and substituting the calculated θ1and θ2in the equation (3) yields the travel range x. As a consequence, the travel range x corresponding to the tangential angle ψ is unconditionally determined, and a split image12A of light reached the point of contact of the luminance distribution generating layer18having the tangential angle ψ is shifted towards the adjacent light source12to as much as x.

As described in the above, the travel range x of the split image12A is determined by the tangential angle ψ, and assuming now that a tangential angle, out of all tangential angles ψ, under which the split image12A of the light source12reaches L/2 is given as b, and a tangential angle, out of all tangential angles ψ, under which the split image12A of the light source12reaches L, that is, the center of the adjacent light source12, is given as c, tangential angle ψ satisfying
x=L/2−D/2  (4)
equals tangential angle b. (L/2−D/2) stands for the middle position between the individual centers of the adjacent light sources12,12. Accordingly, if any tangential angle ψ satisfying the equation (5) below should reside, the individual split images12A,12A of the adjacent light sources12,12should overlap:
x>L/2−D/2  (5).

As described in the above, presence of tangential angle ψ satisfying the equation (5) means overlapping of the individual split images12A,12A of the adjacent light sources12,12, and this means that, if the maximum tangential angle a, allowing thereunder the equation (5) to hold, should reside in the luminance distribution generating layer18, the individual split images12A,12A of the adjacent light sources12,12may overlap.

In the surface emission device10, the optical element15is formed so that the maximum tangential angle a, allowing thereunder the equation (5) to hold, may reside in the luminance distribution generating layer18, and so that the individual split images12A,12A of the adjacent light sources12,12may overlap.

FIG. 4is a graph showing a front luminance distribution given by light emitted from a single light source12after transmitting through the optical element15and before entering the diffuser plate14.

As shown inFIG. 4, the front luminance distribution shows a near-triangle profile maximized in the luminance level at a position straightly above the light source12, and sloping down towards the positions straightly above the adjacent light sources12. The front luminance distribution shown inFIG. 4is slightly deformed as compared with a front luminance distribution having a triangle profile, wherein such deformation is ascribable to action of light reflected on the reflective plate13.

Profile of the front luminance distribution obtained immediately after being emitted from the light source12is not limited to the triangle profile, but may also be a near-triangle profile with a rounded apex (seeFIG. 5), profile having shoulders on the inclined portions thereof (seeFIG. 6), a profile having step-wisely varied slope on the inclined portions (seeFIG. 7), and so forth.

FIG. 8is a graph showing front luminance distribution of light before transmitting through a diffuser plate14under varied distance W, for the case where the split images12A,12A, . . . of the plurality of light sources12,12, . . . overlap, collectively showing a front luminance distribution observed under a designed distance W, and front luminance distributions observed under ±8% shift from the distance W.

The front luminance distributions shown inFIG. 8express results obtained by the Monte Carlo simulation of light emitted from the light sources12,12, . . . reflected, refracted and scattered by the reflective plate13and the optical element15, conforming to probabilities according to the optical characteristics.

InFIG. 8, every single light source12has a front luminance distribution maximized at the position straightly above the light source12, and sloping down towards the positions straightly above the adjacent other light sources, wherein at the middle point between two adjacent light sources12,12, the split images12A,12A of two these light sources12,12overlap, and parts of the individual front luminance distributions overlap.

If the split images12A,12A do not overlap, width of trail (width in the direction of arrangement of the light sources12,12, . . . ) of the front luminance distribution of the individual light sources12,12, . . . may fluctuate, when the distance W between the light sources12and the optical element15varied, and thereby the front luminance distribution may largely vary. For example, when the distance W becomes larger than the designed distance W, the width of trail of the front luminance distribution of the individual light sources12,12, . . . overlap, and on the contrary when the distance W becomes smaller than the designed distance W, the luminance level decreases at a point (middle point) between every adjacent light sources12,12, . . . , largely modifying the front luminance distribution.

In contrast, in the surface emission device10, the split images12A,12A, . . . of the light sources12,12, . . . preliminarily overlap under the designed distance W, and part of the individual front luminance distributions overlap with each other, so that fluctuation in the luminance level relative to changes in the distance W between the light sources12,12, . . . and the optical element15may be suppressed, and as shown inFIG. 8, changes in the front luminance distribution may be small even if the distance W changes, and thereby non-uniformity in luminance may be suppressed.

In addition, because the non-uniformity in luminance ascribable to changes in the distance W between the light sources12,12, . . . and the optical element15may be suppressed, the degree of freedom in the arrangement of the optical element15relative to the housing11may be improved, and thereby workability in the process of assembling the individual components may be improved.

Moreover, overlapping of the split images12A,12A, . . . of the light sources12,12, . . . is synonymous to widening of the width of trail of the front luminance distribution, and this raises a property of making the front luminance distribution per se less susceptible to changes in the distance W.

FIG. 9is a graph showing front luminance distribution of light before transmitting through a diffuser plate14under varied distance W, for a case where the split images12A,12A, . . . of the light sources12,12, . . . slightly overlap at the middle point of between the light sources12,12, collectively showing a front luminance distribution observed under a designed distance W, and front luminance distributions observed under ±8% shift from the distance W.

The front luminance distributions shown inFIG. 9express results obtained by the Monte Carlo simulation for the case of reflection, refraction and scattering conforming to probabilities according to the optical characteristics, similarly to as shown inFIG. 8.

Also for the case where the split images12A,12A, . . . of the light sources12,12, . . . slightly overlap as shown inFIG. 9, it was confirmed that fluctuation in the luminance level relative to changes in the distance W between the light sources12,12, . . . and the optical element15may be suppressed, and that changes in the distance W resulted in only small changes in the front luminance distribution, so that non-uniformity in luminance may be suppressed. As a consequence, as described in the above, the degree of freedom in designing the distance W may be improved, by virtue of overlapping of the split images12A,12A, . . . of the light sources12,12, . . . , and spreading of the width of trail of the front luminance distribution correlative to spreading of the split images12A,12A, . . . .

Paragraphs below will show specific examples of configuration of the surface emission device10, capable of suppressing non-uniformity in luminance (seeFIG. 10toFIG. 21).

In general, in thin-type liquid crystal display devices, the diameter D of the light sources (cold cathode fluorescent lamps) is 3.0 mm to 4.0 mm, the distance L between the centers of every adjacent light sources is 20 mm to 40 mm, and the distance W from the center of the light source to the optical element in the direction of optical axis is 6.0 mm to 16.0 mm. As the optical element, engineering plastics low in price and adapted to mass production are used, wherein the thickness d of the optical element is 0.3 mm to 2.0 mm, and the refractive index n is 1.50 to 1.63. Refractive index n0of air is approximately 1.0.

For example, assuming W=11.7 mm and D=3.0 mm under L=23.7 mm, as shown inFIG. 10, the split images may overlap if the travel range x=L/2−D/2 of the split images of the light sources in the direction normal to the optical axis is 10.35 mm or larger.

In this case, given with d=0.4 mm and n=1.585, the tangential angle ψ of the optical element and the travel range x may be in relation as shown inFIG. 11, based on the equation (1) to the equation (3). FromFIG. 11, the tangential angle b giving x=10.35 mm is found to be approximately 56°. As a consequence, when the parameters adoptable to the general thin-type liquid crystal display devices are given as L=23.7 mm, W=11.7 mm, D=3.0 mm, d=0.4 mm, n=1.585 and n0=1.0, it is necessary for the luminance distribution generating layer of the optical element to have a profile with a maximum tangential angle a equal to or larger than the tangential angle ψ=56°.

As described in the above, by determining all parameters, that is, the distance L between the individual centers of every adjacent light sources, the refractive index n of the optical element, the thickness d of the optical element, the distance W from the center of the light source to the optical element in the direction of optical axis, the refractive index n0of air in the air layer, and the diameter D of the light source, a profile required for the luminance distribution generating layer of the optical element may be determined by the tangential angle calculated using the equation (1) to equation (3).

Maximum values (bmax) and minimum values (bmin) of the tangential angle b calculated within the ranges of L=20 mm to 40 mm, W=6.0 mm to 16.0 mm, D=3.0 mm to 4.0 mm, d=0.3 mm to 2.0 mm, n=1.50 to 1.63, which are the parameters generally adopted to the liquid crystal display devices, are shown in Table 1.

For an exemplary case with L/W=3.0, (L/2−D/2) shows a maximum value under L=40 mm, W=13.3 mm, D=3.0 mm, and a maximum value (bmax) is shown under n=1.50, d=0.3 mm.

If maximum tangential angle a larger than the tangential angle b shown in Table 1 should reside in the luminance distribution generating layer18, the split images12A,12A, . . . of the light sources12,12, . . . may overlap, so that in the surface emission device10, the optical element15is formed so that the maximum tangential angle a larger than the tangential angles b shown in Table 1 may reside in the luminance distribution generating layer18.

Accordingly, in the surface emission device10, the split images12A,12A, . . . of the light sources12,12, . . . overlap, so that changes in the distance W from the center of the light source12to the optical element15in the direction of optical axis P may result in only small changes in the front luminance distribution, thereby the non-uniformity in luminance may be suppressed.

A more preferable front luminance distribution may be exemplified by a case shown inFIG. 12, in which split image12A reaches the position straightly above both adjacent light sources12,12. In order to obtain this sort of front luminance distribution, the tangential angle ψ satisfying x=L−D/2 may be the maximum tangential angle a, so that it is good enough that the tangential angle c under which the split image12A of the light source12reaches the center of the adjacent light source12may agree with the maximum tangential angle a. It is to be noted that straight line T shown inFIG. 12represents the front luminance distribution obtained by summing up the front luminance distributions of the individual light sources12,12, . . . .

Table 2 shows maximum values (cmax) and minimum values (cmin) of the tangential angle c using the parameters same as those used for calculating the values in Table 1, including L=20 mm to 40 mm, W=6.0 mm to 16.0 mm, D=3.0 mm to 4.0 mm, d=0.3 mm to 2.0 mm, and n=1.50 to 1.63.

It may therefore be said that, if the maximum tangential angle a same as the tangential angle c shown in Table 2 should reside in the luminance distribution generating layer18, the split images12A,12A, . . . of the light sources12,12, . . . may overlap over the entire range between every adjacent light sources12,12, . . . .

In addition, in order to obtain the front luminance distribution shown inFIG. 12, it may be necessary that, assuming the luminance level at the position straightly above the light source12as 1, the luminance level of the split image12A of this light source12at the middle point between itself and the adjacent light source12is approximately halved to as low as 0.4 to 0.6 or around, and the luminance level at the position straightly above the adjacent light source12is approximately 0. Accordingly, it may be necessary that the maximum tangential angle a of the optical element15is approximately same with the tangential angle c shown in Table 2, and that the luminance distribution generating layer18contains portions with a tangential angle of b or larger and smaller than c to as much as 40% to 60%. The tangential angle b is an angle under which the split image12A of the light source12reaches L/2, as described in the above, and the tangential angle c is an angle under which the split image12A of the light source12reaches L.

In the surface emission device10, the optical element15is formed so that the maximum tangential angle a almost equal to the tangential angle c such as shown in Table 2 resides in the luminance distribution generating layer18, and that the portions with a tangential angle of b or larger and smaller than c are contained to as much as 40% to 60% in the optical luminance distribution generating layer18.

Accordingly, in the surface emission device10, the split images12A,12A, . . . of the light sources12,12, . . . overlap over the entire region between the individual light sources12,12, . . . , so that changes in the distance W from the center of the light source12to the optical element15in the direction of optical axis P may result in only small changes in the front luminance distribution, thereby the non-uniformity in luminance may be suppressed.

FIG. 13is a graph showing a relation between the maximum tangential angle a and the travel range x of the split image, assuming the distance W from the center of the light source to the optical element in the direction of optical axis as 1, the diameter D of light source as 0.25, giving W/D=4, the refractive index n of the optical element as 1.585, and the thickness d of the optical element as 0.4 mm.

As shown inFIG. 13, rate of change of the maximum tangential angle a increases as the travel range x increases. Since increase in L/W, expressing a ratio of the distance L between every adjacent light sources with respect to the distance W from the center of the light source to the optical element in the direction of optical axis, results in increase also in the travel range x, so that increase in L/W results in increase in the amount of change of travel range x relative to change in the maximum tangential angle a.

As described above, the increase in the rate of change of maximum tangential angle a makes formation of the luminance distribution generating layer difficult. Accordingly, increase in L/W makes formation of the luminance distribution generating layer difficult, and consequently makes control of the maximum tangential angle a difficult.

The maximum tangential angle a may be controllable in the range of L/W of 2.5 or smaller, so that non-uniformity in luminance may appropriately be suppressed in the range of Table 2 with L/W of 2.5 or smaller.

FIG. 14andFIG. 15are graphs showing front luminance distributions in the states where light emitted from the light source12transmitted through the optical element15and the diffuser plate14.FIG. 14is a graph corresponding to a front luminance distribution when the designed distance W shown inFIG. 8is kept, andFIG. 15is a graph corresponded to a front luminance distribution when the designed distance W shown inFIG. 9is kept.

As shown inFIG. 14andFIG. 15, the front luminance distribution is made almost uniform, in the state of light after being transmitted through the diffuser plate14by virtue of diffusing function of the diffuser plate14.

By using the diffuser plate14in this way, the front luminance distribution may be made uniform, and the non-uniformity in luminance may be prevented from occurring.

As described in the above, since the front luminance distribution may be made uniform by using the diffuser plate14, in the state of light emitted from the light sources12,12, . . . and before being transmitted through the diffuser plate14, the front luminance distribution may be made uniform if large difference between the maximum values and the minimum values of the luminance in the front luminance distribution is avoidable.

An allowable range of ratio of the maximum values and the minimum values of the luminance level, under which the front luminance distribution may be made uniform by the action of the diffuser plate14, may be 0.7 or larger, for example, taking the action of the diffuser plate into consideration.

FIG. 16andFIG. 17are graphs plotting on the ordinate the tangential angle ψ, and plotting on the abscissa the ratio of content of such tangential angle ψ in the luminance distribution generating layer, obtained when 23 samples in total were investigated into the state of non-uniformity in luminance.

FIG. 16andFIG. 17are graphs, showing data similarly as inFIG. 13, expressing relations between the tangential angle ψ and the ratio of such tangential angle ψ, assuming the distance W from the center of the light source to the optical element in the direction of optical axis as 1, the diameter D of light source as 0.25, giving W/D=4, the refractive index n of the optical element as 1.585, and the thickness d of the optical element as 0.4 mm. Note that a ratio of the distance L between the centers of every adjacent light sources relative to the distance W herein is given as L/W=2.0, and transmissivity of light of the diffuser plate is 60%.

FIG. 16shows data of 11 samples (A to K) having ratios of the maximum values and the minimum values of the luminance level in the front luminance distribution of light before being transmitted through the diffuser plate of 0.7 or larger, causative of only small incidence of non-uniformity in luminance, and therefore ensured with desirable uniformity in the front luminance distribution.

On the other hand,FIG. 17shows data of 12 samples (L to W) having ratios of the maximum values and the minimum values of the luminance level in the front luminance distribution of light before being transmitted through the diffuser plate of smaller than 0.7, causative of large incidence of non-uniformity in luminance, and therefore failed in ensuring desirable uniformity in the front luminance distribution.

As shown inFIG. 16, data corresponding to small incidence of non-uniformity in luminance indicate that all samples contain the portions with the tangential angle ψ larger than the tangential angle b=56° in the luminance distribution generating layer, and that the portions with the tangential angle b (=56°) are contained approximately to as much as 10% to 30% of all tangential angles ψ (indicated by R inFIG. 16).

On the other hand, the data corresponding to large incidence of non-uniformity in luminance shown inFIG. 17involves samples (L, M, N, O, P, Q) having the maximum tangential angle a smaller than the tangential angle b (=56°), samples (T, V, W) having portions with the tangential angle b or larger contained to as much as exceeding 30%, and a sample (R) having portions with the tangential angle b only to as much as less than 10%, excluding 2 samples.

As shown inFIG. 16andFIG. 17, it was confirmed that the non-uniformity in luminance was suppressed, when the portions with the tangential angle b (=56°) were contained to as much as 10% to 30% of all tangential angles ψ, and the ratio of maximum values and the minimum values of the luminance level in the front luminance distribution of light before being transmitted through the diffuser plate consequently became 0.7 or larger. As a consequence, the non-uniformity in luminance may be suppressed, by forming the luminance distribution generating layer18of the optical element15so as to contain the portions with the tangential angle b or larger to as much as 10% to 30% of the tangential angles ψ.

FIG. 18is a graph showing a front luminance distribution observed in a state where light is emitted from the light sources transmitted through the optical element15and luminance distribution generating layer18. As shown inFIG. 18, when the split image12A of the light source12is positioned straightly above the adjacent light source12(seeFIG. 12), uniformity in the front luminance distribution may be ensured even if the diffuser plate14is not provided. It is necessary herein, particularly for the case where L/W has large values, to form the luminance distribution generating layer18with large tangential angles ψ, for example with a tangential angle b of 56° or larger, in a highly precise manner.

However, as described in the above, by providing the diffuser plate14capable of diffusing light transmitted through the optical element15, the split image12A may be positioned straightly above the adjacent light source12by virtue of the action of the diffuser plate14, in the state of light after being transmitted through the diffuser plate, even if the split image12A of the light source12does not position straightly above the adjacent light source12in the state of light before being transmitted through the diffuser plate14. As a consequence, if the diffuser plate14is provided, it will be not so much necessary to form the luminance distribution generating layer18with large tangential angles ψ irrespective of magnitude of L/W, and thereby manufacturing of the optical element15may be facilitated.

For the case where the diffuser plate14is not provided, the outer surface of the structural portions18a,18a, . . . of the luminance distribution generating layer18of the optical element15may preferably be formed into curved profile, in order to consecutively overlap the split images12aof the light source12with each other, whereas for the case where the diffuser plate14is provided, the travel range x of the split image12A may be reduced, and a smooth luminance distribution may be formed by virtue of effects of the diffuser plate14even under discontinuous split images12A, so that the outer surface of the structural portions18a,18a, . . . may now be formed into a polygonal profile, or as having flat surface in part of the outer surface thereof, for example, and thereby manufacturing of the optical element15may be facilitated.

FIG. 19shows an exemplary optical element15having structural portions18a,18a. . . formed into a flat plane at least in part thereof, when the diffuser plate14is provided.

FIG. 19shows an exemplary case where a structural component is composed of a set of three structural portions18b,18c,18d, and a large number of such structural components are consecutively formed.

In the example shown inFIG. 19, given with L/W=2.0, the luminance distribution generating layer18contains the portions with a tangential angle b of 56° or larger to as much as 10% to 15%, the luminance distribution generating layer18contains the portions with a tangential angle of 0°, that is, the portions formed into a flat plane normal to the optical axis, to as much as 10% to 20%, and the structural portion18cand the structural portion18dare formed into polygonal profiles.

FIG. 20is a graph plotting on the ordinate the tangential angle ψ, and plotting on the abscissa the ratio of content of such tangential angle ψ in the luminance distribution generating layer18, obtained for the optical element15shown inFIG. 19. As shown inFIG. 20, the optical element15shown inFIG. 19contains the portions with a tangential angle of approximately 56° in the luminance distribution generating layer18to as much as 10% to 15%.

FIG. 21is a graph showing a front luminance distribution in a state where light emitted from the light sources12,12, . . . transmitted through the diffuser plate14, for the cases shown inFIG. 19andFIG. 20. Transmissivity of light of the diffuser plate14herein is 60%. As shown inFIG. 21, it was confirmed that the light was diffused by the diffusive action of the diffuser plate14, a desirable level of uniformity in the front luminance distribution was ensured, and thereby the non-uniformity in luminance was suppressed.

Accordingly, the non-uniformity in luminance may be suppressed by using the diffuser plate14, even when the outer surface of the structural portions18a,18a, . . . of the luminance distribution generating layer18of the optical element15was formed into a polygonal profile, or partially into a flat plane.

In the surface emission device10, since the optical element component16such as diffusion sheet, prism sheet, or reflective polarizer, for example, is disposed as being opposed to the optical element15across the diffuser plate14, the light diffused by the diffuser plate14may further be subjected to diffusion, scattering and so forth by the optical element component16, and thereby the suppressive effect on the non-uniformity in luminance may be improved.

Next, an optical element package, which is a structure of integrating the optical element15and the diffuser plate14, will be explained (seeFIG. 22andFIG. 23).

As described in the above, in the surface emission device10, the optical element15, the diffuser plate14and the optical element component16are sequentially disposed as viewed from the side of the light sources12,12, . . . , wherein warping, waving or the like because of its low rigidity may occur due to the thickness of these components, raising a cause for generating non-uniformity in luminance.

In order to prevent such warping and waving from occurring, the optical element15and the diffuser plate14, or the optical element15and the diffuser plate14and the optical element component16may be packaged using a packaging component20such as transparent sheet or transparent film, to thereby configure an optical element package21(seeFIG. 22).

Alternatively, for example, the optical element15and the diffuser plate14may be bonded using a ultraviolet-curable resin or pressure-sensitive adhesive, to thereby configure an optical element package22(seeFIG. 23). In this case, in addition to the optical element15and the diffuser plate14, also the optical element component16may be bonded to the diffuser plate14, to thereby configure the optical element package22.

By configuring the optical element package21or the optical element package22, the rigidity may be enhanced by increasing the thickness, and thereby avoiding the warping, waving and so forth.

Paragraphs below will show exemplary sectional profiles of the luminance distribution generating layer18of the optical element15(seeFIG. 24toFIG. 29).

Although the non-uniformity in luminance may be suppressed by forming the sectional profile (profile of the outer surface) of the luminance distribution generating layer18of the optical element15into a desired curved profile, it may often be difficult to form the luminance distribution generating layer18into the curved profile as described in the above. Formation of the luminance distribution generating layer18with polygonal profiles such as shown below, as approximation of the curved profile, will now successfully suppress the non-uniformity in luminance, while keeping the desirable workability.

The luminance distribution generating layer100is configured by an outer surface101laid in parallel with the direction of arrangement of the light sources, and the outer surfaces102,102,103,103, . . . ,107,107gradually increased, referring to the outer surface101, in the angle of inclination with respect to the direction of arrangement of the light sources towards the light sources. The luminance distribution generating layer100has a profile symmetrical in the direction of arrangement of the light sources about a center line M which falls on the point halving the outer surface101. Assuming now angles of inclination of the individual outer surfaces101,102,103, . . . in the direction of arrangement of the light sources sequentially as s1, s2, s3, . . . , s7, the luminance distribution generating layer100is formed so as to satisfy s1<s2<s3< . . . <s7.

Although the luminance distribution generating layer100herein is configured with 13 outer surfaces (line segments) differed in the angle, the number of outer surfaces is not limited to 13, instead, the number of outer surfaces may arbitrarily be determined while considering the distance L between the light sources, the diameter D of the light source and so forth.

By using the luminance distribution generating layer18having the sectional profile approximated to a curved profile as shown inFIG. 24, it is no more necessary to form such curved profile which may otherwise be difficult to form, and thereby ensuring a desirable workability of the optical element.

FIG. 25andFIG. 26show examples200,300of the luminance distribution generating layer, wherein the polygonal profile shown inFIG. 24was divided to form a plurality of structural portions.

The luminance distribution generating layer200shown inFIG. 25is configured with a plurality of sets of two structural portions200a,200balternately arranged.

The structural portion200ahas seven outer surfaces, for example, and is composed of outer surfaces201,202,202,203,203,204,204, whereas the structural portion200balso has seven outer surfaces, for example, and is composed of outer surfaces205,206,206,207,207,208,208.

The outer surfaces201,205are laid in parallel with the direction of arrangement of the light sources, and respectively have an angle of inclination same as the angle of inclination s1of the luminance distribution generating layer100. The angles of inclination of the outer surfaces202,203,204with respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s3, s5, s7in the luminance distribution generating layer100, and the angles of inclination of the outer surfaces206,207,208with respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s2, s4, s6in the luminance distribution generating layer100.

By using such luminance distribution generating layer200composed of the structural portions200a,200b,200a,200b, . . . derived by division from the profile of the luminance distribution generating layer100, the optical element may readily be processed, by virtue of smallness in the number of outer surfaces of the structural portions200a,200b.

The luminance distribution generating layer300shown inFIG. 26is configured with a plurality of sets of two structural portions300a,300balternately arranged.

The structural portion300ahas six outer surfaces, for example, and is composed of outer surfaces301,301,302,302,303,303, whereas the structural portion300balso has six outer surfaces, for example, and is composed of outer surfaces304,304,305,305,306,306.

The angles of inclination of the outer surfaces301,302,303with respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s3, s5, s7in the luminance distribution generating layer100, and the angles of inclination of the outer surfaces304,305,306with respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s2, s4, s6in the luminance distribution generating layer100.

Between the structural portions300aand300b, there is formed a parallel plane307which is parallel with the direction of arrangement of the light sources. The parallel plane307is a plane corresponding to the outer surface101of the luminance distribution generating layer100.

By using such luminance distribution generating layer300composed of the structural portions300a,300b,300a,300b, . . . derived by division from the profile of the luminance distribution generating layer100, the optical element may readily be processed, by virtue of smallness in the number of outer surfaces of the structural portions300a,300b.

Furthermore, for the case of using the luminance distribution generating layer300, when the optical element is formed by injection molding using a die1000such as shown inFIG. 27, the die1000will have a projection between the portions for forming the structural portion300aand the structural portion300b, so that the die is added with a large rigidity by virtue of the projection1001having a predetermined width in the direction of arrangement of the light sources. Accordingly, the projection1001may be less likely to deform, may allow smooth releasing of the die1000, and may thereby improve accuracy of processing of the molded luminance distribution generating layer300.

FIG. 28andFIG. 29show examples400,500of the luminance distribution generating layer, wherein the polygonal profile was divided into three.

The luminance distribution generating layer400shown inFIG. 28is configured with a plurality of sets of three structural portions400a,400b,400calternately arranged.

The structural portions400a,400b,400crespectively have five outer surfaces, for example, wherein the angles of inclination of the outer surfaces401,402,403of the structural portion400awith respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s1, s3, s6in the luminance distribution generating layer100, the angles of inclination of the outer surfaces404,405,406of the structural portion400bwith respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s1, s4, s7in the luminance distribution generating layer100, and the angles of inclination of the outer surfaces407,408,409of the structural portion400cwith respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s1, s2, s5in the luminance distribution generating layer100.

By using such luminance distribution generating layer400composed of the structural portions400a,400b,400c, derived by division from the profile of the luminance distribution generating layer100, the optical element may readily be processed, by virtue of smallness in the number of outer surfaces of the structural portions400a,400b,400c.

The luminance distribution generating layer500shown inFIG. 29is configured with a plurality of sets of three structural portions500a,500b,500calternately arranged.

The structural portions500a,500b,400crespectively have four outer surfaces, for example, wherein the angles of inclination of the outer surfaces501,502of the structural portion500awith respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s3, s6in the luminance distribution generating layer100, the angles of inclination of the outer surfaces503,504of the structural portion500bwith respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s4, s7in the luminance distribution generating layer100, and the angles of inclination of the outer surfaces505,506of the structural portion500cwith respect to the direction of arrangement of the light sources are respectively set equal to the angles of inclination s2, s5in the luminance distribution generating layer100.

Between the structural portions500a,500b,500c, there are formed parallel planes507,507which are parallel with the direction of arrangement of the light sources. The parallel planes507,507are planes corresponding to the outer surface101of the luminance distribution generating layer100.

By using such luminance distribution generating layer500composed of the structural portions500a,500b,500cderived by division from the profile of the luminance distribution generating layer100, the optical element may readily be processed, by virtue of smallness in the number of outer surfaces of the structural portions500a,500b,500c.

Also for the case where the luminance distribution generating layer500is used, a die will have a highly rigid projection similarly to the case where the luminance distribution generating layer300was used, and thereby accuracy of processing of the molded luminance distribution generating layer500may be improved.

The descriptions in the above showed the exemplary luminance distribution generating layers having a plurality of sets of two or three structural portions sequentially arranged, wherein the number of division of the polygonal profile is not limited to two or three, but may be four or more. These structures may be understood as being obtained by dividing a polygonal profile into a plurality of structural portions, showing optical characteristics not so largely differ from those of the undivided luminance distribution generating layer100, so that the structure may arbitrarily be selected considering the processability.

FIG. 30is a graph showing exemplary results of simulation of front luminance distribution of the optical element having the luminance distribution generating layer300, and corresponds toFIG. 4.

The front luminance distribution shows a mountain-like profile maximized in the luminance level at the position straightly above the light source12, and sloped down towards the positions straightly above adjacent other light sources.

FIG. 31is a graph showing a front luminance distribution observed when all light sources were turned on inFIG. 30, and corresponds toFIG. 18.

The results shown inFIG. 30andFIG. 31are found to contain slight non-uniformity in luminance as a whole, when compared with the results shown inFIG. 4andFIG. 18, but the non-uniformity in luminance is different from non-uniformity in light sources depending on the distance L between the light sources, and may be suppressed to a non-problematic level in practice, by disposing a diffuser plate, diffuser sheet or the like.