JPH10106328A - Surface light source element and liquid crystal display unit usable in both reflective and transmissive modes - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface light source apparatus having excellent visibility in both of reflecting and transmitting modes and easy to be switched by forming a tilting face having a specified polarized state retaining characteristic in the lower face of a light guiding plate provided with a light source which is made possible to be switched to on and off state in the incident side face. SOLUTION: In a light guiding plate 1 comprising an upper face 11, which is light emitting face, a lower face 12 having a reflecting layer 2, an incident side face 13, a side end part 15 with about 50% or thinner thickness of the face 13, and a side face 16, a light source 3 provided with a lighting and extinguishing switching mechanism constituted of a switch 32, an electric power source 33, etc., is installed in the incident side face 13. Consequently, the light source apparatus is made usable in both of reflective and transmissive modes using outside light and back light. Moreover, the reflecting layer 2 is constituted by alternately forming faces with long side at (-10)-10 deg. tilting angle to the upper face and faces with short side at 25-50 deg. tilting angle. As a result, incident light rays are introduced from the faces with long side and having wider surface area and efficiently reflected in polarized state with excellent orientation property and emitted out of the upper face. In the upper side of the upper face 11 of the surface light source apparatus, liquid crystal cells having polarizing plates in the front and rear sides are arranged to give a liquid crystal display unit usable in both reflective and transmissive modes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light guide plate type surface light source device excellent in light use efficiency in both reflection and transmission modes, and a liquid crystal display device using the same, which consumes less power for both reflection and transmission.

[0002]

2. Description of the Related Art In a portable liquid crystal display device such as a mobile phone using a battery such as a storage battery, a reflection type device which is visually recognized using external light such as sunlight becomes invisible in a dark portion such as at night. The transmission type, which is visible through the illumination light from the backlight, consumes a lot of power and requires frequent replacement of batteries. There has been proposed a liquid crystal display device for both reflection and transmission which can be visually recognized even in a reflection mode according to a mold.

Conventionally, as the above-mentioned liquid crystal display device for both reflection and transmission, a transflective type reflector is arranged on a backlight side of a liquid crystal cell having a backlight via a polarizing plate, and the back light is exposed to sunlight or the like. There is known a reflection mode in which the light is turned off, and in a nighttime or a dark room, the light is visually recognized in a transmission mode in which the backlight is turned on. An EL (electroluminescence) lamp with low power consumption is frequently used as the backlight. ing.

However, in the above-mentioned apparatus, the transmissivity and the transmissivity of the semi-transmissive reflector used in the apparatus conflict with each other, and the transmissivity is usually 50 to 80% and the transmissivity is 5%.
The visibility in both the reflection and transmission modes is adjusted by a reflection plate controlled to 0 to 20%. Therefore, the transmittance in the reflection mode and the reflectance in the transmission mode lower the brightness of the display. There is a problem in that it is difficult to obtain a liquid crystal display device having a viewing performance comparable to that of a transmission-type or reflection-type dedicated device. In addition, EL lamps have a drawback that they have poor brightness and maintainability, and have a short life. Even a single-color type lamp has problems that the brightness is not sufficient and the color rendering properties for colorization are poor.

On the other hand, a surface light source device using a side light type light guide plate in which incident light from a side surface is totally reflected by a lower surface on which dots are formed and emitted from an upper surface is also known. When incident external light is reflected by the lower surface, it is greatly scattered and light in a direction effective for visual recognition is hardly emitted,
Also, in a liquid crystal display device, since external incident light is linearly polarized via a polarizing plate and is incident on the lower surface, the polarization state is eliminated when scattered and reflected through the lower surface, and about half when re-entering the polarizing plate. There is a problem that the light amount is absorbed and the reflector is unsuitable for viewing in the reflection mode.

In the above, when the surface light source device is used for visual recognition in the transmission mode, a prism sheet effective for controlling the direction of light emitted from the upper surface is also provided. In the reflection mode, external light enters the light guide plate via the prism sheet. Therefore, a large amount of light, such as vertical incidence, which is advantageous for visual recognition, is reflected on the surface of the prism sheet, and is not effectively used. Therefore, in order to obtain a liquid crystal display device for both reflection and transmission in a conventional surface light source device, it is necessary to arrange a transflective reflector on the upper surface of the light guide plate, which causes the above-mentioned problems.

[0007]

The present invention has a visibility comparable to that of a conventional reflective or transmissive dedicated machine, and therefore, in a reflection mode in which the backlight is turned off when there is external light more than in the lighting room, is sufficient. In a dark room or when a particularly bright view is required, it can be viewed in the transmission mode with the backlight turned on, and it has excellent visibility in both reflection and transmission modes,
It is an object of the present invention to develop a surface light source device capable of forming a liquid crystal display device capable of easily switching between the reflection mode and the transmission mode.

[0008]

According to the present invention, there is provided a light guide provided with a light-on / off switching mechanism on an incident side surface of a light guide plate configured to reflect incident light from a side surface on a lower surface and to emit the light from an upper surface; Has, on the lower surface thereof, a long side surface having an inclination angle of −10 to 10 degrees with respect to the upper surface and a short side surface inclined from the upper surface to the incident side surface side in an angle range of 25 to 50 degrees alternately. A surface light source device, comprising a reflective layer having a polarization maintaining state and a projection area of the long side surface with respect to the upper surface being at least five times that of the short side surface, and the surface light source device. A liquid crystal cell having a polarizing plate on both sides thereof is disposed on the upper side of the liquid crystal display device.

[0009]

According to the present invention, external light passes through the light guide plate and efficiently enters the long side surface of the wide area, and has good directivity through the long side surface portion of the reflective layer disposed on the lower surface, and A surface that emits light that is reflected with good maintainability of the polarization state and is effective for visual recognition from the upper surface with high reflectance, and light that enters from the side surface via the light source efficiently and is emitted from the upper surface in a direction advantageous for visual recognition. A light source device can be obtained.

Therefore, by using the above-mentioned surface light source device, it is possible to obtain a reflection mode with a brightness comparable to that of an illuminating room with excellent reflection characteristics of incident external light including a small absorption of re-incident light to the polarizing plate in the reflection mode. In addition to being able to achieve sufficient visibility, and using a long-life light source that excels in brightness and maintainability even in the transmission mode, good visibility and color rendering with excellent color rendering can be achieved. It is possible to obtain a liquid crystal display device that is comparable to a dedicated mold type device and exhibits good visibility in a transmission mode comparable to a conventional dedicated transmission type device, and that can easily switch between reflection and transmission modes.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface light source device according to the present invention has a light source provided with a light-on / off switching mechanism on an incident side surface of a light guide plate which reflects incident light from a side surface on a lower surface and emits the light from an upper surface.
And the light guide plate has a lower surface, and an inclination angle with respect to the upper surface is −
The long side surface in the range of 10 to 10 degrees and the inclination angle is 25 to 5
It has a short side surface inclined from the upper surface to the incident side surface side in the range of 0 degrees, and has a reflective layer having a polarization maintaining property, and the projected area of the longer side surface with respect to the upper surface is the shorter side surface. It is more than 5 times that. An example is shown in FIG. 1 is a light guide plate, 2 is a reflection layer, 3 is a light-on / off switching mechanism 3
2 and 33.

The light guide plate has an upper surface, a lower surface opposed thereto, and an incident side surface composed of a side surface between the upper surface and the lower surface, and the incident light from the incident side surface is reflected by the lower surface and emitted from the upper surface. A plate is used. Examples are shown in Figs.
As shown in FIG. 11, upper surface, 12, 16, 17 lower surface,
Reference numeral 13 denotes an incident side surface, 14 denotes a side surface, and 15 denotes a side end portion facing the incident side surface 13.

The light guide plate is not limited to the one shown in the figure, but has a side end portion facing the incident side surface having a thickness smaller than that of the incident side surface, particularly 50% or less. Is preferred. The thickness of the opposite side end portion with respect to the incident side surface is reduced by making the light incident from the incident side surface (the thick arrow in FIGS. 5 and 6) reach the opposite side end portion as the transmission end on the short side surface of the lower surface. It has the advantage that it can efficiently enter and exit from the upper surface via its reflection, and efficiently supply incident transmission light to the upper surface. Further, by adopting such a thin structure, the weight of the light guide plate can be reduced. For example, when the lower surface is a straight surface as shown in FIG. 2, the weight can be about 75% of the light guide plate having a uniform thickness.

The lower surface of the light guide plate has an inclination angle of −
The long side surface in the range of 10 to 10 degrees and the inclination angle is 25 to 5
The structure has alternately short sides inclined from the upper surface to the incident side in the range of 0 degrees, and the projected area of the longer side with respect to the upper surface is five times or more that of the shorter side. Examples of the lower surface structure are shown in FIGS. 5 (a) to 5 (d) and FIGS. 6 (a) to 6 (d). In FIG. 5, the long side surface and the short side surface are convex portions 21, 22, 2
FIG. 6 illustrates an example in which the long side surface and the short side surface form the concave portions 25, 26, 27, and 28, respectively. 41, 43, 45, 47, 5
2, 54, 56, 58 are long side surfaces, 42, 44, 46, 4
8, 51, 53, 55, and 57 are short side surfaces, and the convex portions or concave portions are formed periodically in a direction along the incident side surface.

In the above description, the short side surface is a slope inclined from the upper surface to the incident side surface side, but the long side surface only needs to be in a range of -10 to 10 degrees with respect to the upper surface.
The long side surface has a surface parallel to the upper surface (inclination angle of 0 degree), a slope (+ direction) inclined from the upper surface to the incident side surface, and a slope (-direction) inclined to the side end side facing the incident side surface from the upper surface. ) Can be adopted, so that the structure of the lower surface can take various forms that can be classified into a + direction inclined type, a parallel type, and a − direction inclined type based on the long side surface.

In FIGS. 5 and 6, the projections or depressions are the slopes (41 and 4) forming the projections (21, 22, 23, 24) or depressions (25, 26, 27, 28).
2, 43 and 44, 45 and 46, 47 and 48, 51 and 5
2, 53 and 54, 55 and 56, 57 and 58), based on a straight line 20 shown by a virtual line connecting the intersections with the lower surface, is the intersection (vertex) of the slope projecting beyond the straight line 20 (convex)? , Depending on whether it is concave (concave).

The long side surface on the lower surface of the light guide plate is adapted to receive transmitted light from the incident side surface (transmission mode) as shown in FIG.
6A, light transmitted at an angle larger than the inclination angle with respect to the upper surface is applied to the long side surface 4 as illustrated by the broken line arrow in FIG.
7A, the transmitted light is parallelized by being reflected through the first and second light sources 52, and external light incident from the upper surface is reflected (reflection mode).
(B) As shown by a broken arrow in (b), it is an object to emit return light via reflection from the upper surface without changing the angle as much as possible.

As described above, in the transmission mode, the transmission light incident at an angle larger than the inclination angle of the long side surface is converted to the short side surface (42, 5).
1) can be made to enter by increasing the parallelism with the light directly incident thereon, and improve the directivity (parallelism) of the light reflected from the short side surface and emitted from the upper surface to improve the brightness and visibility. Can be done. Even in the reflection mode, reflection of incident external light effective for visual recognition in a direction that does not contribute to visual recognition is suppressed, and the efficiency of use of incident external light for luminance and visibility is improved.

The preferred inclination angle of the long side surface with respect to the upper surface from the above point is -7 to 7 degrees, particularly -5 to 5 degrees, particularly -3 to 5 degrees.
Three degrees. The sign of the angle depends on whether it is inclined from the upper surface to the side of the incident side (+ direction) as described above or to the side end side facing the incident side (-direction). Also, the inclination angle of the long side surface does not need to be constant within the same light guide plate.

A particularly advantageous long side surface in consideration of parallelizing the transmission light in the transmission mode has a tilt angle of -5 to 0 degrees with respect to the upper surface, and the long side surface as illustrated in FIGS. And the short side surface forms a convex portion or a concave portion. Note in front view shows the inclination angle of the long side surface at theta _1. Further, since the incident angle of the transmission light with respect to the long side surface becomes smaller as the distance from the incident side surface becomes smaller, the reflection angle can be sequentially reduced by gradually reducing the inclination angle of the long side surface with respect to the incident side surface,
This is advantageous for making the reflected transmission light parallel.

In the above description, when the inclination angle of the long side surface exceeds 10 degrees in absolute value, the ratio of the projected area to the top surface of the long side surface is reduced, so that the effective reflection area in the reflection mode is reduced, and the visible area is reduced. The reflectance in the direction advantageous to the above also decreases. On the other hand, even in the transmission mode, the ratio of the transmission light whose emission direction can be controlled through the long side surface decreases due to the decrease in the effective area, and the transmission light incident on the short side surface via the long side surface and the short side. The variation in the reflection angle with the transmission light directly incident on the side surface becomes large, and the controllability for making the output light parallel is reduced, and the directivity of the output light becomes poor.

On the other hand, the short side surface on the lower surface of the light guide plate is used for transmitting the transmission light which is incident directly or via the long side surface as illustrated by the broken line arrows in FIGS. 5 (a) and 6 (a). Light that is reflected through the short side surfaces 42 and 51 in a target emission direction such as a direction perpendicular to the upper surface 11 or an angle direction close thereto and has a direction effective for improving visibility in a transmission mode of a liquid crystal display device or the like. It is intended to efficiently emit light.

The inclination angle of the short side surface more preferable than the above point is:
38-45 degrees, especially 40-44 degrees. If the inclination angle of the short side face is out of the range of 25 to 50 degrees, the above-mentioned emission direction is largely deviated from the vertical direction, and it becomes difficult to provide directivity to the emission light, and the emission efficiency (utilization efficiency) of the transmission light.
Also decrease. In FIGS. 5 and 6, the inclination angle of the short side is θ
Indicated by ₂ .

From the viewpoint of controlling the emission direction, it is preferable that the inclination angle of the short side surface is gradually increased based on the incident side surface. This is because the incident light density of the transmission light that is not parallel to the upper surface occupies the light amount of the transmission light directly incident on the short side surface, is unexpectedly large, particularly, the transmission light that is large near the incident side surface and is not parallel to the upper surface can be effectively used. It is an object to improve the light use efficiency as a short side surface of the inclination angle.

That is, it is an object of the present invention to increase the inclination angle of the short side surface as it becomes farther from the incident side surface, and to advantageously cope with the transmitted light having the transmission angle incident at the highest density. The transmission angle incident on the short side surface at the highest density varies depending on the inclination angle and area ratio of the long side surface and the short side surface, the shape of the light guide plate, etc., but by gradually changing within the range of the inclination angle described above. Optimization can be achieved.

The above-described gradual change may be determined as appropriate, such as a continuous change, a gradual change, and an average change (angle difference / number of short sides). From the viewpoint of the visibility of the liquid crystal display device, etc., the viewing angle with respect to the screen, especially the viewing angle in the longitudinal direction (left / right direction) increases with the size of the screen, and the mainstream of image light is higher than the visibility point. Preferably, the direction is toward the viewer. Generally, the control of the viewing position in which the viewing angle is larger than that of the center of the screen, which is about 40 cm away from the viewing position, can be achieved by increasing the tilt angle as the distance from the incident side increases.

In the above case, the ideal emission direction is a direction perpendicular to the upper surface in the central portion, and a direction toward the central portion in the up, down, left and right directions. It is preferable that the closer to the side surface, the closer to the angle in the transmission direction, and therefore, the closer to the upper surface direction. By the way, when viewing a 12-inch diagonal screen at a distance of 40 cm, the expected angle is ± 13 degrees. At that angle, it is better to emit light downward at the upper end of the screen and upward at the lower end. For example, the angle changes.

As described above, the long side surface of the lower surface of the light guide plate functions as a surface for receiving external light used for reflection in the reflection mode, and functions as an optical path control surface for transmission light in the transmission mode. It is formed so as to be five times or more of that of the short side surface. A preferable projection area on the upper surface of the long side surface is 10 times or more, especially 15 to 100 times, that of the short side surface from a point such as a reflectance of light effective for visual recognition in the reflection mode.

Therefore, as in the case of the conventional light guide plate, the slope for reflecting incident transmission light from the side surface and supplying it to the upper surface corresponding to the shorter side surface for the purpose of improving the emission efficiency is projected to the upper surface by increasing the slope. In the lower surface structure in which the area ratio is increased and the inclination angle of the surface corresponding to the long side surface is set to 20 degrees or more in absolute value, the probability of incidence of transmission light on this surface in the transmission mode is extremely small. The light path control function is hardly generated, and the emitted light has poor directivity. Even in the reflection mode, visibility is poor due to a shortage of a light receiving area effective for visual recognition.

In the above description, the shape of the lower surface of the light guide plate may be an appropriate surface shape such as a linear surface illustrated in FIG. 2 or a curved surface illustrated in FIGS. The downwardly protruding parabolic shape whose vertex 19 illustrated in FIGS. 3 and 4 is on the incident side surface 13 or on the incident side surface between the incident side surface 13 and the opposite end surface 15 is formed on the upper surface through the shape. The reflection angle is controlled to be uniform, the emitted light is easily focused in a direction advantageous for visual recognition, and there is an advantage that the light use efficiency in the transmission mode can be improved.

As shown in FIGS. 5A to 5D and FIGS. 6A to 6D, the shape of the long side surface and the short side surface on the lower surface may be a straight surface, a refraction surface, a curved surface, or the like. The shape of the long side surface and the short side surface need not be constant over the entire lower surface of the same light guide plate.

In the light guide plate used in the present invention, the long side surface in the reflection mode and the short side surface in the transmission mode mainly emit light to the upper surface, thereby preventing the emitted light from being formed into a striped bright line. This can be done by reducing the pitch in units of the long side surface and the short side surface of the alternate arrangement.

The preferable pitch is 500 μm from the viewpoint that an upper surface excellent in uniformity of brightness by preventing unevenness in brightness is obtained.
m, especially 300 μm or less, especially 5 to 200 μm. If the pitch is less than 5 μm, the dispersion due to diffraction is large, making it unsuitable for a backlight for a liquid crystal display device. The pitch does not need to be constant for the same light guide plate.

The shape of the incident side surface of the light guide plate is not particularly limited, and can be appropriately determined. Generally, the surface is perpendicular to the upper surface. However, for example, a shape corresponding to the outer periphery of the light source, such as a curved concave shape, can be used to improve the incident efficiency of incident light. Further, a structure having an introduction portion interposed between the light source and the light source may be employed, and the introduction portion may have an appropriate shape according to the light source or the like. Note that the shape of the upper surface of the light guide plate is generally a flat surface or the like, but may be a structure having a diffusion layer on the surface for the purpose of scattering, if necessary.

The light guide plate can be formed of an appropriate material exhibiting transparency according to the wavelength range of the light source. In the visible light region, for example, acrylic resins such as polymethyl methacrylate, polycarbonate resins such as polycarbonate and polycarbonate / polystyrene copolymer, and transparent resins and glass represented by epoxy resins are exemplified. A light guide plate that does not exhibit birefringence or is formed of a material having low birefringence is preferable from the viewpoint of maintaining the polarization state in the reflection mode.

A preferred method for producing a light guide plate from the viewpoint of mass productivity is to fill or cast a liquid resin that can be polymerized by heat, ultraviolet rays, or radiation into a mold capable of forming a predetermined bottom shape, and then polymerize the resin. Or a method of transferring a shape by pressing a thermoplastic resin under heating to a mold capable of forming a predetermined bottom shape, transferring a thermoplastic resin that has been heated and melted or a resin that has been fluidized through heat or a solvent. A method of filling a mold capable of being formed into a predetermined shape, and the like can be given.

In the surface light source device of the present invention, the angle distribution and the in-plane of the outgoing light are controlled based on the control of the shape and curvature of the lower surface of the light guide plate, the area ratio between the short side and the long side, and the inclination angle. Characteristics such as distribution can be adjusted. When the inclination angle of the long side surface is set to a minus angle, the thickness based on the short side surface can be canceled by the angle, so that the light guide plate can be thinned, which is advantageous for increasing the area. It is preferable to satisfy the condition of the area ratio in addition to the above-described inclination angle of the short side surface and the long side surface in order to obtain a thin and practically sized surface light source device for both reflection and transmission that efficiently emits light uniformly over the entire surface. .

The light guide plate may be formed as a laminated body of different materials, such as a light guide portion for transmitting light and a sheet for forming a lower surface adhered thereto. It need not be formed as a single layer. The thickness of the light guide plate can be appropriately determined depending on the size of the light guide plate, the size of the light source, and the like according to the purpose of use.

The general thickness of a light guide plate used for a liquid crystal display device or the like is 20 mm or less, especially 0.1 mm, based on the incident side surface.
10 to 10 mm, especially 0.5 to 8 mm. The general area ratio between the incident side surface and the upper surface is 1/5 to 1 based on the former / the latter.
/ 100, especially 1/10 to 1/80, especially 1/15 to 1
/ 50.

In forming the surface light source device, a reflection layer 2 having a polarization maintaining state is arranged on the lower surface of the light guide plate 1 as illustrated in FIG. The reflective layer may be provided in close contact with the lower surface of the light guide plate, or may be provided in a state where there is a gap between the concave and convex portions of the lower surface as a reflective plate or the like as shown in the figure, or a combination thereof. it can. The former reflection layer provided in close contact is effective in preventing the generation of light leaking from the lower surface, and the latter reflection layer having a gap, particularly a reflection layer having a gap between itself and a long side surface as illustrated in FIG. The layer is effective for suppressing a change in the re-emission angle of the external light in the reflection mode.

As the reflection layer, one having a polarization state maintaining property in which the polarization state in the reflection mode is not eliminated like a dot type diffuse reflection layer is used, and a layer capable of maintaining the polarization state as much as possible is preferable. Therefore, a reflective layer or the like having fine irregularities on the surface that does not largely cancel the polarization state is permissible. Rather, it is preferable because the diffusion effect reduces the formation of stripe-shaped bright lines based on the lower surface structure.

The reflecting layer is preferably formed of a metal containing at least one kind of high-reflectance metal such as aluminum, silver, gold, copper or chromium in terms of reflectance. The metal reflection layer also has an advantage that the polarization plane of linearly polarized light that is incident on the reflection plane at or near an angle close to the reflection plane is reflected with little conversion.

The reflection layer in close contact with the lower surface of the light guide plate can be formed, for example, as a coating layer in which metal powder is mixed with a binder resin, or as an additional layer of a metal thin film by a vapor deposition method or the like. The reflection plate can be formed by an appropriate method such as a metal powder mixed sheet, a metal powder mixed layer or a resin sheet provided with a metal thin film, or a metal foil according to the above. The resin sheet, in addition to the light guide plate forming material described above, for example, triacetyl cellulose and polyvinyl alcohol, polyimide and polyarylate, polyester and polysulfone,
An appropriate transparent film made of a plastic such as polyether sulfone can be used.

The reflection layer having fine irregularities on the surface is
A method based on contaminants such as metal powder, a method of roughening the surface of a metal foil such as aluminum or silver or the above-mentioned metal thin film by a rolling roll method, or a method of attaching a metal thin film to a resin sheet roughened by embossing or the like It can be formed by an appropriate method such as The reflective layer may be provided with an appropriate coat layer for the purpose of improving reflectance, preventing oxidation, and the like, if necessary.

The surface light source device of the present invention has a light source 3 arranged on the incident side surface 13 of the light guide plate 1 as shown in FIG. 1 for the purpose of use as a sidelight type backlight or the like. The switching mechanism is provided for the purpose of arbitrarily turning on and off the light source. In the illustrated example, a power supply 33 and a switch 32 for controlling the supply of the power form a lighting / light-out switching mechanism.

As the light source, any suitable light source can be used. For example, a (cold or hot) cathode tube, a linear light source comprising a linear or planar array of light-emitting diodes and the like are preferably used. A cold cathode tube is particularly preferable from the viewpoint of low power consumption, durability, and color rendering abundance of colorization.

A switching mechanism for turning on and off the light source can also be formed appropriately. In a cold cathode tube or the like, a control method of power supply to the inverter may be employed. Further, a manual switch or an automatic switch based on brightness using an optical sensor can be used.

In the formation of the surface light source device, if necessary, appropriate auxiliary means such as a light source holder 31 surrounding the light source 3 are arranged to guide divergent light from the light source to the side surface of the light guide plate, as shown in the figure. You can also. As the light source holder, a resin sheet or a metal foil provided with a metal thin film having a high reflectance is generally used. In addition, the light source holder can be extended to the lower surface of the light guide plate to serve also as a reflection sheet.

As described above, in the surface light source device of the present invention,
In order to improve the visibility in the reflection mode, it is important to maintain the state of linear polarization through the polarizing plate. For this purpose, in the present invention, birefringence between the upper surface of the light guide plate and the reflective layer on the lower surface is used. Is preferably as small as possible, especially 50 nm or less, especially 30 nm or less.
It is preferably not more than nm. If such a phase difference is large, the incident light that has been converted into linearly polarized light through the polarizer in the reflection mode is converted into a polarized state by the phase difference, is reflected through the reflective layer, and is again incident on the polarizer. And the amount of light available for visual recognition decreases.

In the above, the evaluation based on the total phase difference is as follows:
Although it has the meaning of allowing each phase difference based on the light guide plate or the reflection plate that can be interposed between the upper surface of the light guide plate and the reflection layer on the lower surface to cancel each other out, but in the thickness direction or the oblique direction, etc. Since the degree of the cancellation is different, it is desirable that the phase difference in the thickness direction or the oblique direction is as small as possible based on each layer.

As described above, the surface light source device of the present invention is excellent in the reflection characteristics of external light and also excellent in the efficiency of light emitted from the side surface of the light guide plate to the upper surface, so that it is bright, easy to see, and has low power consumption. For example, as a backlight system for forming various devices such as a liquid crystal display device having excellent operability, a cell phone or an electronic organizer, which may be powered by a battery such as a storage battery, which functions particularly advantageously with low power consumption, It can be preferably used as a device for forming a portable device such as a PDA.

FIG. 8 shows a configuration example of a liquid crystal display device for both reflection and transmission using the surface light source device of the present invention in a backlight system. This is located above the light guide plate 1 of the surface light source device.
It is formed by arranging a liquid crystal cell 5 having a polarizing plate 4 on the front and back. A liquid crystal display device generally includes a liquid crystal cell functioning as a liquid crystal shutter and an associated driving device, a polarizing plate,
It is formed by appropriately assembling components such as a backlight and a compensating retardation plate if necessary.

There is no particular limitation on the liquid crystal cell used in the present invention, and an appropriate liquid crystal cell, such as a liquid crystal cell using a twisted nematic liquid crystal or a super twisted nematic liquid crystal, for performing display by making polarized light incident on the liquid crystal cell. Can be used. Also, there is no particular limitation on the driving method of the liquid crystal.

The polarizing plate provided on the front and back surfaces of the liquid crystal cell is not particularly limited. However, in order to obtain a display with a good contrast ratio by the incidence of highly linearly polarized light, the polarizing plate on the backlight side is preferably used, for example, iodine. It is preferable to use a material having a high degree of polarization, such as a system or a dye-based absorption linear polarizer.

In forming the liquid crystal display device, for example, appropriate optical elements such as a diffusion plate, an antiglare layer, an antireflection film and a protective layer, and a compensating retardation plate can be appropriately arranged. The compensating retardation plate aims at compensating the wavelength dependence of birefringence and improving the visibility of the liquid crystal cell. Compensation phase difference plate 6 as shown in FIG.
Is disposed between the polarizing plate 4 on the viewing side and / or the backlight side and the liquid crystal cell 5 as necessary.

As the retardation plate for compensation, an appropriate retardation plate can be used according to the wavelength range or the like. The retardation plate can be obtained, for example, as a birefringent sheet obtained by stretching a film made of polycarbonate, polysulfone, polyester, polymethyl methacrylate, polyamide, polyvinyl alcohol, or the like. It can also be formed as a laminate of layers or more.

The diffusion plate is intended to improve the visibility by diffusing the display light. However, by providing the diffusion plate on the viewing side of the liquid crystal cell or on the upper surface of the polarizing plate above the liquid crystal cell, the light is reflected in the reflection mode due to the change in the polarization state. Can be prevented or suppressed from affecting visibility. The optical elements or components forming the above-mentioned surface light source device or liquid crystal display device may be wholly or partially laminated and integrated and fixed, or may be arranged in an easily separable state. Good.

[0058]

【Example】

Reference Example 1 One side surface of a transparent polymethyl methacrylate plate having a predetermined dimension is mirror-polished to form an incident side surface, and then the lower surface is ground to have an apex at a position 10 mm from the incident side surface and a thickness of 5.
The light guide plate was obtained by forming a parabolic surface convex below 02 mm and forming a convex portion in the direction of the incident side surface at a predetermined period on the lower surface thereof by a grinding method using a diamond tool using an NC machine tool.

The light guide plate has a width of 80 mm and a depth of 158 mm.
(The convex portion forming portion is 150 mm excluding the 8 mm on the incident side surface side), the thickness of the incident side surface is 5 mm, the thickness of the opposite end is 1 mm, the upper surface is flat, the pitch of the convex portions is 200 μm, and the incident side surface is While the inclination angle of the short side surface with respect to the upper surface continuously changes in a range of 41 to 44 degrees, the inclination angle of the long side surface continuously changes in a range of 1.5 to 3.7 degrees, The projected area ratio on the upper surface (short side / long side, the same applies hereinafter) is 1/14.
It changes continuously in the range of 4 to 1 / 10.9.

Reference Example 2 In accordance with Example 1, the lower surface is formed of a downwardly convex paraboloid having a vertex at a position 30 mm from the incident side surface, and the pitch of the convex portions on the lower surface is 220 to 180 μm based on the incident side surface. , The inclination angle of the short side surface with respect to the upper surface continuously changes in the range of 39 to 43 degrees, and the inclination angle of the long side surface in the range of 1.6 to 3.3 degrees. It changes continuously, and the projected area ratio to the upper surface is 1 / 13.7 to 1 / 11.6.
The light guide plate continuously changing in the range of was obtained.

REFERENCE EXAMPLE 3 In accordance with Example 1, the lower surface has a downwardly convex paraboloid having an apex on the incident side surface, and the lower surface has a shorter side surface having an inclination angle of 45 degrees with respect to the upper surface and a length of 15 degrees. Have the sides at a certain angle,
A light guide plate was obtained in which the projection width of the short side surface on the upper surface varied in the range of 41.5 to 51.3 μm and the projected area ratio varied in the range of 1 / 3.8 to 1 / 2.9.

Reference Example 4 50 parts by weight of methyl methacrylate, 5 parts by weight of triethylene glycol dimethacrylate, 25 parts by weight of titanium oxide powder having an average particle size of 15 μm, 1 part by weight of benzoyl peroxide,
1 part by weight of Parloyl TCP and 100 parts by weight of methylene chloride are mixed and blown with dry nitrogen, followed by defoaming. The resulting mixture is applied to one side of a polymethyl methacrylate plate having a width of 80 mm, a depth of 140 mm and a thickness of 5 mm. The surface was covered with a separator, heated at 50 ° C. for 2 hours, and further heated at 70 ° C. for 2 hours to remove the separator.
Heating was performed at 0 ° C. for 2 hours to obtain a light guide plate having a lower surface having a diffuse reflection layer exhibiting complete concealing properties.

Example 1 A cold-cathode tube having a diameter of 3 mm was arranged on the incident side surface of the light guide plate obtained in Reference Example 1, and the cold-cathode tube was surrounded by a light source holder composed of a silver-evaporated polyester film, and surface irregularities were processed. A reflective sheet made of silver evaporated on a polyester film is arranged on the lower surface of the light guide plate to obtain a sidelight type surface light source device, and a normally white twisted nematic liquid crystal cell having polarizing plates attached to the front and back surfaces is arranged above the light source device. Thus, a liquid crystal display device for both reflection and transmission was obtained. The cold-cathode tube can be turned on / off by controlling the supply of current to the inverter via a switch.

Example 2 A surface light source device and a liquid crystal display device for both reflection and transmission were obtained in the same manner as in Example 1 except that the light guide plate obtained in Reference Example 2 was used.

Comparative Example 1 A liquid crystal display device for both reflection and transmission was obtained in the same manner as in Example 1 except that the same reflection sheet as in Example 1 was interposed between the surface light source device and the liquid crystal cell.

Comparative Example 2 A liquid crystal display device for both reflection and transmission was used according to Example 1, except that a transflective reflection sheet (P3, manufactured by Nitto Denko Corporation) was interposed between the surface light source device and the liquid crystal cell. Obtained.

Comparative Example 3 A surface light source device and a liquid crystal display device for both reflection and transmission were obtained in the same manner as in Example 1 except that the light guide plate obtained in Reference Example 3 was used.

Comparative Example 4 A surface light source device and a liquid crystal display device for both reflection and transmission were obtained in the same manner as in Example 1 except that the light guide plate obtained in Reference Example 4 was used.

Comparative Example 5 Comparative Example 2 was conducted except that a commercially available EL lamp was used for the surface light source device.
A liquid crystal display device for both reflection and transmission was obtained in accordance with the above.

Evaluation Test The reflective and transmissive liquid crystal display devices obtained in Examples and Comparative Examples were in a transmission mode in a non-selected state (white state) with the surface light source device turned on. The luminance was examined in a dark room (CA-1 manufactured by Minolta Co., Ltd.).
000). 30W × 2 with the surface light source device turned off
The same luminance was examined in the reflection mode placed at a position 2 m directly below the indoor lighting of No. 1. The results are shown in the following table.

[0071]

From the table, it can be seen that the luminance in the reflection mode in the example is almost comparable to that in the comparative example 1 according to the reflection-type dedicated device. Further, it can be seen that the luminance in the transmission mode is particularly excellent with respect to all of the comparative examples. Further, in Comparative Example 3, which exhibited the maximum value in the transmission mode of the comparative example, large brightness unevenness occurred, and the maximum luminance direction in the reflection mode was a direction shifted greatly from the vertical direction.

On the other hand, in Example 1, Comparative Example 2 and Comparative Example 5, in Example 1, about 2
In the reflection mode, the values are about 1.5 times higher. Since these values use the same surface light source device, the method according to the embodiment is different from the method using the transflective reflection sheet of Comparative Example 2. It can be seen that the visibility in both the reflection and transmission modes is greatly improved, and that the EL lamp of Comparative Example 5 is difficult to perform in the bright transmission mode.

[Brief description of the drawings]

FIG. 1 is an explanatory side view of an example of a surface light source device.

FIG. 2 is an explanatory perspective view of an example of a light guide plate.

FIG. 3 is a side view illustrating another example of a light guide plate.

FIG. 4 is an explanatory side view of still another example of the light guide plate.

FIG. 5 is an explanatory side view of an example of a bottom structure.

FIG. 6 is an explanatory side view of another example of the lower surface structure.

FIG. 7 is an explanatory diagram of reflected light in a reflection mode.

FIG. 8 is a side sectional view illustrating an example of a liquid crystal display device.

[Explanation of symbols]

1: light guide plate 11: upper surface 12, 16, 17: lower surface 41, 43, 45, 47, 52, 54, 56, 58: long side surface 42, 44, 46, 48, 51, 53, 55, 57: short Side surface 13: incident side surface 2: reflective layer 3: light source 31: light source holder 32: switch 33: power supply 4: polarizing plate 5: liquid crystal cell

Claims (8)

[Claims] A light source having a light-on / off switching mechanism on an incident side surface of a light guide plate configured to reflect light incident from a side surface on a lower surface and to emit the light from an upper surface, and the light guide plate is provided on a lower surface thereof. A long side surface having an inclination angle of −10 to 10 degrees with respect to the upper surface, and a short side surface inclined from the upper surface to the side of the incident side with respect to the upper surface having an inclination angle of 25 to 50 degrees alternately, and maintaining the polarization state. Wherein the projected area of the long side surface with respect to the upper surface is five times or more that of the short side surface. 2. The light guide plate according to claim 1, wherein the projected area of the long side surface on the lower surface of the light guide plate with respect to the upper surface is 10% of that of the short side surface.
Surface light source device that is more than double. 3. The light guide plate according to claim 1, wherein the inclination angle of the short side surface of the lower surface of the light guide plate with respect to the upper surface is gradually increased based on the incident side surface, and the angle is in the range of 38 to 45 degrees. A surface light source device wherein an inclination angle of a side surface with respect to an upper surface is in a range of -5 to 5 degrees. 4. The light guide plate according to claim 1, wherein the long side surface and the short side surface on the lower surface of the light guide plate form a convex portion or a concave portion, and the inclination angle of the long side surface with respect to the upper surface is in a range of -5 to 0 degrees. Surface light source device. 5. The surface light source device according to claim 1, wherein the reflection layer on the lower surface of the light guide plate comprises a reflection plate. 6. The surface light source device according to claim 5, wherein the reflection plate is made of a substrate or a foil provided with a thin film of a metal made of aluminum, silver, gold, copper or chromium, and has fine irregularities on the surface. 7. The method according to claim 1, wherein a total phase difference due to birefringence between the upper surface of the light guide plate and the reflective layer on the lower surface is 5 or more.
A surface light source device of 0 nm or less. 8. A reflective / transmissive liquid crystal display device, wherein liquid crystal cells having polarizing plates on both sides thereof are arranged above the surface light source device according to claim 1.