JPH11232919A - Front light lighting system and reflecting type display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a display bright and enhance display quality by arranging a front light lighting system lighting a reflecting type display body between the reflection type display body and an observer with a front light lighting system. SOLUTION: A lighting system 20 is mainly composed of a light source 21, a light guide plate 22, and a volume hologram 23 and is arranged on the display face 11a side of a reflecting type display body 11. The light guide plate 22 and the volume hologram 23 are formed integrally, a double refraction layer 24 and a low refractive index layer 2 are formed on the observer side face 22a of the light guide plate 22, and a low refractive index layer 26 is formed on a face opposite to the reflection type display body 11 of the volume hologram 23. A ray from the light source 21 is taken into the light guide plate 22, transmitted in a light guide plate 22, diffracted by the volume hologram 23, and the display face 11a of the reflection type display body 11 is lighted by the diffracted light. By the high angle selectivity of the volume hologram 23, high light utilization efficiency can be obtained without sacrificing displaying quality.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lighting device used for a reflection type display such as a printed matter or a reflection type display device.

[0002]

2. Description of the Related Art Portable information devices such as notebook personal computers and personal digital assistants (PDA) are driven by a battery, so that a display device must have low power consumption in order to enable long-term use. .

As a display device used for such a purpose, a backlight type display device in which a light source is arranged on the back surface of a transmissive display element is known, but a device with sufficiently low power consumption has not yet been obtained. . On the other hand, a reflection type display device that performs display using reflection of external light without providing a light source is known. However, there is a problem that a display cannot be seen in a dark place, and a display element having a sufficient reflectance is still available. However, there is a problem that the display is dark even in a bright place, the display color looks dull, and a display having a transparent feeling like a backlight type display device cannot be obtained.

As a means for solving the problem of a reflective display device in which a display cannot be seen in a dark place, an illumination device called a front light illumination device is provided between the display surface of the reflective display element and an observer. , C.I. Y. (SID 95 DIGEST, p. 3).
75-378).

[0005] The front light illuminating device proposed by them, as shown as an illuminating device 2 in FIG. 6, is arranged between a display surface 1a of a reflective display element 1 and an observer.
A light guide plate 4 having a microprism 3 composed of a flat portion 3a and an inclined portion 3b on the surface on the observer side, and a compensator 5 formed so that the microprism 3 and the irregularities are engaged with each other. The light propagating into the reflective display element 4 is reflected by the inclined portion 3b of the microprism 3 in a direction substantially perpendicular to the surface of the light guide plate 4 facing the display surface 1a of the reflective display element 1. Is illuminated, and the reflected light image from the display surface 1a is observed through the flat portion 3a of the microprism 3 and the compensator 5. The compensator 5 is for correcting distortion of a display image due to unevenness of the microprism 3.

[0006]

In this conventional front light illuminating device, the inclination angle of the inclined portion 3b of the microprism 3 is set so as to prevent the light propagating through the light guide plate 4 from leaking to the observer side. The angle is set to be larger than the critical angle for total reflection. As a result, (1) the reflected light from the reflective display element 1 is reflected again by the microprism 3 and confined in the light guide plate 4 to darken the display, and (2) the inclined portion 3b of the microprism 3 has stripes. (3) When the pitch of the microprisms 3 is close to the pixel pitch of the reflective display element 1, moire fringes are observed.

There is also a problem that the use of the front light illuminating device does not significantly improve the lighting efficiency as compared with the backlight illuminating device.

Accordingly, the present invention provides a front light illuminating device for illuminating a reflection type display such as a printed matter or a reflection type display element, in which the display becomes brighter and the display quality becomes higher.

[0009]

SUMMARY OF THE INVENTION A front light illuminating device according to the present invention is arranged on a display surface side of a reflective display to illuminate the reflective display, and comprises a light source and a light beam from the light source. And a volume hologram for diffracting the light and illuminating the reflective display with the diffracted light.

In this case, it is desirable to provide a light guide plate for taking in a light beam from a light source and guiding the light to the volume hologram, on the opposite side of the volume hologram from the reflection type display body side. Further, it is preferable to provide a birefringent layer on the light guide plate or a layer having a lower refractive index than the light guide plate.

[0011]

The volume hologram has high wavelength selectivity and angle selectivity. For this reason, in the front light illuminating device of the present invention configured as described above, the volume hologram is transparent in most of the angle range in which the observer desires the reflective display body, and the fringes are visible or moire is generated. The above-described problem such as dark display does not occur.

In a liquid crystal display device and the like, red, green,
Reflective display elements that perform color display by juxtaposing multiple types of minute pixels that display the primary color of blue, etc., are widely used,
When such a reflective display element is uniformly illuminated with white light, there is a problem that light utilization efficiency is low and display is dark. This is because, for example, green or blue color light incident on a red pixel is absorbed by a color filter without being used at all.

However, according to the front light illuminating device of the present invention configured as described above, the red component of the illuminating light is applied to the red pixels by utilizing the high wavelength selectivity of the volume hologram and the lens function. The light use efficiency can be remarkably improved by dispersing and condensing the illumination light for each color, such as a component for a blue pixel and a green component for a green pixel.

In the case of a reflective display device using polarized light, such as a TN type or STN type liquid crystal display device, the light use efficiency can be increased by illuminating with a specific linearly polarized light component. Even if a volume hologram that selectively diffracts only the component is used, high light use efficiency cannot be obtained unless a polarized component that has not been diffracted is used.

On the other hand, when the birefringent layer is provided for the light guide plate as described above, the polarization state that has not been diffracted gradually changes the polarization state while propagating in the light guide plate. The polarized light component can be used efficiently.

Since the front light illuminating device is placed in front of the reflective display, it is important that the device is both transparent and thin. According to the invention of claim 2, the front light illuminating device is formed by integrally forming the volume hologram with the light guide plate, diffracting the light beam propagating in the light guide plate by the volume hologram, and illuminating the reflective display. It can be made extremely thin.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment] FIG. 1 shows a first embodiment of the present invention.

The front light illuminating device of this embodiment is roughly divided into a light source 21, a light guide plate 22, and a volume hologram 23, as shown as an illuminator 20, and a light beam from the light source 21 is introduced into the light guide plate 22. The volume hologram 23 diffracts the light, propagates through the light guide plate 22, and illuminates the display surface 11a of the reflective display 11 with the diffracted light.

The light guide plate 22 and the volume hologram 23 are integrally formed. For example, a birefringent layer 24 and a low-refractive index layer 25 are formed on the observer-side surface 22a of the light guide plate 22. On the surface facing the reflective display 11,
The low refractive index layer 26 is formed.

As the light source 21, a known light source such as a fluorescent tube, a discharge tube, an incandescent tube, or an LED can be used. The light guide plate 22 is formed of a translucent material such as a resin such as an acrylic resin or a methacrylic resin, or glass. Since the unevenness of the surface of the light guide plate 22 causes scattering and is not preferable, the surface of the light guide plate 22 is made smooth.

The volume hologram 23 is a hologram in which the period of change in the refractive index is sufficiently smaller than the thickness of the medium, and exhibits high diffraction efficiency only when the wavelength and the incident angle satisfy a specific relationship called Bragg diffraction condition. Things.

As shown in FIG. 2, the volume hologram 23
Is that the normal direction 23a of the period of the change of the refractive index is
The second volume hologram 23 is provided so as to form an angle θ with respect to a surface 22b in contact with the volume hologram 23. θ is appropriately determined according to which direction a light beam propagating at an angle in the light guide plate 22 is emitted.

For example, the refractive index of the hologram medium is set to 1.
5. When the period of the change in the refractive index is set to 259 nm and θ is set to 45 °, the light having the wavelength of 550 nm
When the angle φ with the surface 22b is 0 °, the light satisfies the Bragg diffraction condition and exits in a direction perpendicular to the surface 22b of the light guide plate 22. At wavelengths other than 550 nm, the wavelength λ satisfying the Bragg diffraction condition and the propagation angle φ have the relationship shown in FIG.

For this reason, the emission angle θo also changes depending on the wavelength λ. This change in the emission angle θo due to the wavelength λ does not cause any inconvenience when the reflective display body 11 to be illuminated has a scattering characteristic close to a perfect diffusion surface such as a printed matter. In the case of having a scattering characteristic with directivity like a reflection type liquid crystal display element, there is a disadvantage that a display color changes depending on an angle at which a display is viewed.

To avoid this inconvenience, a light source having a bright line spectrum may be used as the light source 21 and a collimated light beam may be incident on the light guide plate 22 so that the distribution of the propagation angle φ becomes narrow. With this configuration, a light beam having a specific wavelength can be emitted at a specific emission angle.

When white light is emitted at a specific angle, the volume hologram 23 is obtained by multiplex-recording holograms at different wavelengths on a single hologram medium, or by laminating a plurality of hologram media having different reflection wavelengths. ,
Alternatively, a hologram in which mosaic holograms are recorded at different wavelengths on one hologram medium may be used.

As the hologram material of the volume hologram 23, a known hologram material such as a silver salt material or a photopolymer material can be used. Volume hologram 23
Is a method of forming a thin film of a hologram material on the light guide plate 22 and exposing it to be integrally formed with the light guide plate 22 or attaching a separately prepared volume hologram to the light guide plate 22 to be integrated with the light guide plate 22. I do.

In the example shown, the volume hologram 23 is connected to the light guide plate 2.
In this case, the volume hologram 23 is provided on the surface 22b of the light guide plate 22 on the observer side.
2a. Alternatively, a volume hologram may be embedded inside the light guide plate 22. Further, the volume hologram itself may also serve as the light guide plate 22.

The volume hologram 23 is characterized by having a high angle selectivity. The present invention realizes a front light illuminating device which does not cause display deterioration by utilizing this property. However, on the other hand, high angle selectivity may cause the following inconvenience in realizing high light use efficiency.

The above propagation angle φ generally has a distribution within a range satisfying the condition of total reflection. In contrast, B
The angle range that satisfies the Ragg diffraction condition is usually narrower than the distribution width of the propagation angle φ. Therefore, if the distribution of the propagation angle φ or the above angle θ does not change in the light guide direction of the incident light, and therefore, if a volume hologram having a uniform θ is provided with respect to the parallel plate light guide plate, the incident light However, there is a disadvantage that only a part of the light can be used for lighting.

One way to avoid this inconvenience is to vary θ along the light guide direction. For example, in the case where θ is changed from 40 ° to 50 ° along the light guide direction, it is possible to diffract and emit the propagating light having φ in the range of −10 ° to + 10 °.

Another method is to devise the sectional shape of the light guide plate 22 so that the distribution of the propagation angle φ changes in the light guide direction. FIG. 4 shows a specific cross-sectional shape. (A)
(B) A shape whose thickness periodically increases and decreases as in (c), and a shape in which the thickness becomes uniform along the light guide direction as shown in (d) And (e) such that the thickness becomes uniform along the light guide direction.

In order not to distort the display of the reflective display 11, it is desirable that the thickness and the shape of the light guide plate 22 in the light guide direction should be gently and uniformly changed. The shape of (e) is preferred. A refractive index distribution may be formed in the light guide plate 22 to have the same effect as a change in thickness or shape.

FIG. 4 schematically shows a change in the φ distribution of the light beam propagating in the light guide plate 22. In the case of a wavy shape with a constant thickness as shown in FIG. 3A, the φ distribution alternately shifts in the positive and negative directions along the light guide direction without changing the distribution width. In the case of the shape in which the thickness periodically increases and decreases as in (b) and (c), the distribution width increases and decreases along the light guide direction.
(D) In the case of a shape whose thickness changes uniformly along the light guide direction as in (e), the φ distribution spreads or narrows uniformly along the light guide direction.

As described above, by changing the φ distribution along the light guide direction of the light guide plate 22, all the angle components of the propagating light beam can be used efficiently.

However, with the change in the φ distribution, the tail of the distribution exceeds the critical angle for total reflection, and the light guide plate 22
Light may leak out. This leakage light is not preferable because it lowers the light use efficiency and also causes unnecessary scattered light to lower the display contrast.

In order to prevent light leakage, the light guide plate 2
Collimating the incident light to the second and reducing the curvature of the undulation so that the propagation angle φ changes near 0 °, or satisfying the Bragg diffraction condition at an angle smaller than the critical angle for total reflection to reduce the leakage light. What is necessary is just to provide the volume hologram 23 so that it may be emitted before it occurs.

For example, as shown in FIG. 1 or FIG. 2, when the light guide plate 22 is formed to be thinner in the light guide direction, the angle between the two reflection surfaces 22a and 22b is θw.
Assuming that g, light rays that propagate in the light guide plate 22 while repeating total reflection propagate while discretely expanding the φ distribution by 2θwg for each reflection. For example, when θwg = 22 °, φ = When the Bragg diffraction condition is satisfied at −46 °, the light is diffracted and emitted before reaching the critical angle of total reflection of −48 °.

In this manner, diffracted light can be efficiently generated without causing leakage light. Since the change in the φ distribution is discrete, it is preferable to set θwg to a sufficiently small value so as not to jump over an angle satisfying the Bragg diffraction condition.

Light leakage due to scratches and dirt on the light guide plate 22 noticeably lowers display quality. As shown in the example of FIG. 1, low-refractive-index layers 25 and 26 are provided on the light guide plate 22 to confine propagation light inside the light guide plate 22, thereby preventing leakage light caused by scratches and dirt on the light guide plate 22. Can be reduced.

The material of the low refractive index layers 25 and 26 is not particularly limited, but the light guide plate 22 usually has a refractive index of 1.45.
Since it used as in the range of 1.6, which from a low refractive index of the material, for example, such as fluorine resin or MgF _2, may be a known low refractive index material. Low refractive index layer 2
The thicknesses 5 and 26 need only be thick enough to prevent the evanescent wave from seeping out, and generally a few to several tens of μm is sufficient.

The low refractive index layer need not always be provided on both sides of the light guide plate 22, but may be provided on only one side as necessary. Since the illuminating device 20 is placed between the reflective display 11 and the observer, it is important to take measures against degradation of display quality due to dirt. By providing the low refractive index layer 25 or 26, deterioration of display quality can be prevented, and handling in the manufacturing process can be made extremely easy.

[Second Embodiment] FIG. 5 shows a second embodiment of the present invention.

This embodiment is an example in which the present invention is applied particularly to illuminating a reflective display element such as a reflective liquid crystal display element.

In a liquid crystal display device or the like, as shown in FIG.
A reflective display element 12 that performs color display by juxtaposing three types of minute pixels 15R, 15G, and 15B that display the primary colors of red, green, and blue, respectively, between two substrates 13 and 14 is widely used. .

When the reflective display element 12 is uniformly illuminated with white light, there is a problem that the light use efficiency is low and the display is dark. This is, for example, the red pixel 15
Green or blue color light incident on R is not used at all,
This is due to being absorbed by the color filter.

In this embodiment, in this case, the light use efficiency is improved to obtain a bright display.

The front light illuminating device of this embodiment, as shown as an illuminating device 20, can be configured basically in the same manner as that shown in FIG. However, FIG.
Does not show the light source.

In this embodiment, as shown in the figure, the illumination light propagating in the light guide plate 22 and diffracted by the volume hologram 23 by utilizing the high wavelength selectivity and the lens function of the volume hologram 23. The red light component 27R to the red pixel 15R, the blue light component 27G to the blue pixel 15G, the green light component 27B to the green pixel 15B, and the like. Can be significantly improved.

As the volume hologram 23 for condensing different color lights at different positions in this manner, a hologram medium in which holograms are multiplex-recorded at different wavelengths in one hologram medium, a hologram medium in which a plurality of hologram media having different reflection wavelengths are laminated, or One in which holograms are recorded in a mosaic shape at different wavelengths on one hologram medium can be used.

In the case of a reflection type display device using polarized light, such as a TN type or STN type liquid crystal display device, the light use efficiency can be increased by illuminating with a specific linearly polarized light component. R. L. Sutherland (SPIE vo
l. 2152, pp 303-313 (1994)), a volume hologram produced by laser exposure of a liquid mixture of a photopolymerizable monomer and a liquid crystal,
Since it shows high diffraction efficiency for a specific linearly polarized light component,
It can be suitably used as the volume hologram 23 of this embodiment.

However, even if the volume hologram 23 that selectively diffracts only one polarized light component is used, high light use efficiency cannot be obtained unless the undiffracted polarized light component is not used.

On the other hand, by providing the light guide plate 22 and the birefringent layer 24 as in this embodiment, the polarization state is gradually changed while the non-diffracted polarized light component propagates in the light guide plate 22. Therefore, the polarized light component can be used efficiently.

As the birefringent layer 24, known birefringent materials such as crystal flakes such as quartz and mica, stretched polymer films such as polyester and polycarbonate, or oriented liquid crystal materials can be used. In the example of the drawing, the birefringent layer 24 is provided on the observer side surface 22a of the light guide plate 22, but the birefringent layer may be provided on the volume hologram 23 side surface 22b. Alternatively, a birefringent film may be embedded in the light guide plate 22. Further, the birefringent film itself may also serve as the light guide plate 22.

[0055]

According to the present invention, by arranging an illuminating device having a volume hologram between a reflective display and an observer, it is possible to perform display without turning on illumination in a light place. In a dark place, display is possible by turning on the illumination. Further, by assisting the darkness of the display caused by the low reflectance of the reflective display body by the illumination, a bright display can be achieved, and the light amount enough to compensate for the shortage of the external light may be provided. Therefore, lower power consumption can be expected compared to a backlight type display device. Further, it is possible to obtain a transparent display which is difficult to realize with the reflective display element itself.

Further, according to the present invention, by using a volume hologram, the above-mentioned effect can be realized without sacrificing display quality, and a higher light use efficiency than before can be obtained. .

According to the second aspect of the present invention, by forming the volume hologram integrally with the light guide plate, the front light illuminating device can be made extremely thin.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram provided for describing an operation of the lighting device in FIG. 1;

FIG. 3 shows a propagation angle φ and a wavelength λ satisfying the Bragg diffraction condition.
FIG.

FIG. 4 is a diagram illustrating a shape of a light guide plate and a change in a propagation angle distribution of a light beam propagating in the light guide plate.

FIG. 5 is a diagram showing another embodiment of the present invention.

FIG. 6 is a diagram showing a conventional front light illumination device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Reflective display body 12 Reflective display element 20 Illumination device 21 Light source 22 Light guide plate 23 Volume hologram 24 Birefringent layer 25, 26 Low refractive index layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shimizu Sagawa 430 Nakai-cho Sakai, Kami-gun, Kanagawa Prefecture Inside Green Tech Nakai Fuji Xerox Co., Ltd. 72) Inventor Seiichi Suzuki 430, Nakai-cho, Ashigara-kami, Kanagawa Prefecture Green Tech Nakai Fuji Xerox Co., Ltd.

Claims (9)

[Claims] 1. A light source, which is arranged on a display surface side of a reflective display and illuminates the reflective display, wherein a light source and light rays from the light source are diffracted, and the diffracted light is used for the reflective display. A front light illumination device, comprising: a volume hologram for illuminating a body. 2. The front light illuminating device according to claim 1, wherein a light beam from the light source is taken in and integrated with the volume hologram on a side of the volume hologram opposite to the reflection-type display body and guided to the volume hologram. A front light illuminating device comprising a light guide plate that emits light. 3. The front light illumination device according to claim 2, wherein the light guide plate has a change in thickness in a light guide direction. 4. The front light illumination device according to claim 2, wherein a birefringent layer is provided on the light guide plate. 5. The front light lighting device according to claim 2, wherein a layer having a lower refractive index than the light guide plate is provided on the light guide plate. 6. The front light illuminating device according to claim 1, wherein the reflective display includes a plurality of types of pixels that display different colors arranged side by side, and the volume hologram includes a plurality of pixels. Wherein an image of color light corresponding to the display color of the pixel is generated at the position of (1). 7. The front light illumination device according to claim 1, wherein the volume hologram mainly diffracts a specific polarization component. 8. The front light illuminating device according to claim 1, wherein the light source has a bright line spectrum. 9. A light-emitting device comprising: a reflective display element; and an illuminating device disposed on a display surface side of the reflective device. The illuminating device diffracts a light source and a light beam from the light source. And a volume hologram for illuminating the display element.