JPH11109336A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow a reflection type liquid crystal display device to reduce the loss of diffusion reflected light and to brighten a white display by the diffusion reflected light. SOLUTION: A transmission axis 29a of a polarizing plate 29 is made nearly parallel to an oriented processing direction 24a of an oriented film 24 of a surface side. A reflection polarizing plate 30 is provided with the transmission axis 30p and a reflection axis 30s nearly orthogonally intersected with each other, and the transmission axis 30p between them is made nearly parallel to the oriented processing direction 25a of the oriented film 25 of a rear surface side. Then, in an off state, linear polarized light made incident transmitting through the polarizing plate 29 from the surface side is polarized by nearly 90 deg. in the twist direction 28a of a liquid crystal 28 in a process transmitting through a liquid crystal cell 21, and then, it transmits through the reflection polarizing plate 30, and then, is diffusion reflected by a diffusion reflection plate 31. Although the polarizing state of this diffusion reflected light is lost, the major part of the light are transmitted through the reflection polarizing plate 30 and emitted to the surface side finally by the reflective action of the reflection polarizing plate 30 and the diffusion reflective action of the diffusion reflection plate 31.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a reflection type liquid crystal display device using a reflection polarizing plate having both a polarization transmission function and a reflection function.

[0002]

2. Description of the Related Art A liquid crystal display device controls the transmission of light by changing the orientation of liquid crystal molecules according to an applied voltage.
FIG. 7A shows a part of an example of such a conventional liquid crystal display device. This liquid crystal display device is of a reflection type and includes a liquid crystal cell 1. The liquid crystal cell 1 has a structure in which a liquid crystal 8 is interposed between alignment films 4 and 5 of a pair of substrates 6 and 7 provided with electrodes 2 and 3 and alignment films 4 and 5 respectively on opposing surfaces. . On the front side of the liquid crystal cell 1, a polarizing plate 9 on the front side is arranged. On the back side of the liquid crystal cell 1, a polarizing plate 10 on the back side is arranged. On the back side of the polarizing plate 10 on the back side, a diffuse reflection plate 11 is arranged.

[0005] Next, referring to FIG.
The relationship between the orientation direction of the alignment films 4 and 5 by rubbing and the optical axes such as the transmission axes of the polarizing plates 9 and 10 will be described from the front side. The alignment processing direction 4a of the alignment film 4 on the front side is a direction from the lower left side to the upper right side. The alignment direction 5a of the alignment film 5 on the back side is a direction from the upper left to the lower right. Therefore, the liquid crystal molecules of the liquid crystal 8 in this case are twist-oriented at a twist angle of approximately 90 ° in the clockwise direction indicated by the arrow 8a from the back side to the front side. Transmission axis 9 of polarizing plate 9 on the front side
a is substantially parallel to the alignment direction 4a of the alignment film 4 on the front surface side. The transmission axis 10a of the polarizing plate 10 on the back side is
The direction is substantially parallel to the alignment direction 5a of the alignment film 5 on the back side.

Next, the display operation of the liquid crystal display device will be described. First, the optical rotation of the liquid crystal 8 will be described. When the liquid crystal molecules are in an initial twist alignment state (a state in which the liquid crystal molecules are aligned by a predetermined pretilt angle with respect to the surfaces of the substrates 6 and 7 with the alignment controlled by the alignment films 4 and 5), When the voltage is not applied between the electrodes 6 and 7, the liquid crystal 8 has the largest optical rotation when in the off state. On the other hand, when the voltage is applied between the electrodes 6 and 7 in the on state, the liquid crystal molecules rise while maintaining the initial twisted state, and the rising angle increases as the applied voltage increases. As a result, the optical rotation of the liquid crystal 8 gradually decreases, and when the liquid crystal molecules rise almost perpendicularly to the surfaces of the substrates 6 and 7, the optical rotation of the liquid crystal 8 is almost eliminated.

As shown in FIG. 8A, when the liquid crystal molecules 8b are in the initial twisted state (off state), as shown by arrows in FIG. The linearly polarized light that has entered through the plate 9 is rotated by approximately 90 ° in the direction opposite to the twist direction 8a of the liquid crystal 8 in the process of passing through the liquid crystal cell 1, and then passes through the polarizing plate 10 on the back side, Next, the light is diffusely reflected by the diffuse reflection plate 11,
The diffusely reflected light passes through the polarizing plate 10 on the back side, the liquid crystal cell 1 and the polarizing plate 9 on the front side, and is emitted to the front side. In this state, white display is performed, that is, the background is displayed in white.

Next, as shown in FIG. 8B, when the liquid crystal molecules 8b are in a state of rising substantially vertically to the surfaces of the substrates 6 and 7 (on state), the liquid crystal molecules 8b are indicated by arrows in FIG. As shown, the linearly polarized light that has passed through the polarizing plate 9 from the front side and entered is transmitted through the liquid crystal cell 1 as it is without being affected by the optical rotation, and is then absorbed by the polarizing plate 10 on the back side.
In this state, black display is performed, and characters and figures are displayed in black display. In this case, although not shown, when the liquid crystal molecules 8b appropriately rise from the initial twist alignment state due to the difference in the applied voltage between the electrodes 2 and 3, the liquid crystal molecules 8b pass through the polarizing plate 9 from the front side to the front side. Incident linearly polarized light is
In the process of transmitting light, the light becomes a light containing both a polarized light component parallel to the transmission axis 10a of the polarizing plate 10 on the back side and a polarized light component orthogonal to the transmission axis 10a. Then, the light of the orthogonal polarization component is absorbed by the polarizing plate 10 on the back surface side. The parallel polarized light component is transmitted through the rear-side polarizing plate 10 and then diffused and reflected by the diffuse reflecting plate 11, and the diffusely reflected light is reflected by the rear-side polarizing plate 10, the liquid crystal cell 1, and the front-side polarizing plate 9 And is emitted to the surface side. Thus, gradation expression is performed.

[0007]

However, in such a conventional liquid crystal display device, as shown in FIG. 8A, when the liquid crystal molecules 8b are in the initial twist alignment state (off state), the liquid crystal molecules 8b face the front side. Is incident on the liquid crystal 8 to rotate the light by approximately 90 °, and passes through the polarizing plate 10 on the rear surface side and is diffusely reflected by the diffuse reflecting plate 11. As a result, the light becomes a light that includes both a polarization component parallel to the transmission axis 10a of the polarizing plate 10 on the back side and a polarization component orthogonal to the transmission axis 10a. Therefore, the transmission axis 10a of the polarizing plate 10 on the back side of the diffuse reflected light
The light having the polarization component orthogonal to the light is absorbed by the rear polarizing plate 10, and the light having the parallel polarization component is transmitted through the rear polarizing plate 10. That is, there is a problem that a part of the diffuse reflection light is absorbed by the polarizing plate 10 on the rear surface side, resulting in a light loss, and as a result, the luminance of the white display on the background is reduced and the image becomes grayish. An object of the present invention is to obtain a white display with high luminance by reducing the loss of diffuse reflection light.

[0008]

According to a first aspect of the present invention, there is provided a liquid crystal display device comprising: a pair of substrates provided with electrodes and an alignment film on opposing surfaces; A liquid crystal cell in which a liquid crystal whose birefringence changes according to an applied voltage, a polarizing plate disposed on the front side of the liquid crystal cell, and a direction substantially orthogonal to each other disposed on the rear side of the liquid crystal cell A reflective polarizing plate having a reflection axis for reflecting the incident light of the polarized light component along one direction and a transmission axis for transmitting the incident light of the polarized light component along the other direction, and the back side of the reflective polarizing plate And a diffuse reflector for diffusing and reflecting incident light. According to the first aspect of the present invention, when the polarization state of the light diffusely reflected by the diffuse reflection plate is disrupted, the light of one predetermined polarization component of the diffuse reflection light passes through the reflection polarization plate, The light of another polarization component is reflected without being absorbed by the reflective polarizer, and the reflected light is diffused and reflected again by the diffuse reflector, and its polarization state is again lost. Is transmitted through the reflective polarizer, and the light of other polarization components is reflected by the reflective polarizer without being absorbed again.By repeating such a process, the light is diffused and reflected first by the diffuse reflector, and the polarization state is changed. Most of the distorted light will be transmitted through the reflective polarizer,
Therefore, the loss of diffuse reflection light can be reduced. Next, in the liquid crystal display device according to the second aspect, in the liquid crystal display device according to the first aspect, the liquid crystal molecules of the liquid crystal are twisted at an angle of 70 ° to 120 ° from one substrate toward the other substrate. And the transmission axis of the polarizing plate is substantially parallel to the alignment processing direction of the alignment film on the front surface side of the liquid crystal cell, and the transmission axis of the reflective polarizing plate is aligned on the rear surface side of the liquid crystal cell. It is characterized by being substantially parallel to the direction. According to the invention described in claim 2, when the liquid crystal is in the off state, that is, when the liquid crystal is in the initial alignment state,
The light transmitted through the liquid crystal is transmitted through the reflective polarizer and diffusely reflected by the diffuse reflector. The loss of the diffusely reflected light is significantly reduced, and a white background with high luminance is obtained. Next, in a liquid crystal display device according to a third aspect, in the liquid crystal display device according to the first aspect, the liquid crystal molecules of the liquid crystal are twisted at an angle of 70 ° to 120 ° from one substrate toward the other substrate. And the transmission axis of the polarizing plate is substantially parallel to the alignment processing direction of the alignment film on the front surface side of the liquid crystal cell, and the transmission axis of the reflective polarizing plate is aligned on the rear surface side of the liquid crystal cell. It is characterized by being substantially orthogonal to the direction. According to the third aspect of the present invention, when the liquid crystal is in the ON state, that is, the liquid crystal is in the aligned state, the light transmitted through the liquid crystal is transmitted through the reflective polarizer and diffused and reflected by the diffuse reflector. The loss of the emitted light is significantly reduced, and a white display with high luminance can be obtained.

[0009]

FIG. 1A shows a main part of a liquid crystal display device according to an embodiment of the present invention. This liquid crystal display device is of a reflection type and includes a liquid crystal cell 21. The liquid crystal cell 21 has a structure in which a liquid crystal 28 is interposed between the alignment films 24 and 25 of a pair of substrates 26 and 27 each having electrodes 22 and 23 and alignment films 24 and 25 provided on opposing surfaces. . A polarizing plate 29 is arranged on the surface side of the liquid crystal cell 21. On the back side of the liquid crystal cell 21, a reflective polarizing plate 30 is arranged. On the back side of the reflective polarizer 30, a colorless diffuse reflector 31 that diffusely reflects all wavelength light in the visible light region is disposed.

Next, FIG. 2 is a perspective view for explaining the reflective polarizing plate 30. As shown in FIG. This reflective polarizing plate 30
Are a reflection axis 30s and a transmission axis 30p that are substantially orthogonal to each other.
And reflects incident light of a polarization component along the reflection axis 30s (hereinafter, referred to as S-polarization component) and transmits incident light of a polarization component along the transmission axis 30p (hereinafter, P-polarization component). It has become. That is, when light including both the S-polarized light component along the reflection axis 30s and the P-polarized light component along the transmission axis 30p is incident from the surface side of the reflective polarizing plate 30, the reflection axis 30s of the incident light Is reflected by the reflective polarizing plate 30, and the P-polarized component light along the transmission axis 30p is transmitted through the reflective polarizing plate 30. It should be noted that the same characteristic is obtained for incident light from the back side of the reflective polarizing plate 30.

Next, FIG. 3 shows an example of a specific structure of the reflective polarizing plate 30. As shown in FIG. This reflective polarizing plate 30
On opposite surfaces, a pair of transparent films 43 and 44 are formed in which a large number of ridges 41 and 42 each having an isosceles triangular cross section and having a minute width are arranged in parallel in the width direction. Each convex part of the convex part 41 is overlapped with each concave part between the adjacent convex parts 42 of the other transparent film 44, and a plurality of transparent films having different refractive indexes are formed between the two transparent films 43 and 44. It has a structure in which laminated films 45 alternately laminated are interposed.

Next, referring to FIG.
A description will be given of a case where the relationship between the alignment processing direction of the two alignment films 24 and 25 and the optical axis such as the transmission axis of the reflective polarizing plate 30 is viewed from the front side. Orientation process direction 2 of orientation film 24 on front side
4a is a direction from the lower left side to the upper right side.
The alignment processing direction 25a of the alignment film 25 on the back side is a direction from the upper left to the lower right. Therefore, the liquid crystal molecules of the liquid crystal 8 in this case are twist-oriented at a predetermined twist angle in the clockwise direction indicated by the arrow 28a from the back side to the front side. Here, as the twist angle,
70 ° to 120 ° is preferable, and among them, approximately 90 ° is more preferable. In this embodiment, it is set to approximately 90 °. The transmission axis 29a of the polarizing plate 29 is substantially parallel to the alignment direction 24a of the alignment film 24 on the front side. The transmission axis 30p of the reflective polarizing plate 30 is substantially parallel to the alignment direction 25a of the alignment film 25 on the back side. Therefore, the reflection axis 30s of the reflective polarizing plate 30 is substantially orthogonal to the alignment direction 25a of the alignment film 25 on the back surface side.

Next, the display operation of the liquid crystal display device will be described. First, as shown in FIG. 4A, when the liquid crystal molecules 28b are in the initial twist alignment state (off state), the liquid crystal molecules 28b pass through the polarizing plate 29 from the front side as shown by arrows in FIG. Incident linearly polarized light (in this case, P
The light is polarized light. ) Undergoes a birefringence effect of the liquid crystal 28 in the process of transmitting through the liquid crystal cell 21 and is rotated by approximately 90 ° in the direction opposite to the twist direction 28 a of the liquid crystal 28, and the linear polarization direction is changed to the transmission axis 30 p of the reflective polarizing plate 30. It becomes polarized light along. This linearly polarized light is then transmitted through the reflective polarizer 30 and then diffusely reflected by the diffuse reflector 31. The diffusely reflected light has its polarization state changed by the diffuse reflection, and becomes light including both the S-polarized light component along the reflection axis 30s of the reflective polarizing plate 30 and the P-polarized light component along the transmission axis 30p.

The transmission axis 30 of the diffuse reflected light is
The light of the P-polarized component along p passes through the reflective polarizer 30,
The S-polarized light component along the reflection axis 30s is reflected by the reflection polarizing plate 30.
Is reflected by The light of the S-polarized light component reflected by the latter reflective polarizing plate 30 is diffused and reflected again by the diffused reflecting plate 31, the polarization state of which is again lost, and the light including both the S-polarized light component and the P-polarized light component. Become. Then, of the diffusely reflected light, light of the P-polarized component along the transmission axis 30p is transmitted through the reflective polarizer 30, and light of the S-polarized component along the reflective axis 30s is reflected by the reflective polarizer 30. By repeating such a process, most of the light that is first diffusely reflected by the diffuse reflection plate 30 and whose polarization state is lost is transmitted through the reflection polarization plate 30, and therefore, the loss of the diffuse reflection light can be reduced. . Then, the light of the P-polarized component transmitted through the reflective polarizing plate 30 is transmitted through the liquid crystal cell 21 and the polarizing plate 29 and is emitted to the front side. In this state, white display is displayed,
That is, the background is displayed in white. In this case, since the loss of light is small, the white display of the background can be brightened.

Next, as shown in FIG. 4B, when the liquid crystal molecules 28b are in a state of rising substantially vertically to the surfaces of the substrates 26 and 27 (on state), an arrow is shown in FIG. As shown in the figure, the light of the P-polarized light component that has passed through the polarizing plate 29 on the front surface side and entered the liquid crystal cell 21 is not subjected to the optical rotation effect, that is, transmitted without change in the linear polarization direction. Therefore, the light is totally reflected (specular reflection) by the next reflective polarizing plate 30, and the reflected light passes through the liquid crystal cell 21 and the polarizing plate 29 and is emitted to the surface side. The display by the totally reflected light is a white display when viewed from a predetermined direction (a direction symmetrical with respect to the normal to the reflection surface), but is a black display when viewed from other directions.

Therefore, according to the liquid crystal display device of this embodiment, a black display with a high contrast can be obtained on a bright white background due to diffuse reflection in a viewing angle direction other than the total reflection direction with respect to the incident angle of external light. In this case, since the monochrome display is performed by the diffuse reflection and the specular reflection, a display different from the conventional monochrome display by the diffuse reflection and light absorption is obtained.

When an intermediate potential between ON and OFF is applied between the electrodes 22 and 23, the liquid crystal molecules 28b rise from the initial twisted state by an angle corresponding to the intermediate potential due to the difference in the applied voltage, and from the surface side. The light of the P-polarized light component that has passed through the polarizing plate 29 on the front side and entered the liquid crystal cell 2
In the process of transmitting light 1, the light becomes a light including both the S-polarized light component along the reflection axis 30 s of the reflective polarizing plate 30 and the P-polarized light component along the transmission axis 30 p.

Then, of this light, the S-polarized light component along the reflection axis 30s is reflected by the reflective polarizing plate 30, and the P-polarized light component along the transmission axis 30p is transmitted through the reflective polarizing plate 30. Among these, the light of the S-polarized component reflected by the former reflective polarizing plate 30 becomes light containing both the S-polarized component and the P-polarized component in the process of transmitting through the liquid crystal cell 21, and the light of the S-polarized component is The light of the P-polarized component is absorbed by the polarizing plate 29 and is transmitted through the polarizing plate 29 and emitted to the surface side. On the other hand, the light of the P-polarized component transmitted through the latter reflective polarizer 30 is diffusely reflected by the diffuse reflector 31, and finally, most of the light is transmitted through the reflective polarizer 30 in the same manner as described above. I do. This transmitted light is light of a P-polarized component, but becomes light containing both the S-polarized component and the P-polarized component in the process of transmitting through the liquid crystal cell 21 as in the case described above. Is absorbed by the polarizing plate 29, and the light of the P-polarized component is transmitted through the polarizing plate 29 and emitted to the surface side. Thereby, gradation expression is performed.

Next, referring to FIG. 5, the optical axis of the liquid crystal cell 21 of another embodiment of the present invention, such as the alignment processing directions of the alignment films 24 and 25 of the liquid crystal cell 21 and the transmission axis of the reflective polarizing plate 30, will be described. The case where the relationship between the axes is viewed from the front side will be described. In this embodiment, the difference from the case shown in FIG. 1B is that the transmission axis 30p of the reflective polarizing plate 30 is substantially orthogonal to the alignment direction 25a of the alignment film 25 on the back side. Therefore, in this case, the transmission axis 3 of the reflective polarizing plate 30
0p is substantially parallel to the transmission axis 29a of the front-side polarizing plate 29, and the reflection axis 30s is the alignment direction 25 of the rear-side alignment film 25.
It is almost parallel to a.

Next, the display operation of the liquid crystal display device will be described. First, as shown in FIG. 6A, when the liquid crystal molecules 28b are in the initial twist alignment state (off state), the liquid crystal molecules 28b pass through the polarizing plate 29 from the front side as shown by arrows in FIG. The incident P-polarized light is transmitted through the liquid crystal cell 21 in the twist direction 28 a of the liquid crystal 28.
The light is rotated by approximately 90 ° in a direction opposite to the direction described above, and then is totally reflected (specular reflection) by the reflective polarizing plate 30, and the reflected light passes through the liquid crystal cell 21 and the polarizing plate 29 and is emitted to the surface side.
In this state, black is displayed except for the specific direction described above,
That is, the background is displayed in black.

Next, as shown in FIG. 6B, when the liquid crystal molecules 28b are in a state of rising substantially vertically to the surfaces of the substrates 26 and 27 (on state), the liquid crystal molecules 28b are shifted from the surface side to the surface side. The light of the P-polarized light component transmitted through the polarizing plate 29 and transmitted through the liquid crystal cell 21 as it is without being affected by the optical rotation, is transmitted through the reflective polarizing plate 30, and then diffused and reflected by the diffuse reflecting plate 31. . Most of the diffusely reflected light finally passes through the reflective polarizer 30 in the same manner as described above. Since this transmitted light is P-polarized component light, it passes through the liquid crystal cell 21 as it is without being affected by optical rotation, and then passes through the polarizing plate 29 and is emitted to the surface side. This state is a white display. Therefore, the display by the liquid crystal display device of this embodiment is a black and white display by a black background part by specular reflection and a white display part by diffuse reflection.

In the above embodiment, the reflective polarizing plate 30
Although the case where the diffuse reflection plate 31 is arranged on the back side of the first embodiment has been described, a diffuse reflection / semi-transmission plate may be arranged on the back side of the reflective polarizing plate 30 and a backlight may be arranged on the back side. In this case, the backlight is turned off in a bright place and the liquid crystal display device is used as a reflection type, and in a dark place, the backlight is turned on and the liquid crystal display device is used as a transmission type. Further, the present invention is not limited to a twisted nematic liquid crystal display device, but can be applied to a super twisted nematic or homogeneous liquid crystal display device.

[0023]

As described above, according to the present invention, even if the polarization state of the light diffusely reflected by the diffuse reflector is lost, the reflection effect of the reflective polarizer and the diffuse reflection effect of the diffuse reflector are achieved. Finally, most of the light is transmitted through the reflective polarizing plate, so that the loss of the diffuse reflected light can be reduced, and the brightness of the white light due to the diffuse reflected light can be increased.

[Brief description of the drawings]

FIG. 1A is a cross-sectional view of a main part of a liquid crystal display device according to an embodiment of the present invention, and FIG. 1B is a diagram illustrating a relationship between optical axes such as alignment processing directions of both alignment films of the liquid crystal cell. FIG.

FIG. 2 is a perspective view illustrating a reflective polarizing plate.

FIG. 3 is a cross-sectional view of a part of a reflective polarizing plate.

4A is a partial cross-sectional view illustrating an off state of the liquid crystal display device illustrated in FIG. 1, and FIG. 4B is a partial cross-sectional view illustrating an on state.

FIG. 5 is an explanatory diagram illustrating a relationship between optical axes such as alignment processing directions of both alignment films of a liquid crystal cell of a liquid crystal display device according to another embodiment of the present invention.

6A is a partial cross-sectional view illustrating an off state of the liquid crystal display device illustrated in FIG. 5, and FIG. 6B is a partial cross-sectional view illustrating an on state.

FIG. 7A is a cross-sectional view of a part of an example of a conventional liquid crystal display device, and FIG. 7B is an explanatory diagram illustrating a relationship between optical axes such as alignment processing directions of both alignment films of the liquid crystal cell.

8A is a partial cross-sectional view illustrating an off state of the liquid crystal display device illustrated in FIG. 7, and FIG. 8B is a partial cross-sectional view illustrating an on state.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 21 Liquid crystal cell 22, 23 Electrode 24, 25 Alignment film 24a, 25a Alignment processing direction 26, 27 Substrate 28 Liquid crystal 28a Twist direction 29 Polarizer 29a Transmission axis 30 Reflective polarizer 30s Reflection axis 30p Transmission axis 31 Diffuse reflector

Claims (4)

[Claims] 1. A liquid crystal cell comprising a pair of substrates provided with electrodes and alignment films on opposing surfaces, respectively, and a liquid crystal having a birefringence that changes according to a voltage applied between the electrodes. A polarizing plate disposed on the front surface side of the liquid crystal cell, and a reflection axis disposed on the back surface side of the liquid crystal cell, which reflects incident light of a polarization component along one direction of directions substantially orthogonal to each other, and the other. A reflective polarizing plate having a transmission axis for transmitting incident light of a polarized component along the direction, and a diffuse reflector disposed on the back side of the reflective polarizing plate for diffusing and reflecting the incident light. Liquid crystal display device. 2. The liquid crystal molecules of the liquid crystal are arranged so as to be twisted at an angle of 70 ° to 120 ° from one substrate to the other substrate, and the transmission axis of the polarizing plate is located on the surface side of the liquid crystal cell. 2. The liquid crystal display device according to claim 1, wherein the alignment direction of the alignment film is substantially parallel to the alignment direction, and the transmission axis of the reflective polarizer is substantially parallel to the alignment direction of the alignment film on the back side of the liquid crystal cell. Liquid crystal display. 3. The liquid crystal molecules of the liquid crystal are arranged so as to be twisted at an angle of 70 ° to 120 ° from one substrate toward the other substrate, and the transmission axis of the polarizing plate is located on the surface side of the liquid crystal cell. 2. The liquid crystal according to claim 1, wherein the liquid crystal cell is substantially parallel to an alignment processing direction of the alignment film, and a transmission axis of the reflective polarizing plate is substantially orthogonal to an alignment processing direction of the alignment film on the back surface side of the liquid crystal cell. Display device. 4. The liquid crystal display device according to claim 1, wherein said diffuse reflection plate comprises a diffuse reflection / semi-transmission plate having a diffuse reflection function and a transmission function.