JP2009192825A - Polarization state conversion method, polarization state conversion device, and liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method etc., of converting completely-polarized light included in input light into completely-polarized light in a desired target polarization state and outputting it by selecting orientation directions of two retardation plates. <P>SOLUTION: The retardation plates 1 and 2 are disposed in parallel, and light including an initial polarization state SA (for example, output light from a prism sheet on a light transmission plate) is made incident vertically on the retardation plate 1 and passed in the order of the retardation plates 1 and 2 to obtain light in a target final polarization state SB (for example, 45° horizontal linearly polarized light to be made incident on a liquid crystal display panel). The retardation plates 1 and 2 are used which satisfy the condition that the sum Δ1+Δ2 of phase difference imparting capabilities Δ1 and Δ2 is ≥π and at least one of Δ1 and Δ2 is equal to π/2. A set of angles θ1 and θ2 representing the orientation directions of the retardation plates 1 and 2 is selected so as to satisfy SB=R(Δ2, θ1)R(Δ1, θ1). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polarization state conversion method, a polarization state conversion device, and a liquid crystal display device using the polarization state conversion device. More specifically, the present invention relates to two pieces of light in an arbitrary polarization state (initial input polarization state). The present invention relates to a method and apparatus for converting light into another desired polarization state (final polarization state) using the above retardation plate, and a liquid crystal display device using the same.

  It is well known that light includes “unbiased light” (non-polarized light) and “some polarized light” (polarized light), and natural light is a typical example of the former. The latter is “light with some polarization”, and can be generally considered as light obtained by combining (superimposing) “non-polarized light” and “perfectly polarized light”. However, there may be a case where the “non-polarized light” component is zero (completely polarized light only). In addition, there are three types of “completely polarized light”: linearly polarized light, circularly polarized light, and elliptically polarized light, and that linearly polarized light and circularly polarized light can be handled as a special case of elliptically polarized light.

  On the other hand, in the field of optical applications, there is a need to efficiently generate light containing a large amount of complete polarized light in a specific polarization state. A typical example is when a liquid crystal display panel of a liquid crystal display is illuminated using a surface light source device. In this case, “linearly polarized light having a polarization direction that coincides with the transmission axis direction of a polarizing plate (hereinafter also referred to as“ light input side polarizing plate ”) disposed on the light input side of the liquid crystal display panel (or an elliptically polarized light close thereto) It is desirable to generate illumination light containing a lot of the same below.

  In many cases, the exit surface of the surface light source device is rectangular, and the transmission axis direction of the light input side polarizing plate of the liquid crystal display panel is set to a direction inclined by 45 degrees with respect to each side of the rectangle. In that case, it is desired that illumination light containing as much linearly polarized light as possible with a polarization direction parallel to the direction inclined by 45 degrees is supplied to the liquid crystal display panel.

  On the other hand, in a surface light source device that is widely used as a lighting means for a liquid crystal display, light introduced from an incident surface provided at a side end of a light guide plate is emitted from an emission surface, and the traveling direction of this emitted light (usually incident light) An inclination of about 65 to 75 degrees on the side away from the surface is corrected in the front direction of the emission surface using a prism sheet (light traveling direction correcting means) and then supplied to the liquid crystal display panel.

  Here, the light (primary light) introduced from the incident surface of the light guide plate is usually cold-cathode tube or LED light, which is substantially “uneven light (light with zero degree of polarization)”. As is well known, the emitted light (obliquely emitted light) from the exit surface of the light guide plate that emits light with strong directivity has a considerable bias. When this obliquely emitted light is divided into non-polarized light and completely polarized light, the polarization state of the completely polarized light is linearly polarized light or elliptically polarized light close thereto. In the light after the direction correction by the prism sheet, the polarization state slightly changes, but it remains the linearly polarized light or the elliptically polarized light close thereto. Note that one cause of the polarization state change by passing through the prism sheet is that the material, particularly the resin material used for the base sheet portion, has birefringence.

  By the way, even if the completely polarized light included in the light whose direction has been corrected by the prism sheet is either linearly polarized light or elliptical polarized light that is close to it, the polarization direction (in the case of elliptically polarized light, the direction of the major axis) is described above. It is usually quite different from the typical case “45 degrees”, and the case of “matching or substantially matching” is rare. Even in the case of substantially coincidence, in the case of elliptically polarized light, it does not conform to the transmission axis of “45 degrees” described above, depending on the size of the ellipticity (the definition will be described later).

  From this situation, in most cases, the completely polarized light included in the light whose direction has been corrected by the prism sheet (hereinafter also referred to as “front illumination light”) is “a bias that matches the transmission axis direction of the polarizing plate on the light input side”. It is not “linearly polarized light with direction”. For this reason, a non-negligible part of this completely polarized light is blocked by the light input side polarizing plate, and is wasted (cannot contribute to the display action by the liquid crystal display panel). As for the “unbiased light” (non-polarized light) included in the front illumination light, as understood in principle, approximately 50% of the light amount (light energy) is transmitted through the light input side polarizing plate, This contributes to the display function of the liquid crystal display panel (the remaining approximately 50% cannot be transmitted through the light input side polarizing plate).

Therefore, when considered in total, if the completely polarized light included in the front light can be corrected so as to easily pass through the input side polarizing plate, the amount of light blocked by the light input side polarizing plate is reduced, and the light as a whole is reduced. It is possible to improve the use efficiency of the.
Several proposals have already been made based on this concept. One of them is disclosed in Patent Document 1 below. That is, in the patent document 1, light (front illumination light) emitted from the deflection sheet 7 (prism sheet) in a substantially front direction through the polarization conversion sheet 6 is used for the polarizing plate 8 (input side polarizing plate) of the liquid crystal display panel. The configuration to supply is shown.

The substance of the polarization conversion sheet 6 is a single retardation plate (a half-wave plate or a quarter-wave plate), and the orientation direction of the retardation plate (the direction of the fast axis) is changed to the polarizing plate 8. It is described that adjustment is performed so that the transmittance of the (input-side polarizing plate) is maximized.
As shown in this example, it is certain that the polarization state can be controlled in accordance with the orientation direction by passing one quarter-wave plate or half-wave plate. However, in such a method using one quarter-wave plate or one half-wave plate, it is possible to freely adjust or select to control the conversion contents of the polarization state. Since only “orientation of the phase difference plate” is used, it is not possible to flexibly cope with the diversity of the relationship between the polarization states before and after the conversion. This can be explained in more detail as follows.

  First, as in the case of the above liquid crystal display, in the case where the polarization state conversion by the retardation plate is desired, the “polarization state before conversion” (before the conversion of the complete polarization component of the input light to the retardation plate) (Polarization state) cannot be freely selected, and the polarization state is not always one type. In the example of the liquid crystal display described above, the input light to the retardation plate is the output light of the prism sheet. The polarization state of the completely polarized component of this output light is roughly the type of the light guide plate and the type of the prism sheet. It is difficult to choose freely. Moreover, it changes if the combination of the type of light guide plate and the type of prism sheet changes.

On the other hand, the “polarization state after conversion” (the polarization state after conversion of the complete polarization component of the output light from the phase difference plate) is generally set according to the situation. is there. In this specification, the polarization state after polarization conversion set in this way is referred to as a “target polarization state”.
One typical target polarization state set in the above liquid crystal display example is “linearly polarized light having a polarization direction parallel to a direction inclined by 45 degrees with respect to the extending direction of each side of the rectangular light guide plate”. It is. Here, the “45 degree inclination” includes a counterclockwise direction (+45 degree inclination) and a clockwise direction (−45 degree inclination) when viewed from the liquid crystal display panel side. Further, depending on the orientation of the input side polarizing plate, another target polarization state may be set.

  Thus, considerable diversity is assumed in the relationship between the polarization states before and after the polarization state conversion, and a polarization conversion technique that can flexibly cope with such diversity is required. The prior art using one quarter wave plate or one half wave plate cannot meet this demand. First, in the method using one quarter-wave plate, “when the polarization state of the completely polarized component of the input light to the retardation plate (hereinafter referred to as“ input polarization state ”) is given, A major limitation on the realizable range of the polarization state of the completely polarized component of the output light from the phase difference plate (hereinafter referred to as “output polarization state”) is due to the small phase difference imparting ability (less than π). It exists in principle (the reason is described later). In addition, as described above, the fact that only the “alignment direction of one phase difference plate” can be freely adjusted also restricts the relationship between the “output polarization state” and the “input polarization state”.

  Next, in the method using one half-wave plate, there is no restriction due to the small phase difference imparting ability as in the case of the quarter-wave plate (the reason will be described later). Since only the “orientation direction of one retardation plate” can be freely adjusted or selected, there is a restriction on the relationship between the “output polarization state” and the “input polarization state”. In other words, it is possible to freely adjust or select either a quarter-wave plate or a half-wave plate “only the orientation direction of one retardation plate” (the retardation imparting ability is fixed). Under such circumstances, the generality of “generating an arbitrarily set target polarization state from a given input polarization state” cannot be obtained.

  As will be described later, while maintaining the degree of polarization of any input light having some bias (the definition will be described later), the output light converted into the “completely polarized light in the target polarization state” in which the completely polarized light included therein is desired In general, two phase difference plates having a total phase difference imparting ability equal to or greater than π (a phase difference corresponding to a half wavelength) are required, and the phase difference Δ between the phase difference plates is required. And the orientation θ must satisfy a “specific relationship” corresponding to the input polarization state and the target polarization state (output polarization state). However, the following Patent Document 1 does not mention “use of two retardation plates”, and as a natural result thereof, there is no description about the “specific relationship”.

  In addition, as a technique for improving the transmittance of the input-side polarizing plate, there is a method described in Patent Document 2 below. This method uses the fact that the optical rotation characteristics of the base material (large size sheet) of the base film of the prism sheet produced by stretch molding differ depending on the location, and cut out from the part with the best optical rotation characteristics A base film is employed to obtain light in a bias direction aligned with the transmission axis direction of the input side polarizing plate. Although this method allows rough control of the polarization state, as in Patent Document 1, it is a general-purpose device that converts completely polarized light in input light that can take various polarization states into completely polarized light of a desired target polarization state. It does not provide a method.

  Even if the optical field other than the liquid crystal display field is expanded in general, the polarization state of the completely polarized light included in any input light having a non-zero degree of polarization is maintained while maintaining the degree of polarization of the input light. No disclosure example of a polarization control technique having a high degree of versatility, such as conversion to the target polarization state of the light and output light, cannot be found.

JP 2006-39056 A JP 2001-166302 A

  One object of the present invention is to convert the polarization state of completely polarized light contained in any input light having a non-zero degree of polarization into another target polarization state while maintaining the degree of polarization. It is an object of the present invention to provide a method and apparatus. Another object of the present invention is to provide a liquid crystal display excellent in light utilization efficiency using the same apparatus.

  In order to achieve the above-mentioned object, the present invention firstly reads: “Input light consisting of completely polarized light and non-polarized light in the initial input polarization state A and having a polarization degree not zero is predetermined as a target polarization state, and the initial input There is provided a polarization state conversion method for converting into output light composed of completely polarized light in a final polarization state B different from the polarization state A and non-polarized light.

  According to the feature of the present invention, the method includes the step of generating the primary converted light by inputting the input light perpendicularly to the first retardation plate and passing the input light through the first retardation plate. 1 stage, and the 2nd stage which produces | generates a secondary conversion light by inputting the said primary conversion light perpendicularly | vertically with respect to a 2nd phase difference plate, and making it pass through this 2nd phase difference plate .

  When the phase difference providing ability of the first phase difference plate is Δ1, and the phase difference providing ability of the second phase difference plate is Δ2, Δ1 + Δ2 ≧ π, and among Δ1 and Δ2, At least one value is equal to π / 2. In other words, a quarter wavelength plate is used as at least one of the first retardation plate and the second retardation plate.

Further, the initial input polarization state A has one set of Stokes parameters as four components, the normalized vector is SA, and the final polarization state B is another set of Stokes parameters as four components, and the normalized vector Is represented by SB,
The Mueller matrix expressing the polarization conversion action of the first retardation plate is R (Δ1, θ1) [where θ1 is the first position in the plane perpendicular to the thickness direction of the first retardation plate. An angle representing the first orientation direction of the phase difference plate],
The Mueller matrix expressing the polarization conversion action of the second retardation plate is R (Δ2, θ2), where θ2 is the second position in the plane perpendicular to the thickness direction of the second retardation plate. Angle representing the second orientation direction of the phase difference plate]
(I) the first phase difference providing ability represented by Δ1,
(Ii) the first orientation direction represented by θ1;
(Iii) the second phase difference imparting ability represented by Δ2, and
(Iv) A combination of the second orientation directions represented by θ2 is a relational expression:
R (Δ2, θ2) {R (Δ1, θ1) SA} = SB
The first alignment direction and the second alignment direction are determined so as to satisfy the above. Thereby, the polarization state of the completely polarized light included in the secondary converted light matches the final polarization state B.

  Here, both the first retardation plate and the second retardation plate may be quarter-wave plates. One of the first retardation plate and the second retardation plate may be a half-wave plate. In general, the first alignment direction and the second alignment direction are different from each other. A liquid crystal display panel including a polarizing plate disposed on the light input side, wherein the input light is emitted obliquely from the exit surface of the light guide plate and then corrected in the traveling direction in the front direction of the exit surface. The final polarization state B can be determined so that the polarization direction of the linearly polarized light component included in the output light is parallel to the transmission axis direction of the polarizing plate.

In order to achieve the above-mentioned object, the present invention also provides that “the input light consisting of completely polarized light and non-polarized light in the initial input polarization state A and having a non-polarization degree of zero is predetermined as the target polarization state, and the initial input There is provided a polarization state conversion device that converts output light composed of completely polarized light in a final polarization state B different from the polarization state A and non-polarized light.
According to a feature of the present invention, the polarization state conversion device includes a first retardation plate and a second retardation plate arranged in parallel with each other, and the first retardation plate transmits the input light. It arrange | positions so that it may input perpendicularly | vertically with respect to the said 1st phase difference plate, and may output primary conversion light.

The second retardation plate causes the primary converted light output by the first retardation plate to be input perpendicularly to the second retardation plate and outputs secondary converted light. Is arranged.
Further, when the phase difference providing ability of the first phase difference plate is Δ1, and the phase difference providing ability of the second phase difference plate is Δ2, Δ1 + Δ2 ≧ π, and among Δ1 and Δ2, At least one value is equal to π / 2. In other words, at least one of the first retardation plate and the second retardation plate is a quarter-wave plate.

Further, the initial input polarization state A has one set of Stokes parameters as four components, the normalized vector is SA, and the final polarization state B is another set of Stokes parameters as four components, and the normalized vector Is represented by SB,
The Mueller matrix expressing the polarization conversion action of the first retardation plate is R (Δ1, θ1) [where θ1 is the first position in the plane perpendicular to the thickness direction of the first retardation plate. An angle representing the first orientation direction of the phase difference plate],
The Mueller matrix expressing the polarization conversion action of the second retardation plate is R (Δ2, θ2), where θ2 is the second position in the plane perpendicular to the thickness direction of the second retardation plate. Angle representing the second orientation direction of the phase difference plate]
(I) the first phase difference providing ability represented by Δ1,
(Ii) the first orientation direction represented by θ1;
(Iii) the second phase difference imparting ability represented by Δ2, and
(Iv) A combination of the second orientation directions represented by θ2 is a relational expression:
R (Δ2, θ2) {R (Δ1, θ1) SA} = SB
The first alignment direction and the second alignment direction are determined so as to satisfy the above. Thereby, the polarization state of the completely polarized light included in the secondary converted light matches the final polarization state B.

  Here, both the first retardation plate and the second retardation plate may be quarter-wave plates. Alternatively, one of the first retardation plate and the second retardation plate may be a half-wave plate. In a typical case, the first alignment direction and the second alignment direction are different from each other.

Furthermore, the present invention provides a “liquid crystal display including a polarization state conversion device as a surface light source device and a liquid crystal display panel including a polarizing plate disposed on the light input side”.
According to a feature of the invention, the surface light source device is disposed along the incident surface, a light guide plate having an incident surface and an output surface, a primary light source that supplies primary light to the light guide plate through the incident surface, and A member provided with a traveling direction correcting member for correcting the traveling direction of the outgoing light obliquely emitted from the emitting surface to the front direction of the emitting surface is used.

  The polarization state conversion device according to claim 9 is adopted as the polarization state conversion device, and the outgoing light from the emission surface whose traveling direction has been corrected to the front direction by the traveling direction correction member, It arrange | positions so that it may input into the said 1st phase difference plate as input light, The said output light is supplied to the liquid crystal display panel containing the polarizing plate arrange | positioned at the light input side.

The final polarization state B is determined to represent linearly polarized light having a polarization direction parallel to the transmission axis direction of the polarizing plate.
In a typical case, the light exit surface of the light guide plate is rectangular, and the transmission axis direction of the polarizing plate forms an angle of 45 degrees with respect to the extending direction of each side of the rectangle. Further, the first retardation plate may be integrated with the traveling direction correcting member. A light diffusing member that diffuses the secondary converted light may be disposed on the light input side of the polarizing plate, and the second retardation plate may be integrated with the light diffusing member.

  According to the present invention, input light that can have various polarization states (for example, front illumination light from a surface light source device) has the same degree of polarization, and a desired target polarization state (for example, a polarization component of a liquid crystal display panel). Like the input-side polarizing plate transmission axis, it is possible to perform highly versatile polarization state control that can convert linearly polarized light (aligned in a direction inclined by 45 degrees with respect to each side of the light guide plate) into output light having a completely polarized component. . In particular, no matter how the combination of the input polarization state and the target polarization state changes, it is only necessary to adjust the orientation direction of both retardation plates without changing the retardation imparting ability Δ1, Δ2 of both retardation plates. It is very advantageous in practice to enable conversion to the target polarization state.

  In addition, it is possible to improve the light use efficiency of the liquid crystal display by incorporating a device for controlling the polarization state into the liquid crystal display. At that time, what is the combination of the input polarization state (completely changing component of the output light of the prism sheet) and the target polarization state (linear polarization having a polarization direction parallel to the transmission axis of the input side polarizing plate of the liquid crystal display panel)? Even if it changes, it is possible to convert to the target polarization state by adjusting the orientation direction of both phase difference plates without changing the phase difference imparting ability Δ1, Δ2 of both phase difference plates. It is very advantageous in manufacturing a liquid crystal display.

  Prior to the description of the embodiments of the present invention, assumptions (general matters when describing a polarization state, a polarization state representation method using Stokes parameters, and a Poincare method used in connection with the representation method) An outline of the sphere and the Mueller matrix expressing the polarization conversion action of the retardation plate will be described.

(I) Items describing the polarization state;
In the present invention, the polarization conversion is performed using the first retardation plate and the second retardation plate, but it is assumed that the input light is “light with some bias”, as described above. In general, it can be considered as light that is a combination (superposition) of “non-polarized light” and “perfectly polarized light”. There may be a case where the component of “non-polarized light” is zero (completely polarized light only), but an input of only non-polarized light is not assumed. Input (incident) to each phase difference plate is performed vertically. Under this circumstance, the polarization conversion action when the combined light of “non-polarized light” and “completely polarized light” is input to the phase difference plate is divided into the action for the “non-polarized light” component and the action for the “completely polarized light” component. Can think.

First, it is known that the action against non-polarized light is neutral (no action) as a whole. That is, even if non-polarized light is perpendicularly incident on the phase difference plate, the light emitted from the phase difference plate is also non-polarized (the theoretical basis is omitted).
Therefore, in the case of the present invention, it is sufficient to pay attention to the “completely polarized light” included in the input light and consider the polarization conversion action of the retardation plate for the completely polarized light.

  FIG. 1 is a diagram for explaining the polarization state of completely polarized light traveling in the z-axis direction (not shown; perpendicular to the xy plane). As is well known, general completely polarized light can be described as elliptically polarized light, and linearly polarized light and circularly polarized light can be treated as a special case of elliptically polarized light. In FIG. 1, an ellipse figure shows a locus drawn by the tip of a completely polarized polarization vector (a vector obtained by combining an x-direction polarization component and a y-direction polarization component) that travels perpendicular to the z-axis direction. It represents. In the figure, symbol ξ represents the major axis of the ellipse, and symbol η represents the minor axis direction of the ellipse.

Θ is the major axis orientation measured with respect to the x-axis direction, and the positive / negative sign is positive for counterclockwise and negative for clockwise. Further, β is an angle (ellipticity angle) in which a rectangle in contact with an ellipse is considered and an angle formed by the diagonal direction with respect to the major axis ξ is represented by positive in the counterclockwise direction and negative in the clockwise direction. However, since there are two diagonal lines (β value is positive and negative), β> 0 for clockwise elliptically polarized light and β <0 for counterclockwise elliptically polarized light. The ellipticity can be defined by Tan β.
Note that β = 0 represents linearly polarized light, and the ellipticity Tan β = 0. In circularly polarized light, the ξ and η directions are arbitrary (undefined). For example, the ξ direction = x direction and the η direction = y direction (Θ = 0 degrees). The ellipticity angle is β = −45 degrees (counterclockwise circularly polarized light) or β = + 45 degrees (clockwise circularly polarized light).

(II) Representation of polarization state using Stokes parameters and Poincare sphere;
As a method for expressing the polarization state of general light, a method using a vector having four components of Stokes parameters (hereinafter referred to as a Stokes vector) is known. Stokes parameters constituting the four components of the Stokes vector are defined as follows.
S0 = intensity S1 = horizontal linearly polarized light component S2 = 45 degree linearly polarized light component S3 = clockwise circularly polarized light component Also, the degree of polarization V is defined by V = [(S1 ² + S2 ² + S3 ² ) / S0 ² ] ^1/2 be able to. In the case of complete polarization, S1 ² + S2 ² + S3 ² = S0 ² and V = 1.

  As described above, since the polarization conversion action of the phase difference plate is considered focusing on complete polarization here, the Stokes vector normalized as S0 = 1 is considered, and the light input to the first phase difference plate is considered. The polarization state of the complete polarization component (= initial input polarization state) will be represented by a normalized Stokes vector SA. Similarly, the polarization state (= final polarization state) of the completely polarized component of the light output after passing through the two retardation plates is expressed by a standardized Stokes vector SB.

As a technique for geometrically or visually expressing the above-mentioned Stokes vector, there is a method using a Poincare sphere. As shown in FIG. 2, the Poincare sphere defines a sphere having a radius S0 in consideration of orthogonal coordinate axes (S1, A2, and S3 axes) corresponding to the Stokes parameters S1, S2, and S3. If this Poincare sphere is used, the Stokes vector and the point on the Poincare sphere correspond one-on-one. Here, since a normalized Stokes vector is considered, S0 = 1. FIG. 2 illustrates points on the Poincare sphere representing horizontal linearly polarized light, points on the Poincare sphere representing 45 degree linearly polarized light, and clockwise circularly polarized light.
With S0 = 1, the coordinate value of horizontal linearly polarized light is (1, 0, 0), and the coordinate value of 45 degree linearly polarized light is (0, 1, 0). The coordinate value of clockwise circularly polarized light is (0, 0, 1).

(III) Relationship between Stokes parameters and major axis azimuth angle Θ, ellipticity angle β, etc .;
As described above, complete polarization can be expressed using the major axis azimuth angle Θ and ellipticity angle β described in (I) above and the Stokes parameter described in (II) above. . It is known that the relationship between these expressions is as follows. Hereinafter, this formula is referred to as “polarization state expression conversion formula” for convenience.
・ S1 ＝ cos2Θcos2β
・ S2 ＝ sin2Θcos2β
・ S3 = sin2β
Here, since a standardized Stokes vector is considered, S0 = 1.

(IV) About Mueller Matrix;
When the polarization state is expressed by the Stokes vector, it is known that the polarization conversion action when the polarization state is perpendicularly incident on the phase difference plate can be expressed by a Mueller matrix. The simulation is a matrix of 4 rows and 4 columns, and the phase difference imparting ability Δ of the phase difference plate and the angle indicating the orientation direction of the phase difference plate in a plane perpendicular to the thickness direction of the phase difference plate are used as parameters in the matrix element. θ is included. The Mueller matrix R (Δ, θ) is generally expressed as follows for convenience. This matrix is sometimes called a linear phaser matrix.

Each matrix element r11 to r44 is as follows.
・ R11 = 1
・ R12 = r13 = r14 = 0
・ R21 = 0
R22 = 1- (1-cosΔ) sin ² 2θ
R23 = (1-cosΔ) sin2θcos2θ
・ R24 = −sinΔsin2θ
・ R31 = 0
R32 = (1-cosΔ) sin2θcos2θ
R33 = 1- (1-cosΔ) cos ² 2θ
・ R34 = sinΔcos2θ
・ R41 = 0
・ R42 = sinΔsin2θ
・ R43 = −sinΔcos2θ
・ R44 = cosΔ

  Here, Δ is the phase difference imparting ability of the phase difference plate (the phase difference given by the passage of light perpendicularly incident on the phase difference plate until it is output from the phase difference plate = the phase difference plate direction, The phase difference between the polarization component and the polarization component in the slow axis direction). As is well known, a retardation plate having this retardation providing ability Δ = π / 2 (a phase corresponding to a quarter wavelength) is a quarter-wave plate, and the retardation providing ability Δ = π (two minutes). Is a half-wave plate. Note that Δ of the phase difference plate is normally expressed in a range of 0 <Δ <2π. Although it is possible to give a phase difference exceeding 2π, in consideration of the periodicity of the light wave, a phase difference that is a positive integer multiple of 2π is ignored and expressed. For example, a phase difference plate that gives a phase difference (3π) for three-half wavelengths is equivalent to a half-wave plate, so that they are not distinguished from each other. Further, when Δ = 0 or Δ = 2π, there is no phase difference imparting ability (thus, such an optical element is not called a “retardation plate”).

  Further, the orientation direction of the retardation plate represented by θ represents “an angle formed by the direction of the fast axis of the retardation plate with respect to the“ reference direction ””. Here, the “reference direction (reference direction)” is usually the horizontal direction (x direction in the drawing of FIG. 1), and the range that θ can take is 0 degree ≦ θ <180 degrees.

If the phase difference plate is a quarter-wave plate (phase difference providing ability Δ = π / 2), each matrix element of the Mueller matrix is as follows.
・ R11 = 1
・ R12 = r13 = r14 = 0
・ R21 = 0
・ R22 = 1−sin ² 2θ = cos ² 2θ
・ R23 = sin2θcos2θ
・ R24 = −sin2θ
・ R31 = 0
・ R32 = sin2θcos2θ
・ R33 = 1−cos ² 2θ = sin ² 2θ
・ R34 = cos2θ
・ R41 = 0
・ R42 = sin2θ
・ R43 = -cos2θ
・ R44 = 0

Further, if the retardation plate is a half-wave plate (retardation imparting ability Δ = π), the following is obtained.
・ R11 = 1
・ R12 = r13 = r14 = 0
・ R21 = 0
・ R22 = 1−2sin ² 2θ = cos4θ
・ R23 = 2sin2θcos2θ = sin4θ
・ R24 = 0
・ R31 = 0
・ R32 = 2sin2θcos2θ = sin4θ
・ R33 = 1−2 cos ² 2θ = −cos 4θ
・ R34 = 0
・ R41 = 0
・ R42 = 0
・ R43 = 0
・ R44 = -1

The polarization conversion action of the retardation plate represented by the Mueller matrix is generally expressed by the equation R (Δ, θ) S obtained by multiplying the Stokes vector S representing the polarization state of the input light by the Mueller matrix R (Δ, θ) from the left side. Can be represented. FIG. 3 is a diagram illustrating this with a geometric image. In the figure, the Stokes vector S representing the polarization state of the input light (here, assuming perfect polarization) is SA. The four components S0 to S4 of SA are expressed as follows.
・ S0 = SA0 (however, here SA0 = 1)
・ S1 = SA1
・ S2 = SA2
・ S3 = SA3
The position on the Poincare sphere represented by the Stokes parameter (polarization state) SA is assumed to be A in FIG.

By the way, performing polarization conversion using the Mueller matrix R (Δ, θ) is equivalent to performing the following geometric operation (i) and then performing the geometric operation (ii) below. It has been.
(I) An axis corresponding to a point obtained by rotating the S1 axis about the S3 axis by 2θ while maintaining the position of the point A on the Poincare sphere (refer to FIG. 3A), with the S3 axis of the Poincare sphere as the rotation axis. Define aa. The axis aa is an axis that is a rotation axis aa for the next operation (ii).
(Ii) As shown in FIG. 3B, the point A is rotated around the axis aa by Δ on the plane P perpendicular to the axis aa determined by the operation (i). Move. Let B be the position on the Poincare sphere after movement.
If the Stokes vector corresponding to this point B is SB, the operations (i) and (ii) can be expressed as follows.
R (Δ, θ) SA = SB

  From the operation contents of the above (i) and (ii), the polarization conversion action of the Mueller matrix R (Δ, θ) is geometrically viewed as “rotation operation (i) by θ” and “rotation operation by Δ. (Ii) ". Here, considering the case of using one retardation plate as in the prior art, the orientation direction θ of the retardation plate can be freely selected, but the retardation providing ability Δ cannot be changed. For example, a half-wave plate is fixed to Δ = π, and a quarter-wave plate is fixed to Δ = π / 2. Therefore, no matter what value (fixed value) is set for Δ, the relationship between the starting point A on the Poincare sphere and the point B after movement is extremely limited.

  That is, in FIGS. 3A and 3B, no matter how much θ is changed, the point after the above operations (i) and (ii) can move only on one closed curve on the Poincare sphere. Absent. Now, let's consider changing Δ from 0 to π (0 degrees to 180 degrees) with Δ fixed. The point where the above geometrical operations (i) and (ii) are applied to the starting point A is as follows: under θ = 0, “a plane passing through the point A and perpendicular to the S1 axis (a plane parallel to the S2S3 plane ) Above, the point A is a point that is rotated about the S1 axis by Δ. This point is indicated by B0 in FIG. Then, when θ is gradually increased to π, the point after the completion of the operations (i) and (ii) is separated from the point B0 and passes through the point B shown in FIG. = Π will return to the original B0. This means that “a single phase difference plate has a strong restriction on the positional relationship between point A and point B and cannot be freely set”. In other words, a single retardation plate with a fixed retardation providing ability cannot move freely on the Poincare sphere no matter how the orientation direction is adjusted.

  The inventor of the present application considers the above-mentioned restrictions, and in short, thinks that there is a fundamental cause in “there is only one factor that can be freely adjusted or selected (the orientation direction of the retardation film)”. We paid attention to the fact that “a situation where there are two factors that can be freely adjusted or selected” is created by combining the phase difference plates. That is, if the orientation direction of each phase difference plate can be selected independently, it can move freely on the Poincare sphere as long as the phase difference providing ability of the two phase difference plates is sufficient.

Here, specifically, the condition that “the retardation providing ability of the two retardation plates is sufficient” is, specifically, “the total of the two retardation providing ability is π (180 degrees) or more”. That is. This can be understood from the content of the polarization conversion action of one retardation plate described with reference to FIGS. 3 (a) and 3 (b). The reason is as follows.
That is, passing through the two retardation plates means that the operations (i) and (ii) can be executed once again by freely selecting θ. This is because the aa axis is freely re-determined again from the point B reached by passing through the first phase plate, and then on a plane perpendicular to the re-determined aa axis. The fixed rotational movement (movement of Δ of the second sheet) is possible.

  FIG. 4 shows a basic form of an embodiment of the present invention along this concept. In FIG. 4, a left retardation plate 1 (first retardation plate) and a right retardation plate 2 are arranged in parallel to each other, and input light (light to be polarization-converted) is a retardation plate. Incident vertically from the left side of 1. This input light is “light whose polarization degree V is not zero” (light including at least some polarization component). As described above, such input light is combined light (superimposed light) of “non-polarized light” and “completely polarized light”, and the action of each phase difference plate is considered to be “non-polarized light” in view of the principle of superposition. ”And“ completely polarized light ”. Even if the non-polarized light passes through the phase difference plates 1 and 2 one after another, it remains unpolarized without being influenced by the combination of the phase difference imparting ability and the orientation angle of each phase difference plate. Therefore, in order to discuss the polarization conversion action by the phase difference plates 1 and 2, only complete polarization needs to be considered.

  In FIG. 4, “initial input polarization state A” is this complete polarization included in the input light, and this is represented by Stokes vector as SA. As shown in the figure, each component (Stokes parameter) of SA is expressed as SA0, SA1, SA2, and SA3 (however, here, SA0 = 1 is considered because a standardized Stokes vector is considered). If the Mueller matrices representing the polarization conversion action of the phase difference plates 1 and 2 are R (Δ1, θ1) and R (Δ2, θ2), respectively, the polarization state of completely polarized light contained in the light output from the phase difference plate 1 ( The primary conversion state A ′) is R (Δ1, θ1) SA.

  The Stokes vector representing the primary conversion state A ′ is denoted by SA ′, and each component (Stokes parameter) is denoted by SA′0, SA′1, SA′2, SA′3 (however, as apparent from the Mueller matrix form). SA′0 = SA0, where SA′0 = SA0 = 1). Further, the light output from the phase difference plate 1 is vertically incident on the phase difference plate 2 to undergo polarization conversion. The state of complete polarization (final polarization state B) contained in the output light obtained is R (Δ2, θ2) SA ′.

The Stokes vector representing the final polarization state B is SB, and each component (Stokes parameter) is expressed as SB0, SB1, SB2, SB3 (provided that SB0 = SA'0 = SA0, as apparent from the Mueller matrix) Here, SB0 = SA'0 = SA0 = 1). From the above, the relationship between the initial input polarization state A and the final polarization state B is expressed by the following basic formula (# 1).
SB = R (Δ2, θ2) R (Δ1, θ1) SA (# 1)

However, in the present invention, in consideration of practical convenience, a quarter-wave plate is adopted for at least one of the phase difference plates 1 and 2. Further, from the discussion of the geometrical image of the polarization conversion action of the Mueller matrix described above, the phase difference imparting ability of the two retardation plates combined is sufficiently large in the range of less than 2π (π or more = 2 minutes). The phase difference corresponding to one wavelength). That is, in the present invention, the following additional conditions (# 2) and (# 3) are imposed on the retardation plates 1 and 2 used.
At least one of Δ1 or Δ2 = π / 2 (# 2)
π ≦ Δ1 + Δ2 <2π (# 3)

  Now, when considering the situation where it is actually desired to perform polarization conversion, it is considered that “final polarization state B” and “initial input polarization state A” can be freely changed, as can be seen in the example of the liquid crystal display described above. hard. That is, in most cases, how to convert a given initial input polarization state A to the desired final polarization state B is an important matter. In order to meet this demand, SA and SB should be known and the Mueller matrices R (Δ1, θ1) and R (Δ2, θ2) should be determined so as to satisfy the basic formula (# 1). As described above, the parameters included in both Mueller matrices are Δ1, θ1, Δ2, and θ2, and the basic equation (# 1) can be considered as an equation that includes four variables. However, in practice, only θ1 and θ2 can be changed without difficulty.

  This is because, in order to change the phase difference imparting ability Δ of the phase difference plate, it is necessary to make the structure of the phase difference plate itself variable or to prepare in advance a phase difference plate having various kinds of phase difference imparting ability Δ. However, it is practically impossible to make the structure of the retardation plate itself variable. In addition, it is difficult to obtain a large number of retardation plates having various retardation providing ability Δ at low cost. Therefore, the basic formula (# 1) is considered as an equation including two variables θ1 and θ2, and Δ1 and Δ2 are regarded as constant parameters. However, in that case, the additional conditions (# 2) and (# 3) are imposed on the constant parameters.

  For the additional condition (# 2), a quarter-wave plate, which can be called the most common and inexpensively available retardation plate, is used for at least one of the two retardation plates. There is a meaning. In addition, in addition condition (# 3), in addition to addition condition 2, the equation represented by basic formula (# 1) can be solved for a very diverse “combination of final polarization state B and initial input polarization state A” ( 0 ≦ θ1 <2π and 0 ≦ θ2 <2π exist).

As described above, the procedure for carrying out the polarization conversion method according to the present invention is as follows, for example.
(1) The retardation plates 1 and 2 are prepared. There are various combinations, but the following three are typical combinations.
(Combination example 1);
-Retardation plate 1 = 1/4 wavelength plate-Retardation plate 2 = 1/4 wavelength plate (Combination example 2);
-Phase difference plate 1 = 1/4 wavelength plate-Phase difference plate 2 = 1/2 wavelength plate (Combination Example 3);
-Retardation plate 1 = 1/2 wavelength plate-Retardation plate 2 = 1/4 wavelength plate

(2) A target final polarization state B is determined. The final polarization state B is usually easily expressed by the major axis azimuth Θ and the ellipticity angle β. For example, in the B = 45 degree direction linearly polarized state, which is often required for a liquid crystal display, Θ = 45 degrees and β = 0 degrees. If B = horizontal polarization in the horizontal direction, Θ = β = 0 degree.
(3) The Stokes vector SB is calculated using the above-described polarization state expression conversion formula.
(4) An initial input polarization state A of input light to be used (for example, light emitted from a prism sheet on the light guide plate) is obtained. Since the measurement method for obtaining this is well known, description thereof is omitted. From the measurement result, the major axis direction Θ and the ellipticity angle β are determined for the completely polarized light included in the input light.

(5) The Stokes vector SA is calculated using the above-described polarization state expression conversion formula.
(6) With respect to SA and SB obtained in the above (3) and (5), the Mueller matrices R (Δ1, θ1) and R (Δ2, θ2) satisfying the basic formula (# 1) are obtained.
Specifically, the following two methods can be employed.

(A) As described above, by substituting the known numerical values (that is, the values of the elements SA and SB and the values of the parameters Δ1, Δ2) into the basic formula (# 1), the solution is obtained by releasing the numerical values (θ1, Find the set of θ2).
(B) Prepare a large number of sets (θ1, θ2), and substitute for each set with the basic values (# 1) together with known values (ie, the values of each element of SA and SB and the values of parameters Δ1, Δ2). The right side R (Δ2, θ2) R (Δ1, θ1) SA of the equation (# 1) is calculated. The solution is a set of (θ1, θ2) that gives what can be substantially regarded as SB from among the obtained many vectors. As a method of preparing a set of (θ1, θ2), for example, θ1 value and θ2 value are prepared in 1 degree increments (0 degree, 1 degree, 2 degrees,... 179 degrees), and for all numerical value pairs The right side R (Δ2, θ2) R (Δ1, θ1) SA of the equation (# 1) is calculated, and a set of (θ1, θ2) that gives what can be substantially regarded as SB may be solved. If a more precise solution is desired, the increments of θ1 value and θ2 value may be made finer (for example, in increments of 0.1 degrees).

  (7) The retardation plates 1 and 2 are arranged in parallel at the obtained orientation angles θ 1 and θ 2 as shown in FIG. 4 so that the light of the initial input polarization state A is perpendicularly incident on the retardation plate 1. Then, the output light of the phase difference plate 2 is substantially in the final polarization state B.

Next, specific examples of generating the final polarization state B from various initial input polarization states A by selecting the orientation direction of the phase difference plates 1 and 2 (see FIG. 4) will be described. The final polarization state B is a “desired polarization state”, and in general, any of the elliptical polarization described with reference to FIG. 1 can be set. As the polarization state B, the following three types of linearly polarized light B (1) to B (3) B (1) to B (3) are considered. As described with reference to FIG. 1, linearly polarized light is “ellipsoidal light with an ellipticity angle β = 0 ° and a major axis azimuth Θ (0 ≦ Θ <180 °)”.
B (1); 45 degree linearly polarized light (β = 0, Θ = 45 degree)
B (2): Horizontal linearly polarized light (β = 0, Θ = 0 degrees)
B (3): 30 degree linearly polarized light (β = 0, Θ = 30 degrees)

As the retardation plates 1 and 2 to be arranged, the following three types of combinations that are easily available and inexpensive are considered in consideration of the above-mentioned additional conditions # 2 and # 3.
● Arrangement 1;
-Phase difference plate 1 = 1/4 wavelength plate (Δ1 = π / 2)
-Phase difference plate 2 = 1/4 wavelength plate (Δ2 = π / 2)
● Arrangement 2;
-Phase difference plate 1 = 1/4 wavelength plate (Δ1 = π / 2)
-Retardation plate 2 = 1/2 wavelength plate (Δ2 = π)
● Arrangement 3;
-Retardation plate 1 = 1/2 wavelength plate (Δ1 = π)
-Phase difference plate 2 = 1/4 wavelength plate (Δ2 = π / 2)

  As described above, the initial input polarization state A cannot generally be freely selected. Therefore, here, the following states 1 to 12 are defined, and for each of the arrangement examples of the phase difference plates 1 and 2 from which the final polarization states B (1) to B (3) are obtained for all or part of them, The combinations of the orientation directions (angles θ1, θ2) of the retardation plates 1 and 2 were determined according to the calculation method described above. The obtained results are shown in table form (Tables 1 to 3) in FIGS.

● State 1 (linearly polarized light);
・ Elliptic angle β = 0 degrees ・ Long axis orientation Θ = −22.5 degrees ● State 2 (elliptical polarization);
・ Elliptic angle β = 15.0 degrees ・ Long axis orientation Θ = 12.1 degrees ● State 3 (elliptical polarization);
・ Elliptic angle β = 15.0 degrees ・ Long axis orientation Θ = 33.0 degrees ● State 4 (elliptical polarization);
・ Elliptic angle β = 22.5 degrees ・ Long axis orientation Θ = −22.5 degrees ● State 5 (elliptical polarization);
・ Elliptic angle β = −22.5 degrees ・ Long axis orientation Θ = −15.0 degrees ● State 6 (elliptical polarization);
・ Elliptic angle β = -22.5 degrees ・ Long axis orientation Θ = -60.0 degrees

● State 7 (elliptical polarization);
・ Elliptic angle β = 26.1 degrees ・ Long axis orientation Θ = 27.4 degrees ● State 8 (elliptical polarization);
・ Elliptic angle β = −30.0 degrees ・ Long axis orientation Θ = 22.5 degrees ● State 9 (elliptical polarization);
・ Elliptic angle β = 30.0 degrees ・ Long axis orientation Θ = 45.0 degrees ● State 10 (elliptical polarization);
・ Elliptic angle β = 30.0 degrees ・ Long axis orientation Θ = 0.0 degrees ● State 11 (elliptical polarization);
・ Elliptic angle β = 30.0 degrees ・ Long axis orientation Θ = −45.0 degrees ● State 12 (circularly polarized light);
・ Elliptic angle β = -45.0 degrees ・ Long axis orientation Θ = 0.0 degrees

As can be seen from the results shown in Table 1 of FIG. 5, in any of the arrangements 1 to 3 in which two quarter wave plates or a quarter wave plate and a half wave plate are combined, A set of θ1 and θ2 (solutions of θ1 and θ2) in which the orientation directions of the phase difference plates 1 and 2 that realize the target final polarization state B (1) are specific is obtained. Note that the solutions of θ1 and θ2 are not limited to one set, and two sets may be obtained. For example, another solution for obtaining B (1) from state 4 is as follows.
・ Another solution in arrangement 1 (θ1, θ2) = (1.3 degrees, 29.3 degrees)
・ Another solution in arrangement 2 (θ1, θ2) = (67.5 degrees, 0.0 degrees)
・ Another solution in arrangement 3 (θ1, θ2) = (0.0 degrees, −67.5 degrees)

  The above is the outline and specific example of the polarization conversion method according to the present invention. As described above, a polarization conversion device that executes this method can be incorporated into a liquid crystal display. FIG. 8 shows the basic arrangement. As shown in the figure, the liquid crystal display LCD includes a liquid crystal display panel LP and a surface light source device that supplies illumination light to the liquid crystal display panel LP. The surface light source device includes a primary light source 10, a reflector 11, a light guide plate 20, a prism sheet 30, and a phase difference plate 33. In addition to this, a reflective sheet is often arranged along the back surface of the light guide plate 20, but the illustration is omitted. Further, for example, a light diffusion plate may be disposed between the phase difference plate 33 and the liquid crystal display panel LP or between the light guide plate 20 and the prism sheet 30.

  Here, the prism sheet 30 also serves as a phase difference plate. Reference numeral 32 denotes a base portion that provides the phase difference plate, and a large number of prism-like projections 31 are formed on the inside thereof. This base portion corresponds to the phase difference plate 1 in the above description, and the phase difference plate 33 corresponds to the phase difference plate 2 in the above description. The base portion 32 of the prism sheet 30 may not be a retardation plate, and the retardation plate may be configured separately from the prism sheet.

  Of the various elements constituting the surface light source device, the portion excluding the base portion (phase difference plate 1) and the phase difference plate (phase difference plate 2) 33 has a known structure and action. In the present embodiment, a base portion (phase difference plate 1) and a phase difference plate (phase difference plate 2) 33 are added to this, and this additional portion corresponds to the polarization conversion device. The material of the light guide plate 20 is well known, and for example, a translucent resin such as PMMA is used. In addition, a light scattering light guide in which scattering power (fine scatterers) is dispersed inside the light guide plate 20 may be used. Further, as shown in the drawing, it is preferable to gradually reduce the thickness from the incident surface 21 a toward the end surface 23.

  As is well known, light (primary light) from a primary light source (for example, a cold cathode tube or an LED light source) enters the light guide plate 20 from the incident surface 21 a of the light guide plate 2 and approaches toward the end surface 23. To propagate internally. In the process, the light is repeatedly reflected from the emission surface 21b and the back surface 22 and gradually emitted from the emission surface 21b. Usually, there is almost no bias in the primary light, but due to the process of repeatedly reflecting on the emission surface 21b and the back surface 22, the polarization characteristics at the time of emission from the emission surface 21b, etc., the emission light has a certain degree of polarization (degree of polarization). ) Occurs. The degree of polarization V of the emitted light varies depending on the type of the light guide plate 20 (construction material, surface shape, size, presence / absence and type of added scatterer, etc.), but may be about V = 0.1 to 0.20. Many. Needless to say, completely polarized light is included in the outgoing light from the outgoing surface 21b according to the degree of polarization.

  As is well known, such outgoing light having a non-polarization degree of 0 is emitted obliquely forward. The light ray 40 depicted in FIG. 8 is the representative light ray, and is inclined forward by about 65 to 75 degrees with respect to the front direction (z direction). The prism-shaped protrusion 31 corrects the direction of the representative light beam 40 in the front direction by a known action. As a result, the representative light beam 40 is perpendicularly incident on the phase difference plate 1 of the base portion 32. The polarization state of the complete polarization component of this light corresponds to “initial input polarization state A” in FIG. This input light is incident on the input-side polarizing plate 50 of the liquid crystal display panel LP via the base portion (phase difference plate 1) 32 and the phase difference plate (phase difference plate 2) 33.

  In many cases, the transmission axis direction of the input-side polarizing plate 50 (bias direction giving the maximum transmittance) is 45 degrees (direction inclined by 45 degrees with respect to the four sides of the light guide plate 20). Therefore, the light utilization efficiency increases as the light amount that includes a large amount of 45-degree linearly polarized light component corresponding thereto. This polarization state is nothing but the final polarization state B (1) described above. Therefore, in this case, the final polarization state B (1) can be realized with the orientation direction (optimum θ1 and θ2 set) of the base portion (phase difference plate 1) 32 and the phase difference plate (phase difference plate 2) 33. It is sufficient to set

  However, as can be understood from the discussion so far, the optimum set of θ1 and θ2 is the combination of the phase difference imparting ability Δ1, Δ2 of the phase difference plate 12 (base portion 32 and phase difference plate 33) to be used, and the initial setting. It varies depending on the input polarization state A. In one typical example, the initial input polarization state A is linearly polarized light with an ellipticity angle β = 0 degrees and a major axis orientation Θ = 90 degrees. If both the phase difference plate 1 (base portion 32) and the phase difference plate 2 (phase difference plate 33) to be used are quarter wave plates, there are two optimum solutions, and (θ1, θ2) = (45 degrees, 90 degrees) or (-45 degrees, 0 degrees).

  Further, the transmission axis direction of the input side polarizing plate 50 may be another angle, for example, 0 degree. In such a case, the light utilization efficiency increases as the amount of light that contains a large amount of the horizontal linearly polarized light component corresponding thereto. This polarization state is nothing but the final polarization state B (2) described above. Therefore, in this case, the basic direction (# 1) described above is solved for the orientation directions (optimum set of θ1 and θ2) of the base portion (phase difference plate 1) 32 and the phase difference plate (phase difference plate 2) 33. In this way, the final polarization state B (2) may be determined. The details of the determination method will be omitted because the same description is repeated.

  In the examples described above, a combination of quarter-wave plates or a quarter-wave plate and a half-wave plate is used, but it is assumed that the additional conditions # 2 and # 3 are observed. Even if other combinations are adopted, the initial input polarization state A can be obtained with the same diversity as in the case of using the quarter wave plates or the combination of the quarter wave plate and the half wave plate. To the final polarization state B is possible. 9 and 10 are diagrams for illustrating this using a Poincare sphere.

That is, in FIGS. 9 and 10, assuming that the initial input polarization state A is 45 degrees linearly polarized light, the values of θ1 and θ2 are set to 0 degrees to 0 degrees under the following combinations (i) and (ii). The result of plotting on the Poincare sphere a number of final polarization states B obtained when changing in the range of 180 degrees is shown.
● Combination (i); Fig. 9
・ Phase difference plate 1 = 1/3 wavelength plate (Δ1 = 2π / 3)
-Phase difference plate 2 = 1/4 wavelength plate (Δ2 = π / 2)
● Combination (ii); FIG.
・ Phase difference plate 1 = 2/5 wavelength plate (Δ1 = 4π / 5)
-Phase difference plate 2 = 1/3 wavelength plate (Δ2 = π / 2)

  The distribution of the plot points in Fig. 9 and Fig. 10 extends almost evenly on the Poincare sphere, and it can be seen that there is no "area on the Poincare sphere that cannot be reached by conversion". Here, (i) and (ii) satisfy the above conditions (# 1) and (# 2), but if the condition (# 3) is not satisfied, the polarization conversion of the two retardation plates The “range of movement of the point on the Poincare sphere” corresponding to the action is limited. 11 and 12 are diagrams for illustrating this using a Poincare sphere.

That is, FIGS. 11 and 12 assume 45-degree linearly polarized light as the initial input polarization state A, and the values of θ1 and θ2 are set to 0 degrees to 0 degrees under the following combinations (iii) and (iv). The result of plotting on the Poincare sphere a number of final polarization states B obtained when changing in the range of 180 degrees is shown.
● Combination (iii); FIG.
-Phase difference plate 1 = 1/6 wavelength plate (Δ1 = π / 3)
-Phase difference plate 2 = 1/4 wavelength plate (Δ2 = π / 2)
● Combination (iv); FIG.
-Phase difference plate 1 = 1/5 wavelength plate (Δ1 = 2π / 5)
-Phase difference plate 2 = 1/3 wavelength plate (Δ2 = π / 2)

  The distribution of plot points in FIGS. 11 and 12 is clearly different from that shown in FIGS. 9 and 10, and a blank area is generated on the Poincare sphere. In both FIG. 11 and FIG. 12, the blank area occurs in a portion far from the point on the Poincare sphere representing the initial input polarization state. Therefore, it is possible to use the retardation plates 1 and 2 so that the sum of the retardation imparting capacities Δ1 and Δ2 of the retardation plates 1 and 2 is π (corresponding to a half wavelength) or more (additional condition # 2). It is understood that it is advantageous in increasing the versatility of polarization conversion.

  In a more general case, in order to check whether or not the polarization conversion by the phase difference plates 1 and 2 is possible, the above-described basic equation # 1 is obtained by using the given parameter values (Δ1, Δ2, SA element values and It is only necessary to check whether θ1 and θ2 can be solved under the condition of each element value of SB). That is, if there is a set (θ1, θ2) that satisfies the basic formula # 1 in the range of 0 ° ≦ θ1 <180 ° and 0 ° ≦ θ2 <180 °, the orientation of the retardation plates 1 and 2 is appropriately selected. If the phase difference plate 1 is oriented and arranged at θ1 and the phase difference plate 2 is oriented at θ2, the conversion from the initial polarization state represented by SA to the final polarization state represented by SB can be realized. That is.

It is a figure explaining the polarization state of the completely polarized light which progresses in az axis direction (illustration omitted; perpendicular to xy plane). It is a figure explaining the method of expressing a polarization state using a Poincare sphere. It is a figure explaining the polarization conversion effect | action of the phase difference plate represented by Mueller matrix R ((DELTA), (theta)) by a geometric image. It is a figure explaining the basic form of embodiment of this invention. An example of (θ1, θ2) that can realize the final polarization state B (1) is shown in a table format (Table 1). An example of (θ1, θ2) that can realize the final polarization state B (2) is shown in a table format (Table 2). An example of (θ1, θ2) that can realize the final polarization state B (3) is shown in a table format (Table 3). 1 is a cross-sectional view of a schematic structure of a liquid crystal display incorporating a polarization conversion device according to the present invention. It is the graph which plotted the point on the Poincare sphere showing the change state implement | achieved when each orientation direction of each phase difference plate 1 and 2 is changed little by little about combination (i). It is the graph which plotted the point on the Poincare sphere showing the change state implement | achieved when each orientation direction of each phase difference plate 1 and 2 is changed little by little about combination (ii). It is the graph which plotted the point on the Poincare sphere showing the change state implement | achieved when each orientation direction of each phase difference plate 1 and 2 is changed little by little about combination (iii). It is the graph which plotted the point on the Poincare sphere showing the change state implement | achieved when each orientation direction of each phase difference plate 1 and 2 is changed little by little about combination (iv).

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Primary light source 11 Reflector 20 Light guide plate 21a Incident surface 21b Output surface 22 Back surface 23 Terminal surface 30 Prism sheet 31 Prism-like protrusion 32 Base part (retardation plate 1)
33 Phase difference plate (Phase difference plate 2)
40 Light Rays Representing Outgoing Light 50 Input Side Polarizer of Liquid Crystal Display Panel LCD Liquid Crystal Display Device LP Liquid Crystal Display Panel

Claims (14)

Input light consisting of completely polarized light and non-polarized light in the initial input polarization state A and having a polarization degree other than zero is predetermined as a target polarization state, and is completely polarized in a final polarization state B different from the initial input polarization state A; A polarization state conversion method for converting to output light comprising non-polarized light;
A first stage of generating primary converted light by inputting the input light perpendicularly to the first phase difference plate and passing through the first phase difference plate, and secondly converting the primary converted light to the second phase difference plate. A second step of generating a secondary converted light by passing the second retardation plate perpendicularly to the retardation plate
When the phase difference imparting ability of the first retardation plate is Δ1, and the phase difference imparting ability of the second retardation plate is Δ2, Δ1 + Δ2 ≧ π and at least one of Δ1 and Δ2 Is equal to π / 2,
Further, the initial input polarization state A has one set of Stokes parameters as four components, the normalized vector is SA, and the final polarization state B is another set of Stokes parameters as four components, and the normalized vector Is represented by SB,
The Mueller matrix expressing the polarization conversion action of the first retardation plate is R (Δ1, θ1) [where θ1 is the first position in the plane perpendicular to the thickness direction of the first retardation plate. An angle representing the first orientation direction of the phase difference plate],
The Mueller matrix expressing the polarization conversion action of the second retardation plate is R (Δ2, θ2), where θ2 is the second position in the plane perpendicular to the thickness direction of the second retardation plate. Angle representing the second orientation direction of the phase difference plate]
(I) the first phase difference providing ability represented by Δ1,
(Ii) the first orientation direction represented by θ1;
(Iii) the second phase difference imparting ability represented by Δ2, and
(Iv) A combination of the second orientation directions represented by θ2 is a relational expression:
R (Δ2, θ2) {R (Δ1, θ1) SA} = SB
The first alignment direction and the second alignment direction are determined so as to satisfy
Thereby, the polarization state of the completely polarized light contained in the secondary converted light coincides with the final polarization state B.   2. The polarization state conversion method according to claim 1, wherein each of the first retardation plate and the second retardation plate is a quarter-wave plate.   2. The polarization state conversion method according to claim 1, wherein one of the first retardation plate and the second retardation plate is a half-wave plate.   The polarization state conversion method according to any one of claims 1 to 3, wherein the first alignment direction and the second alignment direction are different from each other. The input light is emitted obliquely from the exit surface of the light guide plate, and then the traveling direction is corrected in the front direction of the exit surface,
The output light is supplied to a liquid crystal display panel including a polarizing plate disposed on the light input side,
5. The final polarization state B is defined such that a direction of polarization of a linearly polarized light component included in the output light is parallel to a transmission axis direction of the polarizing plate. Polarization state conversion method. Input light consisting of completely polarized light and non-polarized light in the initial input polarization state A and having a polarization degree other than zero is predetermined as a target polarization state, and is completely polarized in a final polarization state B different from the initial input polarization state A; A polarization state conversion device that converts light into non-polarized output light;
A first retardation plate and a second retardation plate arranged in parallel with each other;
The first retardation plate is arranged to input the input light perpendicularly to the first retardation plate and output primary converted light,
The second phase difference plate is arranged so that the primary converted light output by the first phase difference plate is input perpendicularly to the second phase difference plate and a secondary converted light is output. Has been
When the phase difference imparting ability of the first retardation plate is Δ1, and the phase difference imparting ability of the second retardation plate is Δ2, Δ1 + Δ2 ≧ π and at least one of Δ1 and Δ2 Is equal to π / 2,
Further, the initial input polarization state A has one set of Stokes parameters as four components, the normalized vector is SA, and the final polarization state B is another set of Stokes parameters as four components, and the normalized vector Is represented by SB,
The Mueller matrix expressing the polarization conversion action of the first retardation plate is R (Δ1, θ1) [where θ1 is the first position in the plane perpendicular to the thickness direction of the first retardation plate. An angle representing the first orientation direction of the phase difference plate],
The Mueller matrix expressing the polarization conversion action of the second retardation plate is R (Δ2, θ2), where θ2 is the second position in the plane perpendicular to the thickness direction of the second retardation plate. Angle representing the second orientation direction of the phase difference plate]
(I) the first phase difference providing ability represented by Δ1,
(Ii) the first orientation direction represented by θ1;
(Iii) the second phase difference imparting ability represented by Δ2, and
(Iv) A combination of the second orientation directions represented by θ2 is a relational expression:
R (Δ2, θ2) {R (Δ1, θ1) SA} = SB
The first alignment direction and the second alignment direction are determined so as to satisfy the condition, whereby the polarization state of the completely polarized light included in the secondary converted light matches the final polarization state B. The polarization state conversion device characterized by the above.   The polarization state conversion apparatus according to claim 6, wherein each of the first retardation plate and the second retardation plate is a quarter-wave plate.   The polarization state conversion device according to claim 6, wherein one of the first retardation plate and the second retardation plate is a half-wave plate.   The polarization state conversion device according to any one of claims 6 to 8, wherein the first alignment direction and the second alignment direction are different from each other. A liquid crystal display comprising a surface light source device, a polarization state conversion device, and a liquid crystal display panel including a polarizing plate disposed on a light input side;
The surface light source device is disposed along a light guide plate having an entrance surface and an exit surface, a primary light source that supplies primary light to the light guide plate through the entrance surface, and is obliquely emitted from the exit surface. A traveling direction correcting member that corrects the traveling direction of the emitted light to the front direction of the emitting surface,
The polarization state conversion device is the polarization state conversion device according to claim 9, and the emission light from the emission surface whose traveling direction is corrected to the front direction by the traveling direction correcting member, It is arranged so as to be input to the first retardation plate as input light,
The output light is supplied to a liquid crystal display panel including a polarizing plate disposed on the light input side,
The liquid crystal display according to claim 1, wherein the final polarization state B is determined to represent linearly polarized light having a polarization direction parallel to a transmission axis direction of the polarizing plate.   The exit surface of the light guide plate is rectangular, and the transmission axis direction of the polarizing plate forms an angle of 45 degrees with respect to the extending direction of each side of the rectangle. The liquid crystal display as described.   The liquid crystal display according to claim 6 or 7, wherein the first retardation plate is integrated with the traveling direction correcting member.   The liquid crystal display according to any one of claims 10 to 12, wherein a light diffusing member for diffusing the secondary converted light is disposed on a light input side of the polarizing plate.   The liquid crystal display according to claim 13, wherein the second retardation plate is integrated with the light diffusion member.

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