JP3108938B2 - Method for measuring tilt angle of liquid crystal molecules - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a tilt angle of liquid crystal molecules with respect to a substrate surface of a liquid crystal cell in which liquid crystal molecules are twisted between a pair of substrates.

[0002]

2. Description of the Related Art Conventionally, a tilt angle of a liquid crystal molecule with respect to a substrate surface of a liquid crystal cell is measured by tilting the liquid crystal cell with respect to a light transmission direction and measuring the intensity of the transmitted light. The tilt angle is calculated by calculation based on

[0003] This tilt angle measuring method will be described with reference to FIG.
Denotes an optical system for measuring the transmitted light intensity of the liquid crystal cell. This optical system comprises a polarizer 3 and a laser 1 between a laser 1 and a photodetector 2 for detecting light from the laser 1. The analyzer 4 and the analyzer 4 are arranged perpendicular to the optical axis O of the light from the laser 1. The polarizer 3 and the analyzer 4
Are arranged so that their transmission axes are orthogonal to each other.

[0004] Then, the transmitted light intensity of the liquid crystal cell is measured by
The liquid crystal cell 5 is disposed between the polarizer 3 and the analyzer 4 of the optical system as shown in FIG. 6, and the liquid crystal cell 5 is tilted with respect to the optical axis O of the optical system as shown by an arrow in the figure. In this method, the light from the laser 1 is incident on the liquid crystal cell 5 via the polarizer 3 and the intensity of the light transmitted through the liquid crystal cell 5 is detected by the photodetector 2 via the analyzer 4. ing.

The liquid crystal cell 5 is formed by joining a pair of transparent substrates 6 and 7 made of glass or the like via a frame-shaped sealing material 8.
A liquid crystal is sealed between the substrates 6 and 7, and although not shown, a transparent electrode and an alignment film are respectively formed on the surfaces of the substrates 6 and 7 opposed to each other with the liquid crystal layer interposed therebetween. The molecules 9 are arranged in substantially parallel directions due to the alignment control force of the alignment films on the surfaces of the substrates 6 and 7.

FIG. 7 shows the change of the transmitted light intensity T with respect to the tilt angle φ of the liquid crystal cell 5 when the transmitted light intensity of the liquid crystal cell 5 is measured by the above optical system. When the transmitted light intensity is measured while tilting, a transmitted light intensity curve having a plurality of maximum values as shown in the figure is obtained.

On the other hand, the transmitted light intensity T detected by the photodetector 2 is theoretically expressed by using the tilt angle φ of the liquid crystal cell 5 and the tilt angle α of the liquid crystal molecules 9 with respect to the substrates 6 and 7 as variables. The transmitted light intensity T is given by the following equation (1-1) and (1-2)
It is expressed by an equation.

[0008]

(Equation 1) That is, by substituting the value of φ (tilt angle of the liquid crystal cell 5) into the above equations (1-1) and (1-2), the transmitted light intensity curve of FIG. 2 is obtained.

Here, the transmitted light intensity curve becomes a curve in which a local maximum value having a small value of T exists between local maximum values having a large value of T as shown in FIG. As a result, the angle of the major axis of the liquid crystal molecules 9 with respect to the optical axis O of the optical system changes, thereby causing the retardation (Δnd) of the liquid crystal layer.
Is greatly changed.

As shown in FIG. 2, the reason why the local maximum value of the value of T deviates from φ = 0 ° is that the liquid crystal molecules 9 of the liquid crystal cell 5
Are arranged at a certain tilt angle α with respect to the surfaces of the substrates 6 and 7, so that the angle of the liquid crystal cell 5 with respect to the optical axis O of the optical system and the angle of the long axis of the liquid crystal molecules 9 with respect to the optical axis O are different. Because it is different.

Therefore, the above equations (1-1) and (1-2) are
Based on the conditions for satisfying the two conditions of having a plurality of local maxima as shown in FIG. 2 and the presence of a local maximum with a small value of T between local maximums with a large value of T, The liquid crystal cell 5 at the point where the light intensity reaches a local maximum value where T is small.
A tilt angle phi _K of the finding the relationship between the tilt angle α of the liquid crystal molecules 9, this relationship is expressed by the following equation (2).

[0012]

(Equation 2) Therefore, by substituting the value of the phi _K in equation (2), the tilt angle of the liquid crystal molecules 9 alpha is determined.

[0013]

However, the above-mentioned conventional tilt angle measuring method is intended for a non-twist type liquid crystal cell in which liquid crystal molecules are arranged in a substantially parallel direction between both substrates. If the liquid crystal molecules were twisted between the two substrates, the measured values would be significantly out of order. For example, the tilt angle of the liquid crystal molecules of a twist alignment liquid crystal cell such as a TN type or STN type liquid crystal cell could not be measured.

An object of the present invention is to provide a tilt angle measuring method capable of measuring a liquid crystal molecule tilt angle of a liquid crystal cell in which liquid crystal molecules are twisted between a pair of substrates.

[0015]

The tilt angle measuring method according to the present invention comprises:

An optical system in which a polarizer, a photoelastic modulator and an analyzer are arranged between a laser and a photodetector for detecting light from the laser, respectively, perpendicular to the optical axis of the light from the laser. The liquid crystal cell is disposed between the photoelastic modulator of the optical system and the analyzer so that the substrate surface thereof is perpendicular to the optical axis, and the liquid crystal cell or the polarization of the optical system is used. While rotating the polarizer, the photoelastic modulator, and the analyzer around a line along the optical axis, the light from the laser is incident on the liquid crystal cell via the polarizer and the photoelastic modulator. The intensity of the light transmitted through the liquid crystal cell is detected by the photodetector via the analyzer, and the apparent duplication of the liquid crystal of the liquid crystal cell is determined based on the DC component and the frequency component of the light detected by the photodetector. Calculate refractivity,

From the apparent birefringence and the known values of the liquid crystal layer thickness, the liquid crystal molecule twist angle, the extraordinary refractive index and the ordinary light refractive index of the liquid crystal, the tilt of the liquid crystal molecules with respect to the substrate surface of the liquid crystal cell is determined. The angle is calculated.

[0018]

As described above, the intensity of the transmitted light is detected while rotating the polarizer, the photoelastic modulator, and the analyzer of the liquid crystal cell or the optical system about the line along the optical axis. If the apparent birefringence of the liquid crystal of the liquid crystal cell is calculated based on the frequency component, the apparent birefringence and
From the known values of the liquid crystal layer thickness, the liquid crystal molecule twist angle, and the extraordinary refractive index and ordinary light refractive index of the liquid crystal, the tilt angle of the liquid crystal molecules of the liquid crystal cell having the liquid crystal molecules twisted can be calculated.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an optical system for measuring the transmitted light intensity of a liquid crystal cell.
And a photodetector 12 for detecting light from the laser 11, a polarizer 13, a photoelastic modulator 14, and an analyzer 15 are respectively arranged perpendicular to the optical axis O of the light from the laser 11. It is arranged and configured.

The optical axes of the polarizer 13, the photoelastic modulator 14 and the analyzer 15 are oriented as shown in FIG.
That is, in FIG. 2, (a) shows the transmission axis of the polarizer 13, (b) shows the optical axis of the photoelastic modulator 14, (d) shows the transmission axis of the analyzer 15, and the polarizer 13 The transmission axis 13a is arranged in a direction intersecting at an angle of 45 ° with two optical axes 14a and 14b of the photoelastic modulator 14 which are orthogonal to each other.
5a is arranged in a direction crossing the polarization axis 13a of the polarizer 13 at an angle of 45 ° (a direction parallel to one optical axis of the photoelastic modulator 14).

The photoelastic modulator 3 changes the refractive index of light in a predetermined direction by, for example, causing an elastic strain in a crystal. The photoelastic modulator 3 has two components having a vibration plane on surfaces orthogonal to each other. A speed difference is generated between the (normal light and the extraordinary light), and the speed difference is changed at a constant frequency. This photoelastic modulator 14
Is connected to a modulation control unit 16, and the photoelastic modulator 14 is driven by a modulation frequency signal from the modulation control unit 16.

The photodetector 12 is connected to a calculation unit 17 for calculating a tilt angle. The calculation unit 17 includes a modulation frequency transmitted from the modulation control unit 16 to the photoelastic modulator 14. A signal and a light intensity signal detected by the photodetector 12 are input.

The arithmetic unit 17 calculates the apparent birefringence Δn of the liquid crystal of the liquid crystal cell 20 based on the DC component and the frequency component of the light detected by the photodetector 12, and calculates the apparent birefringence Δn. If, from the known thickness d of the liquid crystal layer and the liquid crystal molecules a twist angle which is a value of the liquid crystal of the extraordinary refractive index n _e and ordinary refractive index n _o, to calculate a tilt angle of the liquid crystal molecules of the liquid crystal cell 20.

Next, the liquid crystal cell 20 for measuring the tilt angle of liquid crystal molecules will be described.
0 is a TN type or ST in which liquid crystal molecules are twisted.
As shown in FIG. 1, a pair of transparent substrates 21 and 22 made of glass or the like are joined via a frame-shaped sealing material 23, and a liquid crystal is sealed between the two substrates 21 and 11, as shown in FIG. It is configured. Both substrates 21 and 22 of this liquid crystal cell 20
Although not shown, a transparent electrode and an alignment film are formed on the surfaces opposed to each other with the liquid crystal layer therebetween, and the liquid crystal molecules 24 are twisted between the substrates 21 and 22.

FIG. 2C shows the orientation of the liquid crystal molecules of the liquid crystal cell 20. The orientation of the liquid crystal molecules 24 is the orientation 21a of the light incident side substrate 21 and the orientation 22a of the exit side substrate 22. Are arranged in a twist at a twist angle τ corresponding to the intersection angle with. The substrate 2 of the liquid crystal cell 20
The measurement of the tilt angle of the liquid crystal molecules 24 with respect to the 1, 22 planes is as follows.
Perform the following steps. (First step)

First, as shown in FIG. 1, between the photoelastic modulator 14 of the optical system and the analyzer 15, the liquid crystal cell 20 is connected to the substrate 21 and 22 surfaces by the optical axis of the optical system (from the laser 11). The liquid crystal cell 20 is disposed perpendicularly to the optical axis O of the optical system, and the liquid crystal cell 20 is arranged along the optical axis O of the optical system as indicated by the arrow in the figure (in this embodiment, the line coincides with the optical axis O). While rotating the light from the laser 11 to the liquid crystal cell 20 via the polarizer 13 and the photoelastic modulator 14, and the intensity of the light transmitted through the liquid crystal cell 20 is measured by the analyzer 1.
5, and is detected by the photodetector 12.

The light detected by the photodetector 12 will be described. The output light from the laser 11 passes through the polarizer 13 to become linearly polarized light, and further enters the liquid crystal cell 20 through the photoelastic modulator 14. . The light that has passed through the photoelastic modulator 14 is light whose polarization state has been modulated according to the modulation frequency of the photoelastic modulator 14, and the polarization state of this modulated light is
It changes between linearly polarized light and elliptically polarized light.

When this modulated light enters the liquid crystal cell 20,
The polarization state changes due to the birefringence of the liquid crystal in the process of passing through the liquid crystal cell 20, and the modulated light is a polarization state in which the phase difference due to the birefringence of the liquid crystal is superimposed on the phase difference between the ordinary light and the extraordinary light. Of light.

The light transmitted through the liquid crystal cell 20 is transmitted to the analyzer 15.
, And the light of the polarization component transmitted through the analyzer 15 is incident on the photodetector 12 and its intensity is detected. The intensity of the light detected by the photodetector 12 is a value corresponding to the phase difference between the ordinary light and the extraordinary light caused by the birefringence of the liquid crystal of the liquid crystal cell 20. It is superimposed on the modulation frequency. Then, the analyzer 12 outputs a signal corresponding to the detected light intensity to the calculation unit 17. (Second Step) Next, the arithmetic unit 17 calculates the apparent birefringence Δn of the liquid crystal of the liquid crystal cell 20.

The apparent birefringence Δn is determined by the photodetector 1
A component corresponding to the modulation frequency of the photoelastic modulator 14 is extracted from the light intensity signal input from Step 2, and is calculated based on the DC component and the frequency component.

That is, the liquid crystal cell 2 is provided by the above optical system.
When the transmitted light is detected by the analyzer 15 while rotating 0 around the line along the optical axis O, the transmitted light intensity I detected by the photodetector 12 is expressed by the following equation (3).

[0033]

(Equation 3) On the other hand, the apparent retardation Re of the liquid crystal cell 20 is
It is expressed by the following equation (4).

[0034]

(Equation 4) In the equation (4), f (θ), which is a function of the rotation angle θ of the liquid crystal cell 20, has the relationship of the following equation (5).

[0035]

(Equation 5) When the above equation (3) is solved using the Jones matrix, the DC component I _DC and the frequency component Iω of the transmitted light intensity I can be represented by optical parameters. Therefore, (5)
When | f (θ) | in the equation is solved using the Jones matrix and expressed by optical parameters, the following equation (6) is obtained.

[0036]

(Equation 6) Here, A, B, a, and b are complex numbers, and an asterisk on the right shoulder means complex conjugate.

Therefore, based on the parameters of the DC component I _DC and the frequency component Iω of the transmitted light intensity I detected by the photodetector 12 while rotating the liquid crystal cell 20,
If the value of | f (θ) | is determined from the equation (5), the apparent birefringence Δn of the liquid crystal can be calculated from the relation of u = πΔnd / τ from the above equation (6). (Third Step) Next, the arithmetic unit 17 calculates the tilt angle の of the liquid crystal molecules of the liquid crystal cell 20.

The tilt angle ψ is determined by the apparent birefringence Δn of the liquid crystal of the liquid crystal cell 20 calculated in the second step, and
It can be calculated from the liquid crystal layer thickness d of the liquid crystal cell 20, the liquid crystal molecules twist angle tau, the extraordinary refractive index of the liquid crystal n _e and ordinary index n _o. That is, the apparent birefringence Δn of the liquid crystal of the liquid crystal cell 20 is represented by the following equation (7).

[0039]

(Equation 7)

The thickness d of the liquid crystal layer of the liquid crystal cell 20 and the twist angle τ of the liquid crystal molecules are determined at the stage of designing the liquid crystal cell 20, and the extraordinary refractive index _ne and the ordinary refractive index n
_{Since o} also depends on the liquid crystal used, these d,
tau, since the value of n _e and n _o are known, the liquid crystal cell 2
If the apparent birefringence Δn of the liquid crystal of 0 is known, the above (7)
From the equation, the tilt angle 液晶 of the liquid crystal molecule is obtained.

Therefore, as shown in FIG. 1, while rotating the liquid crystal cell 20 about the line along the optical axis O of the optical system, the intensity of light transmitted through the liquid crystal cell 20 is detected by the photodetector 12. Then, based on the DC component and the frequency component of the transmitted light intensity, from the above equations (5), (6) and (7),
The tilt angle の of the liquid crystal molecules of the liquid crystal cell 20 can be obtained.

FIG. 3 shows the relationship between the apparent retardation Re of the liquid crystal cell 20 obtained from the transmitted light intensity detected by the photodetector 12 by the equation (4) and the finally calculated tilt angle 液晶 of the liquid crystal molecules. the shows, here, the liquid crystal layer thickness d = 5.8 [mu] m, a twist angle tau = 240 °, the liquid crystal of the extraordinary refractive index n _e = 1.55, the tilt angle ψ of the liquid crystal cells of the ordinary refractive index n _o = 1.42 measured The results are shown.

As described above, according to the above measuring method, the liquid crystal molecule tilt angle の of the liquid crystal cell 20 in which the liquid crystal molecules 24 are twisted between the pair of substrates 21 and 22 can be measured.

In the first embodiment, the liquid crystal cell 2
Although the transmitted light intensity is detected while rotating 0, the transmitted light intensity is detected by fixing the liquid crystal cell 20 and the polarizer 13 of the optical system as in the second embodiment shown in FIG. The measurement may be performed while rotating the photoelastic modulator 14 and the analyzer 15 around a line along the optical axis O (a line coinciding with the optical axis O).

When the transmitted light intensity is detected while rotating the liquid crystal cell 20, the liquid crystal cell 20 is moved parallel to the optical axis O of the optical system as in the third embodiment shown in FIG. It may be rotated about the line Oa. In this embodiment, the light transmission point of the liquid crystal cell 20 is defined by a circle centered on the rotation center Oa of the liquid crystal cell 20 (the optical axis O of the optical system and the liquid crystal cell 2).
(A circle having a radius corresponding to the distance from the center of rotation Oa of 0), and if the liquid crystal layer thickness d and the like in the region along this circle are uniform, the liquid crystal molecules are similar to the first embodiment. Can be measured.

[0046]

According to the present invention, the transmitted light intensity is detected while rotating the polarizer, the photoelastic modulator, and the analyzer of the liquid crystal cell or the optical system about the line along the optical axis. The apparent birefringence of the liquid crystal of the liquid crystal cell is calculated based on the DC component and the frequency component of the liquid crystal, and the apparent birefringence, the liquid crystal layer thickness, the liquid crystal molecule twist angle, and the abnormal light of the liquid crystal, which are known values, are calculated. Since the tilt angle of the liquid crystal molecules of the liquid crystal cell is calculated from the refractive index and the ordinary light refractive index, the tilt angle of the liquid crystal molecules of the liquid crystal cell in which the liquid crystal molecules are twisted between a pair of substrates can be measured.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a transmitted light intensity measuring optical system according to a first embodiment of the present invention.

FIG. 2 is a diagram showing optical axes of a polarizer, a photoelastic modulator, and an analyzer of the optical system, and a liquid crystal molecule alignment direction of a liquid crystal cell.

FIG. 3 is a diagram showing a relationship between an apparent retardation of a liquid crystal cell and a tilt angle of a liquid crystal molecule.

FIG. 4 is a configuration diagram of a transmitted light intensity measuring optical system showing a second embodiment of the present invention.

FIG. 5 is a configuration diagram of a transmitted light intensity measuring optical system according to a second embodiment of the present invention.

FIG. 6 is a configuration diagram of a transmitted light intensity measuring optical system showing a conventional tilt angle measuring method.

FIG. 7 is a diagram showing a change in transmitted light intensity with respect to a tilt angle of a liquid crystal cell.

[Explanation of symbols]

11 laser, 12 photodetector, 13 polarizer, 14
Photoelastic modulator, 15: analyzer, 20: liquid crystal cell, 21,
22: substrate, 24: liquid crystal molecules.

Claims (1)

(57) [Claims] 1. A method for measuring a tilt angle of liquid crystal molecules with respect to a substrate surface of a liquid crystal cell in which liquid crystal molecules are twisted between a pair of substrates, comprising: a laser and a photodetector for detecting light from the laser; Using an optical system in which a polarizer, a photoelastic modulator, and an analyzer are arranged perpendicular to the optical axis of light from the laser, respectively, between the photoelastic modulator and the analyzer of this optical system The liquid crystal cell is disposed with its substrate surface perpendicular to the optical axis, and the liquid crystal cell or the polarizer, the photoelastic modulator and the analyzer of the optical system are aligned with the line along the optical axis. While rotating about the light, the light from the laser is incident on the liquid crystal cell via the polarizer and the photoelastic modulator, and the intensity of the light transmitted through the liquid crystal cell is measured via the analyzer. Detected by the detector The apparent birefringence of the liquid crystal of the liquid crystal cell is calculated based on the DC component and the frequency component of the light detected by the detector, and the apparent birefringence and the known values of the liquid crystal layer thickness and the liquid crystal molecule twist are calculated. A tilt angle measuring method for a liquid crystal molecule, comprising calculating a tilt angle of a liquid crystal molecule with respect to a substrate surface of the liquid crystal cell from an angle and an extraordinary refractive index and an ordinary refractive index of the liquid crystal.