JPH0518859A - Method for measuring thickness of liquid crystal layer - Google Patents

Abstract

PURPOSE:To efficiently measure the thickness of a liquid crystal layer by vertically transmitting laser beam through a polarizer, a photoelastic modulator, a liquid crystal cell to be measured rotated around an optical axis and an analyser to detect and calculate the intensity of said beam. CONSTITUTION:A polarizer 13, a photoelastic modulator 14, a liquid crystal cell 20 and an analyser 15 are arranged at right angles to an optical axis O and the liquid crystal cell 20 is rotated around the optical axis O. The output beam of laser 11 passes through the polarizer 13 to become linear polarized beam which is, in turn, modulated in its polarized state corresponding to modulation frequency by the modulator 13 to enter the cell 20. The beam is further changed in its polarized state by the double refraction property of a liquid crystal 24 during the transmission through the cell 20 to be incident to a photodetector 12 through the analyser 15. The photodetector 12 detects the intensity of the beam to output the signal corresponding to the intensity to an operation part 17. The component corresponding to the modulation frequency of the modulator 14 is extracted from a beam intensity signal in the operation part 17 and the thickness of the crystal layer can be calculated from the DC component and frequency component thereof, the double refraction property of the liquid crystal of the cell 20 and the twist angle of a liquid crystal molecule.

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 the liquid crystal layer thickness of a liquid crystal cell.

[0002]

2. Description of the Related Art Conventionally, the liquid crystal layer thickness (cell gap) of a liquid crystal cell is measured by causing white light to enter the liquid crystal cell and calculating the liquid crystal layer thickness based on the spectral distribution of light transmitted through the liquid crystal cell. It is done in.

This liquid crystal layer thickness measuring method will be described with reference to FIG.
Indicates an optical system for measuring the spectral distribution of light transmitted through the liquid crystal cell, and this optical system includes a white light source 1 and a spectroscope 2.

To measure the spectral distribution of transmitted light by this optical system, a liquid crystal cell 20 for measuring the liquid crystal layer thickness is arranged between the white light source 1 and the spectroscope 2 as shown in FIG. The white light from the light source 1 is vertically incident on the liquid crystal cell 20, and the intensity of each wavelength light transmitted through the liquid crystal cell 20 is detected by the spectroscope 2. Figure 7
Shows an example of the spectral distribution measured by the above optical system.

The liquid crystal cell 20 is, for example, of the TN type or the STN type, and as shown in FIG. 6, a pair of transparent substrates 21 and 22 made of glass or the like, with a frame-shaped sealing material 23 interposed therebetween. A liquid crystal 24 is sealed between both substrates 21 and 11 by bonding. Although not shown, a transparent electrode and an alignment film are respectively formed on the surfaces of the substrates 21 and 22 of the liquid crystal cell 20 which face each other across the liquid crystal layer, and the molecules of the liquid crystal 24 are formed on both the substrates 21 and 22. Twisted in between.

The liquid crystal layer thickness b of the liquid crystal cell 20 is calculated from the spectral distribution measured by the optical system as shown in FIG.
The wavelengths λ ₁ and λ ₂ at the two points where the transmitted light intensity is maximum (or minimum) are obtained, and the distance Δλ ₁₂ between these λ ₁ and λ ₂ is
It is calculated from the refractive index n of the liquid crystal layer which is known in advance, using the following equation b = Δλ ₁₂ / 2n.

[0007]

However, in the above-described conventional liquid crystal layer thickness measuring method, in order to measure the spectral distribution of the light transmitted through the liquid crystal cell 20, the spectroscope 2 measures the transmitted light intensity of each wavelength light. Since it has to be measured, there is a problem that it takes time to measure the spectral distribution, so that the liquid crystal layer thickness cannot be measured efficiently. An object of the present invention is to provide a liquid crystal layer thickness measuring method capable of efficiently measuring the liquid crystal layer thickness of a liquid crystal cell.

[0008]

The liquid crystal layer thickness measuring method of the present invention comprises:

An optical system in which a polarizer, a photoelastic modulator, and an analyzer are respectively arranged between a laser and a photodetector for detecting the light from the laser, in a direction perpendicular to the optical axis of the light from the laser. Between the photoelastic modulator and the analyzer of this optical system, the liquid crystal cell is placed with its substrate surface perpendicular to the optical axis, and the liquid crystal cell or the polarization of the optical system is While rotating the child, the photoelastic modulator and the analyzer about a line along the optical axis, while allowing the light from the laser to enter the liquid crystal cell through the polarizer and the photoelastic modulator. The intensity of light transmitted through the liquid crystal cell is detected by the photodetector via the analyzer,

The liquid crystal layer thickness can be calculated from the DC component and frequency component of the light detected by this photodetector, and the known values of the birefringence of the liquid crystal of the liquid crystal cell and the twist angle of the liquid crystal molecules. It is a feature.

[0011]

In this way, if the transmitted light intensity is detected while rotating the polarizer of the liquid crystal cell or the optical system, the photoelastic modulator and the analyzer about the line along the optical axis, the DC component of the detected light is detected. Further, the liquid crystal layer thickness of the liquid crystal cell can be calculated from the frequency component, the birefringence of the liquid crystal of the liquid crystal cell and the twist angle of the liquid crystal molecules, which are known values.

In this measuring method, the intensity of light transmitted through the liquid crystal cell need only be detected for light of a single wavelength. Therefore, it is not necessary to measure the intensity of transmitted light of each wavelength as in the conventional measuring method. Therefore, the liquid crystal layer thickness can be measured efficiently.

[0013]

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 intensity of transmitted light of a liquid crystal cell. This optical system comprises a laser 11
And the photodetector 12 for detecting the light from the laser 11, the polarizer 13, the photoelastic modulator 14, and the 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, and (d) shows the transmission axis of the analyzer 15. The transmission axis 13a is aligned with a direction intersecting the two optical axes 14a and 14b of the photoelastic modulator 14 which are orthogonal to each other at an angle of 45 °, and the analyzer 15 has the transmission axis 1a.
5a is aligned with a direction intersecting the polarization axis 13a of the polarizer 13 at an angle of 45 ° (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 elastic distortion in a crystal body, and is composed of two components of light having oscillating planes orthogonal to each other. A speed difference is generated between (ordinary light and extraordinary light), and this speed difference is changed at a constant frequency. This photoelastic modulator 14
A modulation control section 16 is connected to the photoelastic modulator 14, and the photoelastic modulator 14 is driven by the modulation frequency signal from the modulation control section 16.

Further, the photodetector 12 is connected to an arithmetic unit 17 for calculating the liquid crystal layer thickness, and the arithmetic unit 17 is modulated by the modulation control unit 16 to the photoelastic modulator 14. The frequency signal and the light intensity signal detected by the photodetector 12 are input.

The calculation unit 17 calculates the DC component and the frequency component of the light detected by the photodetector 12, and the known values of the birefringence Δn of the liquid crystal of the liquid crystal cell 20 and the twist angle τ of the liquid crystal molecule. From this, the liquid crystal layer thickness d of the liquid crystal cell 20 is calculated.

Next, the liquid crystal cell 20 for measuring the thickness of the liquid crystal layer will be described. The liquid crystal cell 20 is, for example, of the TN type or the STN type, and the molecules of the liquid crystal 24 are between the substrates 21 and 22. In, the twist arrangement is performed at a predetermined twist angle. Since the liquid crystal cell 20 has the same structure as that shown in FIG. 6, the description thereof will be omitted by giving the same reference numerals to the drawing.

FIG. 2C shows the alignment directions of the liquid crystal molecules of the liquid crystal cell 20. The molecules of the liquid crystal 24 are aligned in the alignment direction 21a of the light incident side substrate 21 and in the exit side substrate 22. The twist arrangement is performed at a twist angle τ corresponding to the intersection angle with 22a. Then, the liquid crystal layer thickness d of the liquid crystal cell 20 is measured in the following steps. (First step)

First, as shown in FIG. 1, between the photoelastic modulator 14 and the analyzer 15 of the optical system, the liquid crystal cell 20 is provided with its substrates 21, 22 on the optical axis of the optical system (from the laser 11). Of the liquid crystal cell 20 is arranged so as to be perpendicular to the optical axis O of the optical system, and the liquid crystal cell 20 is a line along the optical axis O of the optical system as shown by the arrow in the figure (a line that coincides with the optical axis O in this embodiment). The light from the laser 11 is made incident on the liquid crystal cell 20 via the polarizer 13 and the photoelastic modulator 14 while rotating about the center of the light, and the intensity of the light transmitted through the liquid crystal cell 20 is analyzed by the analyzer 1
It is detected by the photodetector 12 via 5.

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 passes through the photoelastic modulator 14 to enter the liquid crystal cell 20. .. The light that has passed through the photoelastic modulator 14 is light whose polarization state is 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 24 in the process of passing through the liquid crystal cell 20, and the modulated light has a phase difference due to the birefringence of the liquid crystal 24 superimposed on the phase difference between the ordinary light and the extraordinary light. The light is polarized.

The light transmitted through the liquid crystal cell 20 is analyzed by the analyzer 15
To the photodetector 12, and the intensity of the light of the polarized component that has passed through the analyzer 15 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 generated by the birefringence of the liquid crystal of the liquid crystal cell 20, and this value is the value of the photoelastic modulator 14. It is superimposed on the modulation frequency. Then, the analyzer 12 outputs a signal according to the detected light intensity to the calculation unit 17. (Second Step) Next, the calculation unit 17 calculates the liquid crystal layer thickness d of the liquid crystal cell 20.

With respect to the liquid crystal layer thickness d, a component corresponding to the modulation frequency of the photoelastic modulator 14 is extracted from the light intensity signal input from the photodetector 12, and its DC component and frequency component and the liquid crystal cell 20 are extracted. It can be calculated from the birefringence Δn of the liquid crystal and the twist angle τ of the liquid crystal molecule.

That is, by the above optical system, the liquid crystal cell 2
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 (1).

[0027]

[Equation 1] On the other hand, the apparent retardation Re of the liquid crystal cell 20 is
It is expressed by the following equation (2).

[0028]

[Equation 2] In this equation (2), f (θ), which is a function of the rotation angle θ of the liquid crystal cell 20, has the relationship of the following equation (3).

[0029]

[Equation 3] Further, if the above equation (1) is solved using the Jones matrix, the direct current component I _DC and the frequency component I ω of the transmitted light intensity I can be expressed by optical parameters. Therefore, above (3)
When | f (θ) | in the equation is solved using the Jones matrix and expressed by optical parameters, the following equation (4) is obtained.

[0030]

[Equation 4] Here, A, B, a, and b are complex numbers, and * is attached to the right shoulder to mean the complex conjugate.

Therefore, based on the parameters of the DC component I _DC and the frequency component I ω 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 obtained from the equation (3), the liquid crystal layer thickness d of the liquid crystal cell 20 can be calculated from the equation (4) based on the relationship of u = πΔnd / τ.

Since the twist angle τ of the liquid crystal molecules of the liquid crystal cell 20 is determined at the design stage of the liquid crystal cell 20 and the birefringence Δn of the liquid crystal 24 is also determined by the liquid crystal used, the values of τ and Δn are Liquid crystal cell 2 because it is known
If the DC component and the frequency component of the light passing through 0 are known,
The liquid crystal layer thickness d of the liquid crystal cell 20 can be calculated.

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 photodetector 12 detects the intensity of the light transmitted through the liquid crystal cell 20. Then, the liquid crystal layer thickness d of the liquid crystal cell 20 can be obtained from the equations (3) and (4) based on the DC component and the frequency component of the transmitted light intensity.

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 (2) and the finally calculated liquid crystal layer thickness d. Here, the result of measuring the liquid crystal layer thickness d of the liquid crystal cell in which the birefringence of the liquid crystal is Δn = 0.1 and the twist angle of the liquid molecule is τ = 240 ° is shown.

In the above measuring method, since the intensity of the light transmitted through the liquid crystal cell 20 only needs to be detected for the single wavelength light, the transmitted light intensity of each wavelength light is measured as in the conventional measuring method. It is not necessary, and therefore the liquid crystal layer thickness d can be measured efficiently.

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 to the polarizer 13 of the optical system as in the second embodiment shown in FIG. The photoelastic modulator 14 and the analyzer 15 may be rotated while rotating about 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 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 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).
Although it is displaced along a circle having a radius corresponding to the distance from the rotation center Oa of 0), if the twist angle τ of the liquid crystal molecules in the region along this circle is uniform, the same as in the first embodiment. The liquid crystal layer thickness d of the liquid crystal cell 20 can be measured.

[0038]

According to the present invention, the transmitted light intensity is detected by rotating the polarizer of the liquid crystal cell or the optical system, the photoelastic modulator and the analyzer about a line along the optical axis, and the detected light intensity is detected. Since the liquid crystal layer thickness of the liquid crystal cell is calculated from the DC component and frequency component of the liquid crystal and the known values of the liquid crystal birefringence of the liquid crystal cell and the twist angle of the liquid crystal molecules, the light transmitted through the liquid crystal cell is calculated. Since it is only necessary to detect the intensity of a single wavelength light, and therefore it is not necessary to measure the transmitted light intensity of each wavelength light as in the conventional measurement method, it is possible to efficiently measure the liquid crystal layer thickness of the liquid crystal cell. it can.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a transmitted light intensity measurement optical system showing 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 liquid crystal molecule alignment directions of a liquid crystal cell.

FIG. 3 is a diagram showing a relationship between an apparent retardation of a liquid crystal cell and a liquid crystal layer thickness.

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 showing a second embodiment of the present invention.

FIG. 6 is a configuration diagram of an optical system showing a conventional liquid crystal layer thickness measuring method.

7 is a diagram showing an example of a spectral distribution of transmitted light measured by the optical system of FIG.

[Explanation of symbols]

11 ... Laser, 12 ... Photodetector, 13 ... Polarizer, 14 ...
Photoelastic modulator, 15 ... Analyzer, 20 ... Liquid crystal cell 21,
22 ... Substrate, 24 ... Liquid crystal.

Claims (1)

Claim: What is claimed is: 1. A method for measuring a liquid crystal layer thickness of a liquid crystal cell, comprising a polarizer, a photoelastic modulator and an analyzer between a laser and a photodetector for detecting light from the laser. And an optical system each of which is arranged perpendicularly to the optical axis of the light from the laser, and the liquid crystal cell is disposed between the photoelastic modulator and the analyzer of the optical system with the substrate surface of the optical axis being the optical axis. The liquid crystal cell, or the polarizer, the photoelastic modulator, and the analyzer of the optical system, which are arranged perpendicular to the optical axis, are rotated about a line along the optical axis while the light from the laser is rotated. Is incident on the liquid crystal cell through the polarizer and the photoelastic modulator, and the intensity of light transmitted through the liquid crystal cell is detected by the photodetector through the analyzer, and detected by the photodetector. The DC and frequency components of the generated light and the known values , The liquid crystal layer thickness measuring method characterized in that the twist angle of the liquid crystal of the birefringence of the liquid crystal molecules of the liquid crystal cell, and calculates the liquid crystal layer thickness.