JP2565147B2 - Method of measuring twist angle and cell gap - 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 twist angle and a cell gap of a liquid crystal cell used for a liquid crystal display device and the like, and more particularly to a method for measuring a twist angle and a cell gap at the same time. Is.

[0002]

2. Description of the Related Art A liquid crystal display device using a twisted nematic (TN) structure or a super twisted nematic (STN) structure has features of thinness and low current consumption, and is a data processing device, an OA device, a measuring device, an in-vehicle device. It is widely used as a display. By the way, the optical characteristics of the TN or STN structure are determined by the cell gap (d), the refractive index anisotropy (Δn), and the twist angle. Therefore, accurately grasping these values is extremely important for research and development of liquid crystal displays and for quality control of products.

Here, the cell gap is TN or S.
This is the thickness of the TN liquid crystal layer and is usually set to about 10 μm or less. The refractive index anisotropy is the TN structure or S
It refers to the difference between the refractive index of the liquid crystal material forming the TN structure for ordinary light and the refractive index for extraordinary light. Further, the twist angle is defined as the difference between the azimuth angle φ _A that can be taken by the liquid crystal in the upper substrate 12 and the azimuth angle φ _B that can be taken by the liquid crystal molecules 14 in the lower substrate 13 as shown in FIG.

Here, the refractive index anisotropy is a quantity that is uniquely determined by the material of the liquid crystal, but the twist angle and the cell gap are not constant due to variations in individual products. Among them, the method described in JP-A-4-307312 is known as a method for obtaining the cell gap. This is because the transmitted light intensity is set with the polarization angle of the polarizer inclined by 45 ° from the alignment direction of the light incident surface of the liquid crystal cell and the polarization angle of the analyzer further inclined by 45 ° from the alignment direction of the light emission surface of the liquid crystal cell. The birefringence phase difference dΔn is calculated based on the measurement result, and the cell gap is obtained from the calculation result and the known birefringence Δn.

Further, as a method for determining the cell gap and the twist angle at the same time, a method using phase compensation by IBM's Lien or the like has been proposed ("Simultaneous Measurement of Twist Angle and"
Cell Gap of Twisted Nematic
Selby Ann Optical Method ”, Lien et al., IDRC '91, page 192). This is a method in which the measurement is repeated while operating the phase compensator to asymptotically approach the desired twist angle and cell gap.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The method for obtaining the cell gap described in Japanese Patent Publication No. 07312 is a method for determining the cell gap only on the assumption that the orientation direction and the twist angle are known. However, as described above, it is difficult to obtain an accurate measured value by this method, because the alignment direction of the liquid crystal and the twist angle include variations in each liquid crystal cell. Further, the twist angle cannot be actually measured by this conventional method.

On the other hand, according to the measuring method proposed by Lien, the twist angle and the cell gap can be determined at the same time, but there is the trouble of manually operating the phase compensator, and , It is necessary to repeat the operation many times to determine the position of the liquid crystal cell. Therefore, this method has a problem that the measurement operation requires skill and takes a long time, and there is a drawback that the cell gap and the twist angle cannot be automatically determined.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a measuring method that can simultaneously determine a twist angle and a cell gap, which is easy to operate and easy to automate. Is to provide.

[0009]

In order to achieve the above object, according to the present invention, (1) between a polarizer and an analyzer set such that their polarization angles form a predetermined angle, A liquid crystal cell as a test object is arranged, and the liquid crystal cell is rotated and fixed at an angle at which the amount of transmitted light is maximum or minimum, and (2) a polarizer or an analyzer is rotated to measure the amount of transmitted light, A method for measuring a twist angle and a cell gap, which comprises simultaneously determining a twist angle and a cell gap of a liquid crystal cell as a test object from the relationship between the rotation angle and the amount of transmitted light corresponding to the rotation angle, is provided. It

[0010]

In the present invention, first, the liquid crystal cell of the test object is arranged between the polarizer and the analyzer arranged at a predetermined angle, and the azimuth angle thereof is determined [( Step 1)]. After that, the analyzer (or the polarizer) is rotated and the amount of transmitted light is measured. The twist angle and the cell gap are determined by using this change in the amount of transmitted light [the second (2) stage]. Hereinafter, a specific procedure will be described in more detail.

FIG. 1 is a schematic device layout view for measuring a twist angle and a cell gap according to the present invention. As shown in the figure, the light emitted from the light source 1 is polarized by the polarizer 2, then enters the liquid crystal cell 3, and is rotated by the liquid crystal. Then, the light detected by the analyzer 4 is detected by the photodetector 5. As shown in the figure, the liquid crystal alignment direction on the light incident side of the liquid crystal cell 3 is the polarizer 2
Is tilted by x ° with respect to the polarization angle of, and the polarization angle of the analyzer 4 is tilted by y ° with respect to the polarization angle of the polarizer 2. The transmitted light state at this time can be expressed as in Equation 1 using the Jones matrix.

[0012]

[Equation 1]

In this Jones matrix, a and b are given by the following equations. Further, a ′ and b ′ are conjugate complex numbers of a and b, respectively. a = sin Θ · sin {Θ √ (1 + u ² )} / √ (1+
u ² ) + cos θ · cos {Θ√ (1 + u ² )} + i ·
u ・ cos Θ ・ sin {Θ√ (1 + u ² )} / √ (1+
u ² ) b = cos Θ · sin {Θ √ (1 + u ² )} / √ (1+
u ² ) −sin Θ · cos {Θ √ (1 + u ² )} + i ·
u · sin Θ · sin {Θ √ (1 + u ² )} / √ (1+
u ² ) where u = Δndπ / Θλ, Δn is the refractive index anisotropy of the liquid crystal, Θ is the twist angle, d is the cell gap, and λ is the wavelength of the incident light.

Here, E _X and E _Y are Jones vectors of light incident on the polarizer, and E _X ′ and E _Y ′ are Jones vectors of transmitted light observed on the analyzer side. Obtaining the transmitted light intensity from the above equation is as follows. T = | E _x ′ | ² + | E _y ′ | ² = f ² cos ² (Θ−
2x + y) + [g · cos (Θ + y) + gsin (Θ +
y)] ²

However, f, g and h are given below. f = u · sin {Θ√ (1 + u ² )} / √ (1 + u ² ) g = cos {Θ√ (1 + u ² )} h = sin {Θ√ (1 + u ² )} / √ (1 + u ² ).

From the above equation, when the liquid crystal cell is rotated with y = 0 (that is, the polarizer and the analyzer are in parallel) (when x is changed), the transmitted light amount takes the maximum value under the following conditions. I understand. Θ-2x _max = nπ where n is an integer. Further, it can be seen that the transmitted light amount has the minimum value under the following conditions. Θ-2 x _min = (2n + 1) · π / 2

The liquid crystal cell is fixed at the angle at which the amount of transmitted light becomes maximum or minimum, that is, when the angle formed by the polarization angle of the polarizer becomes x _max or x _min . In this way, the azimuth angle of the liquid crystal cell can be determined in any way, but since the subsequent measurement can be performed in almost the same way,
In the description below, the azimuth angle of the liquid crystal cell 3 is set so that the amount of transmitted light is maximized. It should be noted that with the polarizer 2 and the analyzer in parallel, the maximum value or the minimum value is searched for,
Although the azimuth angle of the liquid crystal cell 3 is determined, the difference between the azimuth angles of the polarizer and the analyzer is always 0 if the value is known.
It does not have to be °. However, it is preferably 0 ° or 90 ° from the viewpoints of easiness of angle determination and easiness of calculation.

After fixing the azimuth angle of the liquid crystal cell 3, the analyzer 4 is rotated to measure the amount of transmitted light. The amount of transmitted light at this time is as follows. ^{T = f 2 cos 2 (Θ} -2x max + y) + [g · cos (Θ + y) + hsin (Θ + y)] 2 = f 2 cos 2 (nπ + y) + [g · cos (Θ + y) + hsin (Θ + y)] 2 = ^{f 2 cos 2 y + [g} · cos (Θ + y) + hsin (Θ + y)] 2 = cos2y [(g 2 -h 2) cos2Θ / 2 + gh · sin2Θ + f 2/2] + sin2y [gh · cos2Θ- (g 2 -h 2) ^{sin2Θ / 2] + (f 2} + g 2 + h 2) / 2 = T 0 + T C cos2y + T S sin2y ... (1)

From the equation (1), when the azimuth (y) of the analyzer 4 is changed and the measurement is performed, the measured light transmission amount is cos2y.
It can be seen that it is possible to separately consider a component that changes with and a component that changes with sin2y. Ratios T _C / T ₀ and T _S / of the coefficients T ₀ , T _C , and T _S of each of these components
T ₀ can be obtained from the measurement result. From the above equation, the two ratios are determined by the cell gap and the twist angle.
Therefore, conversely, the cell gap and the twist angle can be determined from the ratios T _C / T ₀ and T _S / T ₀ . Therefore, the cell gap and the twist angle can be simultaneously obtained by using the method of the present invention. Furthermore, since the measuring method is simple, automated measurement is possible and can be measured in a short time.

[0020]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 2 is a schematic configuration diagram of a measuring device for explaining an embodiment of the present invention. Wavelength 6 as measurement light source
A 32.8 nm HeNe laser 6 was used. Moreover, the silicon photodiode 7 was used as a photodetector. The photocurrent of the silicon photodiode 7 is supplied to the current-voltage converter 8
After being converted into a voltage at, the voltage is detected by the voltmeter 9.

First, a polarizer 2 composed of a Glan-Thompson prism and an analyzer 4 were arranged, the analyzer 4 was rotated around the ray axis, and a position where the reading of the voltmeter 9 was minimized was searched for. The analyzer 4 was then rotated 90 ° around the ray axis.
In this state, the polarization transmission axes of the polarizer 2 and the analyzer 4 are parallel.

A TN cell 10 was prepared as an object to be measured. In this TN cell 10, after applying an alignment film on both upper and lower substrates, rubbing is performed on the alignment films, and the substrates are bonded via a spacer of 6 μm so that the rubbing directions of both substrates are orthogonal to each other.
The TN liquid crystal is filled in the gap, and the twist angle is estimated to be about 90 degrees.

The TN cell 10 was fixed to a sample rotation stage 11 between the polarizer 2 and the analyzer 4. The TN cell 10 was rotated around the ray axis and fixed at a position where the reading of the voltmeter 9 was maximum. After that, the analyzer 4 was rotated once around the optical axis in steps of 2 degrees, and the value of the voltmeter 9 at that time was directly read.
The result is shown in FIG. From this data, the values of T _C / T ₀ and T _S / T ₀ were obtained using the formula (1). The result is that the value of T _C / T ₀ is -0.535, and the value of T _S / T ₀ is-.
It was 0.576.

The concrete method for obtaining the values of T _C / T ₀ and T _S / T ₀ is as follows. 0 on both sides of equation (1)
Integrate from ° to 360 °. The right side is T ₀ multiplied by a constant, and the left side is the sum of the measured values. This gives T ₀
Is found. Next, multiply equation (1) by cos2y, and
Integrate at 360 °. The right side is T _C multiplied by a constant, and the left side is the sum of the measured values multiplied by cos2y. This gives T _C. Further, the formula (1) is multiplied by sin2y and integrated at 0 ° to 360 °. The right side is T _S multiplied by a constant, and the left side is the measured value si
It is the sum of products multiplied by n2y. This gives T _S. Using these calculated values, T _C / T ₀ and T _S
Calculate / T ₀ .

Separately from this, the twist angle and the cell gap are used as parameters, and calculation is performed using the equation (1).
Find T _C / T ₀ and T _S / T ₀ . The graph of the result is shown in FIG. FIG. 4 shows T _C / T ₀ calculated from the measured values.
And by applying the value of T _S / T _0, the twist angle of the TN cell is about 94 degrees, the cell gap is determined to 5.6~5.7μm.

In the above embodiment, the measurement for the TN cell has been described, but the measurement can be performed for the STN cell by the same method. Generally, in liquid crystal cells, the twist angle and the approximate value of the cell gap are known. Therefore, in the case of measuring the STN cell, calculation is performed for the vicinity of the approximate value and T _C / T ₀
And the values of T _S / T ₀ are obtained in advance, and if necessary, they may be plotted as shown in FIG. In addition, in the examples, the measurement is performed in steps of twice, but the measurement may be performed in finer steps or in coarser steps. The finer the division, the longer the measurement time, but the twist angle and cell gap can be obtained with higher accuracy. Further, the method according to the present invention can be similarly applied to a liquid crystal cell in which a voltage is not applied and a liquid crystal cell in which a voltage is applied.

[0027]

As described above, in the measuring method according to the present invention, the test liquid crystal cell is placed between the polarizer and the analyzer and rotated, and the liquid crystal cell is placed at the position where the transmitted light amount reaches the maximum value or the minimum value. Is fixed, the analyzer (or polarizer) is rotated to measure the amount of transmitted light, and the twist angle and cell gap are determined based on these measured values. The twist angle and the gap value of the liquid crystal cell can be simultaneously determined by time. And since the method according to the invention is a combination of simple operations, it is possible to automate the measurement.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a measuring device for explaining the operation of the present invention.

FIG. 2 is a schematic configuration diagram of a measuring device according to an embodiment of the present invention.

FIG. 3 is a graph showing measurement results according to an example of the present invention.

FIG. 4 is a graph showing a relationship between T _C / T ₀ and T _S / T ₀ according to a calculation result, and a twist angle and a cell gap, which is used in an embodiment of the present invention.

FIG. 5 is an explanatory diagram of a twist angle of a liquid crystal cell.

[Explanation of symbols]

 1 Light Source 2 Polarizer 3 Liquid Crystal Cell 4 Analyzer 5 Photo Detector 6 HeNe Laser 7 Silicon Photodiode 8 Current-Voltage Converter 9 Voltmeter 10 TN Cell 11 Sample Rotation Stage 12 Upper Substrate 13 Lower Substrate 14 Liquid Crystal Molecule

Claims (4)

(57) [Claims] (1) A liquid crystal cell, which is a test object, is arranged between a polarizer and an analyzer whose polarization angles are set to form a predetermined angle, and the liquid crystal cell is rotated. (2) Rotate the polarizer or analyzer to measure the amount of transmitted light, and test the relationship between the rotation angle and the amount of transmitted light corresponding to the rotation angle. And a step of simultaneously determining a twist angle and a cell gap of a liquid crystal cell which is an object. 2. The twist angle and cell gap measuring method according to claim 1, wherein the predetermined angle in the first step is 0 ° or 90 °. 3. In the second step (2), when the rotation angle of the polarizer or the analyzer is y, the component that does not depend on the rotation angle of the transmitted light is T ₀ , and the component that changes with cos2y is T _C. the component varying in sin2y as T _S, T _C /
The method for measuring the twist angle and the cell gap according to claim 1, wherein T ₀ and T _S / T ₀ are obtained, and the twist angle and the cell gap are determined based on the obtained values. 4. T _C / T ₀ and T _S / T ₀ are obtained by calculation using the twist angle and the cell gap as parameters, and in the second step (2), T _C / T ₀ and T _S / T ₀ and calculated T _C / T
4. The method for measuring the twist angle and the cell gap according to claim 3, wherein the twist angle and the cell gap are determined based on the comparison between ₀ and T _S / T ₀ .