JPH05281137A - Double-refraction measuring apparatus - Google Patents

Abstract

PURPOSE:To obtain a double-refraction measuring apparatus, which can accurately measure the double refraction of a sample even if there are polarizer and an analyzer. CONSTITUTION:A light source 10 emits linearly polarized light. A first lambda/4 plate 20 converts the linearly polarized light into circularly polarized light. A second lambda/4 plate 40 converts the luminous flux emitted from a sample 30 into the elliptically polarized light close to the linearly polarized light. An image sensor 60 is provided. A measuring means detects the output of a light receiving means with a light polarizing element being rotated and measures the polarized state of the luminous flux. A memory means stores the intrinsic polarized state of the measuring device wherein the measurement is performed without arranging the sample. Another means subtracts the output of the memory means from the output of the measuring means and analyzes the double refraction of the sample. These means are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring birefringence information due to distortion of molecular orientation of optical parts.

[0002]

2. Description of the Related Art Polymer compounds such as plastics generally have anisotropy similar to that of crystals and cause birefringence.

As a device for measuring the birefringence, a device disclosed in, for example, Japanese Patent Laid-Open No. 63-269045 is known.

In the device described in this publication, a sample having a birefringence is arranged between a polarizer and an analyzer arranged in a crossed Nicols mode, and the light quantity of a light flux passing through these is rotated in the crossed Nicols. While being captured by a video camera. In the measurement, the maximum value of the detected light intensity and the rotation angle when the maximum value is output are obtained, and the birefringence of the sample is obtained as a function of the rotation angle of the orthogonal Nicols.

[0005]

However, in the above-mentioned conventional apparatus, if there is an error in the setting of the polarization element such as the polarizer or the analyzer, the measurement result includes the influence of the error, so that the birefringence of the sample is reduced. There is a problem that it cannot be measured accurately.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and accurately measures the birefringence of a sample even when there is an error in the setting of a polarizing element such as a polarizer or an analyzer. An object of the present invention is to provide a birefringence measuring device capable of performing the above.

[0007]

In order to achieve the above object, a birefringence measuring apparatus according to the present invention changes a polarization state of a light source which makes a specific polarized light incident on a sample and a light beam emitted from the sample. A polarizing element, a light receiving means for receiving the light flux transmitted through the polarizing element, a measuring means for detecting the output of the light receiving means while rotating the polarizing element to measure the polarization state of the light flux, and a measurement without arranging the sample. The present invention is characterized by having storage means for storing the polarization state peculiar to the measuring device and means for analyzing the birefringence of the sample by subtracting the output of the storage means from the output of the measuring means.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of a birefringence measuring device according to the present invention.

This apparatus comprises a light source section 10 which emits linearly polarized light having a predetermined spread, a first λ / 4 plate 20 which converts the light flux emitted from the light source section into circularly polarized light, and a light flux which has passed through a sample 30. Second λ / 4 plate 40 as a phase shifter for converting light into elliptically polarized light close to linearly polarized light, an analyzer 50 rotatably provided in the optical path, and a CCD for receiving the light flux transmitted through the analyzer 50. A two-dimensional image sensor 60 such as a sensor is provided. The output of the image sensor 60 is A / D converted and stored in the frame grabber 61.
Is displayed by the display 63 as birefringence information analyzed by
Displayed in.

A memory 64 is connected to the computer 62 as a storage means for storing birefringence information in the state where the sample is not placed.

The light source unit 10 is a laser device for generating linearly polarized light, or a light source 11 in which a light source for emitting a light beam having a random polarization state and a polarizer are combined, and a beam expander for expanding the diameter of the light beam emitted from this light source. 12 and 12.

The second λ / 4 plate 40 is a computer 62.
It is rotatably provided around the optical axis so that at least two angles can be set via the motor 41 in response to an instruction from.

The analyzer 50 is automatically rotated by the motor 51, and the rotation angle is input to the computer 62 by the angle sensor 52.

When the above device is used, the error of the polarization characteristic peculiar to the device is measured as an initial setting, and the measurement result is stored in the memory 64. Next, the sample is placed, the birefringence of the sample is measured, and the error component unique to the apparatus is subtracted from the measurement result to extract the birefringence information of the sample itself.

The principle of measurement when the sample is placed will be described below. The principle of measurement is the same when the sample is not placed.

First, the sample 30 is placed between the first and second λ / 4 plates 20 and 40, and circularly polarized light is made incident on the sample.
The polarized light emitted from the sample is used as the second λ / 4 plate 40 and the analyzer 50.
Light is received via.

When circularly polarized light is made incident on a sample having birefringence, due to the difference in the refractive index between the slow axis and the fast axis of the sample, the traveling speeds of the two orthogonal axes differ, and the light is emitted as elliptically polarized light. It

This elliptically polarized light is again passed through a phase shifter such as a λ / 4 plate to be elliptically polarized light close to linearly polarized light, and is transmitted through an analyzer. By rotating the analyzer, the amount of light received changes sinusoidally. Therefore, the polarization state can be measured by sampling this change at a plurality of points.

When linearly polarized light is made incident on a linear birefringent crystal such as plastic, when the incident light matches with the intrinsic polarized light, the polarization state is not changed and is transmitted. Cannot be detected. When circularly polarized light is incident, the sample has no insensitive direction, unlike the case where linearly polarized light is used, and the polarization state can be measured in any direction.

Further, the closer the measurement light is to linearly polarized light, the greater the change in the amount of light reaching the light receiving element, and the higher the measurement accuracy. Therefore, as the retarder, a λ / 4 plate is suitable for producing elliptically polarized light closer to linearly polarized light.

In the first stage, the neutral axis of the second λ / 4 plate 40 is set to 45 degrees with respect to the neutral axis of the first λ / 4 plate 20. The computer 60 samples the output of the image sensor 60 from the output of the angle sensor 32 at the time when the rotation angle of the analyzer 30 reaches a predetermined value, and stores it in the frame grabber 61.

In the second stage, the neutral axis of the second λ / 4 plate 40 is set to 0 ° with respect to the neutral axis of the first λ / 4 plate 20. Then, similarly to the first step, the output of the image sensor 60 is sampled at least at three locations where the rotation angle of the analyzer 50 is different, and the polarization state of the light flux is measured for each pixel.

In the case of expressing elliptically polarized light, as shown in FIG. 2, the major radius a, the minor radius b, and the inclination ψ of the ellipse drawn by the tip of the electric field vector in the plane opposed to the traveling direction of light are three. One parameter is required.

The intensity I when elliptically polarized light is received through the analyzer can be obtained by the equation (1), where θ is the rotation angle of the analyzer.

[0026]

[Equation 1]

Since there are three unknown variables α, β and ψ representing the polarization characteristics in the equation (1), the output intensity I when the analyzer 50 is rotated to at least three different angular positions is measured. By doing so, the values of three unknowns can be obtained. Here, in order to simplify the calculation, four measurements are performed every 45 °. Assuming that the intensities obtained by the four measurements are I0, I45, I90, and I135, the variables expressing the polarization characteristics are obtained according to the following equation.

[0028]

[Equation 2]

In the case of measuring the polarization characteristics based only on the intensity, even if the sample is contaminated, it is recognized as a change in intensity, so that noise is added to the signal and the polarization is accurately measured. There is a problem that the characteristics cannot be measured.

Considering the influence of noise, the detected light intensity I can be expressed by the following equation, where the sum noise ns and the product noise nm are given.

[0031]

[Equation 3]

If there is such noise, α and β cannot be accurately obtained. However, the inclination ψ of the main axis of the polarization ellipse can be accurately obtained without being affected by noise.

Here, the sum noise means optically noise caused by mixing of light such as illumination light other than the measuring light into the light receiving means, and electrically, for example, due to a defective offset adjustment of the television signal. Refers to the noise that is generated. Further, the product noise means a noise generated optically due to uneven illumination or the like, and an electric noise means a noise caused by a variation in sensitivity between pixels of the CCD.

In order to measure the birefringence, it is necessary to determine the axial direction of the birefringence and the retardance.

In the apparatus of the embodiment, the above ψ is analyzed at least twice by changing the set angle of the retarder. The computer 62 determines a value ξ corresponding to ψ at the first stage based on the intensity information of each pixel stored in the frame grabber 61.
And a value η corresponding to ψ in the second stage.

Assuming that the retardance is smaller than π / 2, the retardance δ and the axial direction θ can be obtained based on the following principle only by measuring ψ of polarized light close to linearly polarized light.

Left circularly polarized light is made incident on a sample having a birefringence of retardance δ and axial direction θ, and the light flux emitted from the sample is 4
Output light X4 when transmitted through a λ / 4 plate set at 5 degrees
5 can be represented by the following vector.

[0038]

[Equation 4]

The long axis direction ξ of the polarization ellipse of this output light X45
Is as in equation (2).

[0040]

[Equation 5]

When the angle of the λ / 4 plate is 0 degree, the output light X0 is represented by the following vector.

[0042]

[Equation 6]

When the coordinates of the observation system are rotated by 45 degrees, the output light X0-45 becomes the following vector.

[0044]

[Equation 7]

Axis direction η of elliptically polarized light detected at this time
Can be expressed by equation (3).

[0046]

[Equation 8]

Δ and θ can be obtained from the equations (2) and (3) as follows.

[0048]

[Equation 9]

The above calculation is executed for each pixel of the image sensor, and the birefringence information of the entire area of the sample can be analyzed at one time.

When the analysis is completed, for example, the retardance δ
Is converted into light and dark gradation and displayed on the display 63, or converted into dot size and printed out.
Thereby, the variation of birefringence of the sample can be visually grasped as a whole.

However, when the birefringence of the sample is measured by the above method, there is no problem if all the optical elements are set according to the design values, but in reality, the phase of the λ / 4 plate, the analyzer, etc. Since it is possible that the set angle includes an error, if these errors can be canceled, more accurate measurement can be performed.

In the birefringence measuring apparatus according to the present invention, in order to cancel the setting error of each optical element, the bias in the state where the sample is not arranged is obtained in advance, and the bias amount is subtracted in the actual measurement to obtain an accurate measurement. Various measurements are possible.

The phases of the first and second λ / 4 plates, the direction of the neutral axis, and the direction of the analyzer are 1.00 ° from the setting.
If there is an error of, a retardance error as shown in Table 1 is generated.

[0054]

Table 1 Error retardance error Phase of the first λ / 4 plate 1.00 ° 1.00 ° Direction of the first λ / 4 plate 1.00 ° 2.00 ° Second λ / 4 plate Phase 1.00 ° 0.00 ° Second λ / 4 plate direction (0 °) 1.00 ° 2.00 ° Second λ / 4 plate direction (45 °) 1.00 ° 2.00 ° Analyzer direction 1.00 ° 2.00 °

FIGS. 3 to 7 are graphs showing the retardance detected as the value of the entire system when the sample has a retardance of 5 °, plotted by changing the axial direction θ. The radial direction of the polar coordinates shown by the pie chart indicates the retardance δ, and the rotation direction indicates the axial direction 2θ. ● in the figure
The mark indicates the ideal value when each optical element has no error, and the tip of the arrow indicates the measured value when each optical element has an error.

FIG. 3 shows that when the phase of the first λ / 4 plate 20 has a 1 ° error, FIG. 4 shows that the direction of the first λ / 4 plate 20 is 1 °.
When there is an error, FIG. 5 shows the direction of the second λ / 4 plate 40 (0
6) has a 1 ° error, FIG. 6 shows the second λ / 4 plate 40.
7 (45 °) has an error of 1 °, and FIG. 7 shows a case where the direction of the analyzer has an error of 1 °.

As can be seen from the figures, when each optical element has an error, the detected retardance is the sample 30.
Regardless of the retardance axis direction, it changes a certain amount in a certain direction.

Further, since the influence of all the elements is expressed as a vector on the polar coordinates, even when a plurality of optical elements have an error, the system as a whole can be considered as the sum of the vectors of the individual elements. ..

Therefore, the information on the birefringence measured without placing the sample can be grasped as the error of the entire system in which the error of each element is combined.

Therefore, in the apparatus of the embodiment, α without sample in the first measurement step of four times is obtained as a bias, and β without sample in four measurements of the second step is obtained. Calculate as bias and store in memory. Also, for the λ / 4 plate to be rotated, the misalignment between the scale and the actual neutral axis should be known.

By subtracting these errors from the measured values when the sample is placed and measured, the errors of the apparatus itself can be canceled and the birefringence of the sample can be accurately measured.

[0062]

As described above, according to the present invention, by subtracting the birefringence information of the bias which the apparatus itself has due to the phase and angle errors of the polarization element from the birefringence information of the measured sample, The birefringence of the sample can be accurately analyzed.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an embodiment of a birefringence measuring device according to the present invention.

FIG. 2 is a graph showing a polarization ellipse.

FIG. 3 is a graph showing retardance when the phase of the first λ / 4 plate has a 1 ° error.

FIG. 4 is a graph showing retardance when the direction of the first λ / 4 plate has a 1 ° error.

FIG. 5 is a graph showing retardance when the direction (0 °) of the second λ / 4 plate has a 1 ° error.

FIG. 6 is a graph showing retardance when the direction (45 °) of the second λ / 4 plate has a 1 ° error.

FIG. 7 is a graph showing retardance when the direction of the analyzer has an error of 1 °.

[Explanation of symbols]

 10 ... Light source part 20 ... 1st (lambda) / 4 board 30 ... Sample 40 ... 2nd (lambda) / 4 board 50 ... Analyzer 60 ... Image sensor 64 ... Memory

Claims (5)

[Claims] 1. A light source for making a specific polarized light incident on a sample, a polarizing element for changing a polarization state of a light beam emitted from the sample, a light receiving means for receiving a light beam transmitted through the polarizing element, and the polarized light. Measuring means for measuring the polarization state of the light flux by detecting the output of the light receiving means while rotating the element, storage means for storing the polarization state peculiar to the measuring device measured without disposing the sample, and the measuring means. Means for analyzing the birefringence of the sample by subtracting the output of the storage means from the output of the birefringence measurement apparatus. 2. The birefringence measuring apparatus according to claim 1, wherein the light source makes circularly polarized light incident on the sample. 3. The birefringence measuring device according to claim 2, wherein the polarizing element is composed of a phase shifter and an analyzer, and the polarization state is measured while rotating the analyzer. 4. The analyzing means sets the measurement result by setting the retarder at a first angle, and sets the second angle by rotating the retarder by a predetermined angle around an optical axis. The birefringence measuring apparatus according to claim 3, wherein the birefringence of the sample is analyzed based on the measurement result. 5. A light source for making circularly polarized light incident on a sample, a light receiving means for receiving a light flux transmitted through the sample via a phase shifter and an analyzer, and the light receiving means while rotating the analyzer. Measuring means for measuring the polarization state of the light flux from the output of the means, storage means for storing the polarization state peculiar to the measuring device measured by the measuring means without disposing the sample, and a plurality of different phase shifters from each other An angle, and a means for analyzing the birefringence of the sample based on the polarization state measured by subtracting the output of the storage means from the output of the measurement means in each setting state. Refraction measuring device.