JPH04218751A - Apparatus for measuring double refraction - Google Patents

Abstract

PURPOSE:To enhance the measuring accuracy of retardation when cos of retardation becomes a value near to 1 in an optical anisotropic sheet-like sample. CONSTITUTION:A sheet 14 or 17 whose retardation is known is superposed on a sample S and synthesized retardation is made close to pi/2 to substract the known retardation.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a birefringence measurement device suitable for measuring the birefringence of a sample that slightly exhibits birefringence.

0002

2. Description of the Related Art Materials become birefringent due to internal strain or stretching. Conversely, by measuring birefringence, it is possible to determine the internal strain of the material and the degree of elongation of the material. To measure birefringence, linearly polarized light is incident on a sample, and the phase difference (retardation) of the orthogonal two-component polarized light when it passes through the sample is measured.
This is done by detecting the Let's assume that the sample is a thin plate or sheet, and the optical axis is parallel to the sample surface.
If the thickness of the sample is d, the refractive index for each polarized light in the direction of the optical axis and the direction perpendicular to it is n1, n2, and the wavelength of the light used is λ, then when linearly polarized light is incident, the optical axis and The phase difference δ between polarized light components in two directions perpendicular to the optical axis when the sample is emitted is δ=2π(n1−n2)d/λ. Here δ
is called retardation. When the thickness d of the sample is known in advance, n1-n2 can be found from the measurement of δ, and n
When 1-n2 is known in advance, the thickness d can be determined from δ. Measurement of retardation is performed as follows. A polarizer and an analyzer are arranged in parallel, a sample is inserted between them and rotated, and a graph of the relationship between the rotation angle of the sample and the intensity of transmitted light is obtained by passing light of wavelength λ. When the optical axis of the sample is parallel to the polarizer or analyzer or perpendicular to them, the linearly polarized light incident on the sample exits the sample as linearly polarized light, so when the sample does not have dichroism. The intensity of the transmitted light is the same in these two directions, and we will now use this as the reference value for the light intensity. When the optical axis of the sample is not parallel to or perpendicular to the polarizer or analyzer, the light transmitted through the sample is generally elliptically polarized light, and only the component of this elliptically polarized light in the direction of the analyzer is measured. If we now consider the case where the optical axis of the sample is at 45° with respect to the polarizer and analyzer, the amplitudes of both the components of the incident light on the sample in the optical axis direction and in the direction perpendicular thereto are equal. If the amplitude of the light emitted from the polarizer is 1, the amplitude of both of these components is 1/√2, and each of these components in the direction of the analyzer passes through the analyzer. Since there is a phase difference of δ between the components, the light transmitted through the analyzer is (1/2) sin (ωt + δ/2) + (1/2
) sin(ωt−δ/2). The above equation becomes cos (δ/2) sin ωt, and the analyzer transmitted light intensity is the reference value {cos (δ/2)} 2 . If the above relationship is drawn in a graph, it will look like a cross, as shown in Figure 2. In this figure, the ratio of the maximum and minimum light intensity is {c
os(δ/2)}2. In this way, cos(δ/2) is determined by measurement, and δ is calculated from this. When using the method described above, when the retardation δ is large to a certain extent, δ can be accurately determined from cos(δ/2), but when δ is small, cos(
The rate of change of δ/2) with respect to a change in δ is proportional to the square of δ and is very small, so a small measurement error of {cos(δ/2)}2 will cause a large error in δ. For this reason, it has been difficult to measure the anisotropy of samples with extremely small anisotropy using polarized light. On the other hand, the slight anisotropy of the sheet material may cause the material to warp at a later date due to changes in temperature or simply due to changes over time, and the occurrence of such warping can be extremely disadvantageous. Therefore, there is a need for a method that can accurately measure extremely mild anisotropy in sheet materials.

0003

SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus that can easily and accurately measure the retardation of a sample having small retardation.

0004

[Means for solving the problem] Using the conventional retardation measurement method, a plate with known retardation is stacked on the sample,
The retardation of the sample was calculated by determining the retardation of the entire stack of the sample and this plate, and then subtracting the above-mentioned known retardation value from it.

[0005]

[Operation] As a board with known retardation, for example, 1/2
Considering a wave plate, the retardation of a 1/2 wave plate is π. If the retardation of the sample is δ and the overall retardation of both is Δ, what is directly measured is the ratio between the maximum and minimum transmitted light intensity, which is cos2 (Δ/2). be. This is cos {(δ
+π)/2}=cos(δ/2+π/2}=sin(δ
/2) Since we are considering a range in which δ is small, what is directly measured is {sin(δ/2)}2 = (δ/2)2, and δ can be calculated from {cos(δ/2)}2. The effect of measurement error on δ is significantly smaller than when calculated.

In general, the retardation of the plate to be inserted is known and is defined as d, and this is close to 1/2 wavelength (although it is sufficient that the phase angle is within a range of about 180°±90°). shall be. Since there is a wide selection range, it is easy to select the board to be inserted. The overall retardation δ when the sample and this plate are stacked is δ + d, and the ratio R of the maximum and minimum transmitted light intensity is R = {cos (Δ/2)}2, so as the square root of R

[Equation 3] is found. Here, δ is small, so the upper value is cos(d
/2), which is in the range of 45° to 135°, and the value of cos changes approximately linearly from positive to negative in this area, therefore,

(δ+d) can be obtained with high accuracy as follows, and by subtracting the known retardation from this, δ can be obtained.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an apparatus according to an embodiment of the present invention. In this embodiment, instead of rotating the sample, the polarizer and analyzer are rotated integrally. With this configuration, it is easy to continuously test the anisotropy of a strip-shaped continuous sample in the length direction. In the figure, 1 is a polarizer, 2 is an analyzer, and these are rotary tables 11 arranged vertically on the same axis A.
, 12, and each rotary table 11, 12 is rotated simultaneously by a pulse motor 4 via a belt 3 at the same angular velocity. The polarizer 1 and the analyzer 2 are mounted on each rotating table so that their polarization directions are parallel to each other. A sample sheet S is passed between the upper and lower rotary tables 11 and 12. A filter 5 above the polarizer 1 transmits light of a specific wavelength. Reference numeral 6 denotes a halogen lamp as a light source, which is guided onto a filter 5 through an optical fiber 7, and the filter-transmitted light passes through a polarizer 1, a sample S, and an analyzer 2, and is detected by a light receiving element 8. The output of the light receiving element 8 is sent to a data processing device 10 through an amplifier 9. A pulse signal is emitted from the data processing device 10, inputted to the motor drive circuit 13, converted into a motor drive pulse, and inputted to the motor 4. The data processing device counts these pulses and detects the rotation angle of the polarizer and analyzer. A half-wave plate 14 is fixedly inserted between the rotating table 11 and the sample S. 1/2
The optical axis of the wave plate 14 is parallel to or perpendicular to the optical axis of the sample S. Since the optical axis of the sample coincides with its stretching direction, the optical axis of the half-wave plate may be aligned with the longitudinal direction of the sample S. When the direction of the optical axis of a general sample is not known in advance, for example, when the sample is a semi-finished product that has already been punched into a predetermined shape from a sheet material,
Remove the two-wavelength plate 14 and rotate the polarizer 1 and analyzer 2 to find the output of the light receiving element 8 for each polarizer rotation angle. From this output, calculate the difference between the two outputs separated by 45 degrees from the polarizer. When calculated for each rotation angle, there are eight angles where this difference is 0 (positions where the positive and negative are reversed) during one rotation of the polarizer, and four angles where the difference is maximum between them. These four angular positions are 90° apart from each other, and this direction is the optical axis of the sample or a direction perpendicular thereto. You can directly find the angular position of the maximum difference here, but since the change in difference is small near the maximum difference and the angular position cannot be found accurately, it is better to find it from the angular position where the difference is reversed. . Although the method for detecting the optical axis of the sample is not limited to this, these methods can
Even if the existence of optical anisotropy in a sample is known, when trying to quantitatively measure the degree of anisotropy, if the retardation is small, it has not been possible to measure with high precision. This allows anisotropy to be quantified with high precision. In FIG. 1, the 1/2 wavelength plate is attached to an arm 16 that is rotatably held by a support 15, and by rotating the arm 16,
It is designed so that it can be moved in and out of the measurement optical path. In this device, the 1/2 wavelength plate 14 is inserted on the measurement optical path, and while the polarizer and analyzer 1 and 2 are rotated, the output of the light receiving element 8 is input into the data processing device at 1° intervals, for example. This result is illustrated in Figure 2. In a sample with small anisotropy, the flower shape in Figure 2 becomes close to a circle, and the ratio R of the maximum diameter to the minimum diameter 45° away from it is a letter. dation δ is

[Math 5] Calculated by

In the apparatus shown in FIG. 1, a sheet 17 with a known retardation may be attached in place of the 1/2 wavelength plate 14 for measurement. In this case, the optical axis of the sheet 17 is made parallel to the sample S. The sheet 17 may be selected by the person performing the measurement by measuring the retardation of a familiar sample and selecting one having an appropriate value. The value of retardation is selected so that the value combined with the retardation of the sample is approximately 180 degrees in angle. Find the maximum and minimum ratio R of the photometric output, and let the retardation of the sample be δ and the retardation of the sheet 17 be d,

Calculate δ+d using Equation 6, and subtract d from this to find δ. In this way, even when there is no 1/2 wavelength plate, the change in the cos value with respect to the change in retardation is proportional to the change in cos, so the accuracy is improved by 1/2 compared to calculating δ directly by cos δ/2 = √R. This is the same as when using a two-wavelength plate.

[0009]

[Effects of the Invention] In the conventional method, the measurement of retardation δ was obtained using the relationship (measured output) = (cos δ/2), so where δ is small, the rate of change in measured output with respect to change in δ is small. In the present invention, where it was not possible to accurately measure δ, cos δ was converted to sin δ, so
The change in δ and the change in measurement output are at the same rate, making it possible to accurately quantify the anisotropy of a material with small anisotropy. Further, the invention of claim 1 is easy to implement even if a half-wave plate is not available because it is sufficient to search for a plate with a retardation close to π from a sample at hand and use it.

[Brief explanation of the drawing]

[Fig. 1] Perspective view of an apparatus according to an embodiment of the present invention

[Figure 2]
The figure is a graph of measured output in polar coordinates.

[Explanation of symbols]

1 Polarizer 2 Analyzer 3 Belt 4 Pulse motor 5 Filter 6 Light source 7 Optical fiber 8 Light receiving system 9 Amplifier 10　　　　　Data processing device 11, 12 turntable 13 Motor drive circuit 14 1/2 wavelength plate 15 Pillar 16 Rotating arm

Claims (2)

[Claims] Claim 1: A polarizer and an analyzer are arranged with their polarization planes parallel to each other, and a plate with a known retardation is inserted between the polarizer and the analyzer to overlap the sample and a plate with a known retardation. Polarizer, plate with known retardation, sample, when changing the relative angular position between the plate and the pair of polarizer and analyzer.
Measure the maximum and minimum ratio R of the intensity of light transmitted through the analyzer, calculate the overall retardation Δ of the sample and the board with known retardation using [Equation 1], and subtract the known retardation from this. A birefringence measuring device characterized in that the retardation of a sample is calculated by calculating the retardation of a sample. 2. A polarizer and an analyzer are arranged with their polarization planes parallel to each other, and a sample and a 1/2 wavelength plate are inserted in a stacked manner between the polarizer and the analyzer. Polarizer, 1/ when the relative angular position between the plate and the pair of polarizer and analyzer is changed.
Measure the ratio R of the maximum and minimum intensity of light transmitted through the two-wave plate, sample, and analyzer, and calculate the retardation δ of the sample using the formula 2
] A birefringence measurement device characterized by calculating the birefringence.