JPH0325347A - Double refraction measuring method - Google Patents

Abstract

PURPOSE:To easily and speedily measure the retardation of a sample with high accuracy by detecting the phase difference angle of sample transmitted polarized light beams and calculating the phase angle degree of the retardation and the retardation from a specific expression. CONSTITUTION:The two orthogonal polarized light beams incident on the sample Sa are switched between the vertical angle of incidence and a non-vertical angle and when they are made incident vertically and not vertically, the phase angles of the two sample transmitted polarized light beams are detected; and the phase angle degree (m) of the retardation and the retardation D are calculated from m=(alpha'costheta-alpha)/2pi(1-costheta) and D=lambda(2pim+alpha)/2pi where alpha and alpha' are the respective phase difference angles and theta is the angle of refraction of the light incident on the sample surface at the time of the non-perpendicular incidence. In this case, the angles alpha and alpha' are measured by an analyzer A and the angle theta is found from the inclination and refractive index of the light incident on the sample, so (m) and D are accurately calculated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field in Icheon) The present invention relates to a birefringence measuring device for determining the orientation degree of molecules of plastics or fibers, the thickness of a sample, etc.

(Prior Art) The birefringence characteristics of a sample can be expressed by the phase difference, or retardation, between the two polarized lights of the output light when two polarized lights whose polarization planes are perpendicular are incident on the sample in the same phase.

The degree of birefringence can be compared by comparing the retardation of samples of the same thickness.

Conventionally, two methods have been used to measure the retardation of a sample. One method involves inserting a sample between a polarizer and an analyzer, making white light incident thereon, and comparing the interference color of the transmitted light with a pre-calculated interference color table to determine retardation. If the thickness of the sample is d1, the refractive index of the sample for ordinary rays is Nn, and the refractive index for extraordinary rays is Na, then the retardation D for light with wavelength λ is d(N
a-Nn), and if we consider the case where the two polarizers and the analyzer that have passed through the sample are parallel, θ is exactly 2π
Since light with a wavelength λ0 such that m (m is an integer) is best transmitted, the transmitted light exhibits a color determined by the wavelength λ. The vividness of the colors is good as the wavelength range of the transmitted light is narrow. As the phase angle order m becomes larger (that is, the sample becomes thicker), there are multiple wavelengths in the visible light wavelength range where θ is an integer multiple of 2π, and the brightness of the transmitted light decreases, making it difficult to compare. It becomes difficult to measure retardation based on color, and it is said that the limit for this method is usually a thickness of about 2000 nm.Furthermore, since colors are compared, the measurement accuracy of retardation is originally low.

Another method is to insert a wedge-shaped thin plate of quartz crystal over the sample and between the polarizer and analyzer, and slide the quartz plate so that the retardation between the sample and the quartz plate cancels each other out. The retardation date of the sample is determined from the slide length of the quartz plate at that time. For quartz plates, the relationship between thickness and retardation value is known;
Since the crystal plate is wedge-shaped, the letter decimal point at each point in the sliding direction is known in advance.

The retardation between the sample and the quartz plate can be made additive or offset depending on the direction of insertion of the quartz plate. When the crystal plate is inserted in a direction that cancels the retardation of the sample, and the color of the transmitted light disappears, the retardation of the light flux transmission position of the crystal plate is equal to that of the sample, so the retardation of the sample depends on the position of the crystal plate. Seek. Although this method has good accuracy, the dry range at which retardation can be measured is within the range of the retardation width of the quartz plate, and if it exceeds this range, the colorimetric method described above must be used in conjunction with it, resulting in poor accuracy. descend.

<Problems to be Solved by the Invention> An object of the present invention is to provide a method that can accurately measure retardation even when the retardation has a large value.

(!Means for solving problem I) The angle of incidence of orthogonally bipolarized light incident on the sample is changed, and the sample transmission is changed both when the incident light is perpendicular to the sample surface and when it is incident at a certain angle. Calculate by letter when measuring the phase difference angle of two biases, let each phase difference angle be α, α′, and let the refraction angle of the incident light with respect to the sample surface when the light is obliquely incident on the sample be θ. If the retardation is d from the phase difference angle α, and the thickness when slightly thickened is d', then the retardation in each case is D=d (Na-Nn) D'=d' (Na-Nn) D, Multiplying D' by 2π/λ gives the phase difference angle, and the phase difference angle is 2πrn+α and 2πm+α, where α and α° are measured tangently.The above equation is determined.

(Operation) The phase difference angles α and α゛ can be measured with high precision by rotating a polarizer or an analyzer using a measuring device.

The refraction angle θ is determined from the inclination of the light incident on the sample and the refractive index. Therefore, the above m. D can be determined with high accuracy through calculation.

The principle of the above method is to determine the order m from the change in the retardation angle when the thickness is slightly changed for a sample whose phase angle order of retardation is unknown.If the change in thickness is small, the order m is determined before and after. does not change, only the phase difference angle changes. If the original thickness m is found, D can be found from the equation (3) above,
This is the same as equation (2) above.

(Example) The drawing shows an example of an apparatus for carrying out the method of the present invention.
o is a light source that emits a monochromatic light beam, and a laser is used. The laser beam! It is reflected by M and MR and guided onto the optical axis of the polarization measurement system M. The mirror MR is configured to be able to swing around the mirror surface center O with the axis perpendicular to the plane of the drawing. In the polarization measurement system, P is a polarizer, C is a 1/4 wavelength plate, L is a lens, Sa is a sample, L' is a lens, A is an analyzer, and D is a photodetector. The lens L is designed to form an image of the center 0 of the surface of the mirror MR on the sample Sa. The lens L' is designed to re-image the image on the sample Sa onto the light receiving surface of the photodetector D. As a result, when the mirror MR is rotated so that the reflected light beam from the mirror makes an angle φ with the optical axis of the polarization measurement system, the incident angle of the laser light beam on the sample becomes i. The center O of the mirror MR is determined from the distance from the lens L to the sample Sa.
Since the distance from to lens L is larger, φ is smaller than i, and in lens L, the optical path is formed symmetrically with L with sample Sa as the center, so mirror MR is Even if you oscillate it, the polarizer will remain intact. The influence of the inclination of the optical path on the analyzer can be ignored, and there is no need to swing the optical axes of the polarizer and analyzer in conjunction with swinging the mirror MR.

The measurement operation with the above device is performed as follows.

The polarization direction of the polarizer P is made to match the optical axis of the quarter-wave plate C, the direction of the analyzer A is made orthogonal to the polarizer P, and the sample Sa is set. Since the polarization direction of the polarizer P and the optical axis of the quarter-wave plate C match, the light incident on the sample is M-line polarized light polarized in the direction of the polarizer P. Now rotate the sample around the optical axis of the device, find a position where the output of photodetector D becomes O, and set it at that position. In this way, the optical axis of the sample is made parallel or perpendicular to the direction of the incident polarized light. After completing the above preparation operations, measurement operations begin.

The optical axis of the sample is fixed at a direction of 45° with respect to the polarization direction of the polarized light P, and the analyzer A is made perpendicular to the optical axis of the sample. Rotate the optical axis of the quarter-wave plate C by 45 degrees and fix it. Here, if the retardation is O, when the polarizer P is turned 45 degrees to make it parallel to the optical axis of the quarter-wave plate, linearly polarized light parallel to the tip axis will be incident on the sample, and the Since the light is emitted, the output of the photodetector is 0. When the retardation is 1/4π in phase difference angle, the polarizer is replaced by a 174 wavelength plate C.
When the sample is turned by 45 degrees from the parallel direction, circularly polarized light enters the sample, and the sample transmitted light becomes linearly polarized light parallel to the sample optical axis, so the photodetection output is O at this time. Similarly, twice the rotation angle from the optical axis direction of the quarter-wave plate C of the polarizer when the photodetection output becomes O is the retardation phase difference angle, that is, α in the above equation (1). becomes. next! IM
Rotate R and perform the same measurement as above to obtain α゛. Then, from the measured values of α and α′ and the rotation angle of the mirror MR, the phase angle order m and the retardation B are determined according to formulas (l) and ■.
Calculate the

(Effects of the Invention) According to the present invention, unlike a method using interference colors, the method relies on angle measurement, so it has a high degree of aperture and can detect phase angle orders that could not be determined using the conventional phase angle method. It can be calculated.

And the measurement is two angles α. The measurement of α′ is easy to operate, easy to calculate, and easy to automate, making it possible to measure the retardation of a sample easily, quickly, and with high precision.

Note that if the mirror MR is rotated in both positive and negative directions, and the calculated values of m, D, etc. for each are averaged, even if the surface of the sample is set at a slight angle with respect to the optical axis of the device, the influence of This allows accurate measurement values to be obtained even when it is difficult to set samples accurately, such as in online measurements.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing is a plan view of an apparatus according to an embodiment of the present invention. So... light source, M... mirror, MR... rocking mirror, P...
・Polarizer, C ・1/4 wavelength plate, L, L'
"i/ins, Sa...sample, A...analyzer, D.
...Photodetector.

Claims (1)

[Claims] The angle of incidence of orthogonal bipolarized light incident on the sample to the sample is switched between perpendicular and non-perpendicular, and the phase difference angle of the bipolarized light transmitted through the sample is detected at each time of perpendicular incidence and non-perpendicular incidence. When the respective phase difference angles are α and α′, and the refraction angle of the incident light with respect to the sample surface when the light is non-perpendicularly incident is θ, the phase angle order m of the retardation and the retardation are m=(α′ cosθ−α)/2π(1−cosθ)D=
A birefringence measurement method characterized by calculating by λ/2π(2πm+α).