JPS5887445A - Measuring method for degree of multirefraction of clear film and device thereof - Google Patents

Abstract

PURPOSE:To incomplicatedly and reliably measure the degree of multirefraction of a clear film in a non-contact and a non-breakdown manner, by a method wherein the degree of multirefraction of the film is found through detection of a wavelength strength distribution of light transmitting the film. CONSTITUTION:If a film is perpendicular irradiated with a parallel white light, lights II, III... coming out after reflecting several times between the two surfaces of the film, in addition to a directly transmitted lightI, are present as a transmitted light. The two types of lights perform an interference, and as a result, the light having a given wavelength strengthenes and the light having a specific wavelength weakens. This depends on a refractive index n and a thickness D. A linear polarized light is used in light entering the film. A wavelength distribution of a transmitted light having a polarized light surface in parallel with the direction of the one main refraction index of the film and a wavelength strength distribution of the transmitted light having a polarized light surface in parallel with the direction of the other main refraction index of the film are detected, and this finds the refractive index of the film through comparison of the two distribution with each other.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for measuring birefringence of a transparent film.

Generally, when light passes through a substance exhibiting optical anisotropy, a so-called birefringence phenomenon occurs in S-. Birefringence occurs because, in a substance exhibiting optical anisotropy, the refractive index differs depending on the direction of vibration of light. The direction in which the refractive index in a substance shows its maximum value and the direction in which it shows its minimum value are orthogonal to each other, and if we know the refractive indices in the directions that form an angle of 90@ with these two directions, that is, in the three directions that are orthogonal to each other, we can determine the material's refractive index. Optical anisotropy can be expressed. The refractive indices in these three directions are called principal refractive indices, and the five directions are called principal refractive index directions or electrical principal axes.

A polymer film exhibits optical anisotropy as a reflection of its internal molecular orientation. Knowing the degree of this optical anisotropy is important in determining the structure and physical properties of the polymer film, as well as the uniformity of abrasive elongation during the C11 manufacturing process. -It is important to know gender etc. Birefringence has traditionally been used as an index of optical anisotropy.A common method for measuring birefringence is using a polarizing microscope.Method using a polarizing microscope The method calculates the degree of birefringence Δn by measuring the letter derection r, which is the product of the degree of birefringence Δn and the thickness D of the sample.According to this method, the degree of birefringence Δn is obtained by observing the film to be measured using a microscope. The operation was complicated, as it had to be cut to the appropriate size, and only an experienced person could measure it accurately.

The present invention is intended to eliminate such conventional drawbacks, and its purpose is to provide a method and apparatus that can easily and accurately measure the degree of birefringence of a transparent film in a non-contact, non-destructive manner. In other words, the present invention converts white parallel polarized light into a transparent light 5tA by polarizing the light parallel to one of the principal refractive index directions of the film.
c@ Detect the wavelength intensity distribution of the light transmitted through the film when irradiated directly, and further detect the film when irradiated perpendicularly to the film with the plane of polarization parallel to the direction of the principal refractive index of the other film. The specified invention is a method for measuring the birefringence of a transparent film, which is characterized by detecting the wavelength intensity distribution of the transmitted light and determining the birefringence from the two wavelength intensity distributions. A distribution detection section is provided oppositely, an irradiation section of a transparent film is formed between the light source section and the detection section, and the polarization plane of the light source is kept parallel to the directions of the two principal refractive indices of the film. The combined invention is a device for measuring birefringence of a transparent film, which is characterized in that it is formed in such a way that it is possible to perform the following steps.

Hereinafter, nine aspects of the present invention will be described in detail.

In Figure 1, when six parallel color beams are irradiated perpendicularly to the film, in addition to the light (I) that passes through the film directly, the light that comes out after being reflected several times between both surfaces of the film. <IF), (1)... exist. This type 211 light interferes, and as a result, light with a specific wavelength becomes stronger and light with a specific wavelength becomes weaker. In other words, the wavelength intensity distribution of the radiant energy of the light before it enters the film (hereinafter referred to as the light spectrum).
changes as it passes through the film. This change depends on the refractive index n and thickness D of the film. When linearly polarized light is used as the light incident on the film, and the plane of polarization is made to match one of the directions of the principal refractive index of the film, the spectrum of the transmitted light and the plane of polarization are the direction of the other principal index of refraction of the film. Therefore, the difference in the spectra of the transmitted light when matched with , depends on the difference in the respective principal refractive indexes and the degree of birefringence. In other words, the spectrum of transmitted light with a plane of polarization parallel to the direction of one principal refractive index of the film and the spectrum of transmitted light of light with a plane of polarization parallel to the direction of the other principal refractive index of the film. By comparison, the degree of birefringence of the film can be determined.

Hereinafter, the present invention will be explained in detail using mathematical formulas.

In Figure 1, when linearly polarized light is incident perpendicularly to the film with its polarization plane matching the direction of one principal refractive index of the film, the light that enters the film and exits outside the film (
I) and Eye A/4je: There is a light button (6) that is incident, undergoes nine reflections at the interface between the film and the air, and then exits. 9 Originally, if the incidence is normal, the light (11, (If), (
4) passes through the same place, but in the first garden, it was shown shifted for the sake of explanation. Now 9 Light α transmitted without being reflected
) and the light that exits the film after being reflected twice, that is, the light reflected on one side of the film and reflected on the other side @), the difference in optical distance (optical path difference) is shown by the following formula. .

Δ = 2 n++ D ・・・・・・・・・・・・・・・・・・・・・・・・・・・ (However, nl is the refractive index in one optical axis direction of the film, and D is the refractive index in the direction of one optical axis of the film. Indicates the thickness of the film. Therefore, for light of a wavelength such that Δ satisfies the condition shown by the following equation (2), the transmitted light becomes maximum, and the following equation (
For wavelengths that satisfy the conditions shown in 3), the transmitted light becomes minimum O Δ=, q+λ−(2) ! 11 = 1
．． 2. 5-Δ=I! Iλ"-(3' m=
21 21 1 '''where m is the order of interference and λ is the wavelength.

In Fig. 2, the wavelength lλKr indicating the th maximum is taken as the wavelength lλKr, and the maximum is 1, 11, +
2...9M small numbers are numbered +÷, ++, +y..., and the wavelengths are λ and λ+'! ! +1 to λ. If +->-... then the light intensity is at its peak.
Or wavelength 2 that shows the minimum) λj, λ1 (j<11 j=
÷, 1).

2...s 1''2+ 1+÷, 2...), the interference degree of χj of 1 km is expressed by the following equation (4).

Therefore, the optical thickness tl++D of the film can be expressed by the following equation (5).

fluD=10 m)u−・−・−・・−= (5) Therefore, by irradiating the film perpendicularly with white linearly polarized light whose polarization plane coincides with the direction of one principal refractive index of the film, the transmitted light spectrum is If you observe the extremely thick or minimum wavelength of
The optical thicknesses n and D for the two principal refractions can be calculated.

Furthermore, by rotating the polarization plane relative C906 to match the other light intensity of the film and performing similar measurements and calculations, the optical thickness for the other principal refractive index of the film is calculated as shown in the following equation (6). ``LD can be calculated.

nlD = ', filλj・・−・−・・・・
−・(11) Each principal group refractive index # obtained in this way
The frequency Δn of c-night is obtained. To determine the thickness D of the film, it may be measured in advance using a separate square, or it may be determined by substituting an appropriate refractive index value into the optical thickness nD obtained using the aforementioned square. In this case, the accuracy of the refractive index value used will affect the accuracy of the degree of birefringence, but the refractive index value does not change that much once the material is determined, so it may be necessary to use values measured in advance with other samples or #C Sufficient accuracy can be obtained even if the published values are used. In order to match the polarization plane with the direction of the principal refractive index of the film, the polarization plane of the light source is provided to be rotatable.

Alternatively, the film may be provided so as to be rotatable about the normal line of the film surface.

Also, if the direction of the principal refractive index of the film to be measured is not known in advance, add a polarizer between the film and the light receiving section, and make sure that the polarization plane of this polarizer is perpendicular to the polarization plane of the light source. If you rotate the film with the axis normal to the film surface, or rotate the polarization plane of the light source and the polarizer so that they are perpendicular to each other, you can find the position where the amount of light entering the light receiving area is minimized. The direction of the polarization plane is 2 of the film.
coincides with the direction of the two principal refractive indices.

If this film and the polarizer of the light-receiving part are made to coincide with the polarization plane of the light source when measuring birefringence, they will not affect the birefringence measurement at all. To measure the spectrum of 2Mi transmitted light, a spectroscope using a prism or a diffraction grating may be used, for example.

In addition, the most suitable wavelength range of the light used as a light source is a wavelength range in which there is no absorption of light of a specific wavelength by the sample, that is, a visible light region, and the wavelength range is 4000 to 8000 A.
'It's in good condition.

The most common method for obtaining a linearly polarized light source is to use a non-linearly polarized light source as a light source and to hit a polarizer between the light source and the film.

By using the method of the present invention, the degree of birefringence can be measured by touch without cutting the sample unlike the conventional method using a polarization mirror, and information regarding the film thickness can also be obtained at the same time. It is necessary to measure the thickness of the material and the time required for measurement is limited.

In order to carry out the present invention, a linearly polarized light source section is used.

A device is used that includes an irradiation section and a transmittance wavelength distribution detection section. The light source section consists of a light source that emits white light, a polarizer, and an optical system such as a lens for converting the light beam into parallel light beams with a certain width. White light sources that are generally used as white light sources include xenon lamps, etc.
Examples include single-wire halogen lamps, tungsten lamps, and the like. As the polarizer, a polarizing plate, a polarizing prism, etc. are used.

The irradiation section is a section that irradiates the film with light from the linearly polarized light source section, and the space between the linearly polarized light source section and the transmittance wavelength distribution detection section is the irradiation section. In this irradiation section, the film is held perpendicular to the light. The detection section is for measuring the transmittance wavelength distribution of the film and consists of a light receiving element and a spectroscope. Examples of spectrometers include prisms and diffraction gratings. The light-receiving element may be any device that can measure the spectrum of transmitted light, that is, the intensity of light transmitted through the film at each wavelength. However, as mentioned above, if a linear light-receiving element (linear image sensor) is used as the light-receiving element, the movable part This method is most preferable because it reduces the amount of time required for measurement and shortens the time required for measurement.

As a mechanism for aligning the polarization plane of the light source with the optical axis of the film, is a rotatable polarizer provided?

Alternatively, the film may be provided so as to be rotatable within the plane of the film, but since industrially produced films generally have a fairly wide area,9 to take advantage of the non-destructive and non-contact characteristics of the present invention, a polarizer is required. It is preferable to provide it so that it can be rotated.

In the present invention, a transparent film is usually used as the film for measuring the degree of birefringence, but it may be slightly colored as long as the maximum and minimum wavelengths due to interference in the transmittance spectrum μ can be confirmed.

The present invention will be explained in detail below using a specific apparatus according to the present invention shown in nine figures.

In Figure 3, the light emitted from the white light source (1) is converted into parallel light by the lens system, and then passes through the polarizer (2) to become linearly polarized light with a plane of polarization parallel to the direction of one principal refraction of the film. and passes through the film (3).

Furthermore, the light passes through the #C diffraction grating (4) and is separated by the lens (5).
Project the spectrum onto the optical element (6) (diffraction grating (4)).A prism may be used in place of C.The electric signal obtained from the linear light receiving element (61C) is focused at the focusing position corresponding to each wavelength. Since the light intensity is at wavelength −
If converted to light intensity, a relative transmittance spectrum can be obtained. If the light source has a sufficiently uniform spectrum μ, a relative transmittance spectrum is sufficient, but it is usually better to measure the spectrum of only the light source in advance and correct it to the absolute transmittance spectrum (4). desirable. In this way, a transmittance spectrum of polarized light whose plane of polarization coincides with one of the directions in the principal refraction of the film is obtained. Furthermore, add a polarizer of 90
``By rotating, you can obtain a transmittance spectrum of polarized light in which the plane of polarization matches the direction of another principal refraction of the film.The two spectra obtained in this way show the maximum and minimum, respectively. Find the wavelength and calculate the birefringence Δ1 of the film according to the above formula.

As an apparatus for carrying out the present invention, the above-mentioned apparatus is most preferable. Alternatively, the point light receiving element may be fixed and the prism or diffraction grating may be rotated to allow only light of a specific wavelength to enter the point light receiving element.Furthermore, the polarization plane may be rotated. In this case, the film may be rotated to match the plane of polarization with the direction of principal refraction of the film.

As a special case, if the direction of the principal refraction of the film is unknown, the direction of the principal refraction can be determined using the method described above by providing a polarizer between the film and the emissivity index detection section. can be determined.

Since the present invention employs the configuration described above, it has the advantage that birefringence measurement of a film can be easily and accurately performed.

Below are 5I! The present invention will be specifically explained with examples.

EXAMPLE The birefringence of a Daryester film was measured using the apparatus shown in FIG. At that time, a 50W elementary halogen pump was used as a light source, and the light was made into parallel light by the lens of the light source, and then linearly polarized through a polarizing plate (Bolloid plate) and irradiated perpendicularly to the film. In addition, the light transmitted through the film was input into a diffraction grating spectrometer, and the spectrum μ was projected onto the light receiving element. The light-receiving element uses a 512-bit linear imager with a wavelength range of 5.
000 to 5oooX.

First, align the polarization plane of the polarizing plate with one of the pre-measured directions of the film's principal refractive index, irradiate the film with linearly polarized parallel light, and calculate nuD from the spectrum of the transmitted light. did. Next, rotate the polarization plane of the polarizing plate by 90″ and similarly calculate nm D from the spectrum of transmitted light.
The degree of birefringence Δn was calculated. To calculate the thickness of the film, use n
= 1.6 was used.

For comparison, we also conducted measurements using conventional polarized light scanning. Polarized light microscopy requires a film thickness value, and the value obtained when birefringence was measured by the method of the present invention was used as the film thickness.

The measurement results are shown in Table 1. According to the measurement results, it can be seen that the measured values according to the present cabinet method and the measured values according to the conventional method are in good agreement.

Table 1

[Brief explanation of drawings]

FIG. 1 is a schematic diagram for showing the effect of the present invention, FIG. 2 is a spectrum μ diagram obtained by the present invention, and FIG. 3 is a schematic diagram of an example of an apparatus for carrying out the present invention. Patent applicant Unitika Co., Ltd. 111 JirL &

Claims (2)

[Claims] (1) A transparent film is irradiated perpendicularly with a parallel ray of white linearly polarized light with its polarization plane parallel to one of the principal refractive index directions of the film, and the wavelength intensity of the light transmitted through the film is detected, Further, with the plane of polarization parallel to the direction of the other principal refractive index of the film, C11I is irradiated perpendicularly to the wrinkled film, and the 74
The wavelength intensity distribution of the light that has passed through the PM is detected. A method for measuring the degree of birefringence of a transparent film, characterized by determining the degree of birefringence from the intensity distribution of both wavelengths. (2) A light source section for white linearly polarized parallel light and a detection section for wavelength intensity distribution are arranged opposite each other, and an irradiation section of a transparent film is formed between the light source section and the detection section, and the polarization plane of the 9W4 light source is 1. A method for measuring birefringence of a transparent film, characterized in that a method for measuring birefringence of a transparent film is formed, which is capable of keeping #C parallel to the directions of two principal refractive indices.