Method for fabricating polarizing filter matching pairs, and method and apparatus for determining the concentration of birefringent particles using polarizing filter pairs.

Abstract

The present invention relates to a method for making a matched pair of polarizing filters, the method comprising the steps of: mounting a first linear polarizer and a second linear polarizer in a light beam; rotating the second linear polarizer to obtain maximum extinction of the light beam; inserting a first quarter-wave optical retarder in the light beam; rotating the first quarter-wave optical retarder to obtain maximum extinction of the light beam; then rotating the first quarter-wave optical retarder through a 45-degree angle; inserting a second quarter-wave optical retarder; and rotating the second quarter-wave optical retarder through a second angle to obtain maximum extinction of the light beam before fixing the first linear polarizer and the first quarter-wave optical retarder and the second linear polarizer and the second quarter-wave optical retarder. The present invention also relates to a method and apparatus for analyzing a sample containing birefringent particles suspended in a fluid (e.g., suspended in a liquid) using a matched pair of polarizing filters.

Description

[Technical Field]

[0001] The present invention relates to a method for fabricating polarizing filter matching pairs. Polarizing filter matching pairs obtained by such a method enable the optimization of the detection of depolarized light produced by a sample containing birefringent particles. Polarizing filter matching pairs obtained by such a method preferably have 10 -5 It has an extinction ratio of less than . The present invention also relates to a method for analyzing a sample containing birefringent particles suspended in a fluid (e.g., suspended in a liquid) using polarizing filter matching pairs. In particular, the present invention relates to a method for determining the concentration of birefringent particles suspended in a fluid using polarizing filter matching pairs. Furthermore, the present invention relates to an apparatus for analyzing a sample containing birefringent particles suspended in a fluid (e.g., suspended in a liquid) using polarizing filter matching pairs. In particular, the present invention relates to an apparatus for determining the concentration of birefringent particles suspended in a fluid using polarizing filter matching pairs. [Background technology]

[0002] Currently, mitigating climate change induced by carbon dioxide (CO2) emissions is a major challenge. The ocean accounts for a significant portion of these emissions. Until now, the capacity of the deep sea to store carbon dioxide has been largely unknown and underestimated. Carbon storage in the ocean is governed by two mechanisms: the dissolution pump and the biocarbon pump. The biocarbon pump refers to a series of processes in which inorganic carbon (such as carbon dioxide) is fixed into organic matter through photosynthesis and then sequestered from the atmosphere, usually by transport to the deep sea. The biocarbon pump consists of two pumps: the organic carbon pump, driven by phytoplankton fixing CO2 into particulate organic carbon (POC) through photosynthesis and transporting some of it to the deep sea; and the carbonic acid pump, associated with the formation of calcium carbonate CaCO3 (PIC) through biological calcification and its transport to the deep sea.

[0003] Understanding biocarbon pumps hinges on accurately and precisely measuring the concentrations of PIC and POC. Due to inadequate sampling and analytical techniques for measuring PIC concentrations and downflow (particularly the spatial and temporal variability of PIC in the ocean), our understanding of PIC circulation has been limited.

[0004] Previous attempts to measure PIC concentration have been based on methods such as injecting acid into a suspension and measuring the change in pH, or filtering a known amount of seawater and then determining the PIC concentration in a laboratory or through microscopic analysis. Such methods have many drawbacks. They require ship support, are labor-intensive, cumbersome, and do not allow for autonomous in-situ operation.

[0005] US7030981 describes a method and apparatus for measuring the concentration of CaCO3 particles, comprising a first linear polarizer and a second linear polarizer having mutually orthogonal polarization axes. The extreme sensitivity of polarizer alignment in such apparatus is a significant drawback. For example, minute mechanical twists induced by vibration and changes in water pressure due to depth can cause many random fluctuations in the signal, making measurement of CaCO3 concentration impossible. [Overview of the project]

[0006] The objective of the present invention is to achieve an extinction ratio of 10 -5 Less than 5 x 10 -6 Less than, or even 1 × 10 -6 The objective is to provide a method for producing polarizing filter matching pairs that are less than [a certain value].

[0007] The object of the present invention is to provide a method and apparatus for using polarizing filter matching pairs to analyze a sample containing birefringent particles.

[0008] The object of the present invention is to provide a method and apparatus for using polarizing filter matching pairs to determine the concentration of birefringent particles suspended in a fluid such as a liquid.

[0009] An object of the present invention is to provide a method and an apparatus for determining the concentration of calcium carbonate particles or the concentration of polymer particles (e.g., the concentration of calcium carbonate particles or the concentration of polymer particles suspended in a fluid (e.g., water)).

[0010] An object of the present invention is to provide a method and an apparatus that enable detecting even a small concentration of birefringent particles suspended in a fluid such as a liquid and / or enable detecting a small change in the concentration of birefringent particles suspended in a fluid such as a liquid.

[0011] An object of the present invention is to provide a method and an apparatus for determining the PIC concentration in a marine environment.

[0012] An object of the present invention is to provide a method and an apparatus for determining the concentration of birefringent particles suspended in a fluid even when mechanical torsion and / or vibration are induced and the pressure (of water) changes.

[0013] An object of the present invention is to provide a method and an apparatus for determining the concentration of polymer particles in water such as seawater.

[0014] An object of the present invention is to provide a method and an apparatus for determining the concentration of birefringent particles suspended in a fluid using a circular polarizer, the method and the apparatus being insensitive to wavelength.

[0015] Another object of the present invention is to provide a method and an apparatus for determining the concentration of birefringent particles suspended in a fluid, wherein the signal recorded by a detector is obtained only from the light depolarized by a birefringent sample (e.g., a birefringent sample suspended in a fluid).

[0016] Yet another object of the present invention is to provide a non-invasive method for quantitatively or qualitatively analyzing a sample containing birefringent particles.

[0017] Furthermore, an object of the present invention is to provide a method and an apparatus for analyzing in-situ a sample containing birefringent particles that function autonomously, and particularly to provide a method and an apparatus for determining the concentration of birefringent particles suspended in a fluid (for example, in water).

[0018] Also, an object of the present invention is to provide a method and an apparatus for optimizing the detection of depolarized light caused by a sample containing birefringent particles.

[0019] According to a first aspect of the present invention, there is provided a method of fabricating a polarization filter matching pair including a first and a second polarization filter. The first polarization filter includes a first linear polarizer and a first quarter-wave optical retarder (first phase plate), and the second polarization filter includes a second linear polarizer and a second quarter-wave optical retarder (second phase plate). This method makes it possible to fabricate a polarization filter matching pair suitable for detecting depolarized light (for example, depolarized light caused by a sample containing birefringent particles suspended in a fluid). This method includes providing a light beam from a light source along a propagation axis (preferably, providing a parallel light beam), providing first, second, third, and fourth rotation stages (preferably, providing first, second, third, and fourth high-precision rotation stages) oriented perpendicular or substantially perpendicular to the propagation axis of the light beam, attaching a first linear polarizer having a first transmission axis to a first rotation stage having a first transmission axis at a specific position called the first position, attaching a second linear polarizer having a second transmission axis to the fourth rotation stage, rotating the second linear polarizer to obtain maximum extinction of the light beam (maximum extinction is obtained when the second transmission axis of the second linear polarizer is perpendicular to the first transmission axis of the first linear polarizer), The steps include inserting a first quarter-wavelength optical retarder having a first optical axis into a second rotating stage (so that the first optical axis of the first quarter-wavelength optical retarder is preferably oriented perpendicular or substantially perpendicular to the propagation axis of the light beam), To obtain maximum extinction of the light beam, the first quarter-wavelength optical retarder is rotated, A step of rotating the first optical axis of a first quarter-wavelength optical retarder in a first direction by a first angle, wherein the first angle is preferably about 45 degrees, more preferably 45 degrees ± 0.10 degrees, for example 45 degrees ± 0.05 degrees. The steps include: inserting a second quarter-wavelength optical retarder having a second optical axis into a third rotating stage (so that the second optical axis of the second quarter-wavelength optical retarder is preferably oriented perpendicular or substantially perpendicular to the propagation axis of the light beam), To obtain maximum extinction of the light beam, the second optical axis of the second quarter-wavelength optical retarder is rotated by a second angle in a second direction, wherein the second direction is opposite to the first direction of rotation of the first optical axis of the first quarter-wavelength optical retarder as viewed from the light source (it is clear that the signs of the first and second angles are opposite as viewed from the light source. When maximum extinction is reached, the second quarter-wavelength optical retarder is in the correct orientation. At this position, the second angle is 45 degrees (±0.05 degrees), but this method does not require measuring the second angle). The first step is to fix together the first linear polarizer and the first quarter-wavelength optical retarder in order to form the first polarizing filter, and the second step is to fix together the second linear polarizer and the second quarter-wavelength optical retarder in order to form the second polarizing filter (by fixing together the first linear polarizer and the first quarter-wavelength optical retarder, the relative positions of the first linear polarizer and the first quarter-wavelength optical retarder (in particular, the relative positions of the first transmission axis and the first optical axis) are fixed). Similarly, By fixing the second linear polarizer and the second quarter-wavelength optical retarder, the relative positions of the second linear polarizer and the second waveplate (in particular, the relative positions of the second transmission axis and the second optical axis) are fixed. The first linear polarizer and the first quarter-wavelength optical retarder, as well as the second linear polarizer and the second quarter-wavelength optical retarder, can be fixed by any technique known in the art. Preferred techniques for fixing them include bonding.

[0020] The first, second, third, and fourth rotating stages are arranged consecutively with respect to each other, with the first rotating stage being closest to the light source and the fourth rotating stage being furthest from the light source.

[0021] A linear polarizer is a device that selectively allows only a specific direction of plane-polarized light to pass through. In one direction, only vertically polarized light can pass through, and when rotated 90 degrees, only horizontally polarized light can pass through. Preferably, the linear polarizer used according to the present invention is 10 -5 Less than 5 x 10 -6 Less than 3 x 10 -6 Less than, or 1 × 10⁻⁶ -6 It has an extinction ratio of less than 0. The extinction ratio of a linear polarizer is the ratio of the polarizer's minimum transmittance to its maximum transmittance. The minimum transmittance occurs when the polarizer's transmission axis is perpendicular to the polarization plane of the incident polarized beam, and the maximum transmittance occurs when the polarizer's transmission axis is parallel to the polarization plane of the incident polarized beam.

[0022] An optical retarder is a polarization device designed to generate a specific phase difference between two orthogonal incident polarization states of an exit beam. A quarter-wavelength optical retarder converts linearly polarized light to circularly polarized light, or vice versa. The thickness of the optical retarder is preferably adjusted to produce a phase difference of one-quarter of the designed wavelength λ (or wavelength range Δλ). Preferably, the quarter-wavelength optical retarder used according to the present invention is of high quality. Preferably, the quarter-wavelength optical retarder used according to the present invention provides a retardation of λ / 4 ± λ / 350.

[0023] To fabricate a polarizing filter matching pair according to the present invention, a pair of quarter-wavelength optical retarders are selected such that the difference between the retardations of the first and second quarter-wavelength optical retarders does not exceed λ / 1000, preferably less than λ / 10000, with λ being the design wavelength.

[0024] A linear polarizer has a transmission axis, and a quarter-wavelength optical retarder has an optical axis. Preferably, the transmission axis of the linear polarizer and / or the optical axis of the quarter-wavelength optical retarder are oriented in a plane perpendicular or approximately perpendicular to the propagation axis of the light beam from the light source.

[0025] Preferably, the linear polarizer has a planar structure that defines a plane having the transmission axis of the linear polarizer embedded in / directed to the plane of the linear polarizer.

[0026] Preferably, the quarter-wavelength optical retarder of the polarizing filter has a planar structure that defines a plane having the optical axis of the quarter-wavelength optical retarder embedded in / directed to the plane of the quarter-wavelength optical retarder.

[0027] The first polarizing filter includes a first linear polarizer having a first transmission axis and a first quarter-wavelength optical retarder having a first optical axis, and the second polarizing filter includes a second linear polarizer having a second transmission axis and a second quarter-wavelength optical retarder having a second optical axis.

[0028] Preferably, the first transmission axis, the first optical axis, the second transmission axis, and the second optical axis are oriented toward a plane perpendicular or substantially perpendicular to the propagation axis of the light beam from the light source.

[0029] Preferably, the first transmission axis is embedded in the plane of the first linear polarizer, the first optical axis is embedded in the plane of the first quarter-wavelength optical retarder, the second transmission axis is embedded in the plane of the second linear polarizer, and the second optical axis is embedded in the plane of the second quarter-wavelength optical retarder.

[0030] The first optical axis and the first transmission axis define a first angle (first included angle), and the second optical axis and the second transmission axis define a second angle (second included angle). Preferably, the first and second angles are 45 degrees (preferably 45 degrees ± 0.05 degrees), and the first and second angles have opposite signs when viewed from the light source. For example, the first angle is +45 degrees (preferably +45 degrees ± 0.05 degrees), and the second angle is -45 degrees (preferably +45 degrees ± 0.05 degrees). In another embodiment, the first angle is -45 degrees (preferably -45 degrees ± 0.05 degrees), and the second angle is +45 degrees (preferably +45 degrees ± 0.05 degrees).

[0031] Preferably, the first polarizing filter of the polarizing filter matching pair polarizes the incident light to circularly polarized light of a first optical rotation, and the second polarizing filter of the polarizing filter matching pair polarizes the incident light to circularly polarized light of a second optical rotation, with the first and second optical rotations being opposite to those of the light source.

[0032] In the first example, the first polarizing filter can be configured to polarize the incident light to left circular polarization, and the second polarizing filter can be configured to polarize the incident light to right circular polarization. In another example, the first polarizing filter can be configured to polarize the incident light to right circular polarization, and the second polarizing filter can be configured to polarize the incident light to left circular polarization.

[0033] As the light source, any type of light source known in the art can be considered. The light source can include an unpolarized or polarized light source. When a polarized light source is used, it is obvious that it is preferable that the polarization direction of the light source is aligned with the first linear polarizer. The light source preferably includes a pulsed light source. A preferred light source is a (pulsed) light source within a wavelength range with minimal absorption by the fluid itself and / or by particles and other substances present in the sample, for example, a pulsed light source within a wavelength range with minimal absorption by particles and other substances present in a fluid such as water or seawater. The light beam from the light source preferably has a central wavelength within the wavelength range of 530 nm to 650 nm or within the wavelength range of 590 nm to 650 nm. The light beam has an emission wavelength band with a full width at half maximum (FWHM) of 50 nm or less, or more preferably 20 nm or less, but does not require a very narrow wavelength selection.

[0034] Preferably, the light source has a central wavelength of 530 nm to 650 nm (e.g., 590 nm to 650 nm) and a full width at half maximum of 50 mm (e.g., 20 mm). A particularly preferred example of the light source has a central wavelength of 650 nm and a full width at half maximum of 20 nm.

[0035] Preferred light sources include LED (light emitting diode) light sources (e.g., pulsed LED light sources (e.g., pulsed LED light sources having a full width at half maximum of 20 nm and a central wavelength of 650 nm)).

[0036] As the detector, any type of detector suitable for measuring the light from the light source, preferably any type of detector suitable for measuring pulsed light from the light source, can be considered.

[0037] Preferred detectors include silicon photodetectors such as amplified switchable gain silicon photodiode detectors.

[0038] The method for fabricating the polarization filter alignment pair according to the present invention is 10 -5This makes it possible to obtain polarizing filter matching pairs having an extinction ratio of less than 5 × 10. In a particular embodiment of the present invention, the extinction ratio is 5 × 10. -6 Less than 3 x 10 -6 Less than, or even 1 × 10 -6 A polarizing filter matching pair with a value less than is obtained. The extinction ratio of a polarizing filter matching pair is defined as the ratio of the intensity of light transmitted through the polarizing filter matching pair to the intensity of unpolarized (or polarized) light incident on the first polarizer.

[0039] The extinction ratio of circular polarizing filters is generally wavelength-sensitive and therefore cannot be used with light sources having a wavelength band (e.g., a 20 nm wavelength band), such as light-emitting diodes. However, the method for fabricating polarizing filter matching pairs according to the present invention makes it possible to obtain polarizing filter matching pairs that are unaffected by wavelength due to the geometric arrangement of the polarizing filters. The polarizing filter matching pairs preferably achieve a low extinction ratio in at least the wavelength range of the emission wavelength band defined by the full width at half maximum (FWHM) of the light beam of the light source.

[0040] Therefore, a polarizing filter matching pair obtained by the method for fabricating a polarizing filter matching pair according to the present invention can use a light-emitting diode as a light source. Furthermore, a method for analyzing a sample using such a polarizing filter matching pair and / or an apparatus including such a polarizing filter matching pair can also use a light-emitting diode as a light source.

[0041] As described above, the first quarter-wavelength optical retarder is rotated in the first direction by a first angle (more specifically, by an angle of 45 degrees). Preferably, the accuracy of the first angle is ±0.1 degrees, more preferably ±0.05 degrees. The second quarter-wavelength optical retarder is rotated by a second angle in the second direction opposite to the first direction to obtain maximum extinction of the light beam. Upon reaching maximum extinction, the correct orientation of the second quarter-wavelength optical retarder is achieved. In this method, it is not necessary to measure the second angle, but it is assumed that the second angle is 45 degrees. Preferably, the accuracy of the second angle is ±0.1 degrees, more preferably ±0.05 degrees.

[0042] One advantage of the method for fabricating polarizing filter matching pairs is that they have a self-correcting function. Slight deviations in the first angle are corrected by the rotation of the second quarter-wavelength optical retarder, resulting in maximum extinction.

[0043] Preferably, the retardation difference between the first quarter-wavelength optical retarder and the second quarter-wavelength optical retarder is 10 with respect to the wavelength (or wavelength range) under design. -3 It is less than . More preferably, the retardation difference between the first quarter-wavelength optical retarder and the second quarter-wavelength optical retarder is 10 with respect to the wavelength (or wavelength range) under design. -4 It is less than.

[0044] According to a second aspect of the present invention, a method is provided for analyzing a sample containing birefringent particles suspended in a fluid (e.g., a liquid such as water or seawater). This method is particularly suitable for determining the concentration of birefringent particles suspended in a fluid (e.g., a liquid such as water or seawater). This method, The steps include providing a light beam from a light source, The step of providing a polarizing filter matching pair comprising a first polarizing filter and a second polarizing filter, wherein the first polarizing filter comprises a first linear polarizer and a first quarter-wavelength optical retarder (first phase plate) and can be configured to polarize incident light to circularly polarized light having a first optical rotation as viewed from the light source, the second polarizing filter comprises a second linear polarizer and a second quarter-wavelength optical retarder (second phase plate) and can be configured to polarize light to circularly polarized light having a second optical rotation, the first optical rotation and the second optical rotation are opposite as viewed from the light source, and the polarizing filter matching pair comprises at least 10 -5 Having an extinction ratio of (more preferably, the polarizing filter matching pair is 5 × 10 -6 Less than 3 x 10 -6 Less than, or 1 × 10⁻⁶ -6 (Having an extinction ratio of less than ), step, The steps include introducing a sample containing birefringent particles suspended in a fluid (preferably suspended in a liquid) between a first polarizing filter and a second polarizing filter, The steps include passing a light beam through a first polarizing filter to generate a first light beam, The steps include bringing a sample into contact with the first light beam to generate a second light beam, The steps include passing a second light beam through a second polarizing filter to generate a third light beam, The process includes the step of measuring a third light beam with a detector.

[0045] The extinction ratio of a polarizing filter matching pair is defined as the ratio of the intensity of light transmitted through the polarizing filter matching pair to the intensity of unpolarized (or polarized) light incident on the first polarizer.

[0046] Preferably, a polarizing filter matching pair can be obtained by the above method for manufacturing a polarizing filter matching pair. Preferably, the first linear polarizer, second linear polarizer, first quarter-wavelength optical retarder, and second quarter-wavelength optical retarder described above are used as the first linear polarizer, second linear polarizer, first quarter-wavelength optical retarder, and second quarter-wavelength optical retarder in a configuration obtained by the above method for manufacturing a polarizing filter matching pair.

[0047] Any type of light source known in the art can be considered as a light source. The light source can include unpolarized or polarized light sources. When a polarized light source is used, it is obviously preferable that the polarization direction of the light source be aligned with the first linear polarizer. The light source preferably includes a pulsed light source. Preferred light sources include (pulsed) light sources in a wavelength range in which absorption by the fluid itself and / or by particles and other substances present in the sample is minimized, for example, a pulsed light source in a wavelength range in which absorption by particles and other substances present in a fluid such as water or seawater is minimized. The light beam from the light source preferably has a center wavelength in the wavelength range of 530 nm to 650 nm or the wavelength range of 590 nm to 650 nm. The light beam is an emission wavelength band having a full width at half maximum (FWHM) of 50 nm or less, or more preferably 20 nm or less, but does not require very narrow wavelength selection.

[0048] Preferably, the light source has a center wavelength of 530 nm to 650 nm (e.g., 590 nm to 650 nm) and a full width at half maximum of 50 nm (e.g., a full width at half maximum of 20 mm). A particularly preferred example of a light source has a center wavelength of 650 nm and a full width at half maximum of 20 nm.

[0049] Preferred light sources include LED (light-emitting diode) light sources (e.g., pulsed LED light sources (e.g., pulsed LED light sources having a full width at half maximum of 20 nm and a center wavelength of 650 nm)).

[0050] As a detector, any type of detector suitable for measuring light from a light source, preferably any type of detector suitable for measuring pulsed light from a light source, can be considered.

[0051] Preferred detectors include silicon photodetectors such as silicon photodiode detectors with amplified, switchable gain.

[0052] The third light beam corresponds to the depolarization light caused by the sample, and more specifically, to the depolarization light caused by birefringent particles suspended in a fluid (e.g., a liquid). Therefore, when the detector detects the third beam, this means that the sample being analyzed is a birefringent sample (e.g., a sample containing birefringent particles suspended in a fluid (e.g., a liquid such as water or seawater)). In certain embodiments of the present invention, the third beam detected by the detector makes it possible to determine the concentration of birefringent particles suspended in a fluid (e.g., a liquid such as seawater).

[0053] Due to the configuration of polarizing filter matching pairs and / or the low extinction ratio of polarizing filter matching pairs, methods for analyzing birefringent samples (e.g., samples containing birefringent particles suspended in a fluid such as a liquid) can determine the concentration of birefringent particles even when the concentration of birefringent particles suspended in the fluid is low (e.g., less than 50 micromoles / L, less than 10 micromoles / L, less than 1 micromoles / L, 0.1 micromoles / L, less than 0.01 micromoles / L, e.g., 0.005 micromoles / L).

[0054] Birefringent particles include, for example, calcium carbonate, quartz, celestite, barite, kaolinite, chlorite, illite, vermiculite, orthoclase, plagioclase, montmorillonite, plastics (also known as microplastics), or combinations thereof.

[0055] The particles preferably have a size in the range of 1 μm to 5 mm, for example, in the range of 2 μm to 3 mm or 2 μm to 1 mm (for example, 10 μm, 100 μm, 200 μm, or 500 μm).

[0056] This method is suitable for analyzing the concentration of calcium carbonate in water (for example, seawater).

[0057] Furthermore, this method is suitable for analyzing the presence or concentration of polymer particles in water.

[0058] A third aspect of the present invention provides an apparatus for analyzing a sample containing birefringent particles. The apparatus comprises a light source that emits a light beam along a propagation axis, a polarizing filter matching pair including a first polarizing filter and a second polarizing filter, and a detector. The light source, the polarizing filter matching pair, and the detector are arranged such that the light beam emitted by the light source can then pass through the first polarizing filter, collide with the sample to be analyzed, and pass through the second polarizing filter before being detected by the detector. The first polarizing filter is preferably located near the light source, and the second polarizing filter is preferably located near the detector. The first polarizing filter includes a first linear polarizer and a first quarter-wavelength optical retarder and can be configured to polarize incident light to circularly polarized light of a first optical rotation as viewed from the light source. The second polarizing filter includes a second linear polarizer and a second quarter-wavelength optical retarder and can be configured to polarize incident light to circularly polarized light of a second optical rotation, where the first and second optical rotations are opposite as viewed from the light source. The polarizing filter matching pair is preferably 10 -5 It has an extinction ratio of less than 5 × 10. More preferably, the polarizing filter matching pair is 5 × 10 -6 Less than 3 x 10 -6 Less than, or even 1 × 10⁻⁶ -6 It has a quenching ratio of less than 1.

[0059] Preferably, a polarizing filter matching pair can be obtained by the above-described method for manufacturing a polarizing filter matching pair. Preferably, the first linear polarizer, second linear polarizer, first quarter-wavelength optical retarder, and second quarter-wavelength optical retarder described above are used as the first linear polarizer, second linear polarizer, first quarter-wavelength optical retarder, and second quarter-wavelength optical retarder in the configuration obtained by the above-described method for manufacturing a polarizing filter matching pair.

[0060] Any type of light source known in the art can be considered as a light source. The light source can include unpolarized or polarized light sources. When a polarized light source is used, it is obviously preferable that the polarization direction of the light source be aligned with the first linear polarizer. The light source preferably includes a pulsed light source. Preferred light sources include (pulsed) light sources within a wavelength range in which absorption by particles and other substances present in the sample is minimized, for example, pulsed light sources within a wavelength range in which absorption by particles and other substances present in a fluid such as water or seawater is minimized. The light beam from the light source preferably has a center wavelength in the wavelength range of 530 nm to 650 nm or 590 nm to 650 nm. The light beam is an emission wavelength band having a full width at half maximum (FWHM) of 50 nm or less, or more preferably 20 nm or less, but does not require very narrow wavelength selection.

[0061] Preferably, the light source has a central wavelength of 530 nm to 650 nm (e.g., 590 nm to 650 nm) and a full width at half maximum of 50 mm (e.g., a full width at half maximum of 20 mm). A particularly preferred example of a light source has a central wavelength of 650 nm and a full width at half maximum of 20 nm.

[0062] Preferred light sources include LED (light-emitting diode) light sources (e.g., pulsed LED light sources (e.g., pulsed LED light sources having a full width at half maximum of 20 nm and a center wavelength of 650 nm)).

[0063] As a detector, any type of detector suitable for measuring light from a light source, preferably any type of detector suitable for measuring pulsed light from a light source, can be considered.

[0064] Preferred detectors include silicon photodetectors such as silicon photodiode detectors with amplified, switchable gain.

[0065] The apparatus according to the present invention enables the measurement of light depolarized by a sample containing birefringent particles. Therefore, this apparatus is suitable for qualitative or quantitative analysis of samples containing birefringent particles.

[0066] In particular, the apparatus according to the present invention makes it possible to measure the depolarization rate (i.e., the percentage of circularly polarized light transmitted by the first polarizing filter, which is depolarized by birefringent particles suspended in a fluid (e.g., water), and then passes through the second polarizing filter and hits the detector).

[0067] In a preferred embodiment, the apparatus according to the present invention comprises a transmittance meter. A transmittance meter is defined as an instrument for measuring the transmittance of light passing through a fluid (e.g., air or water). The sample to be analyzed typically has a length (path length) of at least 5 cm (e.g., 7 cm, 10 cm, or 15 cm). The volume of the sample to be analyzed is given by path length x π x (beam diameter / 2) 2 The volume is typically at least 3 mL (for example, at least 12 mL). The beam diameter is, for example, 8.5 mm.

[0068] In principle, the apparatus according to the present invention (for example, the transmittance meter according to the present invention) enables the analysis of all types of birefringent samples. The apparatus according to the present invention is particularly suitable for analyzing samples containing birefringent particles (for example, birefringent particles suspended in a fluid such as a gas or liquid). Particularly preferred birefringent samples include those having birefringent particles suspended in a liquid (for example, birefringent particles suspended in water such as seawater).

[0069] For a sample containing birefringent particles suspended in a fluid (e.g., a liquid), the apparatus according to the present invention makes it possible to determine the concentration of birefringent particles in the fluid (e.g., a liquid such as water or seawater).

[0070] The apparatus according to the present invention is suitable for determining the concentration of PIC in seawater. PIC contains biological particles and includes polymorphs of both calcite and aragonite, which are calcium carbonate (CaCO3) with extreme birefringence properties. The apparatus according to the present invention makes it possible to determine the concentration of PIC in global marine environments, including nutrient-poor areas and the deep sea.

[0071] Furthermore, the apparatus according to the present invention is suitable for analyzing the presence or concentration of polymer particles in a liquid (for example, water).

[0072] Due to the configuration of polarizing filter matching pairs and / or the low extinction ratio of polarizing filter matching pairs, instruments for analyzing birefringent samples (e.g., samples containing birefringent particles suspended in a fluid such as a liquid) can determine the concentration of birefringent particles even when the concentration of birefringent particles suspended in the fluid is low (e.g., less than 100 micromoles / L, less than 50 micromoles / L, less than 10 micromoles / L, less than 1 micromoles / L, less than 0.1 micromoles / L, less than 0.01 micromoles / L, e.g., 0.005 micromoles / L).

[0073] Preferably, the first polarizing filter and / or the second polarizing filter of the apparatus according to the present invention are rotatable and / or removable. Preferably, the first polarizing filter and / or the second polarizing filter are removable.

[0074] In a preferred embodiment, the second polarizing filter (i.e., the filter before the detector of the device) is removable, so that the device has the additional advantage of being able to measure polarized and unpolarized transmissions in the same device. In one such embodiment, a gain switch can be used to change the gain of the detector depending on whether polarized or unpolarized transmission is being recorded. Alternatively, a neutral density filter can be inserted in place of the removed polarizer to reduce the unpolarized signal to the same amplitude range as the polarized signal.

[0075] Optionally, the apparatus according to the present invention is: Beam splitters (e.g., polarizing beam splitters), and / or, One or more baffles (for example, for protecting a birefringent sample or a zone containing a birefringent sample from incident light (for example, from direct sunlight and / or skylight and / or diffused underwater background light). In a preferred embodiment, the apparatus comprises one or more baffles for protecting the zone containing the birefringent sample and the window surrounding the zone from incident light.), and / or, One or more pressure windows (preferably one or more pressure windows containing a material that does not depolarize as much as possible under pressure stress; preferred pressure windows include amorphous SiO2 or coated Schott Glass SF57 pressure windows), and / or, One or more spectral filters (for blocking background incident light while allowing a light beam (e.g., a collimated beam from a light source) to pass through), and / or One or more lenses (e.g., one or more collimating lenses), and / or It includes one or more additional components, such as one or more precision pinholes.

[0076] During use (especially in bodies of water exposed to sunlight), the apparatus according to the present invention is preferably oriented so that the propagation axis of the light beam from the light source is vertical and the light source is directed upward. The detector is preferably directed to detect the light beam directed upward.

[0077] The apparatus according to the present invention has many advantages compared to apparatus known in the art.

[0078] The first advantage is low sensitivity to mechanical torsion. The sensitivity of the polarizing filter of the apparatus according to the present invention to mechanical torsion is at least an order of magnitude lower, preferably at least two orders of magnitude lower, than the sensitivity of a linear polarizer to mechanical torsion, such as that described in US7030981.

[0079] A second advantage of the apparatus according to the present invention is that the decirculation signal from a birefringent sample (for example, from birefringent particles) is twice as strong as the decirculation signal from a linearly polarized sample.

[0080] The maximum extinction ratio of circular polarizers cited by commercial manufacturers is 2.0 × 10⁻⁶. -3 Therefore, this does not enable the detection of depolarization of light by birefringent particles in most marine environments, but the apparatus according to the present invention is 1.0 × 10 -5 , 5.0×10 -6 , 3 x 10 -6 , or 1.0 × 10 6 It has a quenching ratio. As a result, the detection limit of the apparatus according to the present invention is increased by at least two orders of magnitude compared to apparatuses known in the art.

[0081] The extinction ratio of circular polarizing filters is generally sensitive to wavelength, and therefore cannot use light-emitting diodes as a light source. However, the apparatus according to the present invention is not sensitive to wavelength due to the geometric arrangement of the polarizing filters. Therefore, the apparatus according to the present invention can use light-emitting diodes as a light source.

[0082] The apparatus according to the present invention can be used as a sensor (for example, as a sensor for detecting the presence, amount, or concentration of birefringent materials (for example, the presence, amount, or concentration of birefringent particles in a fluid (e.g., water or seawater))). A preferred application of the apparatus according to the present invention is as a calcium carbonate sensor. Another preferred application of the apparatus according to the present invention is as a sensor for detecting the presence and / or determining the concentration of plastics (e.g., microplastics in a fluid such as water).

[0083] The apparatus according to the present invention functions as a transmittance meter. The transmittance meter can be used to measure the transmittance and depolarization of light. [Brief explanation of the drawing]

[0084] The present invention will be described in more detail below with reference to the accompanying drawings.

[0085] [Figure 1] This is a schematic diagram of the setup for creating polarizing filter matching pairs. [Figure 2] This is a schematic diagram of the configuration of the apparatus for analyzing birefringent samples according to the present invention. [Figure 3] The extinction ratio as a function of a second angle for two different polarizing filter matching pairs according to the present invention is shown. [Modes for carrying out the invention]

[0086] The present invention is described with reference to specific drawings relating to certain embodiments, but the present invention is not limited thereto and is limited only by the claims. The drawings are for illustrative purposes only and are not limiting. The sizes of some elements in the drawings may be exaggerated and may not be drawn to scale for illustrative purposes. Dimensions and relative dimensions do not correspond to actual reductions for the implementation of the present invention.

[0087] When referring to the end point of a range, the value of the end point of the range is included.

[0088] When describing the present invention, terms used shall be interpreted according to the following definitions unless otherwise indicated.

[0089] When listing two or more items, the term "and / or" means that any one of the listed items can be used alone, or any combination of two or more of the listed items can be used.

[0090] The terms "first," "second," etc., used herein and in the claims are used to distinguish similar elements and are not necessarily intended to describe a temporal, spatial, sequential, or otherwise orderly sequence. The terms used in this manner are interchangeable under appropriate circumstances, and it should be understood that embodiments of the invention described herein may operate in an order other than those described or illustrated herein.

[0091] The term "birefringence" refers to the optical property of a material (corresponding to two different refractive indices in a crystal) that splits a beam of light into two beams with unequal velocities, which then recombine to form a beam of light that is no longer linearly polarized.

[0092] The term "particle" refers to any small fragment of any type of material, regardless of the shape of such fragment. The term "particle" can refer to a single particle or a group of particles.

[0093] The terms "analyze" or "analyze" refer to qualitative and / or quantitative measurements or analyses, such as measuring the presence or absence of birefringent material and / or measuring the amount or concentration of birefringent material. In particular, the term "analyze" refers to measuring the presence or absence of birefringent particles and / or determining the concentration of birefringent particles.

[0094] In preferred embodiments of the present invention, the terms “analyze” or “analyze” refer to determining the concentration of particles suspended in a fluid, for example, determining the concentration of particles suspended in a liquid.

[0095] The term "fluid" refers to a medium such as a gas or a liquid. Preferred liquids include water, such as seawater.

[0096] Figure 1 shows a method for optimizing setup 100 to obtain optimal detection of depolarized light caused by a birefringent sample by fabricating a polarizing filter matching pair with a low extinction ratio (high rejection ratio).

[0097] First, a light beam 104 is generated from a light source 102. The light beam 104 is directed along an axis 101 called the propagation axis of the light beam 104. A preferred light source 102 for generating the light beam 104 includes an LED light source. The light beam 104 preferably has a narrow spectral band (e.g., a spectral band with a full width at half maximum of 20 nm or less). For example, the light beam 104 has a spectral band with a central wavelength of 645 nm and a full width at half maximum of 20 nm or less.

[0098] The light beam 104 is preferably a parallel or substantially parallel light beam obtained, for example, using two lenses 136, 120 and a pinhole 122.

[0099] Setup 100 comprises four high-precision rotating stages, the first, second, third, and fourth high-precision rotating stages 110, 112, 114, and 116, respectively. The high-precision rotating stages 110, 112, 114, and 116 are oriented perpendicular or substantially perpendicular to the propagation axis 104 of the light beam 104. Setup 100 further comprises a detector 118, which includes, for example, a silicon photodiode detector. Preferably, a focusing lens (collimating lens) 140 and / or a precision pinhole 142 are provided between the fourth high-precision rotating stage 116 and the detector 118.

[0100] A first linear polarizer 124 and a second linear polarizer 126 are mounted on the first high-precision rotating stage 110 and the fourth high-precision rotating stage 116, respectively. The angle of the transmission axis of the first linear polarizer 124 is noted. The fourth rotating stage 116 is then rotated until maximum extinction (minimum transmission) is obtained. At that point, the transmission axes of the first linear polarizer 124 and the transmission axes of the second linear polarizer 126 are at a 90-degree angle to each other.

[0101] Subsequently, the first quarter-wavelength optical retarder (first phase plate) 128 is inserted into the second high-precision rotating stage 112. The second high-precision rotating stage 112 is rotated to obtain maximum extinction (minimum transmission). At this point, the optical axis of the first quarter-wavelength optical retarder 128 is precisely parallel to the polarization axis of the first linear polarizer 124. Subsequently, the optical axis of the first quarter-wavelength optical retarder 128 is rotated by 45 degrees with respect to the light source 102. The precision of this rotation depends on the mechanical precision of the high-precision rotating stage and can easily be reduced to less than a few milliradians.

[0102] When the first quarter-wavelength optical retarder 128 is set to 45 degrees, the second quarter-wavelength optical retarder (second phase plate) 130 is mounted on the third high-precision rotating stage 114, directly in front of the second linear polarizer 126. The second quarter-wavelength optical retarder 130 is rotated in the opposite direction to the first quarter-wavelength optical retarder 128 until maximum extinction (minimum transmission) is again obtained. At this point, as viewed from the light source 102, the optical axis of the second quarter-wavelength optical retarder 130 is at a -45 degree angle to the transmission axis of the linear polarizer 126.

[0103] At that point, a setup is obtained that includes circularly polarized filter matching pairs 132 and 134 having the maximum mutual rejection ratio. The linear polarizers and quarter-wavelength optical retarders for each filter pair 132, 134, held on the rotating stage, are then fixed together (e.g., bonded together). Preferably, the axis and sides of the linear polarizers are marked so that each filter assembly can be readjusted when finally mounted (e.g., to a transmittance meter).

[0104] Figure 1A shows a preliminary setup of the first linear polarizer 124 and the first quarter-wavelength optical retarder 128. Figure 1B shows the final setup of the first polarizer 124 and the first quarter-wavelength optical retarder 128 after assembly and fixation (e.g., bonding).

[0105] Since the light-receiving angle of the transmittance meter is at least about 2 degrees, it is preferable that the angular dependence of the polarizing filters 132 and 134 be low. This can be achieved by using a true zero-order waveplate (i.e., a waveplate containing a single layer of polarizing material (either polymer material or uniaxial crystal) bonded to an amorphous substrate). Such a true zero-order waveplate has the smallest achievable retardation variation angle.

[0106] Setup 100 may further include a graduated iris 138 positioned in front of the first polarizing filter 132. Furthermore, Setup 100 may include a lens 136 (imaging lens) between the light source 102 and the precision pinhole 122, and / or a lens 140 (focusing lens) positioned between the fourth high-precision rotating stage 116 and the detector 118, and / or a precision pinhole 142 positioned between the fourth high-precision rotating stage 116 and the detector 118 (preferably between lens 140 and detector 118).

[0107] The assembly process for the polarizing filters 132 and 134 described above has the advantage of self-correction.

[0108] The best removal ratio (extinction ratio) achievable with the proposed process is determined by the maximum removal ratio of the intersecting linear polarizers and the angular precision of the rotating stage used in the fabrication process. Preferably, one or more components of the setup are coated with an anti-reflective coating to minimize potential interactions. Most preferably, all components of the setup are coated with an anti-reflective coating.

[0109] Figure 2 shows a schematic diagram of the apparatus 200 according to the present invention. The apparatus 200 is suitable for measuring the depolarization of circularly polarized light by a birefringent sample 201 (for example, a sample containing birefringent particles suspended in water). The apparatus 200 is particularly suitable as a transmittance meter. The apparatus (transmittance meter) measures the depolarization rate (i.e., the percentage of circularly polarized light transmitted by the first polarizing filter that is depolarized by birefringent particles suspended in the fluid (e.g., water) in the sample section of the apparatus, and therefore passes through the second polarizing filter and hits the detector). The apparatus measures, for example, 0.005 mmol of CaCO3 m for a path length of 15 cm. -3 Approximately equal to 3 × 10 -6 m -1 It has a detection limit for depolarization rates lower than that.

[0110] The apparatus 200 comprises a radiation section, a sample section, and a light-receiving section. The radiation section comprises a light source 202 and a first polarizing filter (first circular polarizer) 204 having a first optical rotation. The sample section comprises a sample holder (e.g., a column for receiving and / or holding a sample (e.g., water containing birefringent particles)). The light-receiving section comprises a second polarizing filter (second circular polarizer) 206 having a second optical rotation opposite to the first optical rotation as seen from the light source, and a detector 208.

[0111] Each of the first and second polarizing filters 204 and 206 comprises a linear polarizer and a quarter-wavelength optical retarder. The first polarizing filter 204 comprises a first linear polarizer having a first transmission axis and a first quarter-wavelength optical retarder having a first optical axis. The second polarizing filter 206 comprises a second linear polarizer having a second transmission axis and a second quarter-wavelength optical retarder having a second optical axis. Preferably, the first linear polarizer, the first quarter-wavelength optical retarder, the second linear polarizer, and the second quarter-wavelength optical retarder are oriented such that their planes are perpendicular to the propagation axis of the light beam. Preferably, the first transmission axis and the second transmission axis are oriented perpendicular to each other. The angle defined by the first optical axis and the first transmission axis (enclosed angle), and the angle defined by the second optical axis and the second transmission axis (enclosed angle) are preferably equal or approximately equal, and most preferably equal to 45 degrees. The angle between the first optical axis and the first transmission axis and the angle between the second optical axis and the second transmission axis are preferably opposite in sign when viewed from the light source. For example, the angle between the first optical axis and the first transmission axis is +45 degrees, and the angle between the second optical axis and the second transmission axis is -45 degrees.

[0112] The light beam 203 emitted from the light source 202 then passes through the first polarizing filter 204, collides with the sample 201, and passes through the second polarizing filter 206 before being detected by the detector 208.

[0113] The light source 202 includes, for example, an LED light source that generates a pulsed light beam 203 along the propagation axis 205. The light beam 202 preferably has a narrow spectral band, preferably a spectral band with a center wavelength of 645 nm and a full width at half maximum of 20 nm or less. The light beam 203 preferably passes through a pinhole 209.

[0114] Preferably, the light beam 203 is collimated into a parallel or substantially parallel beam by the collimating lens 210.

[0115] The parallelism of the beam is determined by the ratio of the diameter of the pinhole 209 to the focal length of the collimating lens 210.

[0116] Subsequently, it may be preferable that a portion of the light be extracted by the beam splitter 212. The portion 213 deflected by the beam splitter 212 can illuminate a reference detector 214, which is used as a monitor for the intensity of the light source 202.

[0117] In contrast to transmittance meters known in the art, the transmittance meter according to the present invention preferably has a polarizing beam splitter 212 that deflects linearly polarized light in the opposite direction to the direction of the first polarizing filter 204. This ensures that the maximum available light is delivered to the water column through the first polarizing filter 204. Since the LED is unpolarized, the light deflected by the beam splitter 212 allows for accurate monitoring of the light transmitted through the first polarizing filter 204.

[0118] The apparatus 200 preferably comprises one or more pressure windows 216, 220. The material of the pressure windows is preferably carefully selected so as to minimize the amount of depolarization caused by stress. Coated SF57 glass stress-relieving amorphous SiO2 (amorphous quartz) is a suitable material for this purpose.

[0119] Unpolarized background light from sunlight and sky light passing through the water surface is preferably reduced to a level at which the detector 208 can operate. Therefore, the apparatus 200 is preferably used in a vertical configuration in which the light source radiates light upward and the detector faces downward. In this orientation, the background light entering the detector 208 comes from sunlight and sky light illuminating the radiating window and support structure, which is within the detector's field of view and diffusely reflected by the window and support structure. For this reason, it is preferable that the metal components of the radiating section within the field of view are black (e.g., black anodized).

[0120] Even with these precautions in place, it is preferable to further shield the area of ​​the radiating section visible to the detector from direct sunlight and skylight. This can be achieved by the careful use and positioning of light baffles 218 that obscure the sensing area. These baffles 218 are preferably kept small and thin enough not to obstruct the free lateral flow of water through the water section. The key here is the existence of a limiting light cone through which radiation coming from above penetrates the water surface. This cone defines the limit of the image of the sky underwater that can illuminate the radiating surface. This limiting angle is 50 degrees. Water waves distort the cone, spreading the illumination to an angle of 60 degrees. By arranging a minimum number of baffles, it is preferably ensured that no portion of this descending light reaches the visible surface of the radiating section. Direct shielding from the sun and sky can be achieved with a small number of baffles placed around the radiating and receiving sections. However, there may be another unpolarized light source in the water column coming from rising light backscattered by particles in the water. Shielding from this light source requires a larger set of baffles spaced along the entire length of the instrument's open water column. Note that even in this case, it is still possible to achieve free lateral flow through the measurement column.

[0121] Preferably, the first polarizing filter 204 is the last element before the pressure window 216. In this way, it is ensured that the depolarization of circularly polarized light originating from the first polarizing filter 204 does not occur anywhere other than the water sample.

[0122] The light-receiving section preferably includes a pressure window 220 (for example, a stress-relieving amorphous quartz window immediately followed by a second polarizing filter 206 having the opposite optical rotation to the first polarizing filter 204 used in the radiating section). The axes of the linear polarization subcomponents of these circular filters must be carefully oriented perpendicular to each other so that the rejection ratio is maintained across the entire wavelength range of the light source.

[0123] Preferably, the apparatus includes a narrow spectral bandwidth optical filter 222 positioned immediately after the second polarizing filter 206. By introducing the spectral bandwidth optical filter 222, the background light passing through the detector 208 is reduced.

[0124] Preferably, the out-of-band optical density of filter 222 is 10 over the wavelength range of 200 nm to 1200 nm or greater. -4 The OD is (OD-4) or higher, ensuring the maximum amount of background light removal. Preferably, the light-receiving section further comprises a lens (collimating lens) 224 and / or a precision pinhole 226. The lens 224 is preferably positioned after the second polarizing filter 206 and after the narrowband spectral filter 222.

[0125] The water column depolarization signal acquisition angle is set by the ratio of the diameter of the pinhole 226 to the focal length of the lens 220.

[0126] Figure 3 shows the extinction ratios obtained using two different polarizing filter matching pairs, each constructed according to the manufacturing method of the present invention. The obtained extinctions are plotted as a function of a second angle (i.e., the angle between the optical axis of the second quarter-wavelength optical retarder and the transmission axis of the second linear polarizer) (in particular, as a function of the deviation of the second angle from 45 degrees (±0.05 degrees)).

[0127] Figure 3 shows that the intended extinction ratio is achieved with the polarizing filter matching pair according to the present invention. The first polarizing filter matching pair is 1.84 × 10⁻⁶ -6 It reaches an extinction ratio of 2.55 × 10⁻¹⁰, and other polarizing filter matching pairs are 2.55 × 10⁻¹⁰. -6 The extinction ratio was reached. The minimum extinction ratio is achieved when the second quarter-wavelength optical retarder is oriented correctly. At this position, the second angle is 45 degrees (±0.05 degrees). As mentioned above, in the method of fabricating polarizing filter matching pairs, it is not necessary to measure the second angle. From Figure 3, it is clear that the minimum extinction ratio is achieved by rotating the second optical axis of the second quarter-wavelength optical retarder before fixing the various components. [Explanation of symbols]

[0128] 100 setups 101 Propagation axis of a light beam 102 Light source 104 Light beam 110 First high-precision rotating stage 112 Second high-precision rotating stage 114 Third high-precision rotating stage 116. Fourth high-precision rotating stage 118 detectors 120 Focusing Lens 122 Precision Pinholes 124 First linear polarizer 126 Second Linear Polarizer 128 First quarter-wavelength optical retarder 130 Second quarter-wavelength optical retarder 132 First Polarizing Filter 134 Second Polarizing Filter 136 Imaging lens 138 Graduated Iris 140 Focusing Lens 142 Precision Pinholes 200 equipment 201 samples 202 Light source 203 Light beam 204 First Polarizing Filter 205 Propagation axis of the light beam 206 Second Polarizing Filter 208 detectors 209 Precision Pinholes 210 Collimating Lens 212 Polarizing Beam Splitter 214 Reference signal detector 216 Pressure window 218 Baffle 220 Pressure window 222 Narrowband Spectral Filter 224 Collimating Lens 226 Precision Pinholes

Claims

1. A method for producing a polarizing filter matching pair including first and second polarizing filters (132, 134), wherein the first polarizing filter (132) includes a first linear polarizer (124) and a first quarter-wavelength optical retarder (128), and the second polarizing filter (134) includes a second linear polarizer (126) and a second quarter-wavelength optical retarder (130), and the method is as follows: The steps include providing a light beam (104) from a light source (102) along the propagation axis (101), The steps include providing first (110), second (112), third (114), and fourth rotating stages (116) oriented perpendicular to the propagation axis of the light beam, A step of attaching a first linear polarizer (124) having a first transmission axis to the first rotating stage at a first position, The steps include attaching a second linear polarizer (126) having a second transmission axis to the fourth rotating stage, To obtain the maximum extinction of the light beam, the second linear polarizer (126) is rotated, The steps include inserting a first quarter-wavelength optical retarder (128) having a first optical axis into the second rotating stage, To obtain the maximum extinction of the light beam, the first quarter-wavelength optical retarder (128) is rotated, A step of rotating the first optical axis of the first quarter-wavelength optical retarder (128) in a first direction over a first angle, wherein the first angle is 45 degrees, preferably 45 degrees ± 0.10 degrees, The steps include inserting a second quarter-wavelength optical retarder (130) having a second optical axis into the third rotating stage, A step of rotating the second optical axis of the second quarter-wavelength optical retarder (130) by a second angle in a second direction in order to obtain the maximum extinction of the light beam, wherein the second direction is opposite to the first direction of rotation of the first optical axis of the first quarter-wavelength optical retarder (128) as seen from the light source (102), A method comprising the steps of fixing together the first linear polarizer (124) and the first quarter-wavelength optical retarder (128) to form the first polarizing filter (132), and fixing together the second linear polarizer (126) and the second quarter-wavelength optical retarder (130) to form the second polarizing filter (134).

2. The method according to claim 1, wherein the first polarizing filter has a first optical rotation, the second polarizing filter has a second optical rotation, and the first and second optical rotations are opposite to those of the light source.

3. The polarizing filter matching pair is 10 －５ The method according to claim 1 or 2, having a lower extinction ratio than [a certain value].

4. A method for analyzing a sample containing birefringent particles suspended in a fluid, A step of preparing a polarizing filter matching pair including a first polarizing filter and a second polarizing filter according to the method of claim 1, The steps include providing a light beam from a light source, The steps include introducing a sample (201) containing birefringent particles suspended in a fluid between the first polarizing filter (204) and the second polarizing filter (206), The steps include passing the aforementioned light beam through the first polarizing filter to generate a first light beam, The steps include bringing the sample into contact with the first light beam to generate a second light beam, The steps include passing the second light beam through the second polarizing filter to generate a third light beam, A method comprising the step of measuring the third light beam with a detector (208).

5. The method according to claim 4, wherein the analysis includes the step of determining the concentration of the birefringent particles suspended in the fluid.

6. The method according to claim 4 or 5, wherein the polarizing filter matching pair is obtained by the method specified in claim 1 or 2.

7. The method according to any one of claims 4 to 6, wherein the birefringent particles include calcium carbonate, quartz, celestite, barite, kaolinite, chlorite, illite, vermiculite, orthoclase, plagioclase, montmorillonite, plastic, or a combination thereof.

8. The method according to any one of claims 4 to 7, wherein the fluid includes water or seawater.

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