# US 2023/0332995 A1 · A method to produce a matched pair of polarizing filters and a method and apparatus to determine the concentration of birefringent particles using a pair of polarizing filters > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2023/0332995 A1 - Document ID: US20230332995A1 - Status: Published application - Kind: A1 - Application number: 18/013,110 - Filed: 2021-06-29 - Published: 2023-10-19 - Earliest claimed: 2020-06-30 - Assignees: Universiteit Gent; His Majesty the King In Right of Canada, AS Represented By the Minister of National Defence - Applicants: Universiteit Gent; His Majesty the King In Right of Canada, AS Represented By the Minister of National Defence - Inventors: Griet NEUKERMANS; Georges FOURNIER - Web page: https://patentgrep.com/patent/US20230332995A1 ## CPC - G01N 15/06: Investigating concentration of particle suspensions (by weighing G01N5/00; investigating sedimentation of particle suspensions G01N15/04; investigating individual particles G01N15/10) - G01N 21/23: Bi-refringence - G01N 33/18: Water - G02B 27/288: {Filters employing polarising elements, e.g. Lyot or Solc filters (G02B5/3016 takes precedence)} - G01J 4/04: Polarimeters using electric detection means (G01J4/02 takes precedence) - G01N 15/075: by optical means - G01N 21/534: {by measuring transmission alone, i.e. determining opacity} - G01N 2015/0053: {in liquids, e.g. trouble} - G01N 2021/216: {using circular polarised light} - G01N 2021/217: {Measuring depolarisation or comparing polarised and depolarised parts of light} - G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q) - G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS - G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N) ## Abstract A method to produce a matched pair of polarizing filters includes the mounting of a first linear polarizer and a second linear polarizer in a beam of light, rotating the second linear polarizer to obtain maximum extinction of the beam of light, inserting a first quarter-wave optical retarder in the beam of light and rotating the first quarter-wave optical retarder to obtain maximum extinction of the beam of light, subsequently rotating the first quarter-wave optical retarder over an angle of 45 degrees, inserting a second quarter-wave optical retarder, rotating the second quarter-wave optical retarder to obtain maximum extinction of the beam of light before securing the first linear polarizer, the first quarter-wave optical retarder and the second linear polarizer and the second quarter-wave optical retarder. A method and apparatus to analyze a sample include birefringent particles suspended in a fluid. ## Claims ### Claim at position 1 1-15. (canceled) ### Claim 16 A method to produce a matched pair of polarizing filters comprising a first and a second polarizing filter, said first polarizing filter comprising a first linear polarizer and a first quarter-wave optical retarder and said second polarizing filter comprising a second linear polarizer and a second quarter-wave optical retarder, said method comprising the steps of: providing a beam of light from a light source along a propagation axis; providing a first, second, third and fourth rotation stage oriented perpendicular to said propagation axis of the beam of light; mounting a first linear polarizer having a first transmission axis in said first rotation stage in a first position; mounting a second linear polarizer having a second transmission axis in said fourth rotation stage; rotating said second linear polarizer to obtain maximum extinction of said beam of light; inserting a first quarter-wave optical retarder having a first optical axis in said second rotation stage; rotating said first quarter-wave optical retarder to obtain maximum extinction of said beam of light; rotating said first optical axis of said first quarter-wave optical retarder in a first direction over a first angle, said first angle being 45 degrees, plus or minus 0.10 degrees; inserting a second quarter-wave optical retarder having a second optical axis in said third rotation stage; rotating said second optical axis of said second quarter-wave optical retarder in a second direction over a second angle to obtain maximum extinction of said beam of light, said second direction being opposite to said first direction of said rotation of said first optical axis of said first quarter-wave optical retarder as viewed from said light source; securing said first linear polarizer and said first quarter-wave optical retarder together to form said first polarizing filter and securing said second linear polarizer and said second quarter-wave optical retarder together to form said second polarizing filter. ### Claim 17 (depends on 16) The method according to claim 16, wherein said first polarizing filter has a first handedness sense and said second polarizing filter has a second handedness sense, with said first handedness sense and said second handedness sense being opposite as viewed from said light source. ### Claim 18 (depends on 16) The method according to claim 16, wherein said matched pair of polarizing filters has an extinction ratio lower than 10−5. ### Claim 19 A method to analyze a sample comprising birefringent particles suspended in a fluid, said method comprising the steps of: providing a beam of light from a light source; providing a matched pair of polarizing filters comprising a first polarizing filter and a second polarizing filter, said first polarizing filter comprising a first linear polarizer and a first quarter-wave optical retarder and being configurable to polarize incident light into circularly polarized light having a first handedness sense viewed from the light source and said second polarizing filter comprising a second linear polarizer and a second quarter-wave optical retarder and being configurable to polarize light into circularly polarized light having a second handedness sense, with said first handedness sense and said second handedness sense being opposite viewed from the light source, said matched pair of polarizing filters having an extinction ratio of at least 10−5; introducing a sample comprising birefringent particles suspended in a fluid between said first polarizing filter and said second polarizing filter; passing said beam of light through said first polarizing filter, thereby creating a first beam of light; contacting said sample with said first beam of light thereby creating a second beam of light; passing said second beam of light through said second polarizing filter, thereby creating a third beam of light; measuring the third beam of light by means of a detector. ### Claim 20 (depends on 19) The method according to claim 19, wherein said analyzing comprises determining the concentration of said birefringent particles suspended in said fluid. ### Claim 21 (depends on 19) The method according to claim 19, wherein said matched pair of polarizing filters is obtainable by the method to produce a matched pair of polarizing filters comprising a first and a second polarizing filter, said first polarizing filter comprising a first linear polarizer and a first quarter-wave optical retarder and said second polarizing filter comprising a second linear polarizer and a second quarter-wave optical retarder, said method comprising the steps of: providing a beam of light from a light source along a propagation axis; providing a first, second, third and fourth rotation stage oriented perpendicular to said propagation axis of the beam of light; mounting a first linear polarizer having a first transmission axis in said first rotation stage in a first position; mounting a second linear polarizer having a second transmission axis in said fourth rotation stage; rotating said second linear polarizer to obtain maximum extinction of said beam of light; inserting a first quarter-wave optical retarder having a first optical axis in said second rotation stage; rotating said first quarter-wave optical retarder to obtain maximum extinction of said beam of light; rotating said first optical axis of said first quarter-wave optical retarder in a first direction over a first angle, said first angle being 45 degrees, plus or minus 0.10 degrees; inserting a second quarter-wave optical retarder having a second optical axis in said third rotation stage; rotating said second optical axis of said second quarter-wave optical retarder in a second direction over a second angle to obtain maximum extinction of said beam of light, said second direction being opposite to said first direction of said rotation of said first optical axis of said first quarter-wave optical retarder as viewed from said light source; securing said first linear polarizer and said first quarter-wave optical retarder together to form said first polarizing filter and securing said second linear polarizer and said second quarter-wave optical retarder together to form said second polarizing filter. ### Claim 22 (depends on 19) The method according to claim 19, wherein said birefringent particles comprise calcium carbonate, quartz, celestite, barite, kaolinite, chlorite, illite, vermiculite, orthoclase, plagioclase, montmorillonite, plastic or combinations thereof. ### Claim 23 (depends on 19) The method according to claim 19, wherein said fluid comprises water or seawater. ### Claim 24 An apparatus for analyzing a sample comprising birefringent particles suspended in a fluid, said apparatus comprising a light source for emitting a beam of light along a propagation axis, a matched pair of polarizing filters comprising a first polarizing filter and a second polarizing filter and a detector, said light source, said matched pair of polarizing filters and said detector being arranged such that said beam of light emitted from said light source subsequently can pass through said first polarizing filter, can impinges on the sample to be analysed and can pass through said second polarizing filter before being detected by said detector, said first polarizing filter comprising a first linear polarizer and a first quarter-wave optical retarder and being configurable to polarize incident light into circularly polarized light of a first handedness sense viewed from the light source and said second polarizing filter comprising a second linear polarizer and a second quarter-wave optical retarder and being configurable to polarize incident light into circularly polarized light of a second handedness sense, with said first handedness sense and said second handedness sense being opposite as viewed from said light source, said matched pair of polarizing filters having an extinction ratio lower than 10−5. ### Claim 25 (depends on 24) The apparatus according to claim 24, wherein said matched pair of polarizing filters is obtainable by the method to produce a matched pair of polarizing filters comprising a first and a second polarizing filter, said first polarizing filter comprising a first linear polarizer and a first quarter-wave optical retarder and said second polarizing filter comprising a second linear polarizer and a second quarter-wave optical retarder, said method comprising the steps of: providing a beam of light from a light source along a propagation axis; providing a first, second, third and fourth rotation stage oriented perpendicular to said propagation axis of the beam of light; mounting a first linear polarizer having a first transmission axis in said first rotation stage in a first position; mounting a second linear polarizer having a second transmission axis in said fourth rotation stage; rotating said second linear polarizer to obtain maximum extinction of said beam of light; inserting a first quarter-wave optical retarder having a first optical axis in said second rotation stage; rotating said first quarter-wave optical retarder to obtain maximum extinction of said beam of light; rotating said first optical axis of said first quarter-wave optical retarder in a first direction over a first angle, said first angle being 45 degrees, plus or minus 0.10 degrees; inserting a second quarter-wave optical retarder having a second optical axis in said third rotation stage; rotating said second optical axis of said second quarter-wave optical retarder in a second direction over a second angle to obtain maximum extinction of said beam of light, said second direction being opposite to said first direction of said rotation of said first optical axis of said first quarter-wave optical retarder as viewed from said light source; securing said first linear polarizer and said first quarter-wave optical retarder together to form said first polarizing filter and securing said second linear polarizer and said second quarter-wave optical retarder together to form said second polarizing filter. ### Claim 26 (depends on 24) The apparatus according to claim 24, wherein said apparatus is a transmissometer. ### Claim 27 (depends on 24) The apparatus according to claim 24, wherein said first linear polarizer has a first transmission axis, said first quarter-wave optical retarder has a first optical axis, said second polarizing filter has a second transmission axis and said second quarter-wave optical retarder has a second optical axis, with said first transmission axis, said second transmission axis, said first optical axis and said second optical axis each being oriented in a plane perpendicular to said propagation axis of said beam of light. ### Claim 28 (depends on 24) The apparatus according to claim 24, wherein said first transmission axis and said second transmission axis are perpendicular to each other. ### Claim 29 (depends on 25) The apparatus according to claim 25, wherein said first optical axis and said first transmission axis define a first angle and said second optical axis and said second transmission axis define a second angle, with said first angle and said second angle being 45 degrees plus or minus 0.10 degrees and with said first angle and said second angle having opposite signs as viewed from said light source. ### Claim 30 (depends on 25) The apparatus according to claim 25, further comprising one or more of the following components: a beam splitter; and/or one or more baffle; and/or; one or more pressure window; and/or one or more spectral filter; and/or one or more lens, and/or; one or more precision pinhole.