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

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.

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.