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

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

1-15. (canceled)

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.

17. 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.

18. The method according to claim 16, wherein said matched pair of polarizing filters has an extinction ratio lower than 10.sup.−5.

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.sup.−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.

20. The method according to claim 19, wherein said analyzing comprises determining the concentration of said birefringent particles suspended in said fluid.

21. 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.

22. 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.

23. The method according to claim 19, wherein said fluid comprises water or seawater.

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.sup.−5.

25. 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.

26. The apparatus according to claim 24, wherein said apparatus is a transmissometer.

27. 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.

28. The apparatus according to claim 24, wherein said first transmission axis and said second transmission axis are perpendicular to each other.

29. 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.

30. 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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0113] The present invention will be discussed in more detail below, with reference to the attached drawings, in which:

[0114] FIG. 1, including FIGS. 1A to 1C, is a schematic illustration of a setup for the fabrication of a matched pair of polarizing filters;

[0115] FIG. 2 is a schematic illustration of the configuration of an apparatus for analyzing a birefringent sample according to the present invention;

[0116] FIG. 3 shows the extinction ratio in function of the second angle for two different matched pairs of polarizing filters according to the present invention.

DESCRIPTION OF EMBODIMENTS

[0117] The present invention will be described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto but only by the claims. The drawings are only schematic and are non-limiting. The size of some of the elements in the drawing may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[0118] When referring to the endpoints of a range, the endpoints values of the range are included.

[0119] When describing the invention, the terms used are construed in accordance with the following definitions, unless indicated otherwise.

[0120] The term ‘and/or’ when listing two or more items, means that any one of the listed items can by employed by itself or that any combination of two or more of the listed items can be employed.

[0121] The terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0122] The term ‘birefringence’ refers to the optical property of a material to split a beam of light into two beams of unequal velocities (corresponding to two different refractive indices of the crystal) which subsequently recombine to form a beam of light that is no longer linearly polarized.

[0123] The term ‘particle’ refers to any type of small fragments of a material independent of the shape of such fragments. The term particle refers to single particles or to a plurality of particles.

[0124] The term ‘analyzing’ or ‘analysis’ refers to any qualitative and/or quantitative measurement or analysis, for example measuring the presence or absence of birefringent material and/or measuring the amount or concentration of birefringent material. In particular the term analyzing refers to measuring the presence or absence of birefringent particles, and/or determining the concentration of birefringent particles.

[0125] In preferred embodiments of the present invention the term ‘analyzing’ or ‘analysis’ refers to determining the concentration of particles suspended in a fluid, for example determining the concentration of particles suspended in a liquid.

[0126] The term ‘fluid’ refers to a medium such as a gas or a liquid. Preferred liquids comprise water such as sea water.

[0127] The method to optimize the setup 100 in order to obtain an optimal detection of depolarized light caused by a birefringent sample through the fabrication of a matched pair of polarizing filters having a low extinction ratio (high-rejection ratio) is illustrated in FIG. 1.

[0128] First a beam of light 104 is generated from a light source 102. The beam of light 104 is oriented along an axis 101, referred to as the propagation axis of the beam of light 104. A preferred light source 102 to generate the beam of light 104 comprises a LED source. The beam of light 104 is preferably centered about a narrow spectral band, for example a spectral band equal or less than 20 nm full width at half height. The beam of light 104 has for example a spectral band equal to or less than 20 nm full width at half height with center wavelength 645 nm.

[0129] The beam of light 104 is preferably a parallel or nearly parallel beam of light, for example obtained using two lenses 136, 120 and a pinhole 122.

[0130] The setup 100 comprises four high precision rotation stages, respectively a first, a second, a third and a fourth high precision rotation stage 110, 112, 114 and 116. The high precision rotation stages 110, 112, 114 and 116 are oriented perpendicular or substantially perpendicular to the propagation axis 104 of the light beam 104. The setup 100 further comprises a detector 118. The detector 118 comprises for example a silicon photodiode detector. Preferably, a focusing lens (collimating lens) 140 and/or a precision pinhole 142 is/are provided between the fourth high precision rotation stage 116 and the detector 118.

[0131] A first linear polarizer 124 and a second linear polarizer 126 are mounted respectively in the first high precision rotation stage 110 and the fourth high precision rotation stage 116. The angle of the transmission axis of the first linear polarizer 124 is noted. The fourth rotation stage 116 is subsequently rotated until maximum extinction (minimum transmission) is obtained. At that point, the transmission axis of the first linear polarizer 124 and the transmission axis of the second linear polarizer 126 are at an angle of 90 degrees with respect to one another.

[0132] Subsequently, a first quarter-wave optical retarder (first phase plate) 128 is inserted in the second high precision rotation stage 112. The second high precision rotation stage 112 is rotated to obtain maximum extinction (minimum transmission). At that point, the optical axis of the first quarter-wave optical retarder 128 is precisely parallel to the polarization axis of the first linear polarizer 124. Subsequently, the optical axis of the first quarter-wave optical retarder 128 is rotated over 45 degrees as viewed from the light source 102. The accuracy of this rotation depends on the mechanical precision of the high precision rotation stage and can be easily less than a few milliradians.

[0133] Once the first quarter-wave optical retarder 128 has been set at 45 degrees, a second quarter-wave optical retarder (second phase plate) 130 is mounted in the third high precision rotation stage 114, just in front of the second linear polarizer 126. The second quarter-wave optical retarder 130 is rotated in the opposite sense as the first quarter-wave optical retarder 128 until once again maximum extinction (minimum transmission) is obtained. At this point, the optical axis of the second quarter-wave optical retarder 130 is at an angle of −45 degrees with the transmission axis of the linear polarizer 126 as viewed from the light source 102.

[0134] At that point a setup comprising a matched pair of circular polarizing filters, 132 and 134 with maximum mutual rejection ratio is obtained. The linear polarizer and the quarter-wave optical retarder for each pair of filter 132, 134 held in the rotation stages are then secured together, for example glued together. Preferably, the axes and side of the linear polarizers are marked to allow each filter assembly to be realigned when finally mounted, for example in a transmissometer.

[0135] FIG. 1A shows the preliminary setup of the first linear polarizer 124 and the first quarter-wave optical retarder 128. FIG. 1B shows the final setup of the first polarizer 124 and the first quarter-wave optical retarder 128 after assembling and securing (for example gluing).

[0136] Since the light acceptance angle of the transmissometer is about at least two degrees the angular dependence of the polarizing filters 132 and 134 should preferably be low. This can be achieved by using true zero order waveplates, i.e. waveplates comprising a single layer of polarizing material (either polymer material or an uniaxial crystal) bonded to an amorphous substrate. Such a true zero order waveplate has the lowest achievable retardance variation angle.

[0137] The setup 100 may further comprise a graduated iris 138 positioned before the first polarizing filter 132. Furthermore, the setup 100 may comprise a lens 136 (imaging lens) between the light source 102 and the precision pinhole 122 and/or a lens 140 (a focusing lens) positioned between the fourth high precision rotation stage 116 and/or a precision pinhole 142 positioned between the fourth high precision rotation stage 116 and the detector 118, preferably between the lens 140 and the detector 118.

[0138] The assembly process of the polarizing filters 132, 134 as described above has the advantage to be self-correcting.

[0139] The best rejection ratio (extinction ratio) that can be achieved with the proposed process is determined by the maximum rejection ratio of the crossed linear polarizers and by the angular accuracy of the rotation stages used in the fabrication process. Preferably, one or more of the components of the setup are provided with an antireflection coating to minimize any potential mutual interaction. Most preferably, all components of the setup are provided with an antireflection coating.

[0140] FIG. 2 shows a schematic illustration of an apparatus 200 according to the present invention. The apparatus 200 is suitable to measure the depolarization of circularly polarized light by a birefringent sample 201, for example of a sample comprising birefringent particles suspended in water. The apparatus 200 is in particular suitable as transmissometer. The apparatus (transmissometer) measures the depolarization fraction, i.e. the fraction of circularly polarized light transmitted by the first polarizing filter that gets depolarized by birefringent particles suspended in a fluid (for example water) in the sample section of the apparatus and thus passes through the second polarizing filter and then impinges onto the detector. The apparatus has for example a detection limit for the depolarization fraction lower than 3.10.sup.−6 m.sup.−1, which is roughly equivalent to 0.005 mmol CaCO.sub.3 m.sup.−3 for a pathlength of 15 cm.

[0141] The apparatus 200 comprises an emitting section, a sample section and a receiver section. The emitting section comprises a light source 202 and a first polarizing filter (first circular polarizer) 204 of a first handedness. The sample section comprises a sample holder, for example a column for receiving and/or holding a sample, for example water comprising birefringent particles. The receiver section comprises a second polarizing filter (second circular polarizer) 206 having a second handedness, opposite from the first handedness as seen from the light source and comprises a detector 208.

[0142] Each of the first and the second polarizing filters 204, 206 comprise a linear polarizer and a quarter-wave optical retarder. The first polarizing 204 filter comprises a first linear polarizer having a first transmission axis and a first quarter-wave 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-wave optical retarder having a second optical axis. The first linear polarizer, the first quarter-wave optical retarder, the second linear polarizer and the second quarter-wave optical retarder are preferably oriented with their plane perpendicular to the propagation axis of the beam of light. The first transmission axis and the second transmission axis are preferably oriented perpendicular to each other. The (included) angle defined by the first optical axis and the first transmission axis and the (included) angle defined by the second optical axis and the second transmission axis are preferably equal or substantially 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 have preferably opposite signs as viewed from the light source. The angle between the first optical axis and the first transmission axis is for example +45 degrees whereas the angle between the second optical axis and the second transmission axis is −45 degrees.

[0143] A beam of light 203 emitting from the light source 202 subsequently passes through the first polarizing filter 204, impinges on the sample 201 and passes through the second polarizing filter 206 before being detected by a detector 208.

[0144] The light source 202 comprises for example a LED source generating a beam 203 of light pulses along propagation axis 205. The beam of light 202 is preferably centered about a narrow spectral band, preferably a spectral band equal to or less than 20 nm full width at half amplitude with center wavelength 645 nm. The beam of light 203 passes preferably through a pinhole 209.

[0145] Preferably, the beam of light 203 is collimated in a parallel or nearly parallel beam by a collimating lens 210.

[0146] The parallelism of the beam is set by the ratio of diameter of the pinhole 209 to the focal length of the collimating lens 210.

[0147] It can be preferred that part of the light is then picked off by a beam splitter 212. The part 213 diverted by the beam splitter 212 may illuminate a reference detector 214 used as a monitor of the intensity of the light source 202.

[0148] Contrary to transmissometers known in the art, the transmissometer according to the present invention has preferably a polarized beam splitter 212 which diverts the linear polarization in the opposite orientation to the one of the first polarizing filter 204. This ensures that the maximum available light is sent through the first polarizing filter 204 and into the water column. Since LED's are unpolarized, the light diverted by the beam splitter 212 allows an accurate monitoring of the light transmitted through the first polarizing filter 204.

[0149] The apparatus 200 preferably comprises one or more pressure windows 216, 220. The material of the pressure window is preferably carefully chosen so that it has a minimum amount of stress-induced depolarization. Stress relieved amorphous SiO.sub.2 (amorphous quartz) of coated SF57 glass are suitable materials for this purpose.

[0150] The unpolarized background light coming from the sun and sky light that penetrates through the water surface is preferably reduced to a level that will allow the detector 208 to operate. Therefore the apparatus 200 is preferably used in a vertical configuration with the light source emitting light in the upward direction and the detector facing in the downward direction. In this orientation the background light that enters the detector 208 comes from the sun and sky light illuminating the emitter window and supporting structure that are within the field of view of the detector and that are diffusely reflected by the window and supporting structures. For this reason it is preferred that any metal component of the emitter section that is in the field of view is black, for example black anodized.

[0151] Even with these precautions, it is preferred to further shield the area of the emitter section that is visible by the detector from direct sun and skylight. This can be achieved by the judicious use and positioning of light baffles 218 that shadow the sensitive area. These baffles 218 are preferably kept small and thin enough so as to not impede the free flow of water laterally through the water section. It is important to note that there is the limiting cone of light at which any radiation coming from above is transmitted through the water surface. That cone defines the limit of the underwater sky image that can illuminate the emitter surface. This limiting angle is 50 degrees. Water waves can distort the cone extend the illumination to an angle of 60 degrees. The disposition of a minimum number of the baffles preferably ensures that no part of this down welling light reaches the visible surface of the emitter. Direct shielding from the sun and sky can be achieved with a small number of baffles disposed around the emitter and receiver sections. There might however be another source of un-polarized light in the water column that comes from the upwelling light backscattered by underwater particles. Shielding from this source requires a larger set of baffles spaced along the entire length of the instruments open water column. Note that even in this case it is possible to achieve free lateral flow through the measurement column.

[0152] Preferably, the first polarizing filter 204 is the last element before the pressure window 216. In this way it is ensured that no depolarization of the circularly polarized light resulting from the first polarizing filter 204 occurs elsewhere than in the water sample.

[0153] The receiver section comprises preferably a pressure window 220, for example a stress-relieved amorphous quartz window followed immediately by a second polarizing filter 206 of the opposite handedness compared to the first polarizing filter 204 used in the emitter section. The axes of the linear polarization subcomponents of these circular filters must be carefully oriented perpendicular to one another in order to ensure that their rejection ratio is maintained over the wavelength range of the light source.

[0154] Preferably, the apparatus comprises a narrow spectral bandwidth optical filter 222 immediately positioned after the second polarizing filter 206. By introducing a spectral bandwidth optical filter 222 any background light coming through the detector 208 is reduced.

[0155] Preferably, the out of band optical density of the filter 222 is 10.sup.−4 (OD-4) or better over a wavelength range from 200 nm to 1200 nm or more to ensure a maximum amount of background light rejection. Preferably, the receiver section further comprises a lens (a collimating lens) 224 and/or a precision pinhole 226. The lens 224 is preferably positioned after the second polarizing filter 206 and after the narrow band spectral filter 222.

[0156] The water column depolarized signal collection angle is set by the ratio of the diameter of the pinhole 226 and the focal length of the lens 220.

[0157] FIG. 3 shows the extinction ratio that is obtained using two different matched pairs of polarizing filters, each built according to the fabrication method of the present invention. The obtained extinction is plotted in function of the second angle (i.e. the angle of the optical axis of the second quarter-wave optical retarder and the transmission axis of the second linear polarizer), in particular in function of the deviation of the second angle from 45 degrees (plus or minus 0.05 degrees).

[0158] FIG. 3 illustrates that the intended extinction ratio is reached with a matched pair of polarizing filters according to the present invention. The first matched pair of polarizing filters reached an extinction ratio of 1.84 10.sup.−6, while the other matched pair of polarizing filters reached an extinction ratio of 2.55 10.sup.−6. The minimum extinction ratio is reached when the correct orientation of the second quarter-wave optical retarder is reached. In this position the second angle is 45 degrees (plus or minus 0.05 degrees). As mentioned above, the method to produce a matched pair of polarizing filters it is not required to measure the second angle. From FIG. 3, it is clear that the minimum value of the extinction ratio is reached by rotating the second optical axis of the second quarter-wave optical retarder before securing the different components.