METHOD AND DEVICE FOR DETERMINING OPTICAL PROPERTIES OF A SAMPLE MATERIAL

Description

[0001] The invention relates to a method and a device for determining optical properties of a sample material.

[0002] Optical properties of a sample material are often measured using spectroscopic measurement techniques, such as spectroscopy based on attenuated and / or frustrated or prevented total reflection ( engl. attenuated total reflection, ATR or engl. frustrated total internal reflection (FTIR). The advantage of such spectroscopy methods, as disclosed, for example, in US Pat. No. 3,902,807, lies in the fact that an absorption spectrum can be recorded despite the strong background absorption of the sample material. It is known that the attenuation of the light wave at the surface between an ATR element and the sample material varies for different polarizations of the incident light wave, and that the different polarizations also exhibit different phase shifts.

[0003] For example, US 7,920,264 B1 describes a HATR process ( engl. horizontal attenuated total reflection (HATR), which uses the phase shift for two different polarization directions to measure the rotation of the polarization plane through the sample material, i.e. for polarimetry.

[0004] Furthermore, EP 3399299 A1 and DE10145719A1 describe hydrogen sensors. Furthermore, WO 2002001203 A1 describes a refractory measurement method.

[0005] Thermal emitters, which are filtered with a monochromator for wavelength selection, are often used as light sources for such spectroscopy methods. However, thermal emitters have the disadvantage of their low spectral power density and therefore low intensity with narrowband filtering. Alternatively, laser sources such as quantum cascade lasers are also used in the above-mentioned spectroscopy methods. While these represent very powerful light sources, particularly in the mid- and far-infrared wavelength range, they exhibit higher intensity noise compared to thermal emitters. This reduces sensitivity, particularly for sample materials with low concentrations of the substance to be analyzed, and therefore these can no longer be measured.Furthermore, the power density of these light sources is not constant across the wavelength, so the measured spectrum must be compared with a reference measurement, especially with or without a known sample material. In this case, it is possible to simultaneously acquire the reference measurement directly through an additional optical path and subtract it from the measurement signal (. engl. A disadvantage of such an additional reference path is that it could also exhibit wave-dependent transmission and be subject to different fluctuations than the measurement path. The reference path should also preferably have a similar attenuation to the measurement path.

[0006] The invention is based on the object of providing a method and a device for determining the optical properties of a sample material that avoids the disadvantages of the prior art. In particular, the invention is also based on the object of providing an improved method and an improved device for determining the optical properties of a sample material that advantageously reduces, suppresses, and / or eliminates the effects of intensity noise from light sources.

[0007] This object is achieved by a method which is an ATR infrared spectroscopy method for determining the optical properties of a liquid sample material, which comprises the following steps: irradiating light from the infrared wavelength range onto the sample material through a reflection element such that the light is totally reflected at the interface formed by the sample material and the reflection element, wherein the reflection element is an ATR element; and further: a) determining a first intensity of the light of a first polarization state reflected at the interface; b) determining a second intensity of the light of a second polarization state reflected at the interface; c) forming a ratio of the first intensity to the second intensity or vice versa; wherein the first polarization state is parallel polarized light and the second polarization state is perpendicularly polarized light.

[0008] This object is further achieved by a device which is an ATR infrared spectrometer for determining optical properties of a liquid sample material, comprising: a reflection element, wherein the reflection element is an ATR element, and a fixing unit which is configured to fix the sample material in such a way that the sample material is arranged directly on the reflection element; - a light source which is configured to radiate light from the infrared wavelength range onto the sample material through the reflection element in such a way that the light is totally reflected at the interface formed by the sample material and the reflection element;- at least one detector device configured to determine a first intensity of light of a first polarization state reflected by the sample material and to determine a second intensity of light of a second polarization state reflected by the sample material, wherein the first polarization state is parallel-polarized light and the second polarization state is perpendicularly polarized light; - at least one computing unit configured to determine the ratio between the first intensity and the second intensity, or vice versa;

[0009] In particular, the method according to the invention is carried out by means of the device, preferably according to one of claims 1 to 9. This is possible if the device serves to carry out a method according to one of claims 1 to 9.

[0010] The invention is based on the finding that by determining a first intensity of the light of a first polarization state reflected by the sample material, by determining a second intensity of the light of a second polarization state reflected by the sample material, and by subsequently forming the ratio of the first intensity to the second intensity, or vice versa, the effects of intensity noise from light sources on the measurement result are reduced, suppressed, and / or eliminated. The invention utilizes the polarization dependence of the light reflected by the sample material, in particular the light reflected at the interface formed by the sample material and a reflective element, wherein, in particular, the different polarizations of the reflected light exhibit different relative changes in reflection.Surprisingly, it was found that despite the ratio formation of the reflected light intensities of the two polarization states, a signal remains, while at the same time, the intensity noise of the light source used is not included in the final signal. This eliminates the need for a reference path. Fluctuations caused by the measurement setup itself are also not included in the signal, especially as long as they affect both polarization states in the same way.

[0011] "Polarization state," "polarization type," "polarization direction," "polarization degree," and / or "polarization" are preferably understood here to mean the direction of oscillation of light. Particularly in the wave model, light is preferably a wave oscillating perpendicular to its propagation direction, in particular a transverse wave, so that these terms are preferably understood here to mean the direction of oscillation of a transverse wave. The direction of oscillation preferably refers to the field vector of the electric field, in particular, with the field vector of the magnetic field correspondingly oscillating perpendicular to it. Especially when the direction of oscillation changes rapidly and in a disordered manner, the light is preferably unpolarized. The terms "polarization state," "polarization type," "polarization direction," "polarization degree," and / or "polarization" therefore indicate, in particular, the ordered component.

[0012] Furthermore, it is provided that the method is an ATR infrared spectroscopy method, preferably a HATR infrared spectroscopy method, a FEWS infrared spectroscopy method ( engl. fiber evanescent wave spectroscopy (FEWS) and / or an FTIR infrared spectroscopy method. Likewise, the device is an ATR infrared spectrometer, preferably an HATR infrared spectrometer, an FEWS infrared spectrometer, and / or an FTIR infrared spectrometer.

[0013] The sample material is liquid. It is therefore possible that the sample material comprises, for example, water and / or bacteria. Furthermore, it is conceivable that the sample material comprises one or more substances selected from the group: water, bacteria, viruses, solvents, solvent mixtures, and body fluids, especially blood.

[0014] Further advantageous embodiments of the invention are described in the subclaims.

[0015] Preferably, the method further comprises the following step, in particular which is carried out before steps a) and b): - splitting the light reflected by the sample material into the first polarization state and the second polarization state, preferably by means of at least one polarizer, more preferably by means of a first polarizer and a second polarizer.

[0016] It is also advantageous if the device further comprises at least one polarizer, in particular for splitting the light reflected by the sample material into the first polarization state and the second polarization state, in particular that the device comprises a first polarizer for splitting the light reflected by the sample material into the first polarization state and a second polarizer for splitting the light reflected by the sample material into the second polarization state.

[0017] "Polarizer(s)" preferably refers to components that filter electromagnetic waves, in particular light, such as light from the infrared wavelength range, of a specific polarization from non-polarized, partially polarized, or differently polarized electromagnetic waves. Polarizers may use mechanisms selected from the group consisting of dichroism, reflection, birefringence, scattering, and / or diffraction to separate the different polarizations of the incoming waves. For example, polarizers that separate a linearly polarized electromagnetic wave are called linear polarizers. Furthermore, polarizers that separate circularly polarized light are called circular polarizers.

[0018] Advantageously, the at least one polarizer, in particular the first polarizer and the second polarizer, is selected from the group: polarizer based on birefringence, preferably polarization prism, more preferably Nicols prism, Rochon prism, Glan-Thomson prism, polarizer based on dichroism, preferably J-foil and / or H-foil, and / or polarizer based on reflection, preferably Brewster window.

[0019] Preferably, the at least one polarizer, in particular the first polarizer and the second polarizer, is a Brewster window. This makes it possible to separate the first and second polarization states of the light reflected from the sample material, particularly while ensuring that further polarization states do not interfere with the measurement result.

[0020] The first polarization state is parallel polarized light, and the second polarization state is perpendicularly polarized light, or vice versa. This makes it possible to obtain the maximum possible signal, preferably for the ratio of the first intensity to the second intensity, particularly since these polarization states can also be easily separated using the aforementioned polarizers.

[0021] It is also possible that in steps a) and / or b) the intensity of the light of the first and / or the second polarization state reflected by the sample material is determined by means of at least one detector device, in particular that in steps a) and / or b) the intensity of the light of the first polarization state reflected by the sample material is determined by means of a first detector device and the intensity of the light of the second polarization state reflected by the sample material is determined by means of a second detector device.

[0022] It is also possible for the device to have a first detector device for determining the first intensity of the light of the first polarization state reflected by the sample material and a second detector device for determining the second intensity of the light of the second polarization state reflected by the sample material.

[0023] Preferably, the at least one detector device, in particular the first and / or second detector device, is a photodetector selected from the group: photocell, photomultiplier, microchannel plate photomultiplier, CMOS sensor, CCD sensor, photodiode, phototransistor, photoresistor, and / or thermal radiation meter, in particular bolometer, pyroelectric sensor, pyrometer, thermocouple, and / or Golay cell. Photomultipliers or photodiodes, such as an HgCdTe photodiode, are preferably used, particularly when the light to be detected is low and high temporal resolutions are required.

[0024] Furthermore, it is possible for the at least one detector device, in particular for the first and / or second detector device, to be a synchronous detector (lock-in). This makes it possible to further improve noise suppression or further filter the noise.

[0025] It is expedient that the method further comprises the following step, in particular which is carried out before steps a) and b): - irradiating light onto the sample material through a reflection element in such a way that the light is reflected, in particular totally reflected, at the interface formed by the sample material and the reflection element.

[0026] Preferably, the light reflected by the sample material, in particular of the first and / or second polarization state, is the light that is reflected, in particular totally reflected, at the interface formed by the sample material and the reflection element. It is thus advantageous if the light reflected at the interface formed by the sample material and the reflection element is the light reflected by the sample material, in particular of the first and / or second polarization state. It is thus advantageous if the light reflected by the sample material corresponds to or is identical to the light reflected at the interface formed by the sample material and the reflection element.

[0027] Preferably, the light is irradiated onto the sample material through the reflection element in such a way that an electromagnetic wave forms on the surface of the reflection element, which penetrates the sample material, in particular with the field strength of the electromagnetic wave decreasing exponentially. Preferably, the electromagnetic wave on the surface of the reflection element, in particular with its field strength decreasing exponentially, is a so-called evanescent wave, which preferably arises during total internal reflection at the interface formed by the sample material and the reflection element.

[0028] Thus, it is possible that, particularly within the reflection element, the angle (α) at which the light is irradiated onto the sample material is equal to the angle (β) at which the light is reflected from the sample material.

[0029] It is possible that, in particular within the reflection element, the angle (α) at which the light is irradiated onto the sample material and the angle (β) at which the light is reflected from the sample material is between 0° and 90°, preferably between 10° and 85°.

[0030] It is also advantageous if the refractive index of the reflection element (n 1 ) is greater than the refractive index of the sample material (n 2 ).

[0031] Thus, it is possible that the refractive index of the reflection element (n 1 ) is between 2.0 and 4.0, preferably between 2.4 and 2.6, and / or that the refractive index of the sample material (n 2 ) is between 1.05 and 1.95, preferably between 1.25 and 1.75.

[0032] Furthermore, it is expedient if the angle (α) is greater than or equal to the angle for which total reflection occurs at the interface formed by the sample material and the reflection element, in particular for the light irradiated onto the sample material.

[0033] In particular, the critical angle (Θ c ) at which total reflection occurs is calculated as follows: Θ c = arscin n 2 / n 1 .

[0034] The critical angle (Θ c ) is also referred to as the critical angle. From the critical angle (Θ c ), the electromagnetic wave, in particular the incident light, can no longer (or no longer predominantly) penetrate the optically less dense medium, in particular the sample material, and is instead (in particular almost completely) reflected, in particular totally reflected, at the interface between the optically denser and the optically less dense medium, in particular at the interface formed by the sample material and the reflection element. In particular, the angle of reflection is equal to the angle of incidence.

[0035] It is therefore possible that the light irradiated onto the sample material is not capable of propagation in the sample material, particularly due to the angle (α) at which the light is irradiated onto the sample material.

[0036] Advantageously, the reflection element is an optical waveguide, in particular an optical waveguide in which light is guided by total internal reflection. For example, such an optical waveguide is a prism or a fiber, in particular one without a cladding.

[0037] According to the invention, the reflection element is an ATR element, in particular an ATR crystal.

[0038] It is possible for the reflection element, preferably the ATR crystal, to comprise zinc selenide (ZnSe), germanium (Ge), thallium bromoiodide (KRS-5), silicon (Si), AMTIR (amorphous material transmitting infrared radiation), in particular AMTIR-1 (GeAsSe), and / or diamond. Such materials are particularly suitable for light in the infrared wavelength range.

[0039] Furthermore, it is also conceivable that, particularly in FEWS infrared spectroscopy methods and / or in a FEWS infrared spectrometer, the ATR element comprises chalcogenide glass fibers.

[0040] Furthermore, it is advantageous if the light irradiated onto the sample material is unpolarized light, preferably if the light irradiated onto the sample material has a polarization of less than 45° to the plane of incidence. This achieves, in particular, uniform excitation in perpendicular and parallel polarization.

[0041] The light irradiated onto the sample material is light from the infrared wavelength range, preferably from the near and / or the middle and / or the far infrared range, more preferably light from the wavelength range between 0.8 µm and 1000 µm, even more preferably light from the wavelength range between 2.5 µm and 25 µm, furthermore even more preferably light from the wavelength range between 8 µm and 12 µm.

[0042] The light irradiated onto the sample material is expediently emitted by a light source, in particular wherein the light source is selected from the group: lasers, preferably semiconductor lasers, more preferably quantum cascade lasers (QCL), and / or thermal radiators, preferably incandescent lamps, Nernst lamps, resistance heating elements made of silicon carbide, or carbon arc lamps. It is also possible to use other light sources. In particular, the method and / or device for determining the optical properties of a sample material is not subject to any restrictions regarding the light sources used.

[0043] Furthermore, it is also expedient for the device to further comprise one of the following units: - a light source, in particular for irradiating light onto the sample material through a reflection element such that the light is reflected, in particular totally reflected, at an interface formed by the sample material and the reflection element; - a reflection element, preferably an ATR element, more preferably an ATR crystal, in particular wherein the sample material forms an interface with the reflection element; - a fixing unit for fixing the sample material to the reflection element; optionally: one or more optical waveguides, preferably one or more fibers, more preferably one or more glass fibers, in particular for guiding the light irradiated onto the sample material and / or the light reflected by the sample material; - one or more mirrors, in particular for deflecting the light; - at least one optical sump; - one or more wavelength selectors, preferably one or more monochromators, in particular for spectrally isolating a predetermined wavelength, preferably of the light irradiated onto the sample material.

[0044] It is also possible if the light irradiated onto the sample material and / or the light reflected by the sample material, in particular of the first and / or the second polarization state, is a light beam, preferably wherein the light beam has a beam diameter of between 10 µm and 10,000 µm at least in some regions.

[0045] Furthermore, it is also possible that the light irradiated onto the sample material and / or the light reflected by the sample material, in particular of the first and / or the second polarization state, is guided at least in regions in an optical waveguide, preferably in a fiber, even more preferably in a glass fiber.

[0046] It is advantageous if, in step c), the effects of the intensity noise of the light source, in particular on the measurement result, are reduced, preferably eliminated, due to the ratio formation of the first intensity and the second intensity or vice versa.

[0047] Thus, it is possible to reduce the effects of the intensity noise of the light source, in particular on the measurement result, by at least a factor of 2.5, preferably by a factor of 5, more preferably by a factor of 10, even more preferably by a factor of 50, and even more preferably by a factor of 100.

[0048] According to the invention, at least steps a) to c) are carried out for light from the infrared wavelength range, preferably from the near and / or the middle and / or the far infrared range, more preferably for light from the wavelength range between 0.8 µm and 1000 µm, even more preferably for light from the wavelength range between 2.5 µm and 25 µm, furthermore even more preferably for light from the wavelength range between 8 µm and 12 µm.

[0049] Furthermore, it is preferred that the light paths in steps a) and b), in particular for the light of the first and second polarization states reflected by the sample material, are substantially identical. Thus, it is also possible for the light paths in steps a) and b), in particular for the light of the first and second polarization states reflected by the sample material, to be identical. This ensures, in particular, that fluctuations affect both measurement paths equally. Furthermore, it is possible for the light paths in steps a) and b) to differ by less than 10 mm, preferably by less than 1 mm. In particular, such small differences in the light paths arise due to the measurement setup with a first and a second detector device, so that small differences in the light paths can arise, in particular at the end of the measurement section.

[0050] Preferably, steps a) and b) are performed simultaneously; in particular, the intensity of the light of the first and second polarization states reflected by the sample material is determined simultaneously. This eliminates the need for an additional reference measurement. Furthermore, this results in a precise measurement result, since possible differences between the signals that may arise due to temporally offset measurements are eliminated.

[0051] Preferably, the method further comprises the step: d) determining at least one optical property of the sample material on the basis of the formed ratio.

[0052] The computing unit is preferably configured to form the ratio between the first intensity and the second intensity or vice versa.

[0053] In a preferred variant, one or more of the above-mentioned steps, in particular at least steps a), b), c) and preferably d), are carried out or repeated for a plurality of wavelengths for light from the infrared wavelength range, preferably from the near and / or mid and / or far infrared range, more preferably for light from the wavelength range between 0.8 µm and 1000 µm, even more preferably for light from the wavelength range between 2.5 µm and 25 µm, and even more preferably for light from the wavelength range between 8 µm and 12 µm. It is also possible that in step d), after repeating steps a) to c) for the plurality of wavelengths, the determination of at least one optical property of the sample material takes place on the basis of the (plurality of) ratios formed for the plurality of ratios.

[0054] Furthermore, it is possible to apply the above-described method, in particular according to one of claims 1 to 12, and / or the above-described device, in particular according to one of claims 13 to 15, for spectral analysis, in particular of the sample material. The spectral analysis, in particular of the sample material, is preferably based on total reflection, preferably frustrated total reflection.

[0055] It is also possible that the method described above, in particular according to one of claims 1 to 12, and / or the device described above, in particular according to one of claims 13 to 15, is used for or for suppressing the intensity noise of light sources, in particular in spectral analysis.

[0056] In the following, embodiments of the invention are described by way of example with the aid of the enclosed, not to scale, Figuren 2 bis 7 explained. Fig. 1 shows schematically a known measuring setup in plan view Fig. 2a and Fig. 2b show schematically a device in side view and in plan view Fig. 3a shows schematically an enlarged section of the Fig. 2a Fig. 3b shows schematically an enlarged section of the Fig. 3a Fig. 4a to Fig. 4c show diagrams Fig. 5a to Fig. 5c show schematic devices in side view and in plan view Fig. 6 and Fig. 7 show schematic process steps for determining optical properties of a sample material

[0057] Fig. 1 shows a schematic plan view of a measurement setup 1000 known from the prior art. Preferably, the optical properties of a sample material 5 are determined using the measurement setup 1000.

[0058] As in Fig. 1 As shown, the measurement setup 1000 comprises a light source 2, a beam splitter 4, the sample material 5, a mirror 6, a variable attenuator 7 and two detectors 8a and 8b.

[0059] Thermal emitters, which are filtered with a monochromator for wavelength selection, are used as light source 2. However, thermal emitters have the particular disadvantage of having a low spectral power density and therefore low intensity with narrowband filtering. Alternatively, laser sources such as quantum cascade lasers are also used as light source 2. Although these represent particularly powerful light sources in the mid- and far-infrared wavelength range, they exhibit high intensity noise compared to thermal emitters. This reduces the sensitivity, particularly for sample material 5 with low concentrations of the substance to be examined, and these can therefore no longer be measured.Also, the power density of such light sources 2 is not constant over the wavelength, so that the measured spectrum should advantageously be compared with a reference measurement, in particular without or with known sample material 5.

[0060] For this purpose, it is preferred, as in Fig. 1 As shown, according to the state of the art, an additional optical path is integrated into the measurement setup so that the reference measurement can be acquired directly through the additional optical path simultaneously and can be subtracted from the measurement signal ( engl. balanced detection). For this purpose, as in Fig. 1 shown, in particular the light 3 is split with the help of the beam splitter 4 and guided into the reference path and the measuring path. In order to achieve as identical an attenuation as possible in the measuring path and in the reference path, the reference path in the prior art preferably comprises a variable attenuator 7. Subsequently, the measuring signal and the reference signal are advantageously detected in the detectors 8a and 8b, which are, for example, photomultipliers, and subtracted from one another. A disadvantage of such an additional reference path, however, is that it could also have a wave-dependent transmission and is also subject to other fluctuations like the measuring path. The reference path should also preferably have a similar attenuation to the measuring path, although this can usually only be achieved roughly using the variable attenuator 7 because the sample material 5 is unknown.

[0061] In the following, preferred embodiments of the invention are described by way of example with the aid of the enclosed, not to scale, Figuren 2 bis 7 explained, in particular which avoid the disadvantages of the prior art and which advantageously reduce, suppress and / or eliminate the effects of intensity noise from light sources.

[0062] Fig. 2a und Fig. 2b show schematically a device 1 in side view and top view.

[0063] As in Fig. 2a and in Fig. 2b As shown, the device 1 comprises a light source 2, mirror 6, a reflection element 9, at least one polarizer 10 and at least one detector device 8 as well as a computing unit 11.

[0064] The Fig. 2a und Fig. 2b The device 1 shown, preferably a spectrometer, more preferably an ATR infrared spectrometer, for determining optical properties of the sample material 5 comprises: - at least one detector device 8 for determining a first intensity of light 3r of a first polarization state reflected by the sample material 5 and for determining a second intensity of the light 3r of a second polarization state reflected by the sample material 5; - at least one computing unit 11 for forming the ratio of the first intensity and the second intensity or vice versa.

[0065] The light source 2 is preferably selected from the group: lasers, preferably semiconductor lasers, more preferably quantum cascade lasers (QCL), and / or thermal radiators, preferably incandescent lamps, Nernst lamps, silicon carbide heating elements, or carbon arc lamps. The light source preferably emits light from the infrared wavelength range, preferably from the near and / or mid and / or far infrared range, more preferably light from the wavelength range between 0.8 µm and 1000 µm, even more preferably light from the wavelength range between 2.5 µm and 25 µm, and even more preferably light from the wavelength range between 8 µm and 12 µm.

[0066] In the Fig. 2a und Fig. 2b For example, the light source 2 shown is a quantum cascade laser that emits light in the wavelength range between 8 µm and 12 µm.

[0067] As in the Fig. 2a und Fig. 2b As shown, in particular the light emitted by the light source 2 is irradiated onto a sample material which is arranged on the reflection element 9. For the sake of better clarity, the sample material is not shown here. In order to irradiate the sample material, the light emitted by the light source 2 is deflected in particular with the aid of the mirror 6. Furthermore, the light 3e irradiated onto the sample material by the reflection element 9 is preferably irradiated in such a way that the light is reflected, in particular totally reflected, at the interface formed by the sample material 5 and the reflection element 9. Preferably, the light reflected, in particular totally reflected, at the interface formed by the sample material 5 and the reflection element 9 is the light 3r reflected by the sample material 5.With regard to the reflection at the interface formed by the sample material and the reflection element 9, reference is made to the explanations below within the framework of the . Fig. 3a and the Fig. 3b referred to.

[0068] The light 3r reflected from the sample material is then, as also shown in Fig. 2a und Fig. 2b shown, directed by means of a further mirror 6 to at least one polarizer 10.

[0069] By means of the polarizer 10, the light 3r reflected by the sample material is preferably split into the first polarization state and the second polarization state.

[0070] Advantageously, the polarizer 10 is selected from the group: polarizer based on birefringence, preferably polarizing prism, more preferably Nicols prism, Rochon prism, Glan-Thomson prism, polarizer based on dichroism, preferably J-foil and / or H-foil, and / or polarizer based on reflection, preferably Brewster window.

[0071] It is advantageous if the first and second polarization states are different, especially if the first and second

[0072] Polarization state are linearly polarized states with mutually perpendicular oscillation planes, wherein preferably the first polarization state is parallel polarized light and the second polarization state is perpendicularly polarized light.

[0073] In the Fig. 2a und Fig. 2b The polarizer 10 shown is, for example, a linear polarizer, in particular which can sequentially filter out light of two linearly polarized states with mutually perpendicular oscillation planes as a function of a controllable state.

[0074] Subsequently, in particular, the intensity of the light 3r of the first polarization state reflected by the sample material and the intensity of the light 3r of the second polarization state reflected by the sample material, as in Fig. 2a and in Fig. 2b shown, determined by means of the detector device 8.

[0075] Preferably, the detector device 8 is a photodetector selected from the group: photocell, photomultiplier, microchannel plate photomultiplier, CMOS sensor, CCD sensor, photodiode, phototransistor, photoresistor, bolometer, pyroelectric sensor, pyrometer, thermocouple and / or Golay cell. Fig. 2a und Fig. 2b The detector device 8 shown is, for example, a photomultiplier.

[0076] In the Fig. 2a und Fig. 2b The ratio of the intensity of the light 3r of the first polarization state reflected by the sample material and the intensity of the light 3r of the second polarization state reflected by the sample material is then formed in the computing unit 11 shown.

[0077] This is preferably carried out for a plurality of wavelengths for light from the infrared wavelength range, preferably from the near and / or the middle and / or the far infrared range, more preferably for light from the wavelength range between 0.8 µm and 1000 µm, even more preferably for light from the wavelength range between 2.5 µm and 25 µm, furthermore even more preferably for light from the wavelength range between 8 µm and 12 µm.

[0078] The Fig. 2a und Fig. 2b The device 1 shown determines, in particular depending on the controllable state of the polarizer 10, for example, first the intensity of the light 3r of the first polarization state reflected by the sample material and then the intensity of the light 3r of the second polarization state reflected by the sample material. In particular, as soon as the intensities of the light 3r reflected by the sample material for both polarization states are determined, the ratio of these intensities is formed by means of the computing unit 11. For this purpose, the computing unit 11 preferably has a (working) memory and / or a microprocessor. In other words, the Fig. 2a und Fig. 2b The device 1 shown preferably operates sequentially, in particular depending on the controllable state of the polarizer 10, which, for example, filters out parallel polarized light or allows it to pass through in a first controllable state and filters out perpendicular polarized light or allows it to pass through in a second controllable state.

[0079] As in Fig. 2a und Fig. 2b As can be seen, in particular the light paths for determining the intensities for the light of the first and second polarization states reflected by the sample material are identical.

[0080] In particular, due to the ratio formation of the intensities of the light 3r reflected from the sample material for both polarization states, the effects of the intensity noise of the light source 2 are reduced, preferably eliminated.

[0081] For example, it is possible for the effects of the intensity noise of the light source 2 to be reduced by at least a factor of 2.5, preferably by a factor of 5, more preferably by a factor of 10, even more preferably by a factor of 50, and even more preferably by a factor of 100.

[0082] Furthermore, it is also possible that the light 3e irradiated onto the sample material and / or the light 3r reflected by the sample material, in particular of the first and / or the second polarization state, is guided at least in regions in an optical waveguide, preferably in a fiber, even more preferably in a glass fiber.

[0083] It is also possible that the light 3e irradiated onto the sample material and / or the light 3r reflected by the sample material, in particular of the first and / or second polarization state, is guided as a free beam at least in some areas.

[0084] It is also possible that the light 3e irradiated onto the sample material 5 and / or the light 3r reflected by the sample material 5, in particular of the first and / or the second polarization state, is a light beam, preferably wherein the light beam has a beam diameter of between 10 µm and 10,000 µm at least in some regions.

[0085] Fig. 3a shows schematically an enlarged section 12a of the Fig. 2a .

[0086] As in Fig. 3a As shown, light 3e is preferably irradiated by the reflection element 9 onto the sample material 5 in such a way that the light 3r is reflected, in particular totally reflected, at the interface 13 formed by the sample material 5 and the reflection element 9.

[0087] Here, as in Fig. 3a shown, in particular within the reflection element 9, the angle (α) at which the light 3e is irradiated onto the sample material 5 is equal to the angle (β) at which the light 3r is reflected from the sample material 5.

[0088] For example, it is possible that, in particular within the reflection element 9, the angle (α) at which the light 3e is irradiated onto the sample material 5 and the angle (β) at which the light 3r is reflected from the sample material 5 is between 0° and 90°, preferably between 10° and 80°. For example, the angles shown in Fig. 3a shown angles (α) and (β) equal to 32°.

[0089] Advantageously, the refractive index of the reflection element 9 (n 1 ) is greater than the refractive index of the sample material 5 (n 2 ). Thus, it is possible for the refractive index of the reflection element 9 (n 1 ) to be between 2.0 and 4.0, preferably between 2.4 and 2.6, and / or for the refractive index of the sample material 5 (n 2 ) to be between 1.05 and 1.95, preferably between 1.25 and 1.75. For example, the Fig. 3a Sample element 5 shown has a refractive index (n 2 ) of 1.33 and the one shown in Fig. 3a The reflection element 9 shown has a refractive index (n 1 ) of 2.59 at a wavelength of 633 nm. Preferably, in the method and device 1, the dependence of the refractive index of the entire measurement setup, in particular the dependence of the refractive index of the sample material 5 and the reflection element 9, on the wavelength is negligible.

[0090] Thus, it is expedient if the angle (α) is greater than or equal to the angle for which total reflection occurs at the interface 13 formed by the sample material 5 and the reflection element 9, in particular for the light 3e irradiated onto the sample material 5.

[0091] In particular, the critical angle (Θ c ) at which total reflection occurs is calculated as follows: Θ c = arscin n 2 / n 1 .

[0092] The critical angle (Θ c ) is also referred to as the critical angle. From the critical angle (Θ c ), the electromagnetic wave, in particular the incident light 3e, can no longer (or almost no longer) penetrate the optically denser medium, in particular the sample material 5, and is instead completely reflected at the interface between the optically denser and the optically thinner medium, in particular at the interface 13 formed by the sample material 5 and the reflection element 9. In particular, the angle of reflection is equal to the angle of incidence.

[0093] Thus, it is possible that the light 3e irradiated onto the sample material 5 is not capable of propagation in the sample material 5, in particular due to the angle (α) at which the light is irradiated onto the sample material 5.

[0094] Advantageously, the reflection element 9 is an optical waveguide, in particular an optical waveguide in which light is guided by total internal reflection. For example, such an optical waveguide is a prism or a fiber, in particular without a cladding.

[0095] According to the invention, the reflection element 9 is an ATR element, in particular an ATR crystal.

[0096] Thus, it is possible that the reflection element 9, preferably the ATR element, more preferably the ATR crystal, comprises zinc selenide (ZnSe), germanium (Ge), thallium bromoiodide (KRS-5), silicon (Si), AMTIR (amorphous material transmitting infrared radiation), in particular AMTIR-1 (GeAsSe), and / or diamond.

[0097] Furthermore, it is also conceivable that, particularly in FEWS infrared spectroscopy methods and / or in a FEWS infrared spectrometer, the ATR element comprises chalcogenide glass fibers.

[0098] Furthermore, it is expedient if the light 3e irradiated onto the sample material 5 is unpolarized light, preferably if the light 3e irradiated onto the sample material 5 has a polarization of less than 45° to the plane of incidence.

[0099] Fig. 3b shows schematically an enlarged section 12b of the Fig. 3a .

[0100] As in Fig 3b As shown, the light is preferably radiated onto the sample material 5 through the reflection element 9 in such a way that an electromagnetic wave forms on the surface of the reflection element 9, which penetrates the sample material 5, in particular wherein the field strength of the electromagnetic wave decreases exponentially. Preferably, the electromagnetic wave on the surface of the reflection element 9, in particular whose field strength decreases exponentially, is a so-called decaying (evanescent) wave, which preferably arises during total reflection at the interface 13 formed by the sample material 5 and the reflection element 9.In particular, when the sample material 5 now absorbs light, the reflection of the light beam, in particular the light 3r reflected at the interface 13 formed by the sample material 5 and the reflection element 9, is weaker, since the decaying (evanescent) wave, in particular the evanescent field, experiences losses through the sample material 5. Thus, an exemplary distribution of the field strength in front of and behind the interface 13 formed by the sample material 5 and the reflection element 9 is shown in . Fig. 3b , especially for an incident plane wave. As shown in Fig. 3b As can be seen, the field strength in sample material 5 decreases exponentially.

[0101] Fig. 4a bis Fig. 4c show diagrams.

[0102] Fig. 4a und Fig. 4b show, as an example, the reflection at an interface between a reflection element made of ZnSe (n 1 = 2.59) and the sample material water (n 2 = 1.33) for the vertical polarization in Fig. 4a and for the parallel polarization in Fig. 4b for the lossless case 15a and the lossy case 15b, in particular where the extinction coefficient (k) for the lossy case is k = 0.0508. The diagram axes 14a show the angle of incidence in degrees and the diagram axes 14b show the reflectivity for the vertical polarization in Fig. 4a and for the parallel polarization in Fig. 4b . The diagram curves 15a and 15b were calculated for the wavelength of 10 µm as an example, since water strongly absorbs light in this wavelength range. As is particularly Fig. 4a und Fig. 4b As can be seen, at an angle greater than the critical angle of total internal reflection, the attenuation for parallel polarized light is greater than the attenuation for perpendicular polarized light. Furthermore, both polarization directions also exhibit a different phase shift (not shown here).

[0103] As already mentioned in Fig. 4a und Fig. 4b As can be seen, the extinction coefficient of the sample material has a greater influence on the reflection for parallel polarized light than for perpendicularly polarized light. Therefore, it is to be expected that a change in the extinction coefficient will have a greater effect on the reflection of parallel polarized light at the reflection element than on the reflection of perpendicularly polarized light. To confirm this relationship, the relative change in reflection at the reflection element is determined below as a function of a relative change in the extinction coefficient of the sample material. In particular, the change in the extinction coefficient (k) is defined as follows: Δk = k 0 ⋅ ϵ für ϵ ≪ 1 .

[0104] The changes in reflection are specifically defined as follows: ΔR = R k 0 − R k 0 + Δk = R k 0 − R k 0 1 + ϵ .

[0105] Furthermore, the relative change in reflection is defined as: R rel = ΔR / R k 0 .

[0106] In Fig. 4c In particular, this polarization dependence of the relative change in reflection for parallel polarized light 17a and perpendicularly polarized light 17b at the reflection element is shown, wherein in particular the diagram axis 14a shows the angle of incidence to the normal on the interface in degrees and the diagram axis 14c shows the relative change in reflection at the reflection element. How Fig. 4c As can be seen in particular, the influence of the extinction coefficient is greater the closer the angle of incidence, in particular the angle (α) at which the light is irradiated onto the sample material, is to the critical angle 16 of total reflection. Fig. 4c It can be seen that, in particular, a variation of the extinction coefficient for the different polarizations of light leads to different strengths of relative changes in reflection. If the ratio between the reflection coefficients of both polarizations is now preferably formed, a dependence on the extinction coefficient remains. This ratio formation is used, as explained above, in the method and device for determining optical properties of a sample material. In particular, it is possible that, although some signal intensity is lost due to the ratio formation, preferably because there is a dependence on the extinction coefficient in both polarization states, an overall signal is retained, preferably with the effects of the intensity noise of the light source and / or fluctuations in the measurement setup being reduced or eliminated, as explained above.

[0107] It should be noted in particular that this dependence advantageously also exists for polarization states other than parallel and perpendicular polarization. However, the maximum difference between the polarizations becomes smaller, particularly for other polarization states, so that signal strength is preferably lost for other polarizations. In other words, the distance of the maximum relative change in reflection for polarization states other than those in Fig. 4c shown, whereby preferably the fundamental dependence is maintained. Furthermore, the Fig. 4c In particular, it can also be seen that the polarization dependence of the relative changes in reflection at the reflection element has an angle dependence, but the angle adjustment is advantageously possible in a relatively large angular range, wherein the angular range is preferably between 0° and 5°, more preferably between 0.5° and 3°, above the critical angle of total reflection.

[0108] It should be noted here that it is also possible, for example, for the sample material 5 to be liquid and / or solid. For example, it is possible for the sample material 5 to comprise water, bacteria, and / or solids. Furthermore, it is conceivable for the sample material 5 to comprise one or more substances selected from the group: water, bacteria, viruses, solids, solvents, solvent mixtures, coating layers, polymer films, thermosets, body fluids, in particular blood, single- or multicellular organisms, fungi, plants, in particular algae.

[0109] Fig. 5a bis Fig. 5c show schematically devices 1 in side view and in plan view.

[0110] The Fig. 5a and in Fig. 5b The device 1 shown comprises a light source 2, the mirrors 6, a reflection element 9, the polarizers 10a and 10b and the detector devices 8a and 8b as well as an optical sump 18.

[0111] By means of the Fig. 5a and the Fig. 5b With the device 1 shown, it is possible to carry out a method, preferably a spectroscopy method, more preferably an ATR infrared spectroscopy method, for determining optical properties of a sample material, in particular wherein the method comprises the following steps: a) determining a first intensity of light 3r of a first polarization state reflected by the sample material; b) determining a second intensity of the light 3r of a second polarization state reflected by the sample material; c) forming the ratio of the first intensity and the second intensity or vice versa. For the sake of clarity, the Fig. 5a and the Fig. 5b the sample material, which is applied in particular in direct contact with the reflection element 9, is not shown.

[0112] Furthermore, it is possible that the device 1 is preferably a spectrometer, more preferably an ATR infrared spectrometer, a HATR infrared spectrometer, a FEWS infrared spectrometer and / or an FTIR infrared spectrometer.

[0113] Regarding the design of the light source 2, the reflection element 9 and the detector devices 8a and 8b, reference is made in particular to the above explanations. Fig. 5a und 5b The device 1 shown essentially corresponds to the device shown in Fig. 2a und 2b shown device with the difference that the device comprises the polarizers 10a and 10b instead of the polarizer 10 and the detector devices 8a and 8b instead of the detector device 8. Furthermore, the device shown in Fig. 5a und 5b The device shown also includes the optical sump 18.

[0114] As in Fig. 5a und 5b As shown, the light 3r reflected by the reflection element 9 and / or the sample material 5, in particular the light 3r reflected at the interface formed by the sample material 5 and the reflection element 9, is preferably separated into the first and second polarization states by means of the polarizers 10a and 10b. The polarizers are preferably Brewster windows. Advantageously, the perpendicular polarization is reflected by the Brewster window 10a and the parallel polarization by the Brewster window 10b, or vice versa. The other polarizations are advantageously transmitted through the Brewster windows 10a and 10b and reach the optical sump 18, in particular whereby this ensures that they do not interfere with the measurement result. Furthermore, as in Fig. 5a und 5b As shown, the intensities of the two polarization states, in particular the first and / or the second polarization state, are then determined by means of the detector devices 8a and 8b. Fig. 5a und 5b The device 1 shown has the detector device 8a for determining the intensity of the light 3r of the first polarization state reflected by the sample material and the detector device 8b for determining the intensity of the light 3r of the second polarization state reflected by the sample material.

[0115] It is then possible to form the ratio of the intensities of the light 3r of the first polarization state and the second polarization state reflected by the sample material, in particular with reference to the above explanations.

[0116] In particular for further noise suppression, it is possible that at least one detector device 8a and / or 8b is a synchronous detector (lock-in).

[0117] By means of the Fig. 5a und 5b With the device 1 shown, it is possible, for example, to simultaneously determine the intensity of the light reflected by the sample material, in particular the light 3r reflected at the interface formed by the sample material and the reflection element 9, of the first and second polarization states. In particular, due to the two detector devices, simultaneous detection of the two polarizations is possible, in particular, wherein the sample material is excited uniformly, in particular with light of a polarization at 45° to the plane of incidence.

[0118] Furthermore, it is preferred that the light paths for the light 3r of the first and second polarization states reflected by the sample material are substantially identical. Preferably, the light paths differ by less than 10 mm, preferably by less than 1 mm.

[0119] The Fig. 5c The device 1 shown corresponds to the device shown in the Fig. 5a und 5b shown device 1 with the difference that, on the one hand, the sample material 5 is shown as an example, wherein preferably the sample material 5 is fixed by means of a fixing unit 19 such that the sample material 5 is arranged directly on or against the reflection element 9. Preferably, no further material, such as air, is arranged between the sample material 5 and the reflection element 9. Furthermore, in the Fig. 5c also a computing unit 11 for calculating the ratio of the intensities of the light 3r of the first and second polarization states reflected by the sample material. Regarding the design of this and the further Fig. 5c For the elements shown, please refer to the above explanations.

[0120] So it is, as in Fig. 5c shown, is possible if the device 1 comprises one of the following units: - a light source 2, in particular for irradiating light onto the sample material 5 through a reflection element 9 such that the light is reflected, in particular totally reflected, at an interface formed by the sample material 5 and the reflection element 9; - a reflection element 9, preferably an ATR element, more preferably an ATR crystal, in particular wherein the sample material 5 forms an interface with the reflection element 9; - a fixing unit 19 for fixing the sample material 5 on the reflection element 9; - one or more mirrors 6, in particular for deflecting the light; - at least one optical sump 18.

[0121] Furthermore, it is expedient if the device 1 further comprises one of the following devices: - one or more optical waveguides, preferably one or more fibers, more preferably one or more glass fibers, in particular for guiding the light radiated onto the sample material 5 and / or the light 3r reflected by the sample material 5 in certain regions; - one or more wavelength selectors, preferably one or more monochromators, in particular for spectrally isolating a predetermined wavelength, preferably of the light radiated onto the sample material 5.

[0122] Fig. 6 and Fig. 7 show schematic process steps for determining the optical properties of a sample material.

[0123] Fig. 6 shows a schematic flow diagram of a method 100 comprising the method steps 101, 102 and 103 for determining optical properties of a sample material. Preferably, the Fig. 6 The method 100 carried out is a spectroscopy method, more preferably an ATR infrared spectroscopy method. Furthermore, it is also possible for the method to be a HATR infrared spectroscopy method, a FEWS infrared spectroscopy method, and / or an FTIR infrared spectroscopy method.

[0124] In step 101, a first intensity of light of a first polarization state reflected by the sample material is determined. In step 102, a second intensity of light of a second polarization state reflected by the sample material is determined. In step 103, the ratio of the first intensity to the second intensity is determined, or vice versa.

[0125] Furthermore, it is advantageous if step 103 is carried out after steps 101 and 102.

[0126] Regarding further possible developments of the Fig. 6 For the method 100 shown, reference is made here to the above explanations, in particular those which were also presented in connection with the device explained above.

[0127] Fig. 7 shows a schematic flowchart comprising process steps 101, 102, 103, 104, 105, and 106 for determining the optical properties of a sample material. Steps 104, 105, and 106 are optional and therefore shown in dashed lines.

[0128] In the optional step 104, light is irradiated onto the sample material by a reflection element in such a way that the light is reflected, in particular totally reflected, at the interface formed by the sample material and the reflection element.

[0129] In the optional step 105, the light reflected by the sample material is then split into the first polarization state and the second polarization state, preferably by means of at least one polarizer, more preferably by means of a first polarizer and a second polarizer.

[0130] Steps 101, 102 and 103 correspond to those in Fig. 6 shown steps, so please refer to the above explanations in this regard.

[0131] It is advantageous if, in step 103, the effects of the intensity noise of the light source are reduced, preferably eliminated, due to the ratio formation of the first intensity and the second intensity or vice versa.

[0132] In the optional step 106, further method steps for further noise suppression, such as synchronous detection, can then be carried out.

[0133] Furthermore, it is possible to apply the method 100 described above and / or the device 1 described above for spectral analysis, in particular of the sample material. Preferably, the spectral analysis, in particular of the sample material, is based on total reflection, preferably frustrated total reflection, as explained above.

[0134] It is also possible that the method 100 described above and / or the device 1 described above is used for suppressing the intensity noise of light sources, in particular in spectral analysis. List of reference symbols

[0135] 1Device 2Light source 3Light, light beam 3eIncident light 3rReflected light 4Beam splitter 5Sample material 6(Deflection) mirror 7Variable attenuator 8, 8a, 8bDetector device(s) 9Reflection element 10, 10a, 10bPolarizer(s) 11Computing unit 12a, 12bSection(s) 13Interface 14a, 14b, 14cDiagram axis(es) 15a, 15bDiagram curve(s) 16Critical angle 17a, 17bDiagram curve(s) 18Optical sump 19Fixation unit 100Procedure 101, 102, 103, 104, 105, 106Procedure steps 1000Measurement setup

Claims

1. An ATR infrared spectroscopy method, for determining optical properties of a liquid sample material (5), comprising the following step: irradiating (104) light (3e) from the infrared wavelength range onto the sample material (5) through a reflection element (9) in such a way that the light (3e) is totally reflected at the interface (13) formed by the sample material (5) and the reflection element (9), wherein the reflection element (9) is an ATR-Element; and characterized by the following steps: a) determining (101) a first intensity of light (3r) reflected at the interface in a first polarisation state; b) determining (102) a second intensity of the light (3r) reflected at the interface in a second polarisation state; c) forming the ratio (103) of the first intensity and the second intensity, or vice versa; wherein the first polarisation state is parallel polarised light and the second polarisation state is perpendicular polarised light.

2. The ATR infrared spectroscopy method according to claim 1, characterised in that the method further comprises the following step, in particular which is carried out before steps a) and b): - splitting (105) the light (3r) reflected from the sample material (5) into the first polarisation state and the second polarisation state, preferably by means of at least one polariser (10, 10a, 10b), further preferably by means of a first polariser (10a) and a second polariser (10b).

3. The ATR infrared spectroscopy method according to claim 2, characterised in that the at least one polariser (10, 10a, 10b), in particular the first polariser (10a) and the second polariser (10b), is selected from the group of: polarisers based on birefringence, preferably polarisation prism, further preferably Nicol prism, Rochon prism, Glan-Thomson prism, polariser based on dichroism, preferably J-film and / or H-film, and / or polariser based on reflection, preferably Brewster window.

4. The ATR infrared spectroscopy method according to any one of the preceding claims, characterised in that, the reflection element (9) is an ATR crystal.

5. The ATR infrared spectroscopy method according to any one of the preceding claims, characterised in that the light (3e) irradiated onto the sample material (5) is unpolarised light.

6. The ATR infrared spectroscopy method according to any one of the preceding claims, characterised in that the light (3e) irradiated onto the sample material (5) is light from the near- and / or the mid- and / or the far-infrared range, preferably light from the wavelength range between 0.8 µm and 1000 µm, further preferably light from the wavelength range between 2.5 µm and 25 µm, still further preferably light from the wavelength range between 8 µm and 12 µm.

7. The ATR infrared spectroscopy method according to any one of the preceding claims, characterised in that the light (3e) irradiated onto the sample material (5) is emitted by a light source (2), in particular wherein the light source (2) is selected from the group of: laser, preferably semiconductor laser, further preferably quantum cascade laser (QCL), and / or thermal emitter, preferably incandescent lamp, Nernst lamp, resistance heating elements made of silicon carbide or carbon arc lamp.

8. The ATR infrared spectroscopy method according to any one of the preceding claims, characterised in that the light paths in step a) (101) and b) (102), in particular for the light (3r) reflected from the sample material (5) in the first and in the second polarisation state, are substantially identical.

9. The ATR infrared spectroscopy method according to any one of the preceding claims, characterised in that the steps a) (101) and b) (102) are carried out simultaneously, in particular in that the intensities of the light (3r) reflected from the sample material (5) in the first and in the second polarisation state are determined simultaneously.

10. An ATR infrared spectrometer, for determining optical properties of a liquid sample material (5) comprising: - a reflection element (9), wherein the reflection element (9) is an ATR element, and a fixing unit (19) which is designed for fixing the sample material (5) in such a way that the sample material (5) is arranged directly on the reflection element (9); - a light source which is designed for irradiating light from the infrared wavelength range onto the sample material (5) through the reflection element (9) in such a way that the light (3e) is totally reflected at the interface (13) formed by the sample material (5) and the reflection element (9); characterized by - at least one detector device (8, 8a, 8b) which is configured for determining a first intensity of light (3r) reflected from the sample material (5) in a first polarisation state and for determining a second intensity of the light (3r) reflected from the sample material (5) in a second polarisation state, wherein the first polarisation state is parallel polarised light and the second polarisation state is perpendicular polarised light; - at least one computing unit (11), which is configured for forming the ratio between the first intensity and the second intensity, or vice versa.

11. The ATR infrared spectrometer according to claim 10, characterised in that, the ATR infrared spectrometer comprises a first detector device (8a) and a second detector device (8b), wherein the first detector device (8a) is configured for determining the first intensity of the light (3r) reflected from the sample material (5) in the first polarisation state and the second detector device (8b) is configured for determining the second intensity of the light (3r) reflected from the sample material (5) in the second polarisation state.

12. The ATR infrared spectrometer according to any one of claims 10 or 11, characterised in that the ATR infrared spectrometer further comprises at least one polariser (10, 10a, 10b), which is designed for splitting the light reflected from the sample material (5) into the first polarisation state and the second polarisation state, in particular in that the ATR infrared spectrometer comprises a first polariser (10a) and a second polariser (10b), wherein the first polariser (10a) is configured for splitting the light (3r) reflected from the sample material (5) into the first polarisation state and the second polariser (10b) is configured for splitting the light (3r) reflected from the sample material (5) into the second polarisation state.