Differential phase polarimeter

Abstract

The invention relates to a differential phase polarimeter (10), comprising a light source (12) for polarized irradiation light (14); a sample chamber (16) for a test object (18), the sample chamber (16) being arranged such that the test object (18) can be irradiated with the irradiation light (14); a linear polarizer (20) arranged downstream of the sample chamber (16) in the light propagation direction; and a detector (22) arranged downstream of the linear polarizer (20) in the light propagation direction and designed to detect a light intensity of the irradiation light (14) which has passed through the linear polarizer (20); wherein the detector (22) is spatially resolving and arranged in such a way that both test object light (32) which has passed through the test object (18) and background light (34) which has not passed through the test object (18) can be detected.

Description

[0001] Differential phase polarimeter

[0002] The invention relates to a differential phase polarimeter comprising (a) a light source for polarized irradiation light, (b) a sample chamber for a test specimen, wherein the sample chamber is arranged such that the test specimen can be irradiated with the irradiation light, (c) a linear polarizer arranged behind the sample chamber in the direction of light propagation, and (d) a detector arranged (i) behind the linear polarizer in the direction of light propagation and (ii) configured to detect a light intensity of the irradiation light that has passed through the linear polarizer.

[0003] Differential phase polarimeters of this type are used to measure the rotation of the polarization axis, also known as optical rotation, as a property of a test object. For example, the test object could be a solid made of quartz glass or a sugary liquid.

[0004] From CN 1 18090532 A, an imaging method is known for creating images or videos of abrasive oil particles, in which the image contrast is increased by filtering out multiple scattering by decomposing each recorded single image into different polarization directions and identifying the most frequently occurring polarization direction as background polarization and calculating the difference between this background light and the recorded image.

[0005] US Patent 5,847,394 A discloses an imaging method in which a turbid sample is irradiated with, in particular, linearly polarized light, and the backscattered light with two opposing linear polarization components is detected, whereby the components are subtracted from each other or a ratio is calculated. It is desirable to achieve the lowest possible measurement uncertainty with a differential phase polarimeter. Furthermore, a short measurement time and spatially resolved measurement are desirable, although these requirements are in conflict with each other.

[0006] The invention is based on the objective of improving difference phase polarimeters.

[0007] The invention solves the problem by means of a generic differential phase polarimeter whose detector is spatially resolved and is arranged in such a way that both test object light that has passed through the test object and background light that has not passed through the test object can be detected, in particular is detected.

[0008] According to a second aspect, the invention solves the problem by a method for determining a polarization plane rotation of a test specimen comprising the steps (i) arranging the test specimen in a sample chamber, in particular a differential phase polarimeter according to the invention, (ii) irradiating the test specimen with polarized irradiation light such that part of the light passes through the test specimen and part of the light does not pass through the test specimen, (iii) detecting test specimen light that has passed through the test specimen and background light that has not passed through the test specimen using a spatially resolved detector, and (iv) determining the polarization plane rotation of the test specimen from a test specimen light intensity and a background light intensity.

[0009] An advantage of the invention is that the polarization plane rotation, which can also be called optical rotation, can be measured quickly. Compared to difference-phase polarimeters, which use a detector that does not measure with spatial resolution, the simultaneous measurement of the background light and the light emitted by the test object results in a shorter measurement time.

[0010] It is also advantageous that local differences in the polarization plane rotation of the test specimen can be detected. Furthermore, the polarization plane rotation can be measured with a lower measurement uncertainty. Preferably, the expanded measurement uncertainty for determining the optical rotation is below 0.001°, with a coverage factor k = 2.

[0011] For the purposes of this description, a spatially resolved detector is understood to be a detector that can simultaneously measure the light intensity at at least 2, preferably a plurality, particularly preferably at least 10, different locations.

[0012] Polarized light refers specifically to linearly polarized light. The light source can be designed to emit linearly polarized light. Alternatively, the light source can emit unpolarized light and possess a primary polarizer for linearly polarizing the light.

[0013] The test object is understood to be an object whose polarization plane rotation is to be determined. The test object is preferably transparent.

[0014] According to one embodiment, the differential phase polarimeter has an evaluation unit configured to automatically calculate, with spatial resolution, a rotation of the polarization plane caused by the test specimen, based on (a) the light intensity of the test specimen and (b) the background light intensity. The test specimen rotates the polarization plane of the light emanating from the light source. Due to the linear polarizer, the light intensity decreases depending on the rotation of the polarization plane. Therefore, the rotation of the polarization plane can be deduced from the light intensity.

[0015] According to a preferred embodiment, the light source comprises a primary light source for emitting divergent irradiation light and a beam bundler for focusing the divergent irradiation light. Preferably, parallelized irradiation light is thus generated. The beam bundler can also be referred to as a collimator.

[0016] According to a preferred embodiment, the light source has a primary polarizer for linearly polarizing the incident light. Preferably, the spatially resolving detector is an area detector. In particular, the area detector has a plurality of pixels arranged in a checkerboard pattern.

[0017] An imaging optic is preferably arranged between the linear polarizer and the detector.

[0018] According to one embodiment, the primary polarizer is rotatable through a rotational angle. In particular, the primary polarizer is rotatable such that a polarization angle can be achieved by rotating the primary polarizer.

[0019]

[0020] The angle of rotation is variable, in particular periodically variable modulo 2n.

[0021]

[0022] describable as a linear function of time, i.e., according to the formula φ1 = ω1 · t. Where t is time and ω1 is the angular velocity.

[0023] Preferably, the primary polarizer is rotated by the angle of rotation

[0024]

[0025] The detector is designed to rotate so that rotating it creates a periodic intensity modulation on the detector. When the polarization angle changes

[0026]

[0027] As a preferred embodiment provides, the evaluation unit can record the periodically intensity-modulated signal, which changes periodically. The optical rotation of the test object can be determined from the simultaneously recorded periodically modulated signal of the test object's light intensity and the background light intensity. Averaging the results from multiple pixels reduces the stochastic measurement uncertainty. The use of periodically modulated intensity signals and the evaluation of their relative phase reduce the influence of statistical noise and other disturbances of the intensity signal on the determined optical rotation. Preferably, the primary polarizer is rotatable about its optical axis.

[0028] Alternatively or additionally, the linear polarizer is designed to be rotatable by a second rotation angle φ2, in particular such that the polarization of background light and test object light incident on the linear polarizer, especially at constant angular velocity, can be rotated. Preferably, the rotation angle φ2 can be described as a linear function of time, i.e., according to the formula φ2 = ω2 · t. Preferably, the linear polarizer is rotatable about its optical axis. Preferably, the linear polarizer is designed to be rotatable by the rotation angle φ2 such that rotation produces a periodic intensity modulation on the detector.

[0029] It is possible for both the primary polarizer and the linear polarizer to be rotatable. However, it is generally advantageous if either the primary polarizer or the linear polarizer is rotatable.

[0030] It is advantageous if the irradiation light in the sample chamber is parallelized light. This means, in particular, that the irradiation light can be described as consisting of parallel light rays. Alternatively, the irradiation light in the sample chamber can then be considered as consisting of plane wavefronts. In this way, light falling on the detector can be unambiguously assigned to a position on or next to the test specimen.

[0031] According to one embodiment, the evaluation unit is designed to automatically perform a procedure comprising the steps (i) rotating the primary polarizer and / or the linear polarizer, (ii) detecting a light intensity of the test specimen (I P (x,y, φ i )) depending on at least one rotation angle, namely the first rotation angle φ1 or the second rotation angle φ2, (iii) detecting a background light intensity (I H (x, y, φ i)) depending on at least one of the rotation angles

[0032]

[0033] 2 and (iv) Calculating a polarization rotation a(x,y) of the test object, in particular spatially resolved, from the test object light intensity (I P ) and the background light intensity (I H ).

[0034] If only one rotation angle is changed, calculating the spatially resolved polarization rotation of the test object a(x,y) preferably includes determining the rotation angle values ​​from the periodically modulated intensity signals I p (x,y,φ i ) and I H (x,y,φ i For this purpose, the Fourier transform can be used, for example. To reduce image noise, it is advantageous to average the intensity over several pixels in the x and y directions.

[0035] Preferably, the evaluation unit is configured to automatically perform a procedure comprising the steps (i) rotating the primary polarizer relative to the linear polarizer at a fixed rotational frequency f1, where preferably the linear polarizer is not rotated, (ii) detecting a light intensity of the test specimen (I P (x,y, t)) as a function of time (t), (iii) detecting a background light intensity (I H (x,y,t)) as a function of time (t) and (iv) Calculating a, in particular spatially resolved, polarization rotation α(x,y) of the test object from the time-dependent test object light intensity (I P ) and the time-dependent background light intensity (I H ).

[0036] Preferably, the evaluation unit is designed to automatically perform a procedure comprising the steps (i) Fourier transforming the light intensity of the test specimen (I P(x,y, t)), such that a test specimen light phase (φ1(x,y) + α(x,y)) is obtained, (ii) Fourier transform of the background light intensity (I H (x,y, t)), such that a background light phase φ1(x,y) is obtained, and (iii) calculate the desired polarization rotation α(x,y) from the difference between the test light phase (φ1(x,y) + α(x,y)) and the background light phase φ1(x,y). In this way, the polarization rotation is obtained quickly and robustly.

[0037] The polarization properties of a test specimen can vary depending on its location.

[0038] If the measurement field is limited to a section of the test object, then a shift in the measurement field can lead to a change in the measured optical rotation. If it is known that the polarization rotation is constant within a region of the test object, it is advantageous to exploit this to reduce the measurement uncertainty. Preferably, the evaluation unit is therefore configured to automatically perform a method comprising the steps (i) determining at least one region within a test object image from pixels that encode the test object's light intensity, (ii) calculating an average value over the region, and / or (iii) calculating an inhomogeneity parameter that encodes an inhomogeneity of the test object's light intensity and / or the polarization rotation a(x,y) within the region. The region can be defined, for example, by user input.

[0039] The inhomogeneity parameter is a measure of the fluctuations in the measured values ​​for the polarization rotation within the range. For example, the inhomogeneity parameter is the variance or the standard deviation. A method according to the invention preferably comprises the steps described above for the procedures that are preferably carried out by the evaluation unit.

[0040] The invention will now be explained in more detail with reference to the accompanying drawing. This drawing shows

[0041] Figure 1 shows a difference phase polarimeter according to the invention for carrying out a method according to the invention.

[0042] Figure 1 shows a differential phase polarimeter 10 according to the invention, comprising a light source 12 for emitting linearly polarized irradiation light 14, a sample chamber 16 for a test specimen 18, a linear polarizer 20 arranged behind the sample chamber 16 in the direction of light propagation z, and a detector 22. The detector 22 is, for example, a CCD chip.

[0043] In this case, the light source 12 comprises a primary light source 24 for emitting the divergent irradiation light 14 and a beam focuser 28, for example a collimating lens or a mirror collimator. The beam focuser 28 parallelizes the irradiation light 14, which strikes a primary polarizer 30.

[0044] The primary light source 24 is, for example, a laser or a light-emitting diode. The primary polarizer 30 is, for example, a film polarizer, a Glan-Thompson prism, or a single-crystal polarizer.

[0045] It can be seen that the test specimen 18 does not occupy the entire sample space 16. Therefore, in the direction of light propagation z behind the test specimen 18, there is test specimen light 32, which has passed through the test specimen 18, and background light 34, which has not passed through the test specimen 18. Both fall onto an optional imaging optic 26, which focuses them onto the detector 22.

[0046] The difference-phase polarimeter 10 preferably has a primary polarizer drive 36 for rotating the primary polarizer 32 by a first rotation angle φ1. Correspondingly, the polarization plane of the irradiated light 14 behind the primary polarizer also rotates by the first rotation angle φ1. The primary polarizer drive 36 rotates the primary polarizer 32 at a fixed rotation frequency f. Therefore, the following applies:

[0047] φ1 = ω1t = 2πf1t Formula 1

[0048] It is possible, but not necessary, for the differential phase polarimeter 10 to have a linear polarizer drive 38 arranged to rotate the linear polarizer 20 at a fixed rotational frequency f2.

[0049] For example, if only the primary polarizer 32 is rotated, this results in an intensity fluctuation I for the background light 34 on the detector 22. H (x,y,t) according to the penalty law

[0050]

[0051] Formula 2

[0052] and with Formula 1 follows

[0053]

[0054] For the test object light 32, an intensity fluctuation I results. P (x,y, t) to

[0055]

[0056] where a(x,y) is the sought-after polarization plane rotation caused by test specimen 18. With

[0057] cos 2 β = 1 / 2(1 + cos 2β) Formula 6

[0058] follows from Formula 3

[0059] Formula 7

[0060]

[0061] and from Formula 4 Formula 8

[0062]

[0063] An evaluation unit 40, which is connected to the detector 22 and optionally the primary polarizer drive 36, detects the rotational frequency f1 and / or f2 as well as the spatially resolved intensity I(x,y) and calculates the phase shift 2α(x,y) between the periodically modulated intensities I H (x,y) and I P (x,y). This is done, for example, by signal processing, such as discrete Fourier transform (DFT), fast Fourier transform (FFT), lock-in detection, sliding DFT, or fitting with a model function such as cos(at+b).

[0064] For example, if only the primary polarizer 32 is rotated and the DFT is used, then the phase of the signals I H (x,y) and IP (x,y) determined as:

[0065]

[0066] Here, arctan() denotes the extended arctangent function on the uniqueness interval [−π, +π], ℑ{} the imaginary part formation, ℜ{} the real part formation, and DFT(I(x,y))| 2f₁ evaluating the DFT at the frequency 2f1.

[0067] The spatially resolved polarization plane rotation a(x,y) is determined from the difference between formulas 9 and 10. Reference symbol list

[0068] 10 Differential Phase Polarimeters

[0069] 12 light sources

[0070] 14 Irradiation light

[0071] 16 Rehearsal room

[0072] 18 examinees

[0073] 20 Linear polarizer

[0074] 22 Detector

[0075] 24 Primary light source

[0076] 26 Imaging optics

[0077] 28 beam bundlers

[0078] 30 Primary Polarizer

[0079] 32 test specimen lights

[0080] 34 Backlight

[0081] 36 Primary polarizer drive

[0082] 38 Linear polarizer drive

[0083] 40 evaluation units

[0084] 01 first rotation angle

[0085] 02 second rotation angle

[0086] / i Rotational frequency of the primary polarizer

[0087] / (x,y) Intensity

[0088] x location coordinates on the detector

[0089] y location coordinates on the detector

[0090] z Direction of propagation of light along the optical axis

Claims

Patent claims 1. Differential phase polarimeter (10) with (a) a light source (12) for polarized irradiation light (14), (b) a sample chamber (16) for a test specimen (18), wherein the sample chamber (16) is arranged so that the test specimen (18) can be irradiated with the irradiation light (14), (c) a linear polarizer (20) arranged behind the sample chamber (16) in the direction of light propagation, and (d) a detector (22) which (i) is arranged behind the linear polarizer (20) in the direction of light propagation and (ii) is designed to detect the light intensity of the irradiation light (14) that has passed through the linear polarizer (20), characterized by the fact that (e) the detector (22) is spatially resolving and is arranged in such a way that both test object light (32) that has passed through the test object (18) and background light (34) that has not passed through the test object (18) can be detected.

2. Differential phase polarimeter (10) according to one of the preceding claims, characterized by an evaluation unit configured for automatically calculating, with spatial resolution, a rotation of the polarization plane caused by the test specimen (18). (a) a test specimen light intensity of the test specimen light (32) and (b) a background light intensity of the background light (34).

3. Differential phase polarimeter (10) according to one of the preceding claims, characterized in that the light source (12) (a) a primary light source (26) for emitting divergent irradiation light (14), (b) a beam bundler (28) for bundling the divergent irradiation light (14) and (c) has a primary polarizer (30) for linearly polarizing the irradiated light (14).

4. Differential phase polarimeter (10) according to one of the preceding claims, characterized in that the spatially resolving detector (22) is an area detector.

5. Differential phase polarimeter (10) according to one of the preceding claims, characterized in that (a) the primary polarizer (30) is designed to be rotatable about an angle of rotation ( and / or (b) the linear polarizer (20) is designed to be rotatable about a second rotation angle <>2) so that a periodic intensity modulation can be set by rotating the primary polarizer (30) and / or the linear polarizer (20).

6. Differential phase polarimeter (10) according to one of the preceding claims, characterized in that the irradiation light (14) in the sample chamber (16) is parallelized.

7. Differential phase polarimeter (10) according to one of the preceding claims, characterized in that the evaluation unit is designed to automatically execute a procedure with the steps (i) Rotating the primary polarizer (30) and / or the linear polarizer (20), (ii) Detecting a light intensity of the test object (I P (x,y, φ)) as a function of at least one rotation angle (φ1,φ2), (iii) Detecting background light intensity (I H (x,y, φ)) depending on at least one rotation angle (φ1, φ2), (iv) Calculating a polarization rotation (a) of the test object (18), in particular spatially resolved, from the test object light intensity (I P ) and the background light intensity (I H ).

8. Differential phase polarimeter (10) according to claim 7, characterized in that the evaluation unit is designed to automatically execute a procedure with the steps (i) Rotating the primary polarizer (30) relative to the linear polarizer (20) at a fixed rotation frequency (ii) Recording a test specimen light intensity (I P (x,y, t)) as a function of time (t), (iii) Detecting background light intensity (I H (x,y, t)) as a function of time (t), (iv) Calculating a polarization rotation (a) of the test object (18), in particular spatially resolved, from the time-dependent test object light intensity (I P ) and the time-dependent background light intensity (I H ).

9. Differential phase polarimeter (10) according to claim 7 or 7, characterized in that the evaluation unit is configured for automatically performing a method comprising the steps (i) Fourier transform of the light intensity of the test object (I P (x,y, t)), so that a test specimen light phase (φ1(x,y) + α(x,y)) is obtained, (ii) Fourier transform of the background light intensity (I H (x,y, t)), so that a background light phase φ1(x,y) is obtained, and (iii) Calculate the desired polarization rotation α(x,y) as the difference between the test specimen light phase (φ1(x,y) + α(x,y)) and the background light phase φ1(x,y).

10. Differential phase polarimeter (10) according to one of claims 7 to 9, characterized in that the evaluation unit is designed to automatically execute a procedure with the steps (i) Determining at least one area within a test object image from pixels that encode the test object light intensity, (ii) Calculating a mean over the range and / or (iii) Calculating an inhomogeneity parameter that accounts for inhomogeneity of the test specimen light intensity and / or polarization rotation a(x,y) is encoded within the range.

11. Method for determining a polarization plane rotation of a test specimen (18) comprising the steps: (i) Arranging the test specimen (18) in a sample chamber (16), in particular a difference phase polarimeter according to one of the preceding claims, (ii) irradiating the test specimen with polarized irradiation light (14) such that part of the light passes through the test specimen (16) and part of the light does not pass through the test specimen (16), (iii) Detecting test object light (32) that has passed through the test object (16) and background light (34) that has not passed through the test object (16) using a spatially resolved detector (22) and (iv) Determining the polarization plane rotation of the test specimen (16) from a test specimen light intensity of the test specimen light (32) and a background light intensity of the background light (34).

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