Device and method for determining three-dimensional particle positions and particle velocities and for determining particle sizes

Abstract

The invention relates to an imaging system, comprising an optical sensor; and an optically birefringent material arranged in an optical path between the optical sensor and a three-dimensional space to be sensed. An optical axis of the optically birefringent material is inclined relative to the optical path by an inclination angle.

Description

[0001] Device and method for determining three-dimensional particle positions and particle velocities as well as for determining particle sizes

[0002] Field of invention

[0003] The invention relates to a device and a method for simultaneously determining the three-dimensional position and size of solid, liquid, and gaseous particles dispersed in a fluid. By tracking individual particles, the invention allows the volumetric flow velocity of the fluid to be measured. Furthermore, the size distribution and size-dependent behavior of particles in suspensions, e.g., the sedimentation rate, can be determined.

[0004] Measuring particle sizes (or the sizes of droplets or cells) is a frequently necessary task in almost all fields, for example, for characterizing aerosols, monitoring and analyzing environmental pollution, and examining biological and medical samples. Current systems require a second optical access point for backlighting, two cameras, and complex evaluation of the particle images to derive the optical point spread function. Therefore, there is a need to improve the determination of structural dimensions or to provide a suitable device for this purpose.

[0005] Object of the invention

[0006] The object of the invention is to overcome the disadvantages of devices known from the prior art and to provide a simple and cost-effective device for simultaneously determining the spatial position and size of particles in a measuring volume, i.e., in a three-dimensional space, as well as a method for operating this device. The method and the device should be suitable for both stationary and moving particles, which may be in a solid, liquid, or gaseous state and are carried by a fluid. The positions of several particles should be detectable simultaneously, and their motion profiles should be continuously traceable in order to ensure a measurement of the volumetric flow velocity of the fluid based on these motion profiles and to obtain a flow velocity field of the fluid.

[0007] Solution to the task

[0008] The object of the invention is solved by an imaging system according to claim 1 and a method according to claim 14. Advantageous embodiments of the invention are disclosed in dependent claims 2 to 13, relating to the imaging system, and 15 to 22, relating to the method.

[0009] Brief description of the drawings

[0010] Figs. 1a and 1b: Schematic representation of an inventive device or imaging system.

[0011] Fig. 1c: Schematic representation of an example of a device for an imaging system. Fig. 2: Schematic representation of the formation of the double image in the camera's image plane due to birefringence. The image plane is perpendicular to the plane of the drawing. To show the double image, the image plane has been rotated to the plane of the drawing. For simplicity, the lens arrangement has not been shown.

[0012] Fig. 3 : Measured particle image diameters in x and y directions for particles with a diameter dp = 2.5 pm for the ordinary and extraordinary partial jet.

[0013] Fig. 4: Measured particle image areas resulting from the ordinary and extraordinary partial jets for two different particle diameters (d P ), applied in the A O -A a -Level.

[0014] Fig. 5: Calibration function as a function of z', measured for two different particle diameters d P .

[0015] Fig. 6: Measured total area (A total = A o + A a ) of the double images depending on the particle position, for two different particle diameters.

[0016] Fig. 7: Exemplary embodiment of the device according to the invention.

[0017] Fig. 8 Flowchart of an example of a method for an imaging system or device.

[0018] Fig. 9: Example of a method for an imaging system or a device.

[0019] Fig. 10: Example of a system comprising a microscope and a computer system.

[0020] Detailed description of the solution

[0021] Device according to the invention for an imaging system

[0022] Fig. 1a shows a schematic representation of a device according to the invention, equipped with a lens arrangement, here consisting of two lenses, a camera, and a birefringent material in the detection path. In a further embodiment, see Fig. 1a, an imaging system according to the invention can be configured as follows: A device has an optical imaging system comprising at least one lens and a camera. The camera has a light-sensitive sensor, e.g., a photodiode array, a CDD sensor, or a CMOS sensor. It can be a color camera or a monochrome camera, i.e., it can have a color sensor or a monochrome light-sensitive sensor. Preferably, the camera detects light of a wavelength or wavelength range in the visible spectral range (400 nm to 700 nm) or in the near-infrared (700 nm to 1100 nm).

[0023] In one embodiment, the optical axis of the at least one lens can define a detection path, i.e., the detection path runs along the optical axis of the lens. As can be seen, for example, in Fig. 7, further optical elements can be arranged in the detection path of the device. This allows the detection path to deviate from the optical axis of the lens, since, for example, the mirror 5 or the lens 10 can cause a deflection of the detection path from the optical axis of the lens (part of the front optics 6). The imaging system can also include a lens arrangement with two or more lenses that are spatially aligned such that they have a common optical axis. To describe the orientation of the device in space, an orthogonal coordinate system with the axes x, y, z is introduced. Its z-axis lies in the direction of the optical axis of the lens or the lens arrangement, also referred to as the depth direction.The camera is positioned outside the lens assembly in the detection path. It thus limits the detection path at one end. The lens or lens assembly defines a focal plane at the opposite end of the detection path. A particle located within this focal plane in a measurement volume is imaged onto the camera. To achieve this image, the optical imaging system preferably comprises a front optic for capturing the light emitted by particles in the measurement volume and another lens for focusing this light onto the camera. The front optic can, for example, comprise a single lens, multiple lenses, or an objective lens. In this prior art device, the camera thus registers an image, i.e., a single image, of the particle. The focal plane is perpendicular to the z-axis, i.e., parallel to the xy-plane.Preferably, the light-receiving surface of the camera, i.e., the image plane, lies in the xy plane.

[0024] According to the invention, as shown in Figs. aa and bb, at least one optically birefringent material 8 is positioned in the detection path 7.

[0025] Furthermore, Fig. 1a shows a lens arrangement, here consisting of two lenses, a particle located in the region of two (explained below) focal planes f0,f a The optical path consists of a light-sensitive sensor of a camera onto which a double image of the particle (explained below) is projected. The double image comprises two individual images separated by a distance Ay. An orthogonal coordinate system (x, y, z) is adapted to the arrangement. The lens arrangement shown is exemplary; more or fewer lenses can be arranged in the optical path.

[0026] As shown in Fig. 2, the optical axis of the optically birefringent material 8, commonly referred to as the c-axis, has a suitable oblique orientation with respect to the z-axis, i.e., with respect to the detection path. It is inclined relative to the detection path by an angle of inclination that must be non-zero and non-90°, whereby light (scattered light, fluorescent light) emanating from a particle in the measurement volume is split upon entering the optically birefringent material into two linearly polarized partial beams perpendicular to each other: an ordinary partial beam, the o-part o, and an extraordinary partial beam, the o-part a. The angle of inclination of the optical axis of the optically birefringent material (c-axis) relative to the detection path (z-axis) is determined by the section of the optically birefringent material and by its orientation within the detection path.An orientation perpendicular to the z-axis, i.e., plane-parallel surfaces to the xy-plane, is preferred. A suitable inclination angle of the optical axis of the optically birefringent material relative to the detection path is in the range of 5° to 85°, preferably in the range of 15° to 50°. The inclination angle can be predetermined by the section of commercially available single crystals. For example, lithium niobate (128° Y-rotated) has an inclination angle of 38°, which is very suitable. At an inclination angle of zero, the o- and o-beams are not separated; the device is ineffective in this special case.

[0027] The o-ray follows Snell's law of refraction. It is not refracted when incident perpendicularly (angle of incidence α = 0) onto the optically birefringent material, since an angle of incidence α = 0 results in an angle of refraction β = 0. It therefore travels in a straight line through the optically birefringent material, i.e., without changing direction upon entering or exiting it, thus taking the shortest possible geometric path. The law of refraction does not apply to the α-ray; it is refracted even when incident perpendicularly onto the optically birefringent material, for example, a thin disk. Consequently, the o-ray is refracted and deflected upon entering the optically birefringent material, and then refracted again upon exiting it, returning to its original direction.The o-beam travels a greater geometric path length in the birefringent material than the o-beam. The o-beam is thus shifted parallel to the o-beam by a distance Ay. The o-beam and the o-beam therefore strike the camera at a distance Ay, resulting in a double image for each particle, consisting of two particle images spatially separated by the distance Ay on the camera. These images are referred to as the a-image and the o-image. The o-image is generated by the o-beam, and the o-image by the o-beam. The optically birefringent material exhibits a direction-dependent optical density, i.e., optical anisotropy. Therefore, the o-beam and the o-beam have different refractive indices n0,n. a .

[0028] The different directions and thus different geometric path lengths of the two partial rays within the optically birefringent material as a result of refraction, as well as the different refractive indices n0,n a The two partial beams in the birefringent material result in different optical path lengths for each partial beam. This leads to the formation of two focal planes in the detection path within the measurement volume, instead of a single focal plane in the absence of an optically birefringent material: a first focal plane for the o-partial beam f0 and a second focal plane for the a-partial beam f. a, which are axially shifted relative to each other in the z-direction. If, in an optically birefringent material, the optical path length of the o-partial beam is shorter than the optical path length of the a-partial beam, then the first focal plane, i.e., the focal plane of the o-partial beam, is at a greater distance from the camera than the second focal plane, i.e., the focal plane of the o-partial beam, as schematically illustrated in Figs. 1a, 1b, and 2. Consequently, depending on the position of a particle in the z-direction, i.e., depending on its distance to the focal planes f0 and f1, the following applies: a The two particle images projected onto the camera, the o-image and the a-image, have different levels of sharpness. If the particle is located in the focal plane / OIf the particle is in the focal plane, the a-image is sharp, while the a-image is blurred. If the particle is in the focal plane, the a-image is sharp, while the o-image is blurred. When the particle is positioned between the two focal planes, the two images show different degrees of blurriness, depending on the particle's distance from the focal planes, as illustrated in Fig. 2. The a-image produced by the extraordinary partial beam (visible at the bottom of Fig. 2 on the camera plane rotated 90° into the plane of the drawing) is also astigmatically distorted.

[0029] The axial distance between the two focal planes f0,f a The lateral distance and direction from the o-image to the a-image in the camera plane, the xy-plane, in the z-direction as well as the lateral distance and the direction of the o-image to the a-image in the camera plane, the xy-plane, depend on the chosen optically birefringent material, i.e. on the refractive indices n0,n aThe axial distance between the two focal planes depends on the section of the single crystal that determines the position of its optical axis, the c-axis, the thickness of the single crystal, its orientation in the optical setup, the refractive index of the surrounding medium, and the wavelength of the light scattered by the particles and captured by the camera. If none of the parameters listed above vary, the axial distance between the two focal planes f is... o ,f a In the z-direction, the lateral distance and the direction from the o-image to the a-image in the xy-plane are constant. These two distances, as well as the orientation of the double image, consisting of the o-image and the a-image, in the xy-plane can therefore be defined and set by a person skilled in the art by a suitable selection of the aforementioned parameters.

[0030] Sufficient separation can be achieved by selecting a suitable non-zero inclination angle between the detection path and the optical axis of the optically birefringent material. Suitable inclination angles are given above. If the material is a single crystal, these angles are realized by appropriately cutting the material. Additional measures include: a) Selecting an optically birefringent material with strong optical anisotropy, e.g., yttrium vanadate, YVO4. Such materials exhibit a particularly large difference in refractive indices [n0-n] compared to other optically birefringent materials at the same inclination angle. a a) The thickness of the optically birefringent material, or in the case of a thin disk, the disk thickness, is chosen to be sufficiently large. Depending on its transmission and possible aberrations, the thickness of the material can be selected within a wide range between 50 pm and 10 mm.

[0031] The optically birefringent material can be positioned at any point in the detection path between the measurement volume and the camera, independent of the lens arrangement. Therefore, the lens arrangement is not shown in Fig. 2. The measurement volume can be configured as free space, with the two axially offset focal planes f0 and f1 / 2 in the z-direction. aParticles can be located within the measurement volume. They can enter the measurement volume from the environment, move freely within the volume in all spatial directions, and exit freely into the environment without their movement being restricted by walls or other barriers. For example, an airflow containing particles can traverse the measurement volume. In this case, the birefringent element is positioned separately in front of the front optics or at another location along the detection path. Alternatively, the measurement volume can contain a channel, such as a mini- or microchannel, through which a fluid containing suspended particles flows. Microchannels typically have an inner diameter between 5 mm² and 900 pm, while minichannels have an inner diameter between 1 mm and 50 mm. Channels with larger inner diameters are also usable. The channel can run in any direction.To ensure the simplest possible image acquisition and analysis, the channel should run parallel to the x-plane. The channel has a wall with at least one transparent section, such as a transparent channel floor, through which the particles can be observed. This transparent section can be formed by the optically birefringent material. Alternatively, the optically birefringent material can be directly bonded to the transparent section, for example, by being glued on. The optically birefringent material can also be positioned independently of the transparent section, between the transparent section and the front optics, or at another location along the detection path.

[0032] The particles to be observed can be easily illuminated by natural ambient light, for example, if the measurement volume is designed as free space, or if a fluid loaded with particles is passed through a transparent channel within the measurement volume. However, for commercial applications, especially for high-throughput applications and / or very small particles, it is recommended to use an external light source. Preferably, it should emit light in the visible wavelength range (VIS, 400 nm to 700 nm) or in the near-infrared range (NIR, 700 nm to 1100 nm). The light source can be pulsed or continuous (as a CW source, continuous wave). It can be monochromatic or broadband. Suitable examples include a laser (monochromatic), an LED or a halogen lamp (broadband), and similar light sources. Nd:YAG lasers with wavelengths of 1064 nm (NIR) and 532 nm (VIS) are particularly suitable.

[0033] In commercial applications, often only optical access is available. To ensure illumination by an external light source even in this case, a semi-transparent mirror is inserted into the detection path. The light incident on the mirror from the light source via an illumination path is deflected into the detection path and directed towards the measurement volume. The light reflected by the particles in the measurement volume, the wavelength of which may vary, passes through the semi-transparent mirror in a straight line and strikes the camera. Thus, coaxial illumination and detection of the particles is achieved. The semi-transparent mirror can be polarizing and / or dichroic, configured as a short-pass or long-pass filter. Its parameters are selected depending on the wavelengths of the light scattered by the particles and the wavelength of the light source.

[0034] If the specific application allows, the particles are preferably illuminated from a different direction via a second, additional optical access point. In this case, the partial overlap of the illumination path with the detection path is avoided. The semi-transparent mirror is no longer needed.

[0035] In both cases, i.e., in applications with one or two optical access points, the illumination path may contain further optical and mechanical components for focusing, optical filtering and / or deflecting the illumination light.

[0036] The detection path can also include further optical and mechanical components for focusing, optical filtering, or deflecting the light from the particles, as well as optical components for correcting aberrations. These components, together with the components already described (lens or lens array, camera, birefringent material), form an imaging system for imaging the particles.

[0037] The camera's light-sensitive sensor can be based on various technologies, such as CCD, CMOS, or sCMOS, and can have any number and size of individual sensor elements (pixels). The camera can be a monochrome or color camera.

[0038] When using an external pulsed light source, the device is equipped with a synchronization unit to synchronize the pulsed illumination of the particles and the acquisition of the particle double images. The device is also equipped with a digital evaluation unit connected to the camera. For example, the evaluation unit could be a device 130 as shown in Fig. 1c. The camera transmits the acquired double images to the evaluation unit, where, using the method described below, the particle positions and particle velocities as functions of time, as well as the particle sizes, are determined from the acquired double images. A current standard personal computer, e.g., with an Intel Core i5 processor, can be used as the evaluation unit.

[0039] Optionally, the device can include an optical filter element for wavelength-dependent filtering, which allows the illumination light to be separated from the detected light. Such a filter element is useful in conjunction with a camera when illumination is provided by short-wavelength light, but the particles emit light of a longer wavelength through fluorescence.

[0040] The device according to the invention has the advantage that only a single optical access point and a single monochrome camera are required to measure double images of stationary and moving particles. The device can therefore be designed very simply and cost-effectively. More complex and expensive designs are possible, as described above (e.g., using a color camera and / or several optically birefringent materials).

[0041] Fig. 1b shows an exploded view of an example of an imaging system 100. The imaging system 100 can be an optical imaging system. The term "imaging system 100" refers to an image processing imaging system that includes optical, electronic, and / or software-based components that convert a physical object into a digital image and / or analyze, process, or visually display this image. The imaging system 100 can be used, for example, in microscopy, but also in other imaging applications. The proposed concept of the imaging system 100 can be applied to various types of microscopy, such as microscopy in a laboratory, microscopy for the purpose of materials testing, or microscopy during a surgical procedure.

[0042] The imaging system 100 comprises an optical sensor 25. In another embodiment shown in Fig. 1a, a camera sensor is shown as the optical sensor 25. The optical sensor 25 is an electronic component that converts incident light into digital image data. For this purpose, the optical sensor 25 comprises light-sensitive elements (pixels) that generate a signal based on the incident light. The optical sensor 25 then converts the signals into digital image points (pixel values), for example, by means of an analog-to-digital converter. It can be any type of optical sensor 25, such as one using CCD or CMOS technology (charge-coupled device; complementary metal-oxide semiconductor). In one embodiment, the optical sensor 25 comprises, for example, a monochrome light-sensitive sensor or a color sensor.

[0043] The imaging system 100 further comprises an optically birefringent material 8. An optically birefringent material is, for example, an anisotropic medium in which the speed of light depends on the direction of propagation and the polarization direction of the incident light. This means that a light ray is split into two components. For example, a light ray can be split into an ordinary ray (o-ray) and an extraordinary ray (a-ray).

[0044] The birefringent material 8 can be a uniaxial birefringent material. Alternatively, any other beam splitting can be implemented, depending on the birefringent material. The two beams have different refractive indices and are refracted in different directions. Alternatively or additionally to a uniaxial optical birefringent material, the optical birefringent material 8 can be a biaxial optical birefringent material, e.g., mica or topaz. In this case, a splitting into ordinary and extraordinary beams does not necessarily occur. Instead, two extraordinary beams can form; these can be referred to as the fast and slow beams, respectively.

[0045] The optically birefringent material 8 can be any material suitable for birefringence. For example, the optically birefringent material 8 can comprise a (solid) single crystal or a liquid crystal. Alternatively or additionally, the optically birefringent material 8 can comprise a solid and / or liquid optically isotropic material, e.g., glass, to which birefringence, referred to as induced birefringence, is induced by applying an electrical or mechanical stress. When using a single crystal, a uniaxially optically birefringent material is preferred, e.g., lithium niobate, calcite (CaCO3), beryl, rutile (TiO2), yttrium vanadate (YVO4), tellurium dioxide (TeO2), or langasite.

[0046] The optically birefringent material 8 can, for example, comprise a thin disk oriented perpendicular to an optical path 7. Two parallel surfaces of the optically birefringent material 8, e.g., the disk, can be oriented such that they are essentially parallel to an xy-plane of a three-dimensional space to be measured. In these cases, the thickness of the disk can be determined by the distance between these two surfaces. They can serve as the entrance surface for the incident light and as the exit surface from the optically birefringent material 8. For example, the optically birefringent material 8 is approximately 100 micrometers to approximately 1 mm thick, but it can also have a thickness of several tens of millimeters, e.g., 10 mm. Alternatively or additionally, the optically birefringent material 8 can comprise one or more optically birefringent lenses.For example, one or more lenses encompassed by the imaging system 100 can consist of or comprise the optically birefringent material. An optical path difference between two rays (also referred to as phase shift) through the optically birefringent material 8 depends, in particular, on the thickness (and the refractive index difference) of the optically birefringent material 8. Accordingly, by appropriately selecting the thickness of the birefringent material 8, an optical path difference between the plurality of rays, e.g., two different extraordinary rays or an ordinary and an extraordinary ray, can be set. This allows, in particular, the focal plane in the measurement volume and / or the orientation of the birefringent material 8 relative to the optical path 7 to be appropriately selected. Likewise, a lateral offset between the structure 35 in the first image and the second image can be influenced.

[0047] The optically birefringent material 8 is arranged in the optical path 7 between the optical sensor 25 and the three-dimensional space to be detected (measuring volume). The optical path 7 corresponds, for example, to the propagation axis of the ordinary beam. The arrangement of the measuring volume, the optically birefringent material 8, and the optical sensor 25 is configured such that light emitted from the three-dimensional space to be detected and striking the optically birefringent material 8 is split into two beams, and these two beams can be detected by the optical sensor 25. In embodiments with more than one birefringent material, more than two beams can be generated accordingly. Any feasible combination of ordinary beam and extraordinary beam(s) is possible, for example, one ordinary beam and three extraordinary beams. Alternatively, only three beams can be generated, e.g.,...For example, three extraordinary rays can be extinguished, such as a potential fourth ray.

[0048] The optical axis of the optically birefringent material 8 is inclined relative to the optical path 7 by an angle of inclination. This angle of inclination is preferably neither 0° nor 90°. The angle of inclination is formed with respect to a portion of the optical path 7 located at the entrance surface of the optically birefringent material 8. In particular, the angle of inclination can refer to an inclination of one of the two parallel surfaces of the birefringent material relative to the optical axis at the point where the surface meets the optical axis. For example, an angle between the surface facing the measurement volume and the optical axis at the point where it intersects this surface can be called the angle of inclination. Alternatively, in a uniaxial crystal, the angle of inclination can also be determined by the inclination of the optical axis (also called the c-axis) of the birefringent material 8 to the optical path 7 (of the imaging system 100).In this case, the inclination angle refers to the optical path 7 (of the imaging system 100) as it passes through the birefringent material.

[0049] The optical path 7 (of the imaging system 100) can be an imaginary connection along which the light in the imaging system 100 passes through all optical components (e.g., objective, tube, eyepiece). That is, the optical path can correspond to a central ray path of a central or principal ray. The optical path can also be referred to as the direction of observation. If the optical axis of the birefringent material 8 is inclined at an angle of approximately 45° to the optical path 7 (of the imaging system 100) (e.g., in a thin section or by a corresponding section), the birefringent material 8 is oriented such that the birefringence (splitting into ordinary and extraordinary rays) is maximally detectable.

[0050] As described below, a detection path can include further optical and mechanical components for focusing, optical filtering, or deflecting the light in imaging system 100, as well as optical components for correcting aberrations. The optical path can correspond to the detection path along these components. In this case, the optical path can therefore be partially or completely identical to the detection path described below.

[0051] Furthermore, the optically birefringent material 8 is designed to split light into two beams, forming a first focal plane and a second, different focal plane in the three-dimensional space to be detected. For example, the optically birefringent material 8 can split the light into an ordinary and an extraordinary beam. Because two different focal planes can be formed in the three-dimensional space to be detected, a double image (comprising a first image and a second image) can be detected or measured with only one optical sensor 25 in only one measurement interval of the optical sensor 25 in the three-dimensional space to be detected. In particular, the use of a second optical sensor 25 can be dispensed with. This improves the determination of the position of a structure 35 in the three-dimensional space to be detected.In particular, the dual image allows for improved position determination based on the two different focal planes. Two different optical sensors, each covering a different focal plane, can thus be replaced by a single optical sensor 25.

[0052] Furthermore, the imaging system 100 is configured to image a detectable area or structure 35 in the three-dimensional space to be detected at least twice, laterally separated, onto the optical sensor 25. In particular, the structure 35 can be imaged onto the optical sensor 25 with varying sharpness and lateral separation depending on its position in the three-dimensional space to be detected relative to the resulting focal planes. That is, the structure 35 can have a different sharpness and lateral position in the first image than in the second image. In the special case where the structure 35 is located midway between the two focal planes, the detectable structure 35 or area can be equally sharp in both images and exhibit only a lateral offset.

[0053] In other words, the imaging system 100 can enable the acquisition or generation of a double image with only a single camera. This acquisition can occur within a single measurement interval of the optical sensor 25. If the illumination of the three-dimensional space to be captured is pulsed, the measurement interval can alternatively or additionally refer to the duration of the illumination pulse. This means that the measurement interval can be defined either by the optical sensor 25 itself (e.g., via an exposure time) or by controlling a light source.

[0054] This means that the first and second images can be captured essentially simultaneously using the imaging system 100. Accordingly, the double image can be captured or determined based on the simultaneous acquisition of two individual images.

[0055] In one embodiment, the light, which is split into two beams by the optically birefringent material, can be unpolarized or at least partially polarized, in particular fully polarized, and exhibit both intrinsic polarizations of the two beams. That is, the light is unpolarized or at least partially polarized when passing through the optically birefringent material and exhibits both intrinsic polarizations of the two beams. The imaging system 100 can include a light source for illuminating the three-dimensional space to be captured. Alternatively, the light source can also be external to the imaging system 100. For example, the light source can be a background light, such as natural light. In this case, the imaging system 100 cannot have its own light source.

[0056] For example, the three-dimensional space to be captured can be illuminated by natural ambient light. In this case, the light, which is split into two beams, can be unpolarized. Alternatively or optionally, a special light source can be used to illuminate the three-dimensional space to be captured. The special light source can be pulsed or continuous (as a CW source, continuous wave). It can be monochromatic or broadband. Suitable examples include a laser (monochromatic), an LED or a halogen lamp (broadband), and similar light sources. Nd:YAG lasers with wavelengths of 1064 nm (NIR) and 532 nm (VIS) are particularly suitable. The special light source can be integrated into the imaging system 100 or be external to it. For example, the special light source can be controlled by a device of the imaging system 100.When using a special light source, at least partially polarized light can be used or generated to measure the first image and the second image, encompassing both intrinsic polarizations of the two beams.

[0057] As described above, the optically birefringent material is designed to split light into two beams. Because the light incident on the optically birefringent material in imaging system 100 is unpolarized or at least partially polarized, exhibiting both intrinsic polarizations, the birefringent property of the optically birefringent material 8 can be exploited to capture two different images within a measurement interval for generating or determining a double image. The intrinsic polarizations of the optically birefringent material 8 can be polarizations along one of its intrinsic directions. This means that at least partially polarized light can exhibit light that is already polarized in one direction of a first polarization, for example, the ordinary polarization, and in one direction of a second polarization, for example, the extraordinary polarization.Accordingly, both unpolarized and at least partially polarized light are split into two beams by the birefringent material. This means that the two beams can be detected by the optical sensor 25. In other words, light can reach the optical sensor 25 that is unpolarized or at least partially polarized, exhibiting both intrinsic polarizations of the optically birefringent material 8. Therefore, no filtering according to a specific polarization can occur by the imaging system 100. For example, no optical element for filtering the light, such as a polarizing filter, can be arranged in the detection path.

[0058] A double image can, in particular, comprise a double representation of the (under investigation) structure 35, e.g., a particle. As described below, the double image can consist of two individual images separated by Ay. An orthogonal coordinate system (x, y, z) is adapted to the arrangement. That is, the double image can be generated or calculated from two individual images, the first image and the second image.

[0059] In one embodiment, the tilt angle can be set between 5° and 85°. This allows a phase shift between the two beams to be adjusted so that it can be detected by the optical sensor 25 (and evaluated by an evaluation unit). As described above, a maximum phase shift can be achieved for a tilt angle of approximately 45°. Increasing the thickness of the birefringent material 8 can further increase the phase shift. Accordingly, thickness and tilt angle can be combined to improve the design of the optical imaging system 100 or the evaluation of the double image.

[0060] In one embodiment, the optical axis of the optically birefringent material 8 (also referred to as the c-axis) can be inclined relative to a surface normal of the optically birefringent material 8. This means that the material can be selectively cut or oriented at a defined angle to the crystal axis. The cut is generally made such that the c-axis is not perpendicular to the surface, but at a specific angle (e.g., 45°) to it. Such an orientation can be chosen to generate specific birefringent effects. Depending on the inclination or cutting angle to the optical axis, the magnitude of the phase shift, the polarization properties, or even the optical path difference between the two beams can be selectively adjusted.

[0061] In one embodiment, a surface normal of the optically birefringent material 8 can be inclined relative to the optical path 7. This means that the birefringent material is not oriented plane-parallel to the optical path 7 of the imaging system 100, but at a specific angle to it. In this case, the optical axis of the optically birefringent material 8 can be parallel to the surface normal. Optionally, the optical axis of the optically birefringent material 8 can also be inclined relative to the surface normal. This can increase the phase shift.

[0062] In general, the inclination of the surface normal to the optical path 7 causes the effective path of light through the birefringent material to lengthen, and the light to travel at an angle through the material rather than perpendicular to the surface. This increases the proportion of light that is not parallel to the optical axis, thus increasing the phase shift. This can lead to a larger phase shift between the two rays, e.g., the ordinary ray and the extraordinary ray, particularly in materials with low birefringence and / or thinness.

[0063] In one embodiment, the imaging system 100 can further comprise a channel in the three-dimensional space to be detected or a lens. The optically birefringent material can be at least partially enclosed by or consist of the channel or the lens. For example, the optically birefringent material can form part of a channel wall. That is, the birefringent material can be part of the channel and / or part of the lens, or be enclosed by the channel or the lens. Accordingly, the integration of the optically birefringent material 8 can be simplified. For example, the lens can be made of or comprise the optically birefringent material 8, so that no further element (for generating birefringence) needs to be arranged in the optical path 7 of the imaging system 100.

[0064] In one embodiment, the optically birefringent material can be configured to split light into an ordinary ray and an extraordinary ray, through which the first focal plane and the second, other focal plane are formed in the three-dimensional space to be detected. In this case, the birefringent material 8 can, in particular, be a uniaxial birefringent material.

[0065] In one embodiment, the imaging system 100 can further comprise a channel arranged in the three-dimensional space to be captured. The channel is designed to guide a fluid flow. The channel can optionally be transparent, thereby improving optical accessibility. Accordingly, the imaging system 100 can be used to determine a structure 35 within a channel. For example, a fluid flow within the channel can be determined based on the determined structure 35.

[0066] Structure 35 can be, for example, a particle, fine dust or pollen, cells, or other organic structures. In particular, the imaging system 100 can determine the position and, optionally, the dimensions of structure 35. For example, the imaging system 100 can allow the investigation of various structures, such as particles in fluids. The fluids can be liquid or gaseous. The particles are generally solid and can be spherical or non-spherical, and may also have complex shapes. The particles can be made of different materials, such as polymers, metal, or glass. They can be functionalized, for example, coated with a luminescent dye and / or a metal. They can also be inorganic or organic particles contained in the ambient air, such as fine dust or pollen.In addition to solid particles in liquids and gases, liquid particles (i.e., droplets) in gases and gaseous particles (i.e., bubbles) in liquids can also be examined. Examples include water droplets in air and air bubbles in water. Liquid / liquid combinations can also be investigated, such as oil droplets in water.

[0067] In one embodiment, the imaging system 100 can comprise a further optically birefringent material arranged in the optical path 7. An optical axis of the further optically birefringent material is inclined relative to the optical path 7 by an angle of inclination. Accordingly, more than two rays can be generated when light passes through the plurality of optically birefringent materials. This enables multiple imaging. In particular, the optically birefringent material 8 and the further optically birefringent material are inclined at different angles of inclination relative to the optical path 7. This reduces or eliminates destructive interference when acquiring a multiple image.

[0068] This means that the imaging system 100 can be equipped with two or more birefringent materials instead of just one. Instead of a double image of a particle, the optical sensor 25, for example a camera, then registers multiple images of the structure 35, for example a particle. This enables multiple measurements and thus reduces measurement uncertainty.

[0069] For example, the double image can be detected using two crossed 128° lithium niobate plates of the same thickness. A 90° rotation around the crystallographic Z-axis shifts the double image in both lateral directions. AK ~ AX. Furthermore, both structures in the double image are now astigmatically distorted. This is because, upon transitioning to the second plate, rotated by 90°, the ordinary ray becomes the extraordinary ray and vice versa. As a result, identical point spread functions are obtained, with the meridional and sagittal planes rotated by 90°. The required similarity of the point spread functions of both rays for deriving the calibration function (as described below) for the depth position, as required by the fundamental principle, is still maintained.

[0070] With this combination, the information content of both images of the structure is initially the same, resulting in "only" a two-part measurement. However, this simple example of the double plate illustrates how the astigmatism, and thus the focus positions in the x and y directions, can be modified. For example, by combining different birefringent materials, such as different plates, with, for instance, different material, c-axis orientation, and / or thickness, asymmetries can be created to axially shift three or four focus positions, preferably uniformly, across the three-dimensional space to be captured. This can improve a measurement of the structure, such as determining a particle diameter, because the focus positions offer higher sensitivity for measuring the structure, such as particle size.This makes it possible to build a multifocal system which, instead of double images, produces multiple images of a structure, e.g., a particle.

[0071] A double image can, in particular, comprise a double representation of the (under investigation) structure 35, e.g., a particle. As described below, the double image can consist of two individual images separated by Ay. An orthogonal coordinate system (x, y, z) is adapted to the arrangement. That is, the double image can be generated or calculated from two individual images, the first image and the second image. The double image can be determined or generated by acquiring and processing a first image and a second image. For example, the first image can depict the first focal plane and the second image the second focal plane. By using the birefringent material 8, which results in two focal planes, two focal positions of the structure 35 can be shifted relative to each other in the three-dimensional space to be captured.This offset of structure 35 in the first image to the second image can be reflected in the double image, see also Fig. 2.

[0072] Figures 1b and 1c show schematic representations of an example of a device 130 for an imaging system 100 and a corresponding imaging system 100, which may comprise the device 130 or be communicatively coupled to it. The device 130 can control various aspects of a microscope of the imaging system 100, which may be an optical imaging system, and of the entire imaging system 100, and / or process various types of sensor data from the imaging system 100. Consequently, the device 130 can be implemented as a computer system that interfaces with the various components of the imaging system 100, e.g., optical sensor 25. The device 130 can be part of the imaging system 100. Alternatively, the device 130 can be communicatively coupled to the imaging system 100. For example, the device 130 can be a ready-to-use module that can be connected to the imaging system 100.

[0073] The device 130 comprises, as shown in Fig. 1c, a data processing circuit 134 and a memory 136. Optionally, the device 130 also comprises one or more interfaces 132. The data processing circuit 134 is connected to the memory 136 and to the optional one or more interfaces 132. In general, the functionality of the device 130 can be provided by the data processing circuit 134 (for example, for determining the position of the structure), in conjunction with the one or more interfaces 132 (for exchanging information), and / or with the memory 136 (for storing and / or retrieving information).

[0074] Computer-implemented method for the three-dimensional determination of particle positions, particle velocities and particle sizes from double images registered by the camera.

[0075] A computer-implemented method is described that uses the double images of particles recorded by the camera to determine their positions, velocities as a function of time, and particle sizes. This measurement is performed for a large number of particles. The number of particles depends, for example, on the measurement volume, the particle concentration in the fluid, and the measurement duration, and can be, for example, 1000 or more. This allows the local flow velocity of a fluid carrying the particles to be determined at a large number of points within a measurement volume. From the multitude of measured local flow velocities, the three-component, vectorial flow velocity field of the fluid, v, can be derived. x (x,y,z), v y (x,y,z) and v z(x,y,z) can be recorded throughout the entire measurement volume. For example, if the measurement volume contains a channel running parallel to the xy-plane and through which a fluid with suspended particles flows, the transverse component of the particle motion (z-component) can be recorded with high accuracy. This allows, for example, the quantitative investigation of turbulent motions in the measurement volume and / or in the channel.

[0076] The starting point for the derivation of the computer-implemented method is a relationship known from the prior art between particle position, particle image diameter, and properties of the imaging optics according to equation (1), which applies to devices for measuring particle sizes that are not equipped with a birefringent material. This includes devices according to the preamble of claim 1 that generate a single image of a particle on a camera. The particle image diameter d, i.e., the diameter of the single image, depends on the spatial position of the imaged particle and the particle size d. p , depends on the refractive index of the medium as well as on the properties of the imaging system (magnification M, numerical aperture or f-number, wavelength) [34,35] and can be described to a very good approximation by a hyperbolic function

[0015] d = a 2 (z — F) + b 2 (1)

[0077] Here, F is the position of the focal plane. The first term under the square root accounts for the defocus. The further the particle is from the focal plane, the larger this term becomes. The particle's position in the z-direction is given by z. The coefficient a is determined by the imaging system. The coefficient b accounts for the geometric component of the imaging with d. p ■ M and the portion of the particle image that is superimposed by diffraction. To a very good approximation, all three components (geometric, diffraction, defocus) can be superimposed quadratically [Error! Reference source could not be found.].

[0078] In the device according to the invention, which is equipped with at least one birefringent material, the camera registers a double or multiple image of each particle instead of a single image. For the sake of simplicity, the following discussion is limited to devices equipped with only one birefringent material, which is uniaxial, so that a double image consisting of an o-image and an a-image is registered. The different intensities of the o-image and the a-image are characterized by the diameter of the respective image or a quantity derived therefrom, e.g., the area of ​​the respective image. For the double images, a system of equations can be given for the o-image and the o-image according to equation (1): where the equations differ essentially only by the axial offset of the two focal planes, where f0 is the z-position of the focal plane for the ordinary partial beam, f aThe z-position of the focal plane for the extraordinary ray. (Note: In the section above on the device, f0 and f0 denote...) a (The reference symbols are the focal planes for the ordinary and extraordinary beams as such; in this section on the procedure, the positions of these two focal planes on the z-axis are now designated as variables.) According to equations (2a) and (2b), the particle images of the o- and a-images have different or the same particle image diameters d0 and d, depending on the position of the particle in the z-direction. a on.

[0079] Initially disregarding optical aberrations, the o- and a-images exhibit the same characteristics, which is why the following holds true for the coefficients: c3 = c2 and c2 = c4. By normalizing the z-position with z' = z / Az, where Az = f a — f0 defines the axial distance between both focal planes, as well as by defining f0 = 0 and f aIf = 1, a functional relationship between the particle image diameter and the z-position can be derived according to the invention:

[0080] The square of the difference in particle diameters is directly proportional to the z-position of the particles, resulting—depending on the assignment of the particle images—in a monotonically increasing or monotonically decreasing, one-to-one calibration function according to equation (3). This function is also independent of the particle size, as illustrated in the figure. The calibration function for measuring the z-position can also be based on quantities derived from the diameters of the particle images, such as the areas A. o , A a of the o and a image.

[0081] The relationship in equation (3) can be derived analytically by knowledge of parameters of the imaging system, e.g. the optical magnification and numerical aperture.

[0082] Alternatively, the relationship in equation (3) can be established by a calibration measurement. This involves positioning particles at known z-positions, recording them, and then using computer-aided analysis to evaluate their particle image diameter. The calibration function according to equation (3) can then be fitted or interpolated based on the measured values, or defined using a lookup table. An advantage of this approach is that optical aberrations, which could lead to non-linearity in the calibration function, can be taken into account, for example, by using a higher-order function that is fitted to the measured calibration values.

[0083] An advantage of a calibration measurement is that the relationship in equation (3) can be reduced to the square of the difference in particle images, so that: z' = — d and the coefficient does not need to be known.

[0084] Furthermore, knowledge of the distance Az = f alone is sufficient a— f0der focal planes on the relationship in equation (3) between the particle positions z and the diameters d o , d a The solution can be concluded by estimating the rise of the calibration function between z' = 0 and z' = 1 during a measurement with a large number of randomly distributed particles in the measurement volume.

[0085] Alternatively, by measuring the particle image diameters of particles at position z' = 0, the coefficient q of the calibration function can be determined with ci = da - di @ z' = 0 (4) and the slope of the calibration function can be estimated according to equation (3).

[0086] Using a large number of particles, whose z or z' position is determined by the calibration function and whose lateral position (x, y) is determined by the position of the particle images on the camera sensor, the flow velocity field of the fluid can be volumetrically recorded by tracking these particles with a series of image recordings (at least 2 images).

[0087] A major advantage is that, unlike the prior art, particles with a monodisperse size distribution are no longer necessary, since the relationship in equation (3) and the calibration function are independent of the particle size. If the depth position z or z' of the particle is known, the measured particle image diameters d0, d can be used to determine the particle size distribution. a Simultaneously, the particle size can be inferred by taking the defocus into account. This results in a quasi-correction of the defocus.

[0088] Taking into account equations (2a) and (2b), the particle diameter d can be determined. p For a measured position z, the following values ​​are specified twice: This takes into account the s Diffraction. If the geometric component of the imaging predominates, e.g., with comparatively large particles, and the particle is located in one of the two focal planes, i.e., z = 0 or z ≤ 0, then the geometric component of the imaging is dominant. r If ' = 1, then the particle size is directly given by the image scale. Consequently, there is a linear relationship between particle diameter and particle image diameter. Even outside the two focal planes, there exists a calibration function that increases monotonically with the particle diameter for any arbitrary position z'. In the case where the diffraction term can be neglected (e.g., for comparatively large particles, such that: M 2 d » d 2The relationship between particle image diameter and particle diameter is linear, independent of the actual position z', and a calibration measurement with at least two particle diameters is sufficient. If the particles are small and diffraction is not negligible, at least three calibration measurements with suitable and small particle diameters are advisable to account for the nonlinear portion of the calibration function. The calibration function can then be created, for example, by fitting it to the measured values. Alternatively, a look-up table could also be used.

[0089] If for each individual position z' a monotonically increasing function of the particle image diameters d0, d a with d p If it exists, this also applies to its areas A o , A aor other derived quantities. The highest sensitivity for measuring particle diameter is found in the focal planes. With increasing distance from the focal planes, the sensitivity decreases due to the increasingly blurred image, as the influence of defocus predominates. Therefore, it is advantageous to consider both images, i.e., the o- and a-images, for measuring the particle diameter, for example, by summing the two diameters or their areas, or by using alternative calculations of the corresponding quantities. The measured total area A serves as an example. tota ] = A o + A a Two different particle diameters are shown in Fig. 6. These do not overlap, which illustrates that the particle size can be determined by knowing the z' position.

[0090] Alternatively, the particle size can also be determined based on the diameter (or a derived quantity) of the cross-correlation of the two particle images. Similar to considering the areas of the two particle images (see Fig. 6), this yields different diameters for the cross-correlation for different particle sizes, so that the particle size can be deduced from a known position z'.

[0091] Considering the exemplary particle images in Fig. 2, the particle image of the extraordinary partial beam is astigmatically distorted and appears elliptical. In this example, the semi-axes of the elliptical particle image lie in the x and y directions. To allow a simple analytical description of the particle images according to equation (1), the diameters (AX,AY) of the particle images in the x and y directions are considered separately. These can be determined, for example, by an intensity threshold, a two-dimensional Gaussian function, or by correlation with known particle images. Thus, in analogy to equation (1), the following results are obtained.

[0092] Equations:

[0093] The coefficients a iand b[ with i = 1 ... 4 are formulated generally. In practice, astigmatism predominates as an aberration, which is why the coefficients a4 = a2 = a3 - a4 and b4 = b2 = b3 = b4 are very good approximations. In Fig. 3, the measured particle image diameters (AX, AY) for the ordinary and extraordinary partial beams are plotted against the axial z-position of 2.5 pm particles used in a calibration measurement. While the two diameters AX0 and AY0 for the ordinary partial beam are very well aligned, the two diameters AX a and AY a The extraordinary partial beam is axially shifted relative to each other. This confirms the astigmatism mentioned above in the imaging of the extraordinary partial beam for this specific implementation. Therefore, when considering the image diameters in the individual directions, there are three focal planes: f0 for the ordinary beam, f1 for the ordinary beam, and f2 for the ordinary beam. a x as well as f a yfor the extraordinary beam. Consequently, by separately evaluating the particle image diameters in the x and y directions, two calibration functions can be derived according to the relationship in equation (3) in order to determine the spatial position of the particles using computer-aided image analysis. It should be noted that Az is direction-dependent; the following applies: Az x = f0— f a x and Az y = f o — f ay These calibration functions also remain independent of the particle size, see Fig. 5.

[0094] If astigmatism is present, it can also be used to determine the spatial position of the particles according to the APTV approaches (see prior art), while the particle image of the ordinary partial beam allows for an undistorted—albeit slightly blurred, depending on the position—observation of the particle. This undistorted observation can be particularly advantageous in flow measurements with particles exhibiting a polydisperse size distribution or with non-spherical particles (cells, droplets) to enable the determination of the spatial position using the Euclidean distance (see prior art, requires particles with a monodisperse size distribution) by correcting the actual particle image size of the extraordinary beam in the 4A-4F plane. Furthermore, by measuring the depth position, the image of the ordinary partial beam can be focused (keyword: autofocus).This would allow for an undistorted observation of the particles, drops or cells in order to optically determine or characterize their size, shape or similar properties.

[0095] Furthermore, the measurement of the spatial position is possible by any combination of the particle diameters AX0 or AY0 with AX a and AY a possible. Likewise, the areas (A0,A) can be used. a The values ​​of both particle images can be used to derive calibration functions. The areas can be determined, for example, via an intensity threshold or by fitting a function, such as a 2D Gaussian function, to the intensity distribution of the particle image. Again using the Euclidean distance (state of the art [Error! Reference source not found.]), the depth position of the particles, i.e., their position in the z-direction, can be determined, but now in the 0-.zl. a-plane. According to Fig. 4, the corresponding calibration function in the 40-4 "-plane depends on the particle size.

[0096] Furthermore, computer-aided image processing based on neural networks and correlation-based evaluation of the double images (o-image, a-image) can also be used to determine the depth position and particle size, similar to those in [Error!

[0097] [Reference source could not be found., Error! Reference source could not be found., Error! Reference source could not be found., Error! Reference source could not be found.] demonstrated approaches. This can be advantageous for evaluating particle images with low particle image intensity, e.g., for particles at the outer ends of the measurement volume (in the z-direction), where, due to the very high degree of blurring, there is also a (partial) superposition of the resulting double images, i.e., the o- and a-images. Correlation can improve the evaluation, as similarities between the two images are highlighted through correlation analysis.

[0098] The determination of particle size at known spatial position is also carried out with an astigmatically distorted a-image of the extraordinary beam analogously to the explanations above without aberration, for example via the measured total area i4. tota ] = Ao + A a of the o- and a-images, as shown in Fig. 6. These do not overlap, which illustrates that the particle size can be determined by knowing the z' position. Alternatively, the particle size can also be determined based on the diameter (or a derived quantity) of the cross-correlation of the two particle images. Similar to considering the areas of the two particle images (see Fig. 6), this yields different diameters for the cross-correlation, so that the particle size can be determined if the z' position is known.

[0099] An embodiment is illustrated in Fig. 7. A fluid flow 2, pumped through a channel 1 and containing individual particles 2a suspended therein, is illuminated continuously or in pulses by a laser 3 with a wavelength of 532 nm via the illumination path 4, which includes the laser 3, optical components such as a mirror 5, and a microscope objective serving as the front optics 6. The detection of the light, which is scattered elastically or inelastically by the particles 2a, is carried out from the rear via the detection path 7, consisting of an optically birefringent base 8 of the channel 1 made of 128°Y lithium niobate, the front optics 6, the mirror 5, a filter element 9, a tube lens 10 of a microscope, and a sensor of a monochrome sCMOS camera 11. A synchronization unit 12 synchronizes the illumination of the particles 2a by the laser 3 with the image acquisition of the particles 2a by the monochrome camera 11.The particles 2a are fluorescent and emit light of a wavelength of approximately...

[0100] 607 nm, which differs from and is longer than the wavelength of the illumination light, which is why the mirror 5 is designed as a dichroic long-pass mirror and the filter element 9 as an optical long-pass filter. The double images of the individual particles 2a imaged onto the camera 11, captured by the camera 11, are transmitted to a personal computer as a digital evaluation unit 13 and evaluated by it, whereby, according to the invention, the three-dimensional position and size of the individual particles 2a in the fluid flow 2 are determined, and by evaluating the data of a plurality of particles 2a, a three-dimensional, three-component velocity field of the fluid flow 2 as well as a particle size distribution, also spatially distributed, in the fluid flow 2 are determined.

[0101] The exemplary embodiment illustrates that the birefringent material 8 does not necessarily have to be connected to a sensor (here, a microscope), but can be located alone, at any position, in the optical detection path 7. Consequently, the invention can be realized simply by combining a standard optical system with image acquisition, here a microscope, with a birefringent material 8. For example, a channel 1 made of birefringent material 8, through which a suspension is pumped, can be used. According to the invention, the three-dimensional position of the particles suspended therein can then be measured. This also enables volumetric flow measurement. The particle sizes can also be measured according to the invention. There is no limitation to a specific microscopy method (brightfield, darkfield, transmitted light, fluorescence, phase contrast, etc.); all methods can be used.Likewise, there is no restriction on the size of the particles and the measurement volume.

[0102] Exemplary embodiments include a method 800 for determining the position of a structure. The structure can be arranged in a region of the measurement volume to be detected. Fig. 8 shows a flowchart of an example of a method 800 for an imaging system. The method 800 can be carried out by an imaging system or a device as described above, for example, as described with reference to Figs. 1a, 1b, and 1c. Alternatively, the method 800 can be carried out by a separate device, e.g., a computer that is communicatively coupled to the imaging system or the device as described above, see Fig. 10. In particular, the method can be carried out by a data processing circuit (also referred to as an evaluation unit).The data processing circuit can be a computer, a processor, a control unit, a field-programmable logic array (FPLA), a field-programmable gate array (FPGA), a graphics processing unit (GPU), an application-specific integrated circuit (ASIC), an integrated circuit (IC), or a system-on-a-chip (SoC). The data processing circuit can be part of the imaging system. Alternatively, the data processing circuit can be external to the imaging system and communicatively coupled to it. The data processing circuit can be contained within the imaging system or be external to it; see also Figures 1c and 10.

[0103] Method 800 comprises measurement 810, using a single optical sensor, of a first signal indicative of a first image of the structure and a second signal indicative of a second image of the structure. The first image and the second image are acquired at (or for) different focal planes. The first signal and the second signal are measured within the same time measurement interval using the optical sensor. Measurement 810 can therefore, in particular, comprise or be a measurement to obtain a dual image based on the first image and the second image. The dual image can be obtained based on a measurement with only a single optical sensor within (only) one measurement interval, since a birefringent material is arranged in the optical path of the device. The dual image can be obtained by generating or calculating it based on the first image and the second image.In particular, the double image can be generated or calculated based on a known distance Ay (and / or Ax) between the first and second images in the image acquisition. Specifically, a structure from the first image can be mapped to a structure from the second image based on the known distance Ay (and / or Ax). This allows the double image for the structure to be generated or calculated.

[0104] The first image can be a photograph of a first focal plane. The second image can be a photograph of a second focal plane. That is, the first and second images can show different sections along different planes of the three-dimensional space being captured. As described above, the first focal plane differs from the second focal plane due to the birefringent material. This means that the first focal plane lies at a different z-position (axial position) in the three-dimensional space being captured than the second focal plane. In particular, the first focal plane can lie in a first xy-plane of the three-dimensional space being captured, and the second focal plane in a second xy-plane that is different from the first. Accordingly, the first and second focal planes can be parallel to each other and shifted relative to each other in the z-direction.Alternatively, the first and second focal planes can be tilted relative to each other in the three-dimensional space to be captured. In this case, any offset between the different focal planes depends on a position in the three-dimensional space being captured.

[0105] By capturing the first and second images, the position determination of a structure within the three-dimensional space to be captured can be improved. Accordingly, the method comprises determining the position of the structure based on the first and second signals. Using the first and second signals, the position of the structure can be determined by means of a double image due to the multiple focal planes and the lateral displacement of the structure in both images. By using the birefringent material, the position of the structure can be determined depending on its position in the z-direction (see Fig. 2), i.e., depending on a distance to the two focal planes, for example, , and / aFor a uniaxial birefringent material, the structure in the first and second images exhibits different sharpness and lateral positions. If the structure is positioned in the focal plane, it is sharp in the o-image (e.g., the first image) and blurry in the o-image (e.g., the second image). If the structure is positioned in the focal plane f aIn the first image, the structure is sharp, while in the second image it is blurred. If the structure is positioned between the two focal planes, it is blurred in both the first and second images (see also the double image determined from the first and second images in Fig. 2). In a special case, the first and second images can show the structure with the same degree of blur. The explanations using the ordinary and extraordinary rays are exemplary. The first and second images can also originate from a first extraordinary ray and a second extraordinary ray. The lateral displacement of the structure in the first and second images is independent of the image sharpness. The lateral displacement is caused by the splitting of the light into the two rays. Accordingly, the lateral displacement results from a path difference between the two rays.For example, the lateral offset may depend on the thickness of the birefringent material.

[0106] The position of the structure can therefore be determined solely by comparing the first image with the second image. In this comparison, the blurring of the structure in the first and second images can be indicative of the z-position of the structure in the three-dimensional space to be captured. Optionally, the z-position can also be determined based on the lateral offset. This increases the accuracy of the z-position determination. Accordingly, in one embodiment, the position of the structure can be determined solely by the difference between a dimension of the structure in the first signal and the second signal. A dimension of the structure in the first or second signal can, in particular, refer to a structure image size, for example, a particle image size. That is, the dimension of the structure in the first signal or...The second signal does not refer to a real dimension of the structure, but rather to a dimension in the first or second image. It can therefore be a dimension of the structure's representation in the first or second image. This dimension of the structure was described above, for example, as the particle image diameter. The dimension could be, for instance, a radius, a diameter, or a circumference. Due to the different focal planes of the two images, the z-position of the structure can be determined based on the sharpness of the structure (and the resulting variations in its dimensions) in the first and second images.

[0107] In one embodiment, the difference can be a squared difference in the dimension of the structure in the first signal compared to the second signal. Preferably, it can be a difference in the squares of the dimension of the structure. In particular, the difference can be a squared difference in the particle image diameter, from which the position z is determined, according to:

[0108] ■z' = — d Az a2 — d ° 2 '

[0109] In one embodiment, method 800 can include determining a dimension of the structure or a structure size distribution, e.g., a particle size distribution, based on the position and size of the structure in the first image and a size of the structure in the second image. The size of the structure in the first image and in the second image can, in particular, refer to a dimension of the structure in the respective image, i.e., a dimension of the structure in the first signal and the second signal, respectively. The determined dimension of the structure can then be the actual dimension of the structure, e.g., a diameter, a circumference, a radius, for example, of a particle.

[0110] A structure size distribution can be indicative of the spatial distribution of a plurality of structures in the three-dimensional space to be captured. Accordingly, the structure size distribution can be determined based on the position and size of the first image and the size of the second image, or from a multitude of these, given by a multitude of depicted structures.

[0111] In one embodiment, the method 800 may further comprise deriving a unique, particle size-independent calibration function for an axial position z described by the following functional relationship between the particle image diameters d a and d o of particle pattern produced by an ordinary jet and by an extraordinary jet and a distance Az = f a — f0 between the focal planes for the extraordinary and ordinary rays normalized axial position z' :

[0112] Details of this derivation are described above.

[0113] In one embodiment, the method 800 can further comprise determining the positions of a plurality of structures at a first time point and determining the positions of the plurality of structures at a second time point different from the first time point. Furthermore, the method 800 can comprise reconstructing a volumetric, three-component velocity field of a three-dimensional space to be detected, e.g., a fluid, based on the positions of the plurality of structures at the first time point and the second time point.

[0114] The determined positions of the multitude of structures can be indicative of their lateral and axial positions, and optionally their changes over time. The positions at two different time points allow the velocity of the multitude of structures to be determined. Based on the velocity and position, the velocity field can then be reconstructed.

[0115] In one embodiment, the method 800 can further include obtaining position data indicative of a z-position of the structure, evaluating the position data, the first image and the second image to determine a calibration function, and determining a calibration function based on the evaluation of the first image and the second image. This "alternative" variant of determining the calibration function is described in detail above.

[0116] In one embodiment, the method 800 can further include obtaining focus plane data indicative of an (axial) offset of the different focus planes and determining a calibration function for the z-position of the structure based on the focus plane data. For example, a subsequent fit to the measured values, i.e., the particle image diameters d, can be performed. a and d0, which are obtained during a flow measurement. This is described in detail above. In particular, the calibration function for the z-position can be determined from the knowledge of the axial distance between the two focal planes.

[0117] Fig. 9 shows a flowchart of an example of a method 900 for an imaging system or device. The method 900 comprises receiving 910 double images of a plurality of particles (2a) captured by the camera (11) by the digital evaluation unit (13), determining 920 lateral particle positions x, y and particle image diameters d0, d a from the measured double images and derivation 930 of a one-to-one, particle size-independent calibration function for an axial position z described by the following functional relationship between the particle image diameters d a and d„ of particle images produced by an ordinary beam and by an extraordinary beam and an axial position z' normalized to a distance Az = fa ~ fo between the focal planes for the extraordinary and the ordinary beam :

[0118] Method 900 is described in detail with reference to Fig. 7.

[0119] Some embodiments relate to a microscope with a device as described in connection with Fig. 1c. Alternatively, a microscope or an optical imaging system can be communicatively connected with a device as described in connection with Fig. 1c. Fig. 10 shows a schematic representation of a system 1000, e.g., an optical imaging system, configured to perform a method described herein, e.g., with reference to Figs. 7-9. The system 1000 consists of a microscope 1010 and a computer system 1020. The microscope can include the device described above, e.g., the one described with reference to Fig. 1c, or the imaging system as described with reference to Figs. 1a or 1b. The microscope 1010 is configured to acquire images and is connected to the computer system 1020.The computer system 1020 is configured to execute at least part of a procedure described herein. The computer system 1020 can be configured to execute a machine learning algorithm. The computer system 1020 and the microscope 1010 can be separate units or integrated in a common housing. The computer system 1020 can be part of a central processing system of the microscope 1010 and / or the computer system 1020 can be part of a subcomponent of the microscope 1010, such as a sensor, an actuator, a camera, or an illumination unit, etc.

[0120] The computer system 1020 can be a local computing device (e.g., a personal computer, laptop, tablet computer, or mobile phone) with one or more processors and one or more storage devices, or a distributed computing system (e.g., a cloud computing system with one or more processors and one or more storage devices distributed across different locations, such as a local client and / or one or more remote server farms and / or data centers). The computer system 1020 can comprise any circuit or combination of circuits. In one embodiment, the computer system 1020 can include one or more processors of any type. The term "processor" as used herein can refer to any type of computing circuit, such as...a microprocessor, a microcontroller, a CISC (Complex Instruction Set Computing) microprocessor, a RISC (Reduced Instruction Set Computing) microprocessor, a VLIW (Very Long Instruction Word) microprocessor, a graphics processor, a digital signal processor (DSP), a multi-core processor, an FPGA (Field Programmable Gate Array), e.g., of a microscope or a microscope component (e.g., camera), or any other type of processor or processing circuit. Other types of circuitry that may be included in the Computer System 1020 may be a custom circuit, an application-specific integrated circuit (AS1C), or similar, such as one or more circuits (e.g., a communications circuit) for use in wireless devices such as mobile phones, tablet computers, laptops, two-way radios, and similar electronic systems.The Computer System 1020 may include one or more storage devices, which may comprise one or more storage elements suitable for the application, such as main memory in the form of random-access memory (RAM), one or more hard disks, and / or one or more drives capable of handling removable media such as compact discs (CDs), flash memory cards, digital video discs (DVDs), and the like. The Computer System 1020 may also include a display device, one or more speakers, and a keyboard and / or control device, which may include a mouse, trackball, touchscreen, speech recognition device, or any other device that enables a system user to input information into and receive information from the Computer System 1020.

[0121] Some or all of the process steps can be performed by (or using) a hardware device, such as a processor, a microprocessor, a programmable computer, or an electronic circuit. In some embodiments, one or more of the key process steps can be performed by such a device. Depending on specific implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be carried out using a non-transient storage medium, such as a digital storage medium, for example, a floppy disk, a DVD, a Blu-ray disc, a CD, a ROM, a PROM, an EPROM, an EEPROM, or a flash memory, on which electronically readable control signals are stored that interact (or can interact) with a programmable computer system to execute the respective process.Therefore, the digital storage medium can be computer-readable.

[0122] Some embodiments of the invention include a data carrier with electronically readable control signals that are capable of cooperating with a programmable computer system so that one of the methods described herein is carried out.

[0123] In general, embodiments of the present invention can be implemented as a computer program product with program code, wherein the program code serves to execute one of the methods when the computer program product runs on a computer. The program code can, for example, be stored on a machine-readable medium.

[0124] Other embodiments include the computer program for carrying out one of the methods described herein, which is stored on a machine-readable medium.

[0125] In other words, an embodiment of the present invention is therefore a computer program with program code for carrying out one of the methods described herein when the computer program is running on a computer.

[0126] Another embodiment of the present invention is therefore a storage medium (or a data carrier or a computer-readable medium) on which the computer program for carrying out one of the methods described herein is stored when executed by a processor. The data carrier, the digital storage medium, or the medium shown is typically tangible and / or non-transferable. Another embodiment of the present invention is a device as described herein comprising a processor and the storage medium. Another embodiment of the invention is therefore a data stream or a sequence of signals representing the computer program for carrying out one of the methods described herein. The data stream or signal sequence can, for example, be configured to be transmitted via a data communication link, e.g., via the Internet.

[0127] Another embodiment comprises a processing means, e.g. a computer or a programmable logic device, configured or adapted to perform one of the methods described herein.

[0128] Another embodiment comprises a computer on which the computer program for carrying out one of the methods described herein is installed.

[0129] Another embodiment of the invention comprises a device or system configured to transmit a computer program for carrying out one of the methods described herein to a receiver (e.g., electronically or optically). The receiver may be, for example, a computer, a mobile device, a storage device, or the like. The device or system may, for example, include a file server for transmitting the computer program to the receiver.

[0130] In some embodiments, a programmable logic device (e.g., a field-programmable gate array) can be used to perform some or all of the functions of the methods described herein. In some embodiments, a field-programmable gate array can cooperate with a microprocessor to perform one of the methods described herein. In general, the methods are preferably executed by any hardware device.

[0131] When certain aspects of a device or system are described, these aspects should also be understood as a description of the corresponding method, and vice versa. For example, a block, device, or functional aspect of the device or system may correspond to a function, such as a process step, of the corresponding procedure. Similarly, aspects described in relation to a method should also be understood as a description of a corresponding block, element, property, or functional feature of that device or system.

[0132] The following are some examples of the proposed concept:

[0133] Example 1 is a device for measuring the position and / or size of particles (2a), comprising an optical imaging system comprising at least one lens defining a detection path (7) and a camera (11) arranged in the detection path (7) such that particles (2a) located in a three-dimensional measurement volume extending in front of the imaging system can be imaged onto the camera (11), characterized in that the optical imaging system is equipped with at least one optically birefringent material (8) positioned in the detection path (7) between the measurement volume and the camera (11) such that an optical axis of the at least one optically birefringent material (8) is inclined relative to the detection path (7) by an angle of inclination that is not equal to zero and not equal to 90°, such that light emitted by a particle (2a) in the measurement volume,is split into at least one ordinary ray and at least one extraordinary ray, such that at least one focal plane (f, ) exists in the region of the measurement volume. o ) for an ordinary beam and at least one focal plane ( ä) for an extraordinary beam, which are spatially axially separated from each other, and each particle (2a) in the measurement volume is imaged at least twice, laterally separated, onto the camera (11), so that a double image or a multiple image of the particle (2a) can be obtained by means of the camera (11).

[0134] Example 2 is a device according to Example 1, characterized in that the inclination angle of the optical axis of the at least one optically birefringent material (8) relative to the detection path (7) is realized by cutting the optically birefringent material (8) in such a way that the optical axis of the optically birefringent material (8) is inclined by the inclination angle relative to a surface normal of the optically birefringent material (8).

[0135] Example 3 is a device according to Example 1, characterized in that the inclination angle of the optical axis of the at least one optically birefringent material (8) relative to the detection path (7) is realized by positioning the optically birefringent material (8) at an inclination in the detection path (7), such that an optical axis of the optically birefringent material (8) is inclined by the inclination angle relative to the optical axis of the imaging system, given by the detection path (7).

[0136] Example 4 is a device according to Example 1, 2 or 3, characterized in that the inclination angle of the optical axis of the at least one optically birefringent material (8) relative to the detection path (7) is in the range of 5° to 85°, preferably in the range of 15° to 50°, particularly preferably at 38°.

[0137] Example 5 is a device according to one of Examples 1-4, characterized in that it has only one detection path (7) and only one camera (11).

[0138] Example 6 is a device according to Example 1, characterized in that the camera (11) has a monochrome light-sensitive sensor or a color sensor.

[0139] Example 7 is a device according to one of Examples 1-6, characterized in that a channel (1) is arranged in the three-dimensional measuring volume, through which a fluid flow (2) containing particles (2a) can be guided, wherein at least one wall region of the channel (1) located in the detection path (7) is made transparent, so that observation of the particles (2a) is possible.

[0140] Example 8 is a device according to one of Examples 1-7, characterized in that the at least one optically birefringent material (8) is arranged at an arbitrary position between the measuring volume and the camera (11) in the detection path (7), wherein the at least one optically birefringent material (8) is connected to the imaging system or is separated from the imaging system or is designed as a lens of the imaging system or is designed as a wall region of the channel (1).

[0141] Example 9 is a device according to one of Examples 1-8, characterized in that the at least one optically birefringent material (8) is selected from a non-exhaustive list of single crystals comprising lithium niobate, calcite, beryl, rutile (T1O2), yttrium vanadate (YVO4), tellurium dioxide (TeOz), langasite, topaz or mica.

[0142] Example 5 is a device according to one of Examples 1-8, characterized in that the at least one optically birefringent material (8) is a liquid crystal or a material with birefringence induced by an electrical or a mechanical stress.

[0143] Example 11 is a computer-implemented method for determining the position and / or size of particles and / or a three-component flow velocity field of a fluid, carried out with a digital evaluation unit (13) connected to a camera (11) of a device according to any one of claims 1 to 10 or to a camera of a prior art device equipped with one or more cameras, comprising the following steps: a) reception by the digital evaluation unit (13) of double images of a plurality of particles (2a) taken by the camera (11), b) determination of lateral particle positions x, y and particle image diameters d0, d a from the measured double images, c) derivation of a one-to-one calibration function independent of particle size for an axial position z', described by the following functional relationship between the particle image diameters da and d o of particle pattern produced by an ordinary jet and by an extraordinary jet and at a distance z = f a — f0 between the focal planes for the extraordinary and ordinary rays normalized axial position z' :

[0144] Example 12 is a computer-implemented method based on Example 11 for reconstructing a volumetric, three-component velocity field of a fluid based on a large number of particles determined lateral and axial particle positions x, y, z and their change over time.

[0145] Example 13 is a computer-implemented method according to Example 11 for determining the size of particles or a spatial particle size distribution from a large number of

[0146] Particle image diameters of the measured double images and the known particle positions.

[0147] Example 14 is a computer-implemented method according to Example 11, characterized in that the calibration function is determined by a calibration measurement in which particles are positioned at known z-positions, double images of them are taken and then sorted according to their

[0148] Particle image diameters are evaluated to fit or interpolate the calibration function according to the equation given in claim 11, step c) for the axial position z' using measured values ​​or to provide a look-up table.

[0149] Example 15 is a computer-implemented method according to Example 11, characterized in that the determination of the calibration function for position z is possible solely by knowing the position of the two focal planes and subsequently fitting it to the measured values.

[0150] Particle image diameter d a and d0, which are obtained during a flow measurement.

[0151] Example 16 is a computer-implemented method according to Example, characterized in that the position z is determined solely by the square of the difference in particle diameters, according to:

[0152] The following claims are hereby included in the detailed description, each claim being capable of standing alone as a separate example. It should also be noted that, although a dependent claim in the claims relates to a specific combination with one or more other claims, other examples may also include a combination of the dependent claim with the subject matter of another dependent or independent claim. Such combinations are hereby expressly suggested unless it is stated in a particular instance that a specific combination is not intended. Furthermore, features of a claim should also be included for each other independent claim, even if that claim is not directly defined as dependent on that other independent claim.

[0153] The aspects and characteristics described in relation to a particular of the preceding examples can also be combined with one or more of the further examples to replace an identical or similar characteristic of that further example or to additionally introduce the characteristics into the further example.

[0154] List of reference symbols

[0155] 1 - Channel

[0156] 2 - Fluid flow

[0157] 2a - Particles

[0158] 3 - Light source, e.g. laser

[0159] 4 - Lighting path

[0160] 5 - Mirror

[0161] 6 - Front optics, e.g. microscope objective

[0162] 7 - Detection path

[0163] 8 - optically birefringent material, e.g., optically birefringent canal floor

[0164] 9 - Filter element

[0165] 10 - lens, e.g. B. tube lens of a microscope

[0166] 11 - Camera, e.g. B. monochrome camera

[0167] 12 - Synchronization unit

[0168] 13 - digital output unit

[0169] 25 - optical sensor

[0170] 35 - Structure

[0171] 100 - Imaging system

[0172] 130 - Device

[0173] 132 - Interface

[0174] 134 - Data processing circuit

[0175] 136 - Memory f0 - Focus plane for a proper beam f a - Focal plane for an extraordinary beam

Claims

Claims 1. Imaging system comprising: an optical sensor; and an optically birefringent material arranged in an optical path between the optical sensor and a three-dimensional space to be detected, wherein an optical axis of the optically birefringent material is inclined relative to the optical path by an angle of inclination, wherein the optically birefringent material is configured to split light into two beams, through which a first focal plane and a second, different focal plane are formed in the space to be detected, wherein the imaging system is configured to image a structure to be detected in the space to be detected at least twice, laterally separated, onto the optical sensor.

2. Imaging system according to claim 1, wherein the value of the inclination angle is between 5° and 85°.

3. Imaging system according to one of the preceding claims, wherein the light which is split into two beams by the optically birefringent material is unpolarized or at least partially polarized and has both intrinsic polarizations of the two beams.

4. Imaging system according to one of the preceding claims, wherein the optically birefringent material comprises at least one of lithium niobate, calcite, beryl, rutile, yttrium vanadate, tellurium dioxide, langasite, topaz and mica.

5. Imaging system according to one of the preceding claims, wherein the optically birefringent material comprises a liquid crystal.

6. Imaging system according to one of the preceding claims, further comprising a further optically birefringent material arranged in the optical path, wherein an optical axis of the further optically birefringent material is inclined relative to the optical path by an angle of inclination.

7. Imaging system according to one of the preceding claims, wherein the imaging system is configured to apply an electrical or mechanical stress to the optically birefringent material.

8. Imaging system according to one of the preceding claims, wherein the optical axis of the optically birefringent material is inclined relative to a surface normal of the optically birefringent material.

9. Imaging system according to one of the preceding claims, wherein a surface normal of the optically birefringent material is inclined relative to the optical path.

10. Imaging system according to one of the preceding claims, further comprising a channel in the three-dimensional space to be detected or a lens, wherein the optically birefringent material is at least partially encompassed by the channel or by the lens.

11. Imaging system according to any one of the preceding claims, wherein the imaging system comprises or is a microscope.

12. Imaging system according to one of the preceding claims, wherein the optical sensor comprises a monochrome light-sensitive sensor or a color sensor.

13. Imaging system according to one of the preceding claims, further comprising a channel arranged in the three-dimensional space to be detected, wherein the channel is configured to guide a fluid flow, and wherein the channel is at least partially transparent.

14. Method (800) for determining the position of a structure, comprising: Measuring (810) using a single optical sensor, a first signal indicative of a first image of the structure and a second signal indicative of a second image of the structure, wherein the first and second images are recorded at different focal planes, and wherein the first and second signals are measured within the same time measurement interval using the optical sensor; and Determine (820) the position of the structure based on the first and second signals.

15. Method according to claim 14, wherein the position of the structure is determined solely by a difference in a dimension of the structure in the first signal from the second signal.

16. Method according to claim 15, wherein the difference is a squared difference in the dimension of the structure in the first signal from the second signal.

17. Method according to claim 16, wherein the difference is a difference of a particle image diameter squared, by which the position z is determined, according to:

18. Method according to one of claims 14-17, further comprising determining a dimension of the structure or a structure size distribution based on the position and size of the structure in the first image and a size of the structure in the second image.

19. The method according to claim 18, further comprising: Deriving a unique, particle size-independent calibration function for an axial position z', described by the following functional relationship between the particle image diameters d a and d0 of particle pattern produced by an ordinary beam and by an extraordinary beam and one at a distance Az = f a — f obetween the focal planes for the extraordinary and ordinary rays normalized axial position z' 20. Method according to any one of claims 14-19, further comprising: Determining the positions of a large number of structures at a first point in time; Determining the positions of the multitude of structures at a second time point different from the first time point; and Reconstructing a volumetric, three-component velocity field of a three-dimensional space to be captured based on the positions of the multitude of structures at the first time point and the second time point.

21. Method according to any one of claims 14-20, further comprising: Obtaining position data indicative of a z-position of the structure; Evaluate, based on the position data, the first image and the second image, to determine a calibration function; and Determining a calibration function based on the evaluation of the first image and the second image.

22. Method according to any one of claims 14-20, further comprising: Obtaining focus plane data is indicative of an offset between the different focus planes; and Determining a calibration function for the z-position of the structure based on the focus plane data, and a subsequent fit to the measured values, the particle image diameter d a and d0, which are obtained during a flow measurement.

Images