CONFIGURABLE RETROREFLEX SENSOR SYSTEM FOR IMPROVED CHARACTERIZATION OF SAMPLE PROPERTIES

Description

I. State of the art

[0001] Below, some reference numerals that are also used for the invention description are introduced; thus, identical reference numerals are used for identical or corresponding parts of the described systems throughout the entire description.

[0002] Retroreflex sensor systems (alternatively also referred to as retroreflex sensors and, in abbreviated form, as sensors) or corresponding arrangements according to the prior art (compare [STT1], [STT2] and [STT3]) are basically as in Figure 1The setup shown is as follows: A transmitter 1 illuminates the sample P. The light incident on the sample either passes through the sample (transmission) or, after reflection from the sample, reaches the retroreflector 2 and is reflected back towards transmitter 1 either along the same path or with a beam offset. For the sensor's operating principle, it is usually irrelevant whether the reflected beams are spread out by the reflector, as is known, for example, from foil reflectors. The receiver 3 detects and analyzes the reflected radiation. It will also be referred to as the detector. If the beam offset or the spread of the reflected light allows, the receiver can be positioned next to the transmitter (see figure). Figure 1aThis is particularly possible if the retroreflector 2 leads to a defined beam offset or is not ideal and reflects the incident radiation back in a cone. Otherwise, the reflected radiation is deflected towards the receiver via a beam splitter 5 between the transmitter and the sample (see Figure 1). Figure 1b In most embodiments, the transmitter and receiver are integrated into one and the same housing 4 (to form a "transceiver").

[0003] During transmission or reflection at the sample or object, polarization changes of the radiation can occur. Depending on the application, these polarization changes are either undesirable or precisely the desired measurement signal. Examples of cases where these polarization changes are undesirable, i.e., where they interfere with the determination of the object's properties, include: Detection of decorative defects (inclusions, scratches, etc.). Detection or measurement of object structures (e.g., outer edges or contours of the object, interfaces inside the object, ...).

[0004] Examples of cases where polarization changes should be evaluated include: Detection of stresses in the material. Detection of the material type or layer thickness using ellipsometry.

[0005] In state-of-the-art arrangements, this (or other) different information about the object is usually superimposed in the measurement signal and cannot be distinguished from each other, or only to a limited extent.

[0006] Furthermore, when using reflectors 2 with microstructures (films with microprisms, micro glass spheres, etc.), increased noise can occur due to the microstructure of the reflector.

[0007] With the state of the art described above, the following limitations arise, for example, in ellipsometric measurements: Limitations of the unambiguous range for determining the ellipsometric parameters ψ and Δ. Limitations when simultaneously checking for decorative defects or superposition of the signals from these defects and the polarimetric or ellipsometric signals. Unknown inclination angles of the sample in and perpendicular to the plane of incidence. Noise due to the microstructure of the reflector and / or speckle. State of the art:

[0008] [STT1] WO 99 / 41568 A1 = Application PCT / EP2011 / 004553 or EP 1056987. [STT2] WO 2012 / 038036 A1 [STT3] "Ellipsometry of Curved Surfaces", Conference Paper, November 2014, Forum Bildverarbeitung 2014, Regensburg, DOI: 10.5445 / KSP / 1000043608. [STT4] "Simultaneous measurement of surface geometry and material distribution by focusing ellipsotopometry", U. Neuschaefer-Rube and W. Holzapfel, APPLIED OPTICS, Vol. 41, No. 22, 1 August 2002. [STT5] RMA Azzam "Division-of-amplitude Photopolarimeter (DOAP) for the Simultaneous Measurement of All Four Stokes Parameters of Light.", Optica Acta: International Journal of Optics, 29(5): 685-689, 1982.[STT6] US 2010 / 290032 A1 [STT7] US 5 898 169 A [STT8] US 2012 / 113423 A1 [STT9] US 2010 / 274501 A1 [STT10] WO 2010 / 026579 A2 [STT11] NO 20 150765A1 [STT12] EP 2 597 451 A2 [STT13] DE 43 43345A1 [STT14] WO 9631752 [STT15] EP 1 619 456 A1 [STT16] US 4 469 443 A [STT14]- [STT16] disclose optical measuring systems with a retroreflector in which the polarization is modulated. II. Introduction to and the object of the present invention

[0009] Starting from the prior art, the object of the present invention is to enable improved optical characterization of a sample or object in a retroreflection arrangement. For this purpose, a retroreflector-based sensor system according to the invention is to be provided.

[0010] This problem is solved by a sensor system according to claim 1. Advantageously realizable embodiments can be found in the dependent claims.

[0011] Corresponding methods (and uses) arise from a modification of the patent claims that is obvious to a person skilled in the art (for example, according to claim 1, in that the sample is irradiated by means of radiation in the transmitter beam path, ... until finally the radiation S RR is detected by the receiver and measured values ​​are obtained and evaluated from it for optical characterization of the sample, wherein said obtaining and / or evaluation is then improved by providing said configuration means or means).

[0012] The present invention is described below, first in general terms, then in detail with reference to several individual embodiments. With regard to the embodiments, it is possible, according to the claim structure, to omit individual features or to combine individual features differently with other features shown (even across multiple embodiments).

[0013] The present invention relates to a configurable sensor for improved optical characterization of a sample in a retroreflective arrangement.

[0014] Such an improved characterization is achieved according to the invention by the configuration means K3, which is designated below with the reference numeral and which, in preferred embodiments, is combined with one or more of the additional configurations or configuration means designated below with the reference numerals K1, K2, K4 to K6: Configuration means K1: Movement of the reflector during measurement by an actuator. Configuration means K2: Arrangement of one or more optical components between the sample and the retroreflector, which changes the polarization, direction, or intensity of the radiation. Alternatively, a retroreflector can often be used that changes the polarization, intensity, direction, and / or offset of the reflected radiation in a defined (i.e., user-known) manner. Configuration means K3: Arrangement of at least one modulator or switch for the polarization state, wavelength, and / or coherence or emission direction of the illumination. In some embodiments of the invention, it is advantageous to perform modulation of several of the aforementioned parameters simultaneously.An example of this is illumination with left-circularly polarized radiation of a first wavelength λ₁, which is alternately switched to right-circularly polarized illumination at a second wavelength λ₂. The wavelengths λ₁ or λ₂ can lie within the sensitive range of the detector or outside this range (so that, for example, (polarized) fluorescence or nonlinear optical effects on the sample can be evaluated). Configuration means K4: Arranging one or more elements for changing the polarization of the entire receiver beam path or for spectral splitting of the receiver beam path, which preferably leaves the polarization and spectral properties of the transmitter beam path unaffected. Configuration means K5: Extending the receiver to a detector or...Use a receiver that determines the complete Stokes or Jones vector of the incident radiation – even without the constraint of coherence. Alternatively, and technically equivalently, a suitable replacement can be made for an existing or planned detector. Configuration means K6: Use of a data source, e.g., a CAD model, or the arrangement of at least one additional detector or sensor system to consider or acquire further sample properties, such as the sample's geometry. From this additional data, the angle of incidence of the radiation on the sample surface and / or the angle of inclination of the sample relative to the sensor coordinate system can be determined. Using this information, for example, the refractive index n and the extinction coefficient k of the sample can be determined from the ellipsometric measurements ψ and Δ.Alternatively, in many cases it is also possible to determine the geometric data from the ellipsometric data (measured quantities of the detector) itself. This method is known as ellipsotopography. See "Simultaneous measurement of surface geometry and material distribution by focusing ellipsotopometry", see [STT4]. In this case, an evaluation stage can be provided according to the invention to determine the geometry of the sample from the measured values, inter alia, using the Fresnel formulas for the reflection coefficients. As a further configuration means (these are also referred to according to the invention as configuration means K7), means for automatic calibration of the measured values ​​can be provided. These configuration means can be, for example, polarization-preserving or depolarizing retroreflector(s) or a reflecting object or reflector with a precisely known reflection characteristic.The retroreflector(s) can be integrated into a portion of the transmitting beam path or be pivotable into a portion of the transmitting beam path. The retroreflector(s) can be positioned within the housing of the transceiver (or a combined transmit and receive unit) or outside of this housing, in the latter case preferably directly adjacent to the sample (in the case of a reflection arrangement; in the case of a transmission arrangement, a region of the retroreflector not in the shadow of the sample can be used).

[0015] Depending on the measurement task to be solved, a configuration (means) alone or a combination of several or all configurations or means may be advantageous in order to satisfactorily (or even better: optimally) solve the problem according to the invention. III. On the scope of protection of the invention

[0016] A retroreflector-based sensor system according to the invention can be found in claim 1. Certain embodiments are specified in the dependent claims. All embodiments, examples, aspects, methods, uses, and the like described herein that are not within the scope of the claims serve only for illustration.

[0017] The sample is not part of the system; that is, the system consists of the other components mentioned and has a sample area in which the sample can be positioned to achieve the effect described in the claim. The terms "optical" and "radiation" generally refer to visible light, i.e., light with wavelength(s) in the range between approximately 350 nm and 780 nm. However, according to the invention, it is also conceivable to use radiation sources (as transmitters) or radiation in other wavelength ranges, particularly in the UV range below 350 nm or in the infrared range above 780 nm.

[0018] Both a transmission and a reflection arrangement of the sample are possible (see also the exemplary embodiments below). The measured values ​​can be, in particular, intensity values ​​and / or polarization values ​​of the radiation.

[0019] Unless otherwise stated, the transmit beam path is the path of radiation from the transmitter to the point where it reaches the retroreflector. Similarly, the receive beam path is the path of radiation from the retroreflector (after reflection from it) back towards the transmitter until it reaches the receiver. This includes the possibility that only a portion of the radiation actually reaches the next optically effective stage (or the next optically effective means, such as the sample, the retroreflector, a retroreflector, a configuration means, or the like). For example, typically only a portion of the radiation incident on the sample will actually reach the retroreflector after its reflection from or transmission through the sample (radiation components can be lost at / in the sample, for example, through absorption, scattering, etc.).It is also possible that parts of the radiation reflected by the sample do not reach the retroreflector at all, as they pass by or are radiated past the retroreflector.

[0020] The transmitter can illuminate a point-like or one- or two-dimensional area of ​​the sample (preferably a point, a line, or a surface of this sample). The receiver (detector) thus receives the radiation or radiation components that are reflected back by the retroreflector and that—considering the entire beam path, i.e., the transmitter beam path plus the receiver beam path—are reflected (at least) twice by the sample (or that have passed through the sample twice in transmission).

[0021] As a rule, the optical axes in the illumination beam path (i.e., the transmitting beam path) and in the receiving beam path (i.e., from the retroreflector back to the detector) coincide at least partially, or have at least partially approximately the same orientation. The light reflected by and / or transmitted through the sample can, in principle, be reflected back by the retroreflector (in the receiving beam path) along the same, i.e., identical, optical path. This occurs, for example, when a foil reflector is used as the retroreflector. However, it is also possible that the light reflected back by the retroreflector is reflected with a parallel offset (relative to the transmitting beam path), meaning that the receiving beam path runs at least partially at an angle to and parallel with the transmitting beam path (or sections thereof).In this process, a usually slight widening may also be present in the receiving beam path (again, relative to the transmitting beam path).

[0022] Advantageously achievable features can be found in claim 2.

[0023] Preferably, only exactly one (particularly preferably the retroreflector) of the elements described in the claim is movable. The movement of said element (or elements), such as the retroreflector, is preferably effected by an actuator. For example, a motor, a vibrator, or the like can be used as the actuator. The movement can be rotation, vibration, or rapid reciprocating motion along at least one axis (or a combination of such motions).

[0024] Further advantageously realizable features can be found in claim 3.

[0025] The placement of such an element(s) is generally at a defined position(s) in the transmitter beam path and / or the receiver beam path. For example, such an element can be placed in the transmitter beam path between the transmitter and the sample. Alternatively, another element can be placed in the transmitter beam path and in the receiver beam path between the sample and the retroreflector. Alternatively, another such element can be placed in the receiver beam path (leaving the transmitter beam path and / or its radiation unaffected) between the sample and the receiver.

[0026] One or more such element(s) can also be an additional light source(s), which is / are arranged, for example, in one or more partial beam paths of the transmitter or is coupled into such partial beam path(s) via one or more beam splitters.

[0027] Such an optical element can also be the retroreflector itself. An example of this is a retroreflector that defines (i.e., changes in a way known to the user of the retroreflector-based sensor system) the polarization state or polarization of the incident and reflected radiation. Another example is a retroreflector that causes a defined offset between the radiation in the transmitter beam path and the radiation in the receiver beam path.

[0028] Further advantageously realizable features can be found in claim 4.

[0029] Here (as for all other embodiments of the sensor system according to the invention) it applies that the sensor system, in particular its evaluation unit, can operate in a computer-aided manner, i.e. computer-aided or microprocessor-aided.

[0030] The sensor system, in particular its evaluation unit, can therefore include or be a corresponding server PC or microcontroller (with suitable program and data storage).

[0031] For evaluation purposes, suitable evaluation programs can be stored (permanently) in the program memory or loaded into the program memory. These evaluation programs are designed to evaluate the measured values ​​or the data generated from them (which can be stored in the data memory).

[0032] Further advantageously realizable features can be found in claim 5.

[0033] The information storage medium is also referred to as the data source below.

[0034] Further advantageously realizable features can be found in claim 6.

[0035] Such means could include, for example: 1. If the transmitter and receiver are positioned in a common housing (in particular, if the transmitter and receiver are designed as a laser scanner in / with such a common housing), a further retroreflector is located within this common housing, wherein the further retroreflector reflects a portion of the radiation emitted by the transmitter (in particular, a portion of the scan line of said laser scanner) directly back into the receiver (generally before it leaves the common housing) without contacting the sample. 2. A portion of the retroreflector is positioned behind the sample in the transmitter beam path, wherein said portion is not obscured by the sample. 3.In systems according to the invention in a reflection arrangement (generally not in transmission arrangement sensor systems), one or more reference object(s) are arranged in the area of ​​or next to the sample such that a portion of the radiation from the transmitter beam path falls on these reference object(s), is reflected by them, and the latter, reflected portion strikes the retroreflector.

[0036] This method(s) can be used to normalize intensities or polarization values ​​as measured values.

[0037] Further advantageously realizable features can be found in claim 7.

[0038] Sampled configuration tools can include, in particular, the reference objects already mentioned.

[0039] Further advantageously realizable features can be found in claim 8.

[0040] The beam offset can be achieved, in particular, such that the radiation propagates parallel to the transmitter and receiver beam paths, at least in sections. Provided that said beam offset and / or any existing beam expansion that is finite along the combined length of the transmitter and receiver beam paths allows it, the receiver can be positioned next to or (usually slightly) away from the transmitter.

[0041] Further advantageously realizable features can be found in claim 9.

[0042] According to the first version of this claim, it is also possible (for example by a suitable arrangement of additional beam splitters) to reflect the radiation more than twice, for example four times or even six times, at the sample before it is detected as radiation S RR.

[0043] According to the second variant of this claim, it is also possible (for example, with a suitable arrangement of additional beam splitters, which are each arranged slightly tilted towards each other) to transmit the radiation more than twice, for example four or six times, through the sample before it is detected as radiation S RR.

[0044] In both variants, it is generally the case that only a portion of the reflected or transmitted radiation is reflected or transmitted per reflection or transmission process. In other words, (minor) losses of radiation components due to random scattering, absorption, etc., typically occur per process.

[0045] Further advantageously realizable features are described in claim 10.

[0046] The housing can have a common beam inlet and outlet. This is usually designed to allow both the emission of radiation from the transmitter and the reception of radiation reflected back from the sample by the retroreflector.

[0047] Further advantageously realizable features can be found in claim 11.

[0048] The scanning system can be, in particular, a laser scanning system or a laser scanner. Scanning can be performed using one or more movable mirrors. For example, one- or two-dimensional galvanometer scanners can be used as mirror systems for scanning.

[0049] The figures – as well as the description – show and describe the essential features of the invention. For the sake of clarity, non-essential optical elements, which are self-evident to a person skilled in the art and do not affect the fundamental function (purely imaging elements or elements that deflect one or more partial beam paths), are neither shown nor mentioned in the description.

[0050] The beam paths shown in the following exemplary embodiments already function in implementations without special imaging or beam-shaping elements (such as lenses, apertures, filters, or the like) – for example, when using a collimated laser (as a beam source) and a relatively large detector (receiver). It is obvious that even the use of a simple lens can positively influence the size of the measured surface or volume element of the samples as well as the efficiency of the beam path. The advantage of additional apertures and filters in the beam paths is equally obvious. It is therefore clear to those skilled in the art that the principle according to the invention can also be implemented in a form in which the beam paths shown in the following exemplary embodiments are merely partial beam paths of more complex optical arrangements or optical systems (e.g.,additional beam-shaping elements of comprehensive and / or linear or planar scanning or imaging systems - e.g., one-dimensional or two-dimensional laser scanners).

[0051] Example (with reference to the embodiment described below from Figure 3The deflection unit can comprise several elements (rotatable or rotatable mirrors, imaging mirrors, one or more lenses, one or more holographic elements, etc.) to allow, for example, one-dimensional or two-dimensional scanning (laser scanning, preferably time-division multiplexing) or imaging of the sample surface. It is equally clear to those skilled in the art that additional lenses and apertures in the beam path can influence the efficiency of the beam paths and the size of the scanned surface elements of the sample. Nevertheless, the same effect of the configurations would be obtained for each light beam, thus the basic concept of the invention remains recognizable. The incorporation of filters is also possible, for example, for compensation of ambient light.

[0052] In short: According to the invention, almost any beam path can be realized, almost any beam-shaping elements can be additionally introduced into the beam path and / or almost any retroreflector configurations and shapes can be used. IV. Exemplary embodiments, possible embodiments and advantages of the invention IV.1. Fundamental principles regarding the system structures possible according to the invention

[0053] The following will be based on the Figures 2 to 10 Various configuration examples, i.e., embodiments of sensor systems according to the invention, are described.

[0054] This shows Fig. 2 a possible basic structure of the invention in which, in addition to the configuration means K3, another configuration means, several further configuration means or even all configuration means K1 to K6 as described below can be positioned, for example, in the beam path and / or integrated as part of individual elements of the system.

[0055] The sensor system(s) according to Fig. 2The device has / have a radiation-emitting transmitter 1 (here: a laser emitting with wavelength λ). The transmitter 1 is hereinafter also referred to as the illumination. The third configuration means K3 can be formed on or integrated into it, i.e., according to Fig. 2 A configuration K3 can be implemented (alone or together with one, several or all of the other configurations K1 to K2 as well as K4 to K6).

[0056] The laser 1 emits light into the transmitter beam path 1-S, which falls onto a beam splitter 5 and is transmitted through it. The transmission properties of the beam splitter (transmittance, change in polarization, etc.) can be adapted to the measurement task. For accurate evaluation of the measurement signals, they simply need to be known or determined by measurement.

[0057] In the transmitting beam path 1-S, the radiation then falls on the sample P and is reflected specularly at it according to the law of reflection (angle of incidence = angle of reflection).

[0058] This reflected radiation falls in beam path 1-S onto a second configuration medium K2 (if this, as in Fig. 2The light passing through the configuration K2, or the radiation S e incident on the retroreflector 2, is reflected back into itself at the retroreflector 2, i.e., back into the receiving beam path 1-E, the latter 1-E being identical to the transmitting beam path 1-S until it reaches the beam splitter 5. In other words, the radiation reflected back from the reflector 2 travels in the opposite direction along the transmitting beam path 1-S, strikes the sample P a second time, is reflected specularly a second time according to the law of reflection, and thus, after entering the common housing 4 of the transmitter 1 and the receiver 3, again strikes the beam splitter 5.

[0059] The beam splitter 5 reflects a portion of the incoming received radiation 1-E towards the receiver 3, according to its optical properties. The reflection and transmission properties of the beam splitter (transmittance, reflectance, changes in polarization properties in transmission and reflection, etc.) can be adapted to the measurement task. For accurate evaluation of the measurement signals, these properties simply need to be known or determined by measurement.

[0060] Beam splitter 5, transmitter 1, and receiver 3 are arranged here in housing 4 rotated 90° relative to each other. The received radiation 1-E reflected at splitter 5 strikes the fourth configuration device K4, provided that it is configured as shown in Fig. 2 The implemented system is shown, and the received radiation modified by the means K4 hits the receiver 3 as radiation S RR to be detected by the receiver 3.

[0061] How Fig. 2This shows that, in addition to the configuration tool K3, the following configuration tools can also be provided or configurations can be implemented: Configuration device K1 (first configuration device) in / on the retroreflector or as a retroreflector (see in particular the following description). Configuration device K5 in / on receiver 3 (see in particular the following description). Configuration device K6, which here uses a camera to image the impact of the radiation S e on the reflector 2 and / or the radiation 1-S on the configuration device K2 (see in particular the following description).

[0062] The internal structure of receiver 3 depends on the measurement task to be solved. In the simplest case of a point sensor for detecting decorative defects, it can be a photodiode or a photomultiplier. However, it can also follow the design shown in Fig. 20.6 from STT3 (see in this figure the beam path to the right of the non-polarization-changing beam splitter onto which the dashed radiation, labeled "input beam") falls).

[0063] The exact optical properties (transmission, reflection, polarization properties, etc.) of the optical elements in Fig. 2 They can be optimized within wide limits for the measurement task at hand. Generally, for a satisfactory solution, they only need to be known with sufficient accuracy so that they can be appropriately considered in the signal analysis.

[0064] Fig. 2Figure 1 shows, by way of example, configurations K1 to K6 for the optimal adaptation of a retroreflex sensor system to a measurement task or for the extended characterization of samples, using a point measurement system as an example. In addition to configuration K3, the additional configurations K1, K2 and / or K4 to K6 according to the invention can be used: K1: Actuator for moving the retroreflector. K2: Additional optical element that changes the polarization, direction, or intensity of the radiation. K3: Modulator or switch for the polarization state, wavelength, coherence, or emission direction of the illumination. K4: One or more optical elements for changing the polarization and / or for spectral splitting of the received beam path and / or for selecting the detector wavelength (e.g., AOTF). K5: Extended detector that determines the Jones vector or the complete Stokes vector of the incident radiation—even without constraints due to coherence or complete polarization—and may include diffractive elements for spectral splitting as well as point, line, or area sensors. K6: Additional sensor for determining the object geometry or...the angle of incidence of the radiation to the object surface and / or the angle of rotation of the sample surface relative to the detector's preferred direction and / or the height of the sample surface under investigation.

[0065] Fig. 3 shows an alternative system according to the invention, which is basically the same as in Fig. 2 The setup shown is as follows, so only the differences will be described below. Shown is a transceiver (i.e., transmitter 1 and receiver 2 in the same housing 4) equipped with a deflection unit (here, for example, comprising a movable, here: rotatable, mirror 6) that scans the object P line by line or row by row.

[0066] In Fig. 3A camera is provided and drawn as configuration means 6, which images the points of impact of the illuminating radiation on the sample. Using the known beam direction of the transceiver, the height of the object points under consideration can be determined from the image of the points of impact. In combination with a determination of the points of impact of the beam path on the retroreflector (for example, using a camera like K6 in K6), the following can be determined: Fig. 2 In addition to an ellipsometric determination of the beam angles in the sensor itself, a measurement of the surface geometry of the sample can also be performed.

[0067] In the Figures 2 and 3 (This also applies to the other figures) Optics used for imaging or shaping the beam path (e.g. lenses or mirrors for generating a special scan or image) are not shown.

[0068] The transmitter or laser 1 can have the following properties: Retroreflex sensor systems can, in principle, be implemented across the entire electromagnetic spectrum. The only requirement is that suitable retroreflectors, detectors, and "optical" elements with appropriate properties (beam splitting, polarization, refraction, or reflection) exist in the respective range. Probable embodiments operate in the visible or adjacent spectral range. In preferred embodiments, for example, a white light source, one or more lasers, or a broadband IR or UV light source can be contained in the transmitter or coupled into the transmitter via an optical fiber. The receiver can contain one or more photodiodes, one or more line sensors, photomultipliers, etc.

[0069] Suitable samples transmit or reflect (speculently, i.e., according to the law of reflection) a sufficient proportion of the incident radiation. Very suitable samples include, for example, optical (including coated) elements, metals, preferably thin-coated materials, solar cells, and nanoimprints.

[0070] To illustrate the beneficial effects of the individual configurations or configuration means K of these configurations, they are explained below using exemplary application examples. IV.2. The individual configurations, which can be implemented independently or together, and their use cases IV.2.1. Configuration K1

[0071] According to one embodiment of the invention, means K1 can be provided to move the retroreflector 2 during the measurement. This can be achieved by means of a rotational movement or a vibration of the reflector. The movement can be brought about by an actuator as means K1. The actuator can, for example, be a freely rotating motor or a motor or vibrator that is optimally controlled to solve the measurement task.

[0072] The movement can be performed, for example, in a circular or vibratory manner, in order to minimize the noise of the measurement signals through a microstructure of the reflector 2. Fig. 4 This configuration shows an initial beneficial effect. The in Fig. 4 The measurement curves shown were obtained in a setup according to Fig. 3 recorded. As a means of distraction (6 in Fig. 3A polygon mirror was used so that the sample was scanned line by line over time. Using a detector (here, for example, comprising three individual detectors), the intensity profile for three polarization directions (90°, 0°, 45°) was recorded over time and synchronized with the start of the sample scanning so that the measurement data could be displayed as image signals (pixels = x-axis of the Fig. 4 ) can be evaluated. Fig. 4a shows the signals of individual image lines that were recorded without moving the reflector. In contrast, shows Fig. 4b . the result of an evaluation in which individual image lines were recorded with a moving reflector and appropriately averaged in a preprocessing stage so that Fig. 4b represents the course of the mean intensities across the image line.

[0073] How Fig. 4As shown, a recording method optimized for the purpose of configuration K1 can include an averaging of the measurement signals that is adapted to both the measurement task and the movement of the reflector.

[0074] Fig. 4 This shows the improvement in the signal-to-noise ratio through configuration K1 by comparing the signals without configuration K1, i.e., with a static reflector (left) versus a reflector 2 moved according to K1 with averaging of the signals accordingly (right, b).

[0075] In embodiments of the invention in which, according to configuration K2 (see below), a reflector 2 realizing this configuration K2 (which, for example, changes the polarization of the incident radiation in a known manner depending on the point of impact or angle of incidence) or a reflector 2 together with a reflector-external element realizing this configuration K2 is used, the movement can also be controlled in such a way that the detected radiation is modulated with a signal from which the angle of incidence of the measurement signal on the reflector surface or the geometry of the test object P can be calculated in an evaluation unit of the system (not shown). This can be achieved, for example, with an actuator that receives a setpoint value which moves the reflector, for example, into a position or angular orientation dependent on that setpoint. IV.2.2. Configuration K2

[0076] According to one embodiment of the invention, optically effective means K2 can be placed between the sample and the reflector. Examples include delay elements, optical rotators, and attenuators. These can also be variable (for example, selectively controllable, rotatable, or tiltable) so that their optical effect can be changed and leads to a modulation in the measured signals with a desired or evaluable signal component.

[0077] In some embodiments of the invention, K2 may also be a coating of the retroreflector 2 itself, for example a layer that acts as a quarter-wave retarder.

[0078] In further embodiments of the invention, it can involve a targeted construction of a retroreflector 2 itself by means of appropriately aligned microstructures.

[0079] In addition to retroreflectors (hereinafter sometimes referred to simply as "reflectors") consisting of microspheres, reflectors made of microprisms can be used in other embodiments. These microprisms usually consist of three mirrored planes, each forming a 90° angle with each other. The mirrored planes can be either triangular or square. Besides lower beam divergence and higher intensity, these reflectors have polarization-modifying properties—unlike reflectors with microspheres. With a suitable choice of microstructure, the multiple reflections at the micromirrors cause a change in the polarization of the incident light, which depends on both the angle of incidence and the rotation angle of the retroreflector or the microelements and can be determined using a mathematical model.If multiple images are taken at different retroreflector rotation angles, it is possible to directly calculate both the sample's rotation relative to the camera coordinate system and the angle of incidence. This is independent of the sample under investigation, as long as the layer structure consists of isotropic materials. After calculating the angle of incidence and rotation for each pixel, further model parameters such as refractive index and layer thickness can be calculated using conventional ellipsometric methods, since the surface normal is known. IV.2.2.1. First example of the K2 configuration

[0080] An example of the K2 configuration is the suppression of polarization effects during the reflection of the rays at the sample P.

[0081] The combined effect of configuration K2 can be described using the Müller formalism. In this approach, the combined effect of a separate, reflector-external configuration element K2 on the one hand and the retroreflector 2 on the other is described by a resulting Müller matrix Mres as Mres = Mreverse, Mreflector, Minward. Here, Minward is the Müller matrix for the optical path of radiation 1-S from sample P through element K2, Mreflector is the Müller matrix of reflector 2, and Minward is the Müller matrix for the return path of radiation 1-E through the optical element K2 back to sample P.

[0082] To suppress polarization effects during the reflection of radiation at sample P, the optical element K2 is matched to the properties of the retroreflector 2 such that M res is as close as possible to the Müller matrix that is optimal for this application. M res , optimal = 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 comes close.

[0083] In this optimal scenario, polarization effects during reflection from the sample are eliminated or at least minimized. This makes this configuration ideal, for example, for detecting decorative defects on coated or painted surfaces. Otherwise, the dips in reflectance caused by these defects might be masked by polarization effects and cannot be reliably distinguished from them.

[0084] As an optically effective means for this application, K2 can be a depolarizer or a depolarizing coating of the actual reflector.

[0085] This configuration also offers the advantage of eliminating polarization signals that would otherwise superimpose and distort the measurement signal, particularly in applications where structural edges of the test specimen P or interface profiles in the test specimen are to be determined. IV.2.2.2. Further examples of the K2 configuration

[0086] Another example of an application of this configuration is in Fig. 5 depicted. Fig. 5 shows alternative systems according to the invention, which are basically like those in Fig. 2 The structures shown are set up differently, so only the differences will be described below. The first difference is that the ones shown are structured differently. Figs. 5a and 5b The example systems shown are transmission systems.

[0087] Furthermore, in this case, transmitter 1 emits circularly polarized light. Receiver 3 detects the oppositely circularly polarized light. The means K2 according to the invention is an element in the form of a half-wavelength plate or half-wavelength film (rotated arbitrarily to the optical axis). The common detection unit in the housing 4 can be constructed either as shown in Fig. 5b shown (with beam splitter 5 and separate realization of the polarization of illumination 1 and detector 3) or as in Fig. 5aAs shown, a polarizing beam splitter is used there. The radiation coming from the light sources is linearly polarized, and after passing through the quarter-wavelength (λ / 4) delay element 7, the radiation is circularly polarized. The receiver beam path passes through the same element 7 onto the polarizing beam splitter 5, so that element 7 results in a total delay of half a wavelength (λ / 2) over the entire beam path.

[0088] In both configurations, the sensor 3 detects a signal proportional to cos 2< (Δ) with the desired delay Δ in the sample.

[0089] In further embodiments of the invention, the means or element K2 is modulatable or adjustable and allows additional or improved measurements for characterizing the sample. In other embodiments of the invention, K2 is fixedly connected as an optical element to a rotatable reflector 2 according to configuration K1, for example as a coating of the reflector 2 or as a separate optical element rotating with it.

[0090] As an example of an application with a rotating optical element K2, the retroreflex transmission arrangement according to [reference to relevant document] is described (without limiting it to this embodiment). Fig. 5bIt consists of (or contains the following elements): the transmitter 1, which emits circularly polarized light; the receiver 3, which detects the intensity of the incident circularly polarized radiation in a single channel; and a quarter-wavelength plate or film K2 rotating with the polarization-preserving retroreflector 2 according to the invention. When a retroreflector 2 is used without beam offset, the receiver 3 detects an intensity signal I, even with objects P having strong refractive powers, which has the following characteristics: I = 0.5 Cos 2 α − θ 2 Sin Δ 2 with the angle α of the direction of the (possibly stress-induced) delay in the sample, the rotation angle θ of the quarter-wavelength plate or foil and the delay or stress-induced phase shift Δ on the beam paths through the sample P.

[0091] In this embodiment, the delay plate K2 can rotate alone or together with the reflector 2. In other embodiments of the invention, a modulatable delay element is used as K2. In these cases, the angle θ is changed by modulation.

[0092] Fig. 5 This shows examples of systems or sensors with optical elements or configuration means K2 between probe P and reflector 2. Figure 5a Is the optical element K2 attached alone, in Figure 5b Element K2 is firmly connected to reflector 2 and is moved (by means of K1) together with the reflector.

[0093] In the examples mentioned, a polarization-preserving retroreflector is used. The circular polarization of the transmitter beam path is achieved either by using a circularly polarized radiation source 1 and a non-polarizing beam splitter 5 (see...). Fig. 5b) or by using a polarizing beam splitter (5 in Fig. 5a ) together with a delay element (7 in Fig. 5a ). IV.2.3. Configuration K3

[0094] By modulating the polarization state of the illumination 1 with at least two different states by a modulating means or element K3 in the transmitter beam path 1-S, it becomes possible to additionally determine the inclination angle of the sample surface relative to the optical axis. In systems or sensors according to the invention that scan the sample surface (e.g., laser scanners), the inclination angle can thus be determined separately for each image point or pixel. One such modulation according to the invention is the modulation of the illumination 1 such that left- and right-circularly polarized radiation is emitted alternately. For this purpose, an electronically adjustable delay element K3 is installed in the transmitter beam path 1-S, e.g., directly on the beam output side of the transmitter 1 (see Figure 1). Fig. 6a ).

[0095] Alternatively (see above). Fig. 6bA further light source 8 with the same emission wavelength λ as the transmitter 1 (or with an emission wavelength that typically differs only slightly from that of the transmitter 1) can be arranged in the second light path 1-S2 of an inserted polarizing beam splitter 9. (The radiation from the source 8, or rather the light path, is coupled via the beam splitter 9 into the light path 1-S1 of the transmitter 1 to form the total radiation 1-S at the output side of the splitter 9.) A quarter-wavelength or A / 4 delay element 10 can also be arranged in the transmitter beam path 1-S downstream of the splitter 9, i.e., between the polarizing beam splitter 9 and the object P (not visible here).

[0096] The modulation occurs when Fig. 6bby alternately switching on the light sources 1 and 8. For example, within the scope of the present invention, a laser scanner according to STT1 can be used as a transceiver or as a combined transmitting and receiving unit (i.e., as transmitter 1 together with receiver 3 in a common housing 4) (e.g., according to the one described therein). Figures 2 , 3 or 7The setup shown is as follows: If receiver 1 of transceivers 1, 2, 4 can determine at least the components U and V of the incident radiation, which has a Stokes vector {S,U,V,W}, then the angle ϕ of each measurement or image point of the sample P relative to the detector's preferred direction can be calculated from these measured values ​​of the linear polarization components of the Stokes vector. When measuring isotropic samples, the angle ϕ is the rotation of the sample's surface normal relative to the detector's preferred direction (directions of the U polarizations). The detector values ​​U[W+] and V[W+] are determined under positive circularly polarized illumination. The values ​​U[W-] and V[W-] are determined under oppositely circularly polarized illumination. In these notations, the equation for determining the angle ϕ is given by: tan 2 ϕ = V W + + V W − U W + + U W − .

[0097] This allows ellipsometric measurements to be corrected with the arrangement and, according to the invention, a rotational position of the object or each object point relative to the sensor coordinate system to be determined.

[0098] If the illumination beam path 1-S is modulated at the wavelength λ, the system can record the optical properties at multiple spectral measurement points and thus – in a manner known per se for single-point measuring devices according to the prior art – obtain multiple support points for model-based measurement of samples P. Wavelength modulation can also be performed with one- or two-dimensional (imaging) sensors or systems. In this case, the modulation occurs synchronously with the acquisition of partial images – for example, line by line, frame by frame, or in a fixed pixel grid.

[0099] The wavelength modulation can be combined with the modulation of the polarization state of the illumination 1 according to the invention. In an advantageous embodiment of this combination, the two light sources 1 and 8 emit Fig. 6b (each as a laser) different wavelengths, with a spectral separation that is small in relation to each of the two wavelengths of the lasers.

[0100] Fig. 6 Figure 1 shows two embodiments of a circularly polarized illumination modulation (according to K3) by a polarized transmitter 1 and a modulatable or rotatable delay element K3 ( Fig. 6a ) or equivalently by means of a device K3 comprising an additional beam source 8, a polarizing beam splitter cube 9 and a fixed delay element 10 ( Fig. 6b ). In the Fig. 6b In the illustrated version, modulation is achieved by switching the two light sources 1 and 8 on and off. IV.2.4. Configuration K4

[0101] Due to the retroreflection measurement, the optical properties of the object P are quadratically incorporated into the measurement signals (multiplicatively on the outward path 1-S via / through the sample P to the reflector 2 and on the return path 1-E from the reflector 2 via / through the sample P). While the ellipsometric parameters Δ and ψ are incorporated into the measured quantities as arguments of trigonometric functions in systems with simple reflection, they are introduced with a prefactor of 2 in comparable retroreflection measurement systems. Consequently, retroreflection systems have only half the range in which these quantities can be unambiguously determined. Furthermore, depending on the system design, the measurement accuracy for determining the quantities Δ and ψ, including their absolute values, can vary considerably.

[0102] To solve these problems in configuration K4, a fixed, adjustable or modulatable optical means or element (retardation element and / or dichroic element) is arranged to optimally adapt the unique measurement areas to the value ranges of the samples of interest.

[0103] If, according to configuration K4, an additional delay element is placed directly in front of the receiver 3 in the receiving beam path 1-E (i.e., in housing 4 in 1-E on the beam output side of the splitter 5, i.e., between the splitter 5 and the receiver 3), the unique measuring range of the system can be adapted to the range of the samples to be measured. Fig. 7This shows the determination of the phase shift Δ with a retroreflex ellipsometer for a delay element introduced according to configuration 4 or as K4, which in the example shown is set to a phase shift of δ = -π / 2, δ = 0 or δ = +π / 2.

[0104] Depending on the application, the delay element can be fixed in the beam path, a variable element (e.g. electrically controllable), or pivotable into the beam path.

[0105] Further embodiments of the K4 configuration can modulate the quantities Δ and ψ and determine more accurate or supplementary measured values ​​using a corresponding evaluation method.

[0106] Fig. 7 This shows the unique ranges (Δ real -Δ m =0) of the determination of Δ for different phase shifts δ of an additional delay element (as mean K4) in front of receiver 3. IV.2.5. Configuration K5

[0107] Additional object information about P can be obtained if the system, or rather its detector 3, determines all four Stokes parameters simultaneously. This can be achieved, for example, by extending the detector of a retroreflex sensor to a "division of amplitude photopolarimeter" (e.g., according to [STT5]). Alternatively, the detector of a retroreflex sensor can be replaced by a suitable detector.

[0108] For example, if the detector of a retroreflex sensor according to STT1 is extended accordingly and used within the scope of the present invention, it can be verified from the measured values ​​themselves whether the object P or each pixel of the object P is depolarizing or not. If no depolarization is detected, a further ellipsometric investigation of the anisotropy of the sample can be carried out.

[0109] In the present invention, additional light sources can also be introduced into the optical path. These light sources can be arranged in addition to or as an alternative to the light source(s) in the transmitter part 1. With a suitable configuration of the arrangement, this allows the determination of the complete Müller matrix of each investigated object point in a retroreflex setup. IV.2.6. Configuration K6

[0110] For precise ellipsometric measurements – for example, of the thickness of coatings or paints – knowledge of the angle of incidence of the radiation on the sample P is necessary – both in the reflection plane and against the beam axis or preferred direction of the detector 3.

[0111] In many cases, this information is available or accessible in principle (for example, with planar objects P that are precisely guided through the measurement field or in CAD data). If the object geometry is derived from CAD data (one of the possibilities for a configuration tool K6), the system's evaluation unit must compare the CAD data with the movement of the object P through the measurement field and, from the data of the object's movement through the measurement field (motion data) and the CAD data, provide the corresponding angle information for each measurement point.

[0112] The simplest case for such a comparison is the object moving at a constant speed through the measurement field. Under this condition, only a trigger signal needs to be activated when the object begins moving through the measurement field, and the speed of the movement must be known. Then the CAD data can be directly compared with the image data. Otherwise, signals (motion data, e.g., from a motor controller for the movement of object P, or additional position sensor signals, etc.) must be provided to make this comparison possible.

[0113] In cases where the beam angles cannot be derived from the measured values ​​themselves or from external data sources, they must be measured additionally for a more detailed characterization of the optical properties of object P (or of its pixels). This can be done simply by mapping the measurement point onto object P (see camera K6 in [reference]). Fig. 3 ) and / or an image of the point of impact of the radiation 1-S or S e on the reflector 2 is taken (see camera K6 in Fig. 2 Alternatively, an additional measuring system can be arranged which determines the inclinations of the surface of P (relative to the guide or support) using state-of-the-art methods (e.g., mechanical scanning, triangulation, deflectometry, etc.). IV.2.7 Other configuration means that can be combined with the invention

[0114] For extended or improved evaluation of the measured values, it is advantageous to provide additional configuration means with which the measured intensity values ​​can be normalized at the lowest level of the evaluation. These can be integrated into the common housing 4 or located outside of it.

[0115] Examples of this are: For systems with laser scanners (e.g., according to STT1), a further retroreflector (or a combination of a further retroreflector with optical means such as neutral density filters or polarization-modifying optical components) can be arranged in the housing 4 such that a portion of the scan line is reflected directly back into the receiver 3. This is always possible when the entire scan line is not required to detect the sample P. In this case, fluctuations in the illumination 1 or the sensitivity of the detector 3 (or parts thereof) can be compensated for by normalization. In transmission systems according to the invention with laser scanners (e.g., according to STT1), an area of ​​the retroreflector 2 that is not obscured by the object P can be used for normalization. This normalization can compensate for additional fluctuations in the measured intensities that arise outside the housing 4 (e.g., condensation of the retroreflector 2).In reflection systems according to the invention, for corresponding standardization with the same advantages, one or more reference object(s) can be arranged next to the sample P such that part of the illumination 1 is reflected by this / these reference object(s) and the reflected rays strike the retroreflector 2. IV.3 Exemplary combinations of configurations IV.3.1. First combination example

[0116] The unified overall concept of the invention will now be illustrated using the example of a universal system for the improved classification and characterization of objects P or points on the object surface. It will be demonstrated that the addition of each individual described configuration (i.e., several or all of the configuration means K1 to K6) provides an improvement that already represents a sufficient or even the optimal solution for defined measurement tasks. One example according to the invention requires the addition of configuration means K3.

[0117] The combined example is based on an imaging retroreflex laser scanner according to the basic setup in STT1, which is extended according to the invention by appropriate (additional) configuration means to form a comprehensive system for classifying samples with regard to decorative defects, material defects, material type, coating, and coating defects. With this system, all ellipsometric parameters can be determined – even on non-planar surfaces and in many manufacturing processes. Previously, these parameters could often only be determined in the laboratory on flat areas of samples using ellipsometry.

[0118] The basic setup from STT1 utilizes the retroreflection of the radiation reflected from the sample: After reflection off the surface of object P, the rays strike a retroreflector 2. With reflectors 2 without beam offset, the rays are reflected back to the combined transmitting and receiving unit 1, 3, and 4 along precisely the same optical path. Therefore, this system concept yields evaluable signals from the investigated samples P over a wide angular range. (In contrast, with conventional ellipsometers, even small angular deviations on the order of 1° between components and sample in the plane of incidence result in no detectable measurement signal.) The basic setup from STT1 allows decorative defects in the samples to be detected as local dips in reflectance.

[0119] If such a system is extended according to the invention with configuration 1 by a moving reflector 2, the signal-to-noise ratio can be improved. This correspondingly improves the detection of finer or weaker local surface defects.

[0120] If the detector 3 of such a system is extended according to the invention in accordance with configuration K5 (i.e., the combination K1 and K5 is present) so that the polarization degree of the radiation is also detected, then a rough classification can already be carried out for each pixel or imaged point of the sample according to the following criteria: Depolarizing with simultaneous determination of the degree of (de)polarization. The object P (or object point) exhibits a purely circularly anisotropic Müller matrix. The object P (or object point) shows no phase shift. The Müller matrix of the object P or object point shows linear or mixed linear and circular anisotropy.

[0121] For objects without phase shift, the rotation angle relative to the sensor's optical axis can also be determined. Furthermore, the reflectance R (in reflection setups) or the transmission coefficient T (in transmission setups) and the ellipsometric parameter ψ can be calculated from the measured values ​​for these objects. In many cases, the refractive index and the extinction coefficient can then be determined. If only these values ​​need to be determined for these objects, the optimal solution to the problem is already achieved at this stage.

[0122] Further characterization of the sample (measurement points) is possible with the combination of K1 with K3 (according to the invention) and K5. If the sample P is illuminated alternately with left- and right-circularly polarized light, the rotation angle of a sample anomaly relative to the sensor coordinate system (i.e., the receiver coordinate system) can be determined for each pair of image points, regardless of the object type. From this information and the other measured values, it can also be unambiguously determined whether the Müller matrix of the corresponding point on the sample exhibits purely linear anisotropy. For each object point with a Müller matrix exhibiting purely linear anisotropy, the ellipsometric quantities R or T, Δ, and ψ can also be calculated. Anomalies such as local coating defects or material defects can thus be detected as local disturbances, even if they are potentially hidden from the human eye.If, according to the invention K3, the modulation is realized by switching two light sources 1 and 8 which emit spectrally at a small distance, the complex refractive index of the corresponding point on the surface can often be determined for each pixel of a substrate P.

[0123] To improve and / or adjust the measurement accuracy or the unambiguous range for evaluation, an optical element can also be placed directly in front of the sensor according to configuration K4 (this then results in the combination K1 with K3, K4 and K5).

[0124] Further improvement in the accuracy of the system is possible if more precise values ​​of the rotation angle of the sample and / or the angle of incidence are used from external data sources according to configuration K6 (the combination K1 with K3, K4, K5 and K6 is then available).

[0125] In another embodiment of such a system, a spectrally broadband light source can be used in combination with a diffractive optical element in front of the receiver beam path (according to configuration K4). In this case, it is advantageous to modulate the illumination with a modulatable delay element (according to configuration K3) and to use line sensors as detectors in the receiver. In this embodiment, the spectral profile of the ellipsometric parameters for a given polarization is obtained at each point during surface scanning. By scanning the measurement point at least twice with differently polarized illumination, pairs of spectra are obtained from which the properties of the object point and its angular position can be derived using the methods of spectroscopic ellipsometry.

[0126] The configuration combinations are described here as examples based on the basic setup from STT1 (i.e., as extensions of systems according to STT1 according to the invention), specifically for systems operating in reflection mode. The explanations apply analogously to systems operating in transmission mode. Individual configurations can also be omitted if necessary. IV.3.2. Second combination example

[0127] For this example, a retroreflex sensor is extended as follows (e.g., system according to the invention). Fig. 2 , where at least configurations K2 and K3 are provided): The optically effective means K2 (between object P and a polarization-preserving retroreflector 2) is a quarter-wave delay element rotating at the angular velocity ω. According to K3, an alternating left- and right-circular illumination is chosen, which is modulated rapidly compared to the angular velocity ω.

[0128] Fig. 8aFigure 1 shows such an embodiment of the invention in a transmission arrangement with a light source arranged next to the detector, which determines the ellipsometric parameters in transmission. The transmitter S and receiver E can be implemented as point, line, or area illumination and detector, respectively.

[0129] In a particularly advantageous embodiment of this inventive concept, the delay element K2 is rigidly coupled to the retroreflector 2 (for example, as a coating or a film applied over it) and additionally rotates together with it according to K1. In this case, a further improvement in the signal-to-noise ratio can be achieved simultaneously.

[0130] Fig. 8b Figure 1 shows such a design for determining the characteristic parameters in reflection with a rotatable retroreflector 2 according to K1. In an advantageous embodiment, the optical element K2 is directly connected to the reflector 2.

[0131] Fig. 8 This shows two embodiments of the retroreflex ellipsometer according to the invention for determining the ellipsometric parameters independently of the sample adjustment.

[0132] Figure 9 and 10 show (to Fig. 8 ) the measurement signal generated by an embodiment with a single-channel intensity detector during a rotation of the delay element K2. Shown is the sum ( Fig. 9 ) and the difference ( Fig. 10 The intensity signals are measured under alternating left- and right-circular illumination. As this illustrates, the parameters R, T, Δ, and ψ can be easily derived from the signal waveforms. Furthermore, the rotation angle of the sample P (against the preferred direction of the retarding element K2) can be directly deduced from the angular position of the maxima and minima.

[0133] A spectroscopic ellipsometer can be realized by additionally configuring a diffractive optical element in front of the receiver beam path (according to configuration K4) in combination with a linear white light source as the transmitter and a matrix detector as the receiver. This can be used to measure objects that cannot be measured with state-of-the-art spectroscopic ellipsometers.

Claims

1. Retroreflector-based sensor system for optical characterisation of a sample (P), having a transmitter (1) for irradiating the sample (P) which is positioned in the transmitter beam path (1-S), a retroreflector (2) which is positioned behind the sample (P) in the transmitter beam path (1-S) such that it reflects radiation (Se) in the transmitter beam path (1-S) coming from the sample (P) and incident thereon in the receiving beam path (1-E) back onto the sample (P), the transmitter (1), the sample (P) and the retroreflector (2) being positioned such that radiation reflected back from the retroreflector (2) in the receiving beam path (1-E) is again incident on the sample (P) and is reflected back from the latter (P) in the direction towards the transmitter (1), and a receiver (3) which is positioned in the receiving beam path (1-E) such that it detects radiation (SRR) reflected back from the retroreflector (2), incident again on the sample (P) and reflected back from the latter (P), in the direction towards the transmitter (1), from the radiation (SRR) detected by the receiver (3), measured values being obtainable and evaluable for optical characterisation of the sample (P), characterised in that at least one configuration means (K1 to K7) being provided, by means of which improved obtaining and / or evaluation of the measured values is possible, for improved optical characterisation of the sample (P), wherein - one (K3) of the configuration means (K1 to K7) being configured to modulate or switch a polarisation state of the illumination in the transmitter beam path (1-S) between positively circularly polarised and negatively circularly polarised states - the retroreflector-based sensor system is configured to correct ellipsometric measurements by determining, from the values measured by the receiver under positively and negatively circularly polarized illumination for an isotropic sample, a rotational position of a surface normal of the sample (P) relative to a preferred direction of the receiver (3).

2. Retroreflector-based sensor system according to the preceding claim, characterised in that one (K1) or more of the configuration means (K1 to K7) is / are configured and / or positioned in order to move the transmitter (1), the sample (P), the retroreflector (2) and / or the receiver (3), in particular mechanically, whilst the radiation (SRR) is detected by the receiver (3).

3. Retroreflector-based sensor system according to one of the preceding claims, characterised in that one or more (K2 to K4) of the configuration means (K1 to K7) include / s (an) optical element(s) which change / s, modulate / s and / or switch / es one or more parameters of the radiation, preferably which change / s, modulate / s, switch / es and / or effect / s a / the polarisation state, a / the direction, a / the coherence, a / the wavelength, an / the intensity and / or a / the spectral splitting state in the transmitter beam path (1-S) and / or in the receiving beam path (1-E), and / or an offset between the transmitter beam path (1-S), on the one hand, and the receiving beam path (1-E), on the other hand.

4. Retroreflector-based sensor system according to one of the preceding claims, characterised in that one (K5) of the configuration means (K1 to K7) is the receiver (3) itself, by the latter (3) being configured such that, from the radiation (SRR) detected by it (3) and also by it (3) or by an evaluation unit of the sensor system, the entire polarisation state of this radiation (SRR) or the complete Stokes vector of this radiation (SRR) can be determined, in particular can be calculated.

5. Retroreflector-based sensor system according to one of the preceding claims, characterised in that one (K6) or more of the configuration means (K1 to K7) is / are (a) means for obtaining additional information about the sample (P) and / or about the radiation of the transmitter beam path (1-S), preferably the means for obtaining additional information being (a) sensor(s), (a) camera(s) and / or (an) information memory(ies), preferably (an) information memory(ies) of an / the evaluation unit of the sensor system, comprising the additional information, and / or preferably the additional information being or comprising: • the pose, i.e. both the position and the orientation, the position or the orientation, of the sample (P) relative to the transmitter (1), retroreflector (2), receiver (3) and / or to the radiation incident on it (P), • data about the property of the sample or the surface thereof, in particular geometric data of the sample, • the angle of incidence of the radiation which is incident on the sample (P) in the transmitter beam path (1-S), and / or • one or more angle(s) of inclination of one or more defined axis / axes of the sample (P) relative to a defined coordinate system of the receiver (3).

6. Retroreflector-based sensor system according to one of the preceding claims, characterised in that one or more of the configuration means (K1 to K7) is / are (a) means for standardisation of the measured values.

7. Retroreflector-based sensor system according to one of the preceding claims, characterised in that one or more of the configuration means (K1 to K7) is / are positioned or configured in the transmitter beam path (1-S), in the receiving beam path (1-E), in / on the transmitter (1), in / on the retroreflector (2), in / on the receiver (3), in / on a housing (4) including or comprising both the transmitter (1) and the receiver (3), or is / are provided such that the sample (P), the retroreflector (2) or one of the configuration means can be scanned optically by it / them, for example can be imaged by means of an image sensor, in particular a camera.

8. Retroreflector-based sensor system according to one of the preceding claims, characterised in that the transmitter (1), the retroreflector (2) and the receiver (3) are configured and positioned such that (the) radiation which is reflected back at the retroreflector (2), viewed relative to the transmitter beam path (1-S), widens in at least portions of the receiving beam path (1-E) on the identical path or widens on a path offset thereto, i.e. with a beam offset relative to the transmitter beam path (1-S).

9. Retroreflector-based sensor system according to one of the preceding claims, characterised in that the transmitter (1), the sample (P) and the retroreflector (2) are positioned in reflection arrangement, the light coming therefore from the transmitter (1) in the transmitter beam path (1-S) and radiated onto the sample (P) is reflected at the sample (P), coming from there (P) is incident (Se) on the retroreflector (2), is reflected back from the retroreflector (2) into the receiving beam path (1-E) and onto the sample (P), is reflected again at the sample (P) and finally is detected by the receiver (3) as radiation (SRR) which has been reflected twice at the sample (P), or in that the transmitter (1), the sample (P) and the retroreflector (2) are positioned in transmission arrangement, the light coming therefore from the transmitter (1) in the transmitter beam path (1-S) and radiated onto the sample (P) is transmitted through the sample (P), coming from there (P) is incident (Se) on the retroreflector (2), is reflected back by the retroreflector (2) into the receiving beam path (1-E) and onto the sample (P), is transmitted again through the sample (P) and finally is detected by the receiver (3) as radiation (SRR) which has been transmitted twice through the sample (P).

10. Retroreflector-based sensor system according to one of the preceding claims, characterised in that the transmitter (1) and the receiver (3) are positioned adjacent to each other, directly adjacent to each other or as closely adjacent to each other as possible, or in that the receiver (3) is positioned in a spatial region in which the transmitter (1) is also positioned, or in that the transmitter (1) and the receiver (3) are integrated or positioned in(inside) one and the same housing (4).

11. Retroreflector-based sensor system according to one of the preceding claims, characterised in that the transmitter (1) and the receiver (3) are configured as a point-measuring system, i.e. scan a point-like region of the sample (P) or in that the transmitter (1) and the receiver (3) are configured as a scanner system, i.e. scan a one-dimensional, i.e. linear, region of the sample (P) or a two-dimensional, i.e. planar, region of the sample (P).