Method and device for determining the refractive index in the surface area of ​​an object

Description

[0001] The invention relates to a method and a device for determining the refractive index in the surface region of a strand-shaped or plate-shaped object which comes heated from a production plant and has not yet completely cooled to ambient temperature.

[0002] For example, in extrusion lines, strand- or sheet-shaped plastic objects are produced and conveyed along a conveying direction, for example, through cooling sections, until they have completely cooled to ambient temperature and are therefore fully cured. Immediately after exiting the extrusion line and continuing along a further section of the conveyor line, such objects are not yet completely cooled and therefore not fully cured.

[0003] WO 2016 / 139155 A1 and DE 10 2018 128 248 A1 disclose methods and devices for determining the refractive index of strand- or plate-shaped plastic objects by irradiating the objects with terahertz radiation and receiving the terahertz radiation reflected by the objects. This represents an average refractive index across the cross-section of the object or the irradiated section of the object. On this basis, geometric parameters of the object, such as the wall thickness of pipes, can be reliably determined even if the refractive index is initially unknown.

[0004] However, especially when measuring objects shortly after they leave the extrusion line, the geometric parameters determined in this way may deviate from the actual geometric parameters when the object is completely cooled. More reliable results could be obtained by later determination of the refractive index and geometric parameters, once the object has already completely cooled and hardened. On the other hand, there is a desire to determine the geometric parameters as quickly as possible after they leave an extrusion line, for example, in order to be able to intervene in the production process as quickly as possible in the event of faulty parameters and thus minimize scrap.

[0005] There is therefore a need to enable an accurate determination of the refractive index even when measuring the object in a state that is not yet completely cooled and thus hardened.

[0006] Furthermore, the methods for determining the refractive index discussed above, which are known from the state of the art, are not ideally suited for reliably determining the refractive index, for example, in portable measuring devices, so-called handheld devices. Therefore, there is a further need to enable reliable refractive index determination in such applications as well.

[0007] Based on the explained prior art, the invention is therefore based on the object of enabling a reliable determination of the refractive index even when the object is not yet completely cooled down.

[0008] WO 2017 / 000933 A1 describes a method and a device for determining a layer property of a layer in an extrusion process. An extruded product is irradiated with electromagnetic radiation, and an electromagnetic measurement is performed of at least one radiation passing through the layer to be determined. Furthermore, at least one feed rate or feed quantity of a feed material to an extrusion device is measured, and at least one layer property of the layer to be determined is determined from the measured feed quantity and the electromagnetic measurement.

[0009] The invention solves the problem by the independent claims 1 and 11. Advantageous embodiments can be found in the dependent claims, the description and the figures.

[0010] For a method of the type mentioned above, the invention solves the problem by the steps a) Terahertz radiation is emitted onto the surface of the object at an angle of incidence, b) terahertz radiation reflected from the surface of the object is received, c) the refractive index in the surface area of ​​the object is determined from the ratio of incident and reflected terahertz radiation, d) when determining the refractive index, the influence of the surface properties of the object on the proportion of reflected terahertz radiation is taken into account.

[0011] A device of the type mentioned above for solving the problem comprises: a transmitter for transmitting terahertz radiation at an angle of incidence onto the surface of the object, a receiver for receiving the terahertz radiation reflected from the surface of the object, an evaluation device which is designed to determine the refractive index in the surface region of the object from the ratio of radiated and reflected terahertz radiation, wherein the evaluation device is further designed to take into account an influence of the surface properties of the object on the proportion of reflected terahertz radiation when determining the refractive index.

[0012] The object examined according to the invention is in a heated state, having come from a production facility and not yet completely cooled to ambient temperature. Accordingly, it may contain viscous components in its interior that are not yet fully cured and will only harden upon further cooling. The surface area of ​​the object, however, is already cured. During further cooling and thus hardening, the material shrinks. The production facility may, for example, be an extrusion facility. The object may accordingly be an object extruded in an extrusion facility. The object may, for example, be a plastic object. The object may, for example, be a pipe. Furthermore, the object may be conveyed along a conveying direction during the terahertz irradiation according to the invention, for example, a strand-shaped object along its longitudinal axis.For this purpose, the device according to the invention can comprise a conveying device. The device according to the invention can also comprise the object itself.

[0013] In the invention, terahertz radiation is emitted onto the surface of the object at an angle of incidence assumed to be known. The surface of the object reflects the incident terahertz radiation, at least partially, depending on its reflectivity. The reflected terahertz radiation is received by a receiver. The terahertz radiation can, for example, be in a frequency range from 10 GHz to 3 THz. It can be so-called millimeter waves. A transmitter emitting the terahertz radiation and a receiver receiving the reflected terahertz radiation can be arranged essentially at the same location. They can, for example, be integrated together into a transceiver.

[0014] The invention is based on the idea that a reliable refractive index determination is possible in the surface area of ​​the object that has already largely cooled and thus hardened. This, in turn, is based on the realization that an object that has not yet completely cooled still contains viscous components, with a narrow transition zone between these viscous components and the already hardened components, the so-called recrystallization zone. In total, therefore, there are three different types of layers, whose properties differ significantly in terms of density and strength, and thus also in terms of refractive index.Therefore, if an average refractive index is determined across the entire cross-section of an object still containing viscous components in the manner explained above regarding the prior art, this can lead to refractive index values, particularly with regard to the still viscous components, that do not correspond to the refractive index of the material in its fully cured state. This can have a correspondingly distorting effect on the determination of geometric parameters, such as the wall thickness of pipes.

[0015] Investigations by the applicant have shown that, as a tube still contains viscous components, both its length and diameter change only insignificantly, namely by less than 1%, during further cooling to ambient temperature. Nevertheless, a shrinkage of approximately 10% of the tube material can be observed during complete cooling and curing. It can therefore be concluded that this shrinkage is essentially entirely caused by the still viscous component of the object's material. Since the refractive index of a material changes with the material's density, determining an average refractive index for objects still containing viscous components leads to potential measurement errors.This can be avoided by measuring the refractive index in the already completely cooled and thus hardened surface area of ​​the object, as provided for in the invention.

[0016] To measure the refractive index in the surface area, the well-known Fresnel formulas are used. With these formulas, the refractive index in the surface area, in particular at the surface of the object, can be calculated from the ratio of the terahertz radiation reflected from the surface of the object to the terahertz radiation emitted onto the surface, given a known angle of incidence. The required reflectance of the object, i.e. the ratio of reflected radiant power to incident radiant power, can be determined from the proportion of received reflected radiation, given a known incident radiant power. The reflectance describes a mathematical relationship between the refractive indices in the surface area of ​​the object and the object's surroundings. The refractive index of the object's surroundings is known and can be assumed to be 1 for air.In the simplest case, the angle of incidence can be 0°, i.e., the normal incidence of the radiation. In this case, the Fresnel formulas simplify to: . R = n 1 − n 2 n 1 + n 2 2 with R: Reflectance n 1 : Refractive index of the surroundings of the object (for air = 1) n 2 : Refractive index of the surface area of ​​the object.

[0017] However, further investigations by the inventors have shown that the application of this method to determine the refractive index of a heated, not yet completely cooled, strand- or plate-shaped object using terahertz radiation is very complex. This applies in particular to determining the proportion of reflected radiation relative to the incoming radiation. It has been found that the magnitude of the incoming terahertz radiation intensity varies significantly with the distance from the object, and the portion of the terahertz radiation reflected back from the surface varies considerably depending on the orientation and, in particular, the surface of the object.

[0018] The distance to the object and any curvature can be easily adjusted or taken into account with a careful measurement setup. This is different for the surface of the object, which can hardly be influenced within the scope of the measurement setup. To counteract this problem, which exists particularly when using terahertz radiation, the invention provides for the influence of the surface properties of the object on the proportion of reflected terahertz radiation to be taken into account in the determination of the refractive index. This allows influences of the surface that change the reflected proportion to be taken into account and corresponding falsifications of the result of the refractive index determination to be avoided. For example, in this way it is always possible to achieve the most vertical irradiation possible and thus reflection of the terahertz radiation, i.e. at an angle of incidence of 0°.As an input variable in the determination of the refractive index using Fresnel's formulas, a reliably adjusted angle of incidence is of great importance, even in the case of difficult or unknown surface conditions of the object.

[0019] It should be noted that the refractive index also depends on the frequency of the terahertz radiation used. Therefore, whenever reference is made to the refractive index, this refers to the refractive index for the frequency of the terahertz radiation used.

[0020] In the invention, the influence of the surface topography of the object is taken into account in step d). This embodiment is based on the idea that, in particular, irregular surface topography, such as dents, bumps, grooves, or similar, can lead to the assumed known angle of incidence, for example, 0°, deviating from the actual angle of incidence and thus reflection. This can be avoided by taking the surface topography into account.

[0021] According to a further embodiment, in step d), calibration can be carried out against an object with a known reflectance, preferably a reflectance of substantially 1. The surface of the object can be provided with a coating with a known reflectance, preferably a reflectance of substantially 1, for the calibration. The object used for calibration can be the object itself whose surface refractive index is to be determined, i.e. the object coming from the production plant. It is then assumed that the surface topography of, for example, an object conveyed along a conveying direction during terahertz irradiation does not change significantly in its conveying direction, for example its longitudinal direction. This is possible to a good approximation, for example, for objects coming from an extrusion plant.For calibration purposes, for example, the surface of the object to be measured can be treated so that it has a defined, preferably as high as possible, reflectance, for example, a reflectance of 1. For example, the coating can be a film coating or a spray coating. A film could be, for example, a thin metal foil. A spray coating could be, for example, a water spray coating or a coating with another liquid with a high reflectance.

[0022] If a measurement according to the invention is carried out with a specific orientation of the transmitter (and receiver) for the terahertz radiation, the (actual) angle of incidence of the terahertz radiation can be deduced from the reflected radiation component, since the reflected radiation component at normal incidence should correspond to the known reflectance of, for example, 1. If deviations occur here, a deviation from the assumed angle of incidence of 0° can be deduced accordingly. It would also be conceivable to find the normal incidence using the surface thus provided by a corresponding realignment of the transmitter (and receiver) by searching for the orientation of the transmitter (and receiver) at which the reflected radiation component best corresponds to the known reflectance, for example, approximately 1.Calibration on the actual object to be measured delivers the best results, as the calibration is based on the actual surface topography of the object. However, if it can be assumed with a good approximation that the object's shape, including its surface topography, corresponds to another object with a known reflectance, for example a metal strand, in particular a metal tube or a metal plate, an object different from the object to be measured could also be used for calibration. The alignment of the transmitter and receiver to the object used for calibration (distance, etc.) must match as closely as possible to that of the actual object to be measured. The calibration described can be carried out both before and after the inventive determination of the refractive index of the untreated object.To avoid any influence of surface treatment on the result of the refractive index determination, it may be preferable to perform the calibration after the inventive determination of the refractive index of the untreated object. In this case, naturally, no realignment of the transmitter and receiver is required.

[0023] In the invention, in step d), terahertz radiation is emitted to different locations on the surface of the object and / or at different angles of incidence onto the surface of the object. The respective reflected terahertz radiation is received, and the refractive index is determined from the portion of reflected terahertz radiation with the greatest intensity. This results in a search for the maximum reflected radiant power at different orientations of the transmitter (and receiver) for the terahertz radiation, i.e., a search for the normal incidence. It is then assumed that in the region of the maximum reflected radiant power, the angle of incidence essentially corresponds to 0°.According to a further embodiment, a transmitter (and a receiver) for the terahertz radiation can be pivoted about at least one pivot axis and / or rotated around the object while the terahertz radiation is being emitted. In principle, this could include circular orbits around a tubular object, one-dimensional or two-dimensional changes to the angle of incidence, or, for example, pendulum movements of the transmitter and receiver. In each case, the intensity maximum can be determined and used as the basis for determining the refractive index. This could involve, for example, electronic or mechanical (rapid) tilting or spiral circling of the transmitter and receiver. So-called galvo scanners are conceivable in this regard. The intensity maximum can generally be determined by a fit, for example, the fitting of an envelope curve.This configuration is particularly conceivable for handheld devices, where, by using devices for quickly scanning the surface of the object—especially quickly compared to the unavoidable wobbling of the hand holding the handheld device—a corresponding support for the handheld device can be dispensed with. Of course, it would also be conceivable in principle to provide multiple transmitters and receivers, all of which emit terahertz radiation onto the surface of the object and receive reflected radiation, for example, arranged at different locations or distributed over the circumference of the object. Again, the invention then uses the reflected radiation with the highest intensity for further analysis.

[0024] In addition or alternatively to determining the maximum, it would also be possible to determine the refractive index from the mean value of the received reflected terahertz radiation or from the root mean square value of the received reflected terahertz radiation.

[0025] According to a further embodiment, the average refractive index can also be determined across the cross-section of the object. The average refractive index can also be determined from terahertz radiation emitted by the object and reflected by the object, wherein in this case, radiation components that have at least partially passed through the object and were reflected by the internal boundary surfaces of the object are also received and evaluated. The refractive index determination in this regard can be carried out, for example, as described in WO 2016 / 139155 A1 or DE 10 2018 128 248 A1. Reference is made to these documents in this regard. It is advantageous that the same measuring and evaluation device can be used, in particular the same transmitters and receivers, as for determining the refractive index in the surface region of the object. It is understood that reflected signals from different surfaces can be temporally or spatially separated.can be discriminated against based on frequency so that they can be evaluated individually for measurement. Terahertz radiation can therefore be emitted in particular in FMCW mode (frequency modulated continuous wave radar) or in pulsed mode. According to a further embodiment, a comparison of the average refractive index with the refractive index in the surface area of ​​the object can be used to determine whether the object has shrunk as it cools down completely to ambient temperature. The refractive index of a material depends, among other things, on temperature, and in particular on the density of the material. Parts that are still viscous have a lower density than completely cooled areas, so that the refractive index of such parts that are still viscous is also lower.Accordingly, the difference between the mean refractive index, which also takes the viscous regions into account, and the surface refractive index, which does not take these regions into account, can be used to reliably determine the presence and proportion of viscous regions and, from this, the expected shrinkage. This applies even if the measurement is taken essentially immediately after exiting a production facility, such as an extrusion line.

[0026] According to a further embodiment, in step d), the expected intensity of the terahertz radiation reflected by the object can be determined using a finite element method, taking into account the radiation characteristics of a transmitter for terahertz radiation as well as the distance of the transmitter from the object. This expected proportion of reflected terahertz radiation corresponds in particular to the proportion at normal incidence, i.e., angle of incidence = angle of reflection = 0°. If the intensity of the actually measured reflected radiation deviates from the expected value, a deviation from the assumed angle of incidence of 0° can be concluded and this can be taken into account mathematically or corrected in the measurement setup.

[0027] According to a further embodiment, the temperature in the surface area of ​​the object can be determined from the determined refractive index in the surface area of ​​the object. As explained, the refractive index is temperature-dependent. Thus, the determined refractive index can be used to determine the temperature at the position or location where the refractive index was determined.

[0028] According to a further embodiment, the object can be a pipe, wherein terahertz radiation reflected from an inner surface of a wall section of the pipe is further received, and wherein the refractive index at the inner surface of the wall section is determined from the ratio of incident terahertz radiation to that reflected from the inner surface of the wall section. It is understood that any attenuation of the terahertz radiation on its path through the material must also be taken into account.

[0029] According to a further related embodiment, the temperature at the inner surface of the wall section and / or a temperature difference and / or a temperature gradient between the temperature of the inner surface of the wall section and the temperature in the (outer) surface area of ​​the object can be determined from the refractive index at the inner surface of the wall section. Through such temperature observations, for example, material (batch) changes can be detected and mathematically eliminated, so that the refractive index determination according to the invention becomes independent of such changes. Furthermore, the consideration of a temperature difference or a temperature gradient is of particular interest for materials whose composition and thus refractive index changes in cross-section, for example through a pipe wall. This is the case, for example, with foamed material with more highly reflective interfaces.In principle, the invention could also be used to determine the refractive indices of several different material layers within a single material, particularly in the case of a layered object. For this purpose, terahertz radiation reflected at the interfaces between the layers can be evaluated for the refractive index determination.

[0030] It is also conceivable to determine an average temperature of the object from the determined temperature in the surface area of ​​the object and the determined temperature on the inner surface of the wall section. It would also be conceivable to determine an average temperature of the object from the determined average refractive index across the cross-section of the object. The two determined average temperatures of the object can then be compared. From the comparison, conclusions can be drawn about changed material properties of the object, for example changes in the material, the addition of components, a change in batch, etc. It would also be conceivable to measure the temperature in the surface area of ​​the object using a material-independent temperature measuring device, for example a pyrometer.From the measured temperature and the determined temperature difference between the temperature of the inner surface of the wall section and the temperature in the surface area of ​​the object, the temperature at the inner surface of the wall section can be determined. This allows the absolute, material-independent internal temperature to be determined without having to provide a corresponding temperature measuring device in the interior, which is often not possible in practice.

[0031] From the absolute surface temperatures thus determined on the outer and inner surface, for example of a wall section of a pipe, it is possible to draw conclusions about material changes, such as admixtures of other materials, new batches, etc., by comparing them with an average temperature, determined for example from the average refractive index determined as explained above, if differences occur.

[0032] In principle, to take into account the surface properties, especially the surface topography, of the object, it would also be possible to additionally measure the surface optically, for example, using laser radiation. Any topographical structures of the surface could then be taken into account in the inventive determination of the surface refractive index. Laser triangulation, for example, could be used for the measurement.

[0033] The provision of a reflector on the side of the object opposite the transmitter and receiver can also be advantageous in order to keep the measurement result unaffected by fluctuating intensity of the transmitter or fluctuating sensitivity of the receiver.

[0034] According to a further embodiment, the device according to the invention can be a handheld device, i.e., a manually portable device. In this way, the refractive index can be reliably determined externally in a particularly simple manner, whereby the invention also enables reliable measurement when using a handheld device.

[0035] The device according to the invention, in particular its evaluation device, can be designed to carry out the method according to the invention. Accordingly, the method according to the invention can be carried out using the device according to the invention.

[0036] An embodiment of the invention is explained in more detail below with reference to the figures. They show schematically: Figure 1 shows a device according to the invention in a first view, Figure 2 shows the device according to the invention Figure 1in a second view, and Figure 3 a diagram illustrating the influence of the surface topography of an object under investigation.

[0037] Unless otherwise stated, the same reference symbols in the figures refer to the same objects.

[0038] In Fig. 1 a device according to the invention is shown in a vertical sectional view. In Fig. 2 the device is made of Fig. 1 shown in a horizontal sectional view. In the Figures 1 and 2 In the exemplary embodiment shown, a heated object 10 coming from a production plant, for example an extrusion plant, which has not yet completely cooled down to ambient temperature and which is already largely cooled down and completely hardened in the area of ​​its surfaces, but still has viscous components in its interior, is conveyed by means of a suitable conveyor device (not shown in detail) along its Fig. 1pointing vertically into the plane of the drawing and in Fig. 2 from bottom to top pointing central longitudinal axis (cylinder axis) through the device. The object 10 in this case is a circular cylindrical plastic pipe 10. The circular cross section of the pipe 10 is in the Fig. 1 This is clearly visible in the cross-sectional view shown. In particular, the wall 12 of the tube 10 has an outer surface 16 with a circular cross section and an inner surface 18 with a likewise circular cross section, which delimits the cavity 14.

[0039] The Figures 1 and 2The device according to the invention shown further comprises a transmitter for emitting terahertz radiation and a receiver for receiving the terahertz radiation emitted by the transmitter and reflected from boundary surfaces of the tube 10, wherein the transmitter and receiver in the illustrated example are formed by a terahertz transceiver 20. Of course, spatially separated transmitters and receivers could also be provided, for example, opposite one another. Reference numeral 22 schematically shows a radiation optics, in this case comprising a biconvex lens 21 and a cylindrical lens 23 for the terahertz radiation. Of course, other radiation optics are also conceivable. For example, a combined biconvex / cylindrical lens could be used. Furthermore, in the Figures 1 and 2It can be seen that the optical axis 25 of the optical system formed by the transceiver 20 and the radiation optics 22 is perpendicular to the longitudinal axis of the tube 10. The transmitter and receiver, or the transceiver 20, are further connected to an evaluation device 30 via a line 28. It should be noted that the radiation does not have to be focused on the center of the tube 10, but can have any other beam characteristic; in particular, parallel radiation could also be emitted. Furthermore, it is possible to change the direction of the emitted radiation in a controlled manner using a suitable adjustment device.

[0040] Reference numeral 26 denotes a cylindrically curved reflector for the terahertz radiation, the longitudinal axis of which extends in the direction of the longitudinal axis of the pipe 10 guided through the device. The center of curvature of the reflector 26 coincides with the center of curvature of the pipe 10 to be measured, so that the focal line of the cylindrical reflector 26 coincides with the longitudinal axis of the pipe 10. The reflector 26 amplifies the measurement signal and allows even better discrimination of the different measurement signals received by the receiver. It also helps compensate for any fluctuations in the transmission power or reception sensitivity of the transmitter and receiver. It should be noted that the reflector 26 can also have a different geometry. It would of course also be possible to dispense with the reflector 26 entirely.

[0041] With the Figures 1 and 2By means of the device according to the invention shown, in particular the transceiver 20, terahertz radiation is emitted onto the outer surface 16 of the tube 10, if possible at an angle of incidence of 0°. The transceiver 20, in turn, receives terahertz radiation reflected from the surface 16 of the object. By means of the evaluation device 30, the refractive index in the surface region, in particular the outer surface 16 of the tube 10, is determined from the ratio of the incident and reflected terahertz radiation. The refractive index can be calculated in particular in the manner explained above using the Fresnel formulas.

[0042] The Figure 3 The diagram shown is intended to illustrate the fundamental problems of this procedure. For this purpose, the test was carried out for a completely cooled and hardened circular cylindrical plastic pipe 10, Figures 1 and 2The device shown rotates 360° around the plastic tube 10, wherein terahertz radiation is emitted onto the outer surface 16 of the plastic tube 10 and terahertz radiation reflected from this surface 16 is received, again with the transceiver 20. Assuming that the plastic tube 10 has a largely homogeneous refractive index, the measured intensity amplitude of the reflected terahertz radiation should be only minimally affected by the rotation.

[0043] In Figure 3The intensity amplitude of the reflected terahertz radiation actually received by the transceiver 20 during rotation is shown. The intensity of the reflected terahertz radiation is plotted in arbitrary units on the Y-axis, and the angle of rotation of the device, in particular of the transceiver 20, is plotted on the X-axis. It can be seen that intensity fluctuations of more than 20% occur, which cannot be explained by any irregularities in the refractive index in the material of the plastic pipe. In fact, these fluctuations are apparently due to surface properties of the plastic pipe, in particular an irregular surface topography. This causes the angle of incidence and thus also the angle of reflection of the terahertz radiation to vary depending on the topography of the surface during the rotation of the transceiver 20, which in turn leads to corresponding fluctuations in the received intensity of the reflected terahertz radiation.

[0044] To solve this problem, the invention determines a maximum of the radiation intensity received over the entire rotation and uses this as the basis for the mathematical determination of the refractive index, assuming an angle of incidence of 0°.

[0045] This evaluation is based on the assumption that at the occurrence of the intensity maximum the angle of incidence is essentially 0°.

[0046] However, the device according to the invention also allows for other procedures to consider the influence of surface properties of the tube 10, in particular the surface topography of the tube 10. For example, the transceiver 20 can be pivoted or rotated in a spiral, so that terahertz radiation is emitted to different locations on the outer surface 16 of the tube 10 and / or at different angles of incidence onto the outer surface 16 of the tube 10. The maximum can again be determined from the received reflected radiation components and used as the basis for further evaluation to determine the refractive index. For movement, the transceiver 20 can have an adjustment device integrated therein.

[0047] Calibration can also be performed with respect to the pipe 10 to be measured, which has a known reflectance, preferably a reflectance of 1. For this purpose, the surface of the pipe 10 can be provided with a coating with a known reflectance, preferably a reflectance of essentially 1, for example, a film coating or a spray coating, after the inventive determination of the refractive index, and the inventive measurement of the reflected terahertz radiation can be performed again. If a deviation from the reflected radiation intensity expected based on the known reflectance arises, this can be subsequently taken into account by means of a correction calculation in the previously performed determination of the refractive index.

[0048] Since the pipe 10, as shown in the Figures 1 and 2shown, is at least partially transparent to the emitted terahertz radiation, and the radiated terahertz radiation is thus also reflected at the further boundary surfaces of the tube 10 and these further reflected radiation components are also received by the transceiver 20, can be Figures 1 and 2 Using the device shown, an average refractive index across the cross-section of the tube 10 can also be determined, for example as described in WO 2016 / 139155 A1 or DE 10 2018 128 248 A1. Reference is made to the cited publications in this regard.

[0049] On this basis, it is also possible to determine the temperature, for example, at the outer surface 16 and / or the inner surface 18 of the tube 10, or even the average temperature across the cross-section of the tube 10, as explained above. If, for example, the temperature of the outer surface 16 is additionally measured using a material-independent temperature measuring device, such as a pyrometer, a material-independent absolute value for the temperature at the inner surface 18 of the tube 10 can also be calculated in the manner explained above.

[0050] Furthermore, a comparison of the determined mean refractive index with the determined refractive index in the region of the outer surface 16 of the tube 10 can be used to conclude that the tube 10 shrinks during its complete cooling and thus complete hardening.

[0051] Based on the refractive indices thus determined, further geometric parameters of the tube 10 can be determined, for example, based on time-of-flight measurements of the radiation components reflected at the various interfaces of the tube 10. This concerns, for example, the outer diameter shown in the figures at reference numeral 32 or the wall thicknesses of the tube 10 shown at reference numerals 34, 36. The values ​​for the optical wall thicknesses or the optical diameter determined based on the time-of-flight measurements can be converted into the geometric wall thicknesses or diameters in a conventional manner using the determined values ​​for the refractive index.

[0052] The device shown in the figures can be either permanently installed, for example designed to be rotatable around the tube 10, or a portable handheld device. List of reference symbols

[0053] 10 Object / Tube 12 Wall 14 Cavity 16 Outer surface 18 Inner surface 20 Transceiver 21 Biconvex lens 22 Radiation optics 23 Cylindrical lens 25 Optical axis 26 Reflector 28 Cable 30 Evaluation device 32 Outer diameter 34 Wall thickness 36 Wall thickness

Claims

1. A method for determining the refractive index in the surface region of a strand-shaped or planar object (10) coming out of a production line in a heated state and not yet completely cooled to ambient temperature, comprising the following steps: a) terahertz radiation is emitted onto the surface (16) of the object (10) at an angle of incidence, b) terahertz radiation reflected by the surface (16) of the object (10) is received, characterized by the following steps: c) the refractive index in the surface region of the object (10) is determined from the ratio of irradiated to reflected terahertz radiation, d) an influence of the surface properties of the object (10) on the proportion of reflected terahertz radiation is taken into account when determining the refractive index, wherein, in step d), terahertz radiation is emitted onto different locations on the surface (16) of the object (10) and / or at different angles of incidence onto the surface (16) of the object (10), wherein the respectively reflected terahertz radiation is received, and wherein the refractive index is determined from the proportion of reflected terahertz radiation with the highest intensity and / or from the mean and / or the root mean square of the received reflected terahertz radiation.

2. The method according to claim 1, characterized in that, in step d), an influence of the surface topography of the object (10) is taken into account.

3. The method according to one of the preceding claims, characterized in that, in step d), a calibration takes place against an object (10) with a known reflectance, preferably a reflectance of substantially 1.

4. The method according to claim 3, characterized in that, for the calibration, the surface of the object (10) is provided with a coating with a known reflectance, preferably a reflectance of substantially 1, preferably in that the coating is a film coating or a spray coating.

5. The method according to one of the preceding claims, characterized in that a transmitter (20) for the terahertz radiation is pivoted about at least one pivot axis and / or is rotated about the object (10) when the terahertz radiation is being emitted.

6. The method according to one of the preceding claims, characterized in that the mean refractive index over the cross-section of the object (10) is additionally determined, preferably in that the mean refractive index is also determined from terahertz radiation emitted onto the object (10) and reflected by the object (10) and / or in that a shrinkage of the object (10) during complete cooling thereof to ambient temperature is concluded from a comparison of the mean refractive index with the refractive index in the surface region of the object (10).

7. The method according to one of the preceding claims, characterized in that, in step d), the expected intensity of the terahertz radiation reflected by the object (10) is determined using a finite element method and taking into account the radiation characteristics of a transmitter (20) for the terahertz radiation and the distance from the transmitter (20) to the object (10).

8. The method according to one of the preceding claims, characterized in that the temperature in the surface region of the object (10) is determined from the determined refractive index in the surface region of the object (10).

9. The method according to one of the preceding claims, characterized in that the object (10) is a tube (10), and in that terahertz radiation reflected by an inner surface (18) of a wall portion of the tube (10) is further received, and in that the refractive index at the inner surface (18) of the wall portion is determined from the ratio of irradiated terahertz radiation to terahertz radiation reflected by the inner surface (18) of the wall portion, preferably in that the temperature at the inner surface (18) of the wall portion and / or a temperature difference and / or a temperature gradient between the temperature of the inner surface (18) of the wall portion and the temperature in the surface region of the object (10) is determined from the refractive index at the inner surface (18) of the wall portion.

10. The method according to one of the preceding claims, characterized in that the object (10) is a strand-shaped or planar plastic object (10) coming out of an extrusion line and / or in that the terahertz radiation is pulsed terahertz radiation or FMCW terahertz radiation.

11. A device for determining the refractive index in the surface region of a strand-shaped or planar object (10) coming out of a production line in a heated state and not yet completely cooled to ambient temperature, comprising: - a transmitter (20) for transmitting terahertz radiation at an angle of incidence onto the surface (16) of the object (10), - a receiver (20) for receiving the terahertz radiation reflected by the surface (16) of the object (10), - characterized by an evaluation apparatus (30), which is designed to determine the refractive index in the surface region of the object (10) from the ratio of irradiated to reflected terahertz radiation, - wherein the evaluation apparatus (30) is further designed to take into account an influence of the surface properties of the object (10) on the proportion of reflected terahertz radiation during determination of the refractive index, - wherein an adjustment apparatus is further provided, by means of which the transmitter (20) can be adjusted to emit terahertz radiation onto different locations on the surface of the object and / or at different angles of incidence onto the surface of the object, wherein the receiver is designed to receive the respectively reflected terahertz radiation, and in that the evaluation apparatus (30) is designed to determine the refractive index from the proportion of reflected terahertz radiation with the highest intensity and / or from the mean and / or the root mean square of the received reflected terahertz radiation.

12. The device according to claim 11, characterized in that the adjustment apparatus comprises an apparatus for pivoting the transmitter (20) about at least one pivot axis and / or for rotating the transmitter (20) about the object (10) when the terahertz radiation is being emitted.

13. The device according to one of claims 11 or 12, characterized in that the object (10) is a tube (10), and in that the receiver (20) is designed to further receive terahertz radiation reflected by an inner surface (18) of a wall portion of the tube (10), and in that the evaluation apparatus (30) is designed to determine the refractive index at the inner surface (18) of the wall portion from the proportion of reflected terahertz radiation and the known angle of incidence.

14. The device according to one of claims 11 to 13, characterized in that a material-independent temperature measuring apparatus, preferably a pyrometer, is further provided for measuring the temperature in the surface region of the object (10), and in that the evaluation apparatus (30) is designed to determine the temperature at the inner surface (18) of the wall portion from the measured temperature and the determined temperature difference between the temperature of the inner surface (18) of the wall portion and the temperature in the surface region of the object (10).

15. The device according to one of claims 11 to 14, characterized in that the device is a handheld device, and / or in that the terahertz radiation is pulsed terahertz radiation or FMCW terahertz radiation.

16. The device according to one of claims 11 to 15, characterized in that the device is designed to carry out the method according to one of claims 1 to 10.