Device and method for determining the refractive index and / or wall thickness of an object

Abstract

The invention relates to a device for determining the refractive index and / or wall thickness of an object, preferably a planar object, comprising a first transmitter for terahertz radiation and a first receiver for terahertz radiation, the first transmitter being designed to transmit terahertz radiation to the object in a first main beam direction and the first receiver being designed to receive the terahertz radiation transmitted by the first transmitter after said radiation has passed through the object. The invention also relates to a corresponding method.

Description

[0001] Device and method for determining the refractive index and / or wall thickness of an object

[0002] The invention relates to a device for determining the refractive index and / or the wall thickness of an object, preferably a plate-shaped object, comprising a first transmitter for terahertz radiation and a first receiver for terahertz radiation, wherein the first transmitter is designed to emit terahertz radiation along a first main beam direction onto the object, and wherein the first receiver is designed to receive the terahertz radiation emitted by the first transmitter after radiating through the object.

[0003] The invention also relates to a method for determining the refractive index and / or the wall thickness of an object, preferably a plate-shaped object, in which terahertz radiation is emitted onto the object along a first main beam direction using a first transmitter, and the terahertz radiation emitted by the first transmitter is received by a first receiver after radiating through the object.

[0004] For example, terahertz radiation can be used to determine the optical wall thickness of objects that are at least partially transparent to terahertz radiation based on time-of-flight measurements. The terahertz radiation is reflected at the object's interfaces, and the reflected radiation is measured by a corresponding receiver. In order to determine the geometric wall thickness from the optical wall thickness, the refractive index of the object's material must be known. Often, the refractive index is not known or is not known with sufficient accuracy. This applies, for example, to objects extruded in an extrusion device. Various additives are added to the material to be extruded before extrusion in order to optimize the properties of the manufactured object. The exact composition of the material to be extruded, including the additives, is often not known with sufficient accuracy. At the same time, the refractive index changes with changing composition.

[0005] WO 2016 / 139155 A1 describes a device and a method for determining the refractive index of an object whose wall thickness is to be measured using terahertz radiation. This allows for a precise determination of the geometric wall thickness of the object, even if no precise prior knowledge of the refractive index is available.

[0006] The direct, simultaneous determination of the refractive index and wall thickness described in the prior art requires transmitters and receivers for terahertz radiation with a relatively wide bandwidth, especially for small wall thicknesses. For example, with a wall thickness of 1 mm and a refractive index of 1.5, a terahertz radiation bandwidth of approximately 100 GHz is required. Broadband terahertz sensors are complex and correspondingly expensive.

[0007] Based on the prior art explained above, the object of the invention is therefore to provide a device and a method of the type mentioned at the outset with which the refractive index and / or the geometric wall thickness of an object can be determined in a simple and cost-effective manner.

[0008] The invention solves the problem by independent claims 1 and 9. Advantageous embodiments can be found in the dependent claims, the description and the figures. For a device of the type mentioned above, the invention solves the problem by

[0009] • that a second transmitter for terahertz radiation and a second receiver for terahertz radiation are provided, wherein the second transmitter is designed to emit terahertz radiation along a second main beam direction onto the object, wherein the first and second main beam directions run at different angles with respect to a measurement plane receiving the object during a measurement, and wherein the second receiver is designed to receive the terahertz radiation emitted by the second transmitter after radiating through the object,

[0010] • that an evaluation device is provided which is designed to determine a first measured path of the terahertz radiation between the first transmitter and the first receiver from the measurement signals received by the first receiver after irradiation through the object and to compare it with a first reference path of the terahertz radiation between the first transmitter and the first receiver without irradiation through the object, and to determine a second measured path of the terahertz radiation between the second transmitter and the second receiver from the measurement signals received by the second receiver after irradiation through the object and to compare it with a second reference path of the terahertz radiation between the second transmitter and the second receiver without irradiation through the object, and

[0011] • that the evaluation device is further designed to determine the refractive index and / or the wall thickness of the object on the basis of the comparisons. For the method of the type mentioned above, the invention solves the problem by the steps

[0012] • a second transmitter emits terahertz radiation along a second main beam direction onto the object, the first and second main beam directions extending at different angles with respect to a surface of the object opposite the first transmitter and a surface opposite the second transmitter, and a second receiver receives the terahertz radiation emitted by the second transmitter after passing through the object,

[0013] • from the measurement signals received by the first receiver after irradiating the object, a first measured path of the terahertz radiation between the first transmitter and the first receiver is determined and compared with a first reference path of the terahertz radiation between the first transmitter and the first receiver without irradiating the object, from the measurement signals received by the second receiver after irradiating the object, a second measured path between the second transmitter and the second receiver is determined and compared with a second reference path between the second transmitter and the second receiver without irradiating the object, and

[0014] • Based on the comparisons, the refractive index and / or wall thickness of the object is determined.

[0015] The object to be measured according to the invention can, for example, be a plastic object or a glass object. The object can, for example, have a plate-shaped configuration, for example with two mutually parallel surfaces. It is at least partially transparent to the terahertz radiation of the first or second transmitter, so that it can be penetrated by the terahertz radiation. The object can, for example, have been produced in an extrusion device. During the measurement, the object can be conveyed through a measuring region of the device by means of a conveyor device, which can be part of the device according to the invention. The terahertz radiation emitted by the first transmitter and the second transmitter can, for example, have a frequency in a range from 1 GHz to 10 THz. This is the fundamental frequency around which the terahertz radiation has a certain bandwidth.As explained in more detail below, this bandwidth can be small according to the invention compared to known measuring devices.

[0016] According to the invention, terahertz radiation is directed onto the object, in particular onto approximately the same location on the surface of the object, from a first transmitter and a second transmitter. The angles of the first and second main beam directions of the terahertz radiation from the first transmitter and the second transmitter differ relative to a measurement plane receiving the object during a measurement and relative to the surface of the object facing the first or second transmitter. During a measurement, the object is arranged in the device such that its surface facing the first or second transmitter corresponds to the measurement plane. If the device according to the invention comprises the object, the measurement plane is therefore formed by the surface facing the first or second transmitter. The measurement plane can accordingly be a two-dimensional plane if, for example, the object is plate-shaped with a surface facing the first or second transmitter.a flat surface facing the second transmitter, or for example a curved plane if the object has, for example, a cylindrical outer surface. In this case, the measuring plane corresponds to the cylindrical outer surface of the object. The angles can in particular be measured relative to a normal on the measuring plane or on the surface of the object. The difference in the angles can then, for example, be in a range between 10° and 80°, preferably between 30° and 60°. The terahertz radiation emitted by the first transmitter and the second transmitter each radiates through the object and, after radiating through, is received as measurement signals by the first and second receivers, respectively. As explained in more detail below, the first transmitter and the first receiver can be arranged on opposite sides of the object and / or the second transmitter and the second receiver can be arranged on opposite sides of the object.However, it is also possible for the first transmitter and the first receiver and / or the second transmitter and the second receiver to be located on the same side of the object. In this case, a first or second reflector is located on the opposite side, which reflects the emitted terahertz radiation back to the respective receiver after it has passed through the object. The terahertz radiation received as measurement signals has thus passed through the object twice, which can be accounted for mathematically.

[0017] According to the invention, in particular by means of the evaluation device of the device according to the invention, a first measured path between the first transmitter and the first receiver is determined from the measurement signals received by the first receiver after the terahertz radiation emitted by the first transmitter has radiated through the object (at least once). Correspondingly, a second measured path between the second transmitter and the second receiver is determined from the measurement signals received by the second receiver of the terahertz radiation emitted by the second transmitter after the object has been radiated through (at least once). The first or second path can be determined, for example, by measuring the propagation time of the terahertz radiation between the transmission by the first or second transmitter and the reception by the first or second receiver.The first measured path is compared with a first reference path between the first transmitter and the first receiver, as it results without radiating through the object, i.e. without an object arranged in the beam path of the terahertz radiation. The second measured path is correspondingly compared with a second reference path between the second transmitter and the second receiver, as it results again without radiating through the object, i.e. without an object arranged in the beam path of the terahertz radiation. The reference paths therefore correspond to the paths between the first transmitter and the first receiver or between the second transmitter and the second receiver if no object is arranged in the measuring range. From these comparisons between the first and second measured paths and the first and secondAccording to the invention, the refractive index and / or the wall thickness of the object is determined during the second reference travel path, in particular by the evaluation device of the device according to the invention. The determined wall thickness is in particular the geometric wall thickness.

[0018] The first and second measured paths are longer than the first and second reference paths because the propagation speed of the terahertz radiation is slower in the object's medium, which is denser than the surrounding medium, e.g., air. This results in longer propagation times for the terahertz radiation penetrating the object when the object is located in the measurement area than when the object is not penetrating the object. The path between the first transmitter and the first receiver, or between the second transmitter and the second receiver, therefore appears longer when the object is located in the measurement area than when the object is not located in the measurement area.

[0019] The invention exploits the fact that the difference between the first and second measured paths and the first and second reference paths is proportional to the wall thickness of the object, independent of the angle of the main beam direction of the terahertz radiation radiated onto the object, whereas the dependence on the refractive index of the object's material depends on the angle at which the terahertz radiation is incident on the object. This is because the refractive index of the object not only determines the propagation velocity of the terahertz radiation in the object, but also the propagation direction of the terahertz radiation in the object is a function of the refractive index and the angle of the incident terahertz radiation.Thus, from the comparisons made according to the invention, a system of equations consisting of two equations and two unknowns, namely the refractive index and the geometric wall thickness, can be established, which can be solved unambiguously provided that the absolute values ​​of the angles of the first and second principal beam directions differ with respect to the measurement plane or the surface of the object facing the first or second transmitter. On this basis, it is possible according to the invention to determine the refractive index and the geometric wall thickness of the object solely from path or distance measurements between transmitters and receivers, and in particular without a direct measurement of the wall thickness based on terahertz radiation reflected at the boundary surfaces of the object. The path or distance measurements made according to the inventionDistance measurements are possible with narrowband terahertz transmitters and receivers, which would not allow for direct, simultaneous wall thickness and refractive index determination. Thus, the bandwidth of the terahertz radiation used according to the invention only needs to be sufficient for path measurements, which, as explained, can be achieved using time-of-flight measurements. This allows for the use of simpler and more cost-effective transmitters and receivers. The evaluation of the measurement signals is also comparatively simple.

[0020] If the terahertz radiation emitted by the first or second transmitter has an aperture angle, the main beam direction corresponds to the center or central beam or the center direction of the emitted radiation angle or radiation cone. For particularly simple analysis, the first transmitter can, for example, emit terahertz radiation perpendicular to the measurement plane or the surface of the object.

[0021] The evaluation unit of the device according to the invention can be designed separately from the first and / or second transmitter and / or receiver. It can also be partially or completely integrated into the first and / or second transmitter and / or receiver. The evaluation unit can accordingly be constructed in one or more parts. The device according to the invention can also comprise the object to be measured.

[0022] According to one embodiment, the evaluation device can be further designed to determine the first reference travel path from measurement signals received by the first receiver without an object arranged in the beam path of the terahertz radiation emitted by the first transmitter and / or to determine the second reference travel path from measurement signals received by the second receiver without an object arranged in the beam path of the terahertz radiation emitted by the second transmitter. In this case, the reference travel paths are determined metrologically by performing a terahertz measurement without the object in the measuring range of the first or second transmitter and receiver. This measurement of the reference travel paths can accordingly take place before or after the object has been measured. The reference travel paths are thus particularly precise. The measurement of the reference travel paths can in turn be carried out using time-of-flight measurements.

[0023] According to a further embodiment, it is possible for the first transmitter and the first receiver to be arranged on the same side of a measuring area which receives the object during a measurement, wherein a first reflector is arranged on an opposite side of the measuring area, which is designed to reflect terahertz radiation emitted by the first transmitter back to the first receiver after it has passed through the object, and / or for the second transmitter and the second receiver to be arranged on the same side of a measuring area which receives the object during a measurement, wherein a second reflector is arranged on an opposite side of the measuring area, which is designed to reflect terahertz radiation emitted by the second transmitter back to the second receiver after it has passed through the object. In this embodiment, the terahertz radiation emitted by the first orThe terahertz radiation emitted by the second transmitter, after passing through the object for the first time, travels back to the first or second receiver arranged on the same side as the first or second transmitter. Thus, the object is passed through twice by the terahertz radiation before it is received by the respective receiver. As explained, this can be taken into account mathematically when determining the respective path. As explained above, it is alternatively also possible for the first or second transmitter and the first or second receiver to be arranged on opposite sides of the object, so that the terahertz radiation is received by the first or second receiver after passing through the object once.

[0024] The first transmitter and the first receiver can be located substantially at the same location. Accordingly, the second transmitter and the second receiver can be located substantially at the same location. According to one embodiment, the first transmitter and the first receiver can be formed by a first transceiver and / or the second transmitter and the second receiver can be formed by a second transceiver.

[0025] According to a further embodiment, the evaluation device can be further configured to determine a first travel path difference between the first measured travel path and the first reference travel path, and a second travel path difference between the second measured travel path and the second reference travel path. The evaluation device can be further configured to take into account the different angles of the first main ray direction and the second main ray direction when determining the refractive index and / or the wall thickness. Furthermore, the evaluation device can be configured to take into account the angle of the first main ray direction to the measuring plane or the surface of the object opposite the first transmitter, and the angle of the second main ray direction to the measuring plane or the surface of the object opposite the second transmitter, when determining the refractive index and / or the wall thickness.

[0026] If first or second reflectors are provided to reflect the terahertz radiation emitted by the first or second transmitter after it has passed through the object, these reflectors can be flat. For the first path difference A a between the first measured path and the first reference path, the following equation results:

[0027] The following equation results for the second travel path difference Aß between the second measured travel path and the second reference travel path:

[0028] Where n is the refractive index of the object material, d is the geometric wall thickness of the object, α is the angle of the first principal ray direction relative to the normal on the surface of the object facing the first transmitter, and β is the angle of the second principal ray direction relative to the normal on the surface of the object facing the second transmitter. This system of equations consisting of two equations and two unknowns can be uniquely solved for the refractive index n and the geometric wall thickness d, and for known angles α and β.

[0029] As already explained, the invention allows the use of simple and cost-effective narrowband terahertz transmitters and receivers while simultaneously precisely determining the refractive index and the geometric wall thickness of the object. Thus, according to the invention, it is possible for the first transmitter and the second transmitter to emit terahertz radiation with a bandwidth of less than 10 GHz, preferably less than 5 GHz, and more preferably less than 1 GHz. The first and second receivers can also have a correspondingly narrow bandwidth. For example, the easily implemented ISM band between 122 and 123 GHz can be used according to the invention.

[0030] The method according to the invention can be carried out using the device according to the invention. Accordingly, the device according to the invention can be designed to carry out the method according to the invention.

[0031] An embodiment of the invention is explained in more detail below with reference to a drawing. The single figure schematically shows a device according to the invention.

[0032] The device according to the invention comprises a first transceiver 10 with a first transmitter and a first receiver for terahertz radiation, as well as a second transceiver 12 with a second transmitter and a second receiver for terahertz radiation. The first transmitter of the first transceiver 10 emits terahertz radiation along a first main beam direction 14. The second transmitter of the second transceiver 12 emits terahertz radiation along a second main beam direction 16. The second main beam direction 16 runs at an angle β to the first main beam direction 14. In the present example, the terahertz radiation emitted by the first transmitter impinges in the normal direction on a surface 18 of an object 20 to be measured, which is plate-shaped in the example shown and faces the first transceiver 10.In the example shown, this surface 18 simultaneously forms a measurement plane (18) that accommodates the object during the measurement. The object 20 is at least partially transparent to the terahertz radiation emitted by the first and second transmitters and can be made of a plastic or glass, for example. The terahertz radiation emitted by the first transmitter of the first transceiver 10 strikes a flat first reflector 22 after passing through the object 20 and is reflected by the reflector back to the transceiver 10 and thus to the first receiver, which receives the terahertz radiation as measurement signals after passing through the object 20 twice.The terahertz radiation emitted by the second transmitter of the second transceiver 12 strikes the surface 18 of the object 20, which also faces the second transceiver 12, at approximately the same location as the terahertz radiation emitted by the first transmitter, in this case at an angle ß relative to the normal on the surface 18. The terahertz radiation emitted by the second transmitter also radiates through the object 20 and, in doing so, experiences refraction as it enters and exits the object 20, depending on the refractive index of the material of the object 20. After radiating through the object 20, the terahertz radiation from the second transmitter also reaches a planar reflector 24 and is reflected by this back to the second transceiver 12 and thus to the second receiver, which receives it as measurement signals.

[0033] The measurement signals received by the first and second receivers are applied to an evaluation device 26, which uses the received measurement signals to determine a first measured path between the first transmitter and the first receiver. The first path is determined from the propagation time of the terahertz radiation from its transmission by the first transmitter to its reception by the first receiver. This is therefore twice the propagation time between the first transceiver 10 and the first reflector 22. Similarly, the evaluation device 26 uses the measurement signals received by the second receiver, based on a propagation time measurement, to determine the path between the second transmitter and the second receiver; this path, in turn, corresponds to twice the propagation time between the second transceiver 12 and the second reflector 24.Due to the greater density corresponding to the refractive index of the material of the object 20 compared to the surrounding medium, the measured first and second travel times of the terahertz radiation emitted by the first transmitter and received by the first transmitter, or of the terahertz radiation emitted by the second transmitter and received by the second receiver, are longer when the object 20 is arranged in the measuring area than without the object 20 in the measuring area between the transceivers 10, 12 and the reflectors 22, 24. On the basis of a previously carried out reference measurement, a first reference travel path between the first transmitter and the first receiver, in this case twice the travel path between the first transceiver 10 and the first reflector 22, can be measured, in particular by measuring the travel time of the terahertz radiation emitted by the first transmitter until it is received by the first receiver without the object 20 arranged in the measuring area.In a corresponding manner, a second reference path between the second transmitter and the second receiver, in this case the double path between the second transceiver 12 and the second reflector 24, can be measured, again without the object 20 arranged in the measuring area.

[0034] On this basis, the evaluation device 26 forms a first travel path difference A a between the first measured path and the first reference path and a second path difference Aß between the second measured path and the second reference path. As explained above, the path differences A a and Aß the following equations:

[0035] The angle α of the first principal ray direction 14 to the surface normal on the surface 18 of the object 20 is known and, in this case, is 0°. Accordingly, it is not shown in the figure. The angle β shown in the figure between the first and second principal ray directions 14, 16 is correspondingly equal to the angle between the second principal ray direction 16 and the surface normal on the surface 18 of the object 20. This angle is also known and, in this case, is 45°, for example. Thus, the refractive index n and the geometric wall thickness d of the object 20 can be unambiguously determined using the above system of equations.

[0036] Since only path or propagation time measurements need to be performed in this case, the first transceiver 10 with the first transmitter and the first receiver, and the second transceiver 12 with the second transmitter and the second receiver, can be designed with narrowband bandwidth. For example, they can each have a bandwidth of less than 10 GHz, preferably less than 5 GHz, more preferably less than 1 GHz.

[0037] List of reference symbols

[0038] 10 First Transceiver

[0039] 12 Second transceiver

[0040] 14 First main beam direction

[0041] 16 Second main beam direction

[0042] 18 Surface / measuring plane

[0043] 20 Item

[0044] 22 First reflector

[0045] 24 Second reflector

[0046] 26 Evaluation device

Claims

Claims 1. A device for determining the refractive index and / or the wall thickness of an object (20), preferably a plate-shaped object (20), comprising a first transmitter (10) for terahertz radiation and a first receiver (10) for terahertz radiation, wherein the first transmitter (10) is designed to emit terahertz radiation along a first main beam direction (14) onto the object (20), and wherein the first receiver (10) is designed to receive the terahertz radiation emitted by the first transmitter (10) after radiating through the object (20), characterized • that a second transmitter (12) for terahertz radiation and a second receiver (12) for terahertz radiation are provided, wherein the second transmitter (12) is designed to emit terahertz radiation along a second main beam direction (16) onto the object (20), wherein the first and second main beam directions (14, 16) run at different angles with respect to a measuring plane (18) receiving the object during a measurement, and wherein the second receiver (12) is designed to receive the terahertz radiation emitted by the second transmitter (12) after radiating through the object (20), • that an evaluation device (26) is provided which is designed to determine a first measured path of the terahertz radiation between the first transmitter (10) and the first receiver (10) from the measurement signals received by the first receiver (10) after irradiation of the object (20) and to compare this with a first reference path of the terahertz radiation between the first transmitter (10) and the first Receiver (10) without penetrating the object (20), and to determine a second measured path of the terahertz radiation between the second transmitter (12) and the second receiver (12) from the measurement signals received by the second receiver (12) after penetrating the object (20) and to compare it with a second reference path of the terahertz radiation between the second transmitter (12) and the second receiver (12) without penetrating the object (20), and • that the evaluation device (26) is further designed to determine the refractive index and / or the wall thickness of the object (20) on the basis of the comparisons.

2. Device according to claim 1, characterized in that the evaluation device (26) is further designed to determine the first reference travel path from measurement signals received by the first receiver (10) without an object (20) arranged in the beam path of the terahertz radiation emitted by the first transmitter (10) and / or to determine the second reference travel path from measurement signals received by the second receiver (12) without an object (20) arranged in the beam path of the terahertz radiation emitted by the second transmitter (12).

3. Device according to one of the preceding claims, characterized in that the first transmitter (10) and the first receiver (10) are arranged on the same side of a measuring area receiving the object (20) during a measurement, wherein on a a first reflector (22) is arranged on the opposite side of the measuring area, which is designed to reflect terahertz radiation emitted by the first transmitter (10) back to the first receiver (10) after it has passed through the object (20), and / or that the second transmitter (12) and the second receiver (12) are arranged on the same side of a measuring area receiving the object (20) during a measurement, wherein a second reflector (24) is arranged on an opposite side of the measuring area, which is designed to reflect terahertz radiation emitted by the second transmitter (12) back to the second receiver (12) after it has passed through the object (20).

4. Device according to claim 3, characterized in that the first transmitter (10) and the first receiver (10) are formed by a first transceiver (10) and / or that the second transmitter (12) and the second receiver (12) are formed by a second transceiver (12).

5. Device according to one of the preceding claims, characterized in that the evaluation device (26) is further designed to determine a first travel path difference between the first measured travel path and the first reference travel path and a second travel path difference between the second measured travel path and the second reference travel path.

6. Device according to one of the preceding claims, characterized in that the evaluation device (26) is further designed to determine the refractive index and / or the wall thickness different angles of the first main beam direction (14) and the second main beam direction (16) must be taken into account.

7. Device according to one of the preceding claims, characterized in that the evaluation device (26) is further designed to take into account the angle of the first main ray direction (14) to the measuring plane (18) and the angle of the second main ray direction (16) to the measuring plane (18) when determining the refractive index and / or the wall thickness.

8. Device according to one of the preceding claims, characterized in that the first transmitter (10) and the second transmitter (12) emit terahertz radiation with a bandwidth of less than 10 GHz, preferably less than 5 GHz, more preferably less than 1 GHz.

9. A method for determining the refractive index and / or the wall thickness of an object (20), preferably a plate-shaped object (20), in which terahertz radiation is emitted onto the object (20) along a first main beam direction (14) by a first transmitter (10), and the terahertz radiation emitted by the first transmitter (10) is received by a first receiver (10) after radiating through the object (20), characterized by the steps: • terahertz radiation is emitted onto the object (20) along a second main beam direction (16) by a second transmitter (12), the first and second main beam directions (14, 16) being directed at different angles with respect to a surface (18) of the object opposite the first transmitter (10) and a surface opposite the second transmitter (12) object (20), and with a second receiver (12) the terahertz radiation emitted by the second transmitter (12) is received after radiating through the object (20), • from the measurement signals received by the first receiver (10) after irradiating the object (20), a first measured path of the terahertz radiation between the first transmitter (10) and the first receiver (10) is determined and compared with a first reference path of the terahertz radiation between the first transmitter (10) and the first receiver (10) without irradiating the object (20), from the measurement signals received by the second receiver (12) after irradiating the object (20), a second measured path between the second transmitter (12) and the second receiver (12) is determined and compared with a second reference path between the second transmitter (12) and the second receiver (12) without irradiating the object (20), and • on the basis of the comparisons, the refractive index and / or the wall thickness of the object (20) is determined.

10. The method according to claim 9, characterized in that the first reference path is determined from measurement signals received by the first receiver (10) without an object (20) arranged in the beam path of the terahertz radiation emitted by the first transmitter (10) and / or that the second reference path is determined from measurement signals received by the second receiver (12) without an object (20) arranged in the beam path of the terahertz radiation emitted by the second transmitter (12).

11. Device according to one of claims 9 or 10, characterized in that a first travel path difference between the first measured travel path and the first reference travel path and a second travel path difference between the second measured travel path and the second reference travel path are determined from the comparisons.

12. Method according to one of claims 9 to 11, characterized in that the different angles of the first main beam direction (14) and the second main beam direction (16) are taken into account when determining the refractive index and / or the wall thickness.

13. Method according to one of claims 9 to 12, characterized in that in determining the refractive index and / or the wall thickness, the angle of the first main beam direction (14) to the surface (18) of the object (20) opposite the first transmitter (10) and the angle of the second main beam direction (16) to the surface (18) of the object (20) opposite the second transmitter (12) are taken into account.

14. Method according to one of claims 9 to 13, characterized in that the first transmitter (10) and the second transmitter (12) emit terahertz radiation with a bandwidth of less than 10 GHz, preferably less than 5 GHz, more preferably less than 1 GHz.

15. Method according to one of claims 9 to 14, characterized in that it is carried out with a device according to one of claims 1 to 8.

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