DEVICE AND METHOD FOR DETERMINING THE REFRACTIONAL INDEX AND / OR THE WALL THICKNESS OF AN OBJECT

Description

[0001] 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 configured to emit terahertz radiation along a first principal beam direction onto the object, and wherein the first receiver is configured to receive the terahertz radiation emitted by the first transmitter after passing through the object.

[0002] 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 principal beam direction by a first transmitter, and the terahertz radiation emitted by the first transmitter is received by a first receiver after passing through the object.

[0003] Terahertz radiation can be used, for example, to determine the optical wall thickness of objects that are at least partially transparent to terahertz radiation by measuring the time of flight. In this process, the terahertz radiation is reflected at the object's interfaces, and the reflected radiation is measured by a suitable receiver. 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 unknown or not known with sufficient accuracy. This is the case, for example, with objects extruded in an extrusion process. Various additives are mixed into the material to be extruded before extrusion 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 precision. At the same time, the refractive index changes with varying composition.

[0004] 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 an accurate determination of the object's geometric wall thickness, even if the refractive index is not known beforehand.

[0005] The direct, simultaneous determination of the refractive index and wall thickness described in the prior art requires terahertz transmitters and receivers with relatively large bandwidths, 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 necessary. Broadband terahertz sensors are complex and correspondingly expensive.

[0006] DE 10 2022 105 479 B3 describes a method for determining the dimensional data of a plate-shaped or strand-shaped object, in which a combined measurement of the object is carried out using a first measuring system with a first transmitter and first receiver, and a second measuring system with a second transmitter and second receiver. The first transmitter emits high-bandwidth terahertz radiation, and the second transmitter emits low-bandwidth terahertz radiation at several points in time and / or at several locations on the surface of the object. While the first measuring system determines absolute values ​​of the object's dimensional data, optionally including the refractive index, the second measuring system specifically measures only a dimensional change of the object, for example, a change in thickness. The combination of the two measuring systems allows, for example, changes in thickness to be reliably determined at any time.The second measuring system does not evaluate the direct radiation reflections at the object's interfaces, but rather the delay of the terahertz radiation caused by the object as it passes through.

[0007] US Patent 4,553,841 A describes a measurement system for determining the refractive index and thickness of an optical fiber preform using a helium-neon laser. Monochromatic radiation of two closely adjacent frequencies is generated by the Zeeman effect and passed through the object. Another monochromatic beam at one of the laser frequencies is passed by the transparent object. By comparing phases at different angles of incidence, the refractive index and thickness are calculated.

[0008] CN 108020165 A describes another method for measuring the thickness of non-metallic materials using terahertz radiation.

[0009] Based on the prior art described above, the invention is therefore based on the objective of providing 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.

[0010] The invention solves the problem through independent claims 1 and 9. Advantageous embodiments can be found in the dependent claims, the description and the figures.

[0011] For a device of the type mentioned above, the invention solves the problem, among other things, by: that a second transmitter for terahertz radiation and a second receiver for terahertz radiation are provided, wherein the second transmitter is configured to emit terahertz radiation along a second principal beam direction onto the object, wherein the first and second principal beam directions are at different angles with respect to a measuring plane recording the object during a measurement, and wherein the second receiver is configured to receive the terahertz radiation emitted by the second transmitter after passing through the object, that an evaluation device is provided which is configured toto 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 the radiation has passed through the object, and to compare this with a first reference path of the terahertz radiation between the first transmitter and the first receiver without the radiation passing 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 the radiation has passed through the object, and to compare this with a second reference path of the terahertz radiation between the second transmitter and the second receiver without the radiation passing through the object; and that the evaluation device is further configured to determine the refractive index and / or the wall thickness of the object based on these comparisons.

[0012] For the method of the type mentioned at the outset, the invention solves the problem, among other things, by the following steps: Terahertz radiation is emitted onto the object by a second transmitter along a second principal beam direction, the first and second principal beam directions being at different angles with respect to a surface of the object opposite the first transmitter and a surface opposite the second transmitter, respectively. The terahertz radiation emitted by the second transmitter is received by a second receiver after passing through the object. From the measurement signals received by the first receiver after passing through 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 passing through the object.From the measurement signals received by the second receiver after the object has been irradiated, 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 irradiation of the object. Based on these comparisons, the refractive index and / or the wall thickness of the object are determined.

[0013] The object to be measured according to the invention can be, for example, a plastic or glass object. The object can, for example, have a plate-like shape, such as two parallel surfaces. It is at least partially transparent to the terahertz radiation of the first and second transmitters, so that it can be penetrated by the terahertz radiation. The object can, for example, have been manufactured in an extrusion apparatus. During the measurement, the object can be conveyed through a measuring area of ​​the apparatus by means of a conveying device, which can be part of the apparatus according to the invention. The terahertz radiation emitted by the first and second transmitters has a frequency in the range of 1 GHz to 10 THz. This is the fundamental frequency around which the terahertz radiation exhibits a certain bandwidth.As explained in more detail below, this bandwidth can be small according to the invention compared to known measuring devices.

[0014] According to the invention, terahertz radiation is directed from a first transmitter and a second transmitter onto the object, in particular onto approximately the same location on the object's surface. The angles of the first and second principal beam directions of the terahertz radiation from the first transmitter and the second transmitter differ relative to a measurement plane that receives the object during a measurement, or relative to the surface of the object facing the first or second transmitter, respectively. 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 encompasses the object, the measurement plane is thus 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.The measuring plane can be the flat surface facing the second transmitter, or, for example, a curved plane if the object has a cylindrical outer surface. In this case, the measuring plane corresponds to the cylindrical outer surface of the object. The angles can be measured, in particular, relative to a normal on the measuring plane or on the surface of the object. The difference of the angles can then, for example, lie in a range between 10° and 80°, preferably between 30° and 60°. The terahertz radiation emitted by the first and second transmitters each passes through the object and 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 that the first transmitter and receiver, and / or the second transmitter and receiver, are located on the same side of the object. In this case, a first or second reflector is located on the opposite side, reflecting the emitted terahertz radiation back to the respective receiver after it has passed through the object. The terahertz radiation received as measurement signals has therefore passed through the object twice, which can be taken into account in the calculations.

[0015] According to the invention, a first measured path between the first transmitter and the first receiver is determined, in particular by means of the evaluation unit of the device according to the invention, from the measurement signals received by the first receiver after the terahertz radiation emitted by the first transmitter has passed through the object (at least once). Similarly, 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 it has passed through the object (at least once). The first and second paths can be determined, for example, by measuring the time of flight of the terahertz radiation between its emission by the first and second transmitters and its reception by the first and second receivers, respectively.The first measured path is compared with a first reference path between the first transmitter and the first receiver, as it would be without any object passing through it, i.e., without any object positioned in the path of the terahertz radiation. The measured second path is similarly compared with a second reference path between the second transmitter and the second receiver, again as it would be without any object passing through it, i.e., without any object positioned in the path of the terahertz radiation. The reference paths thus 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 present in the measurement area. From these comparisons between the first and second measured paths and the first and second reference paths, respectively, the following results are obtained:According to the invention, the refractive index and / or the wall thickness of the object are determined using the second reference path, in particular by the evaluation unit of the device according to the invention. The determined wall thickness is, in particular, the geometric wall thickness.

[0016] The first and second measured path lengths are longer than the first and second reference path lengths because the propagation speed of the terahertz radiation is slowed down in the object's denser medium compared to the surrounding medium, such as air. Therefore, with an object positioned within the measurement area, the travel times of the terahertz radiation passing through it are longer than when the object is not present. Thus, the path length between the first transmitter and the first receiver, and between the second transmitter and the second receiver, appears longer when an object is present within the measurement area than when no object is present.

[0017] According to the invention, it is exploited that the difference between the first and second measured propagation paths and the first and second reference propagation paths is proportional to the wall thickness of the object, independent of the angle of the principal beam direction of the terahertz radiation incident on 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 speed of the terahertz radiation within the object, but also the propagation direction of the terahertz radiation within the object, which is a function of both the refractive index and the angle of the incident terahertz radiation.This allows a system of two equations with two unknowns, namely the refractive index and the geometric wall thickness, to be formulated from the comparisons made according to the invention. This system has a unique solution, provided that the magnitudes of the angles of the first and second principal beam directions differ with respect to the measuring plane or the surface of the object facing the first and second transmitters, respectively. Based on this, it is possible according to the invention to determine the refractive index and the geometric wall thickness of the object solely from path length or distance measurements between transmitters and receivers, and in particular without a direct measurement of the wall thickness using terahertz radiation reflected at the object's interfaces. The path length or distance measurements made according to the invention...Distance measurements are possible with narrowband terahertz transmitters and receivers, which would not allow for direct, simultaneous determination of wall thickness and refractive index. Therefore, the bandwidth of the terahertz radiation used according to the invention only needs to be sufficient for path length measurements, which, as explained, can be achieved by 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.

[0018] Provided that the terahertz radiation emitted by the first or second transmitter has an opening angle, the main beam direction corresponds to the central beam or the center direction of the emitted radiation angle or radiation cone. For particularly simple evaluation, the first transmitter can, for example, emit terahertz radiation perpendicularly onto the measurement plane or the surface of the object.

[0019] The evaluation unit of the device according to the invention can be configured 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 be constructed as a single unit or in multiple parts. The device according to the invention can also include the object to be measured.

[0020] In one configuration, the evaluation unit can be further designed to determine the first reference path from measurement signals received by the first receiver without an object positioned in the beam path of the terahertz radiation emitted by the first transmitter, and / or to determine the second reference path from measurement signals received by the second receiver without an object positioned in the beam path of the terahertz radiation emitted by the second transmitter. In this case, the reference 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 paths can be performed either before or after the object has been measured. The reference paths are thus determined with particular precision. The measurement of the reference paths can, in turn, be performed via time-of-flight measurements.

[0021] In 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 that records the object during a measurement, with a first reflector arranged on an opposite side of the measuring area, which is configured 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 that records the object during a measurement, with a second reflector arranged on an opposite side of the measuring area, which is configured 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 reflection of the radiation emitted by the first transmitter and the second receiver thus occurs.The terahertz radiation emitted by the second transmitter, after passing through the object once, returns to the first or second receiver, respectively, which are located on the same side as the first or second transmitter. Thus, the terahertz radiation passes through the object twice before being received by the respective receiver. As explained, this can be taken into account when calculating the respective path length. Alternatively, as explained above, it is also possible for the first or second transmitter and the first or second receiver to be located on opposite sides of the object, so that the terahertz radiation is received by the first or second receiver after passing through the object only once.

[0022] The first transmitter and the first receiver can be located essentially in the same place. Similarly, the second transmitter and the second receiver can be located essentially in the same place. In one embodiment, the first transmitter and the first receiver can be formed by a single transceiver, and / or the second transmitter and the second receiver can be formed by a single transceiver.

[0023] In a further embodiment, the evaluation device can be configured to determine a first path difference between the first measured path and the first reference path, and a second path difference between the second measured path and the second reference path. The evaluation device can also be configured to take into account the different angles of the first and second principal beam directions when determining the refractive index and / or wall thickness. Furthermore, the evaluation device can be configured to take into account the angle of the first principal beam direction to the measuring plane or the surface of the object opposite the first transmitter, and the angle of the second principal beam direction to the measuring plane or the surface of the object opposite the second transmitter, when determining the refractive index and / or wall thickness.

[0024] If first or second reflectors are provided to reflect the terahertz radiation emitted by the first or second transmitter after passing through the object, these reflectors can be designed to be flat.

[0025] The following equation results for the first distance difference Δ α between the first measured distance and the first reference distance: Δ α = d ⋅ n 2 + sin 2 α − cos α

[0026] The following equation results for the second path difference Δβ between the second measured path and the second reference path: Δ β = d ⋅ n 2 + sin 2 β − cos β

[0027] It contains n the refractive index of the object's material, d the geometric wall thickness of the object, αThe angle of the first principal beam direction relative to the normal on the surface of the object facing the first transmitter, and β the angle of the second principal beam direction relative to the normal on the surface of the object facing the second transmitter. This system of two equations with two unknowns can be uniquely solved for the refractive index n and the geometric wall thickness d, and for known angles α and β.

[0028] As already explained, the invention allows the use of simple and cost-effective narrowband terahertz transmitters and receivers while simultaneously enabling precise determination of the refractive index and geometric wall thickness of the object. The first and second transmitters emit terahertz radiation with a bandwidth of less than 10 GHz. Preferably, the first and second transmitters emit terahertz radiation with a bandwidth of less than 5 GHz, and more preferably less than 1 GHz. The first and second receivers can also have a correspondingly small bandwidth. For example, the readily available ISM band between 122 and 123 GHz can be used according to the invention.

[0029] The method according to the invention can be carried out with the device according to the invention. Accordingly, the device according to the invention can be configured to carry out the method according to the invention.

[0030] 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.

[0031] The device according to the invention comprises a first transceiver 10 with a first transmitter and a first receiver for terahertz radiation, and a second transceiver 12 with a second transmitter and a second receiver for terahertz radiation. Terahertz radiation is emitted from the first transmitter of the first transceiver 10 along a first principal beam direction 14. Terahertz radiation is emitted from the second transmitter of the second transceiver 12 along a second principal beam direction 16. The second principal beam direction 16 is at an angle β to the first principal beam direction 14. In the present example, the terahertz radiation emitted by the first transmitter strikes a surface 18 of a plate-shaped object 20 to be measured, facing the first transceiver 10, in a normal direction.In the illustrated example, this surface 18 simultaneously forms a measurement plane 18 that receives 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, for example, be made of plastic or glass. After passing through the object 20, the terahertz radiation emitted by the first transmitter of the first transceiver 10 strikes a flat first reflector 22 and is reflected 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, specifically at an angle β relative to the normal on the surface 18. The terahertz radiation emitted by the second transmitter also passes through the object 20 and undergoes refraction upon entering and exiting the object 20, according to the refractive index of the object's material. After passing through the object 20, the terahertz radiation from the second transmitter also reaches a planar reflector 24 and is reflected back to the second transceiver 12 and thus to the second receiver, which receives it as measurement signals.

[0032] The measurement signals received by the first and second receivers are presented to an evaluation unit 26, which determines a first measured path between the first transmitter and the first receiver from the received measurement signals. This first path is determined from the travel time of the terahertz radiation from transmission by the first transmitter to reception by the first receiver. It is therefore twice the path between the first transceiver 10 and the first reflector 22. Similarly, the evaluation unit 26 determines the path between the second transmitter and the second receiver from the measurement signals received by the second receiver, based on a time-of-flight measurement. This path again corresponds to twice the path between the second transceiver 12 and the second reflector 24.Due to the higher density of the material of object 20 compared to the surrounding medium, corresponding to the refractive index of the material, the measured first and second propagation times of the terahertz radiation emitted by and received by the first transmitter and of the terahertz radiation emitted by and received by the second transmitter are longer when object 20 is located in the measurement area than when object 20 is not located in the measurement area between the transceivers 10, 12 and the reflectors 22, 24. Based on a previously performed reference measurement, a first reference path between the first transmitter and the first receiver, in this case twice the path between the first transceiver 10 and the first reflector 22, can be measured, in particular by measuring the propagation time of the terahertz radiation emitted by the first transmitter until its reception by the first receiver without object 20 located in the measurement area.Similarly, a second reference path between the second transmitter and the second receiver, in this case twice the path between the second transceiver 12 and the second reflector 24, can be measured, again without an object 20 being placed in the measuring area.

[0033] Based on this, the evaluation unit 26 calculates a first path difference Δα between the first measured path and the first reference path, and a second path difference Δβ between the second measured path and the second reference path. As explained above, the following equations apply to the path differences Δα and Δβ: Δ α = d ⋅ n 2 + sin 2 α − cos α Δ β = d ⋅ n 2 + sin 2 β − cos β

[0034] 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 is 0° in this case. It is therefore not shown in the figure. The angle β shown in the figure between the first and second principal ray directions 14, 16 is 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 is, for example, 45° in this case. Thus, the refractive index n and the geometric wall thickness d of the object 20 can be uniquely determined using the above system of equations.

[0035] Since only path length and transit time measurements are required, the first transceiver 10 with the first transmitter and the first receiver, and the second transceiver 12 with the second transmitter and second receiver, can be configured as narrowband transceivers. They each have a bandwidth of less than 10 GHz, preferably less than 5 GHz, and more preferably less than 1 GHz. Reference symbol list

[0036] 10 First transceiver 12 Second transceiver 14 First main beam direction 16 Second main beam direction 18 Surface / measuring plane 20 Object 22 First reflector 24 Second reflector 26 Evaluation unit

Claims

1. A device for determining the refractive index and / or wall thickness of an object (20), preferably a planar 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 having a frequency in a range of from 1 GHz to 10 THz in 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 said radiation has passed through the object (20), wherein • 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 having a frequency in a range of from 1 GHz to 10 THz in a second main beam direction (16) onto the object (20), wherein the first and second main beam directions (14, 16) extend at different angles with respect to a measurement plane (18) that encompasses 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 said radiation has passed through the object (20), wherein the first transmitter (10) and the second transmitter (12) emit terahertz radiation having a bandwidth of less than 10 GHz, • an evaluation apparatus (26) is provided, which is designed to determine a first measured travel distance 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 said radiation has passed through the object (20) and to compare said first measured travel distance with a first reference travel distance of the terahertz radiation between the first transmitter (10) and the first receiver (10) without said radiation having passed through the object (20), and to determine a second measured travel distance 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 said radiation has passed through the object (20) and to compare said second measured travel distance with a second reference travel distance of the terahertz radiation between the second transmitter (12) and the second receiver (12) without said radiation having passed through the object (20), and • the evaluation apparatus (26) is further designed to determine the refractive index and / or wall thickness of the object (20) on the basis of the comparisons.

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

3. The 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 measurement region that encompasses the object (20) during a measurement, wherein a first reflector (22) is arranged on an opposite side of the measurement region and is designed to reflect terahertz radiation emitted by the first transmitter (10) back to the first receiver (10) after said radiation has passed through the object (20), and / or in that the second transmitter (12) and the second receiver (12) are arranged on the same side of a measurement region that encompasses the object (20) during a measurement, wherein a second reflector (24) is arranged on an opposite side of the measurement region and is designed to reflect terahertz radiation emitted by the second transmitter (12) back to the second receiver (12) after said radiation has passed through the object (20).

4. The 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 in that the second transmitter (12) and the second receiver (12) are formed by a second transceiver (12).

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

6. The device according to one of the preceding claims, characterized in that the evaluation apparatus (26) is further designed to take into account the different angles of the first main beam direction (14) and the second main beam direction (16) when determining the refractive index and / or wall thickness.

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

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

9. A method for determining the refractive index and / or wall thickness of an object (20), preferably a planar object (20), in which terahertz radiation having a frequency in a range of from 1 GHz to 10 THz is emitted in a first main beam direction (14) onto the object (20) by means of a first transmitter (10), and the terahertz radiation emitted by the first transmitter (10) is received by means of a first receiver (10) after passing through the object (20), comprising the following steps: • terahertz radiation having a frequency in a range of from 1 GHz to 10 THz is emitted by means of a second transmitter (12) in a second main beam direction (16) onto the object (20), wherein the first and second main beam directions (14, 16) extend at different angles with respect to a surface (18) of the object (20) that is opposite the first transmitter (10) and opposite the second transmitter (12), and the terahertz radiation emitted by the second transmitter (12) is received by means of a second receiver (12) after passing through the object (20), wherein the first transmitter (10) and the second transmitter (12) emit terahertz radiation having a bandwidth of less than 10 GHz, • a first measured travel distance of the terahertz radiation between the first transmitter (10) and the first receiver (10) is determined from the measurement signals received by the first receiver (10) after said radiation has passed through the object (20) and is compared with a first reference travel distance of the terahertz radiation between the first transmitter (10) and the first receiver (10) without said radiation having passed through the object (20), a second measured travel distance between the second transmitter (12) and the second receiver (12) is determined from the measurement signals received by the second receiver (12) after said radiation has passed through the object (20) and is compared with a second reference travel distance between the second transmitter (12) and the second receiver (12) without said radiation having passed through the object (20), and • the refractive index and / or wall thickness of the object (20) is determined on the basis of the comparisons.

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

11. The method according to one of claims 9 or 10, characterized in that a first travel distance difference between the first measured travel distance and the first reference travel distance and a second travel distance difference between the second measured travel distance and the second reference travel distance is determined from the comparisons.

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

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

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

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