Device and method for determining refractive index and / or wall thickness of article

Abstract

The invention relates to a device for determining the refractive index and / or the wall thickness of an article, preferably a plate-shaped article, comprising a first transmitter for terahertz radiation and a first receiver for terahertz radiation, the first transmitter being designed to emit terahertz radiation onto the article in a first main beam direction and the first receiver being designed to receive terahertz radiation from the article in a second main beam direction. The first receiver is configured to receive the terahertz radiation emitted by the first transmitter after penetrating the article. The invention further relates to a corresponding method.

Description

Technical Field

[0001] The present invention relates to an apparatus for determining the refractive index and / or wall thickness of an article, preferably a plate-shaped article, the apparatus comprising a first transmitter for terahertz radiation and a first receiver for terahertz radiation, wherein the first transmitter is configured to emit terahertz radiation onto the article along a first main beam direction, and wherein the first receiver is configured to receive the terahertz radiation emitted by the first transmitter after penetrating the article.

[0002] The present invention further relates to a method for determining the refractive index and / or wall thickness of an article, preferably a plate-shaped article, wherein terahertz radiation is emitted onto the article by a first transmitter along a first main beam direction, and the terahertz radiation emitted by the first transmitter and passing through the article is received by a first receiver. Background Technology

[0003] Terahertz radiation can be used to determine, for example, the optical wall thickness of an article that is at least partially transparent to terahertz radiation, based on propagation time measurements. Here, the terahertz radiation is reflected at the boundary surfaces of the article, and the reflected radiation is measured by a corresponding receiver. For the geometric wall thickness to be determined from the optical wall thickness, the refractive index of the article's material must be known. Often, the refractive index is not known or not known accurately enough. This applies, for example, to articles extruded in an extrusion apparatus. Thus, the material to be extruded is mixed with different additives before extrusion to optimize the properties of the resulting article. The exact composition of the material to be extruded and the additives is often not known accurately enough. Simultaneously, the refractive index changes with the changing composition.

[0004] WO 2016 / 139155 A1 describes an apparatus and method for determining the refractive index of an article whose wall thickness is to be measured by means of terahertz radiation. This allows for the accurate determination of the geometric wall thickness of an article, even without prior knowledge of the refractive index.

[0005] The direct simultaneous determination of refractive index and wall thickness described in the prior art is particularly relevant for transmitters and receivers of terahertz radiation with relatively large bandwidths, especially for small wall thickness requirements. For example, a bandwidth of, for instance, 100 GHz for terahertz radiation is necessary with a wall thickness of 1 mm and a refractive index of 1.5. Broadband terahertz sensors are complex and correspondingly expensive. Summary of the Invention

[0006] Based on the prior art as described, the present invention is therefore intended to provide an apparatus and method of the type described above, thereby enabling the determination of the refractive index and / or geometric wall thickness of an article in a simple and cost-effective manner.

[0007] This invention achieves this objective through independent claims 1 and 9. Advantageous design solutions are described in the dependent claims, the specification, and the drawings.

[0008] The present invention thus solves this objective for devices of the type described at the beginning.

[0009] --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 onto an article along a second main beam direction, wherein the first and second main beam directions extend at different angles relative to the measuring layer containing the article in the case of measurement, and the second receiver is configured to receive the terahertz radiation emitted by the second transmitter after penetrating the article.

[0010] - An analysis and processing mechanism is provided, configured to: determine a first measured propagation path of terahertz radiation between a first transmitter and a first receiver using a measurement signal received by a first receiver after penetration of an article, and compare it with a first reference propagation path of terahertz radiation between the first transmitter and the first receiver in the absence of penetration of an article; and determine a second measured propagation path of terahertz radiation between a second transmitter and a second receiver using a measurement signal received by a second receiver after penetration of an article, and compare it with a second reference propagation path of terahertz radiation between the second transmitter and the second receiver in the absence of penetration of an article.

[0011] - The analysis and processing mechanism is further configured to determine the refractive index and / or wall thickness of an article based on comparison.

[0012] For methods of the type described above, the present invention achieves this objective through the following steps.

[0013] - A second transmitter emits terahertz radiation onto an article along the direction of a second main beam, wherein the directions of the first and second main beams extend at different angles relative to the surfaces of the article opposite the first and second transmitters, and a second receiver receives the terahertz radiation emitted by the second transmitter after penetrating the article.

[0014] - A first measured propagation path of terahertz radiation between a first transmitter and a first receiver is determined by a measurement signal received by a first receiver after penetration of an article, and compared with a first reference propagation path of terahertz radiation between the first transmitter and the first receiver without penetration of an article; a second measured propagation path between a second transmitter and a second receiver is determined by a measurement signal received by a second receiver after penetration of an article, and compared with a second reference propagation path between the second transmitter and the second receiver without penetration of an article; and

[0015] - Determine the refractive index and / or wall thickness of the item based on comparison.

[0016] The article to be measured according to the invention can be, for example, a plastic or glass article. The article can have, for example, a plate-like design, having, for example, two mutually parallel surfaces. The article is at least partially transparent to the terahertz radiation from the first or second transmitter, so that the article can be penetrated by the terahertz radiation. The article can be manufactured, for example, in an extrusion apparatus. The article can be conveyed through the measurement area of ​​the apparatus during measurement by means of a conveying mechanism, which can be part of the apparatus according to the invention. The terahertz radiation emitted by the first and second transmitters can, for example, have a frequency in the range of 1 GHz to 10 THz. Referring here to a fundamental frequency, the terahertz radiation has a certain bandwidth around this fundamental frequency. As further explained below, this bandwidth according to the invention can be smaller compared to known measuring devices.

[0017] According to the invention, a first transmitter and a second transmitter each direct terahertz radiation toward the article, particularly close to the same location on the surface of the article. The first and second main beam directions of the terahertz radiation from the first transmitter and the second transmitter are different relative to the measuring layer containing the article during measurement or relative to the angle of the surface of the article facing the first or second transmitter. The article is positioned in the device during measurement such that its surface facing the first or second transmitter corresponds to the measuring layer. Whenever the device according to the invention includes an article, the measuring layer is formed by the surface facing the first or second transmitter. If the article is, for example, plate-shaped with a planar surface facing the first or second transmitter, then the measuring layer can be correspondingly a two-dimensional layer; or if the article, for example, has a cylindrical outer surface, then the measuring layer can be, for example, a curved layer. In this case, the measuring layer corresponds to the cylindrical outer surface of the article. Angles can be measured, particularly relative to the normal on the measuring layer or on the surface of the article. The difference in angles can then be, for example, in the range of 10° to 80°, preferably between 30° and 60°. Terahertz radiation emitted by the first and second transmitters respectively penetrates the article and is received as a measurement signal by the first or second receiver after penetration. As further explained below, the first transmitter and the first receiver can be located on opposite sides of the article and / or the second transmitter and the second receiver can be located on opposite sides of the article. However, it is also possible that the first transmitter and the first receiver and / or the second transmitter and the second receiver can be located on the same side of the article. In this case, there is a first or second reflector on the opposite side, which reflects the emitted terahertz radiation back to the corresponding receiver after penetration of the article. Therefore, the terahertz radiation received as a measurement signal penetrates the article twice, which can be taken into account in calculations.

[0018] According to the invention, particularly by means of the analysis and processing mechanism of the apparatus according to the invention, a first measurement propagation path between the first transmitter and the first receiver is determined by a measurement signal received by the first receiver—after the terahertz radiation emitted by the first transmitter has (at least once) passed through an article. Accordingly, a second measurement propagation path between the second transmitter and the second receiver is determined by a measurement signal received by the second receiver of the terahertz radiation emitted by the second transmitter after (at least once) passing through an article. The first or second propagation path can be determined, for example, by measuring the propagation time of the terahertz radiation between the emission by the first or second transmitter and the reception by the first or second receiver. The first measurement propagation path is compared with a first reference propagation path between the first transmitter and the first receiver, such that it is generated in the absence of passing through an article, i.e., without an article placed in the radiation path of the terahertz radiation. The second measurement propagation path is correspondingly compared with a second reference propagation path between the second transmitter and the second receiver, such that it is also generated in the absence of passing through an article, i.e., without an article placed in the radiation path of the terahertz radiation. Therefore, the reference propagation path corresponds to the propagation path between the first transmitter and the first receiver or between the second transmitter and the second receiver when no article is placed in the measurement area. The refractive index and / or wall thickness of the article are determined according to the invention by this comparison between the first or second measured propagation path and the first or second reference propagation path, particularly by the analysis processing mechanism of the apparatus according to the invention. The determined wall thickness is particularly the geometric wall thickness.

[0019] The propagation paths of the first and second measurements are longer than the first and second reference propagation paths because terahertz radiation propagates slowly in a medium denser than air. Therefore, when an object is placed in the measurement area, the terahertz radiation passing through that object has a longer propagation time compared to when no object is present. Consequently, the propagation path between the first transmitter and the first receiver, or between the second transmitter and the second receiver, is longer when an object is placed in the measurement area compared to when no object is present.

[0020] According to the present invention, 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 article, independent of the angle of the main beam direction of the terahertz radiation incident on the article, while the refractive index of the article's material is related to the angle at which the terahertz radiation is incident on the article. This is because the refractive index of the article not only determines the propagation speed of the terahertz radiation in the article, but also, additionally, the propagation speed of the terahertz radiation in the article is a function of the refractive index and the angle of the incident terahertz radiation. Thus, a system of equations consisting of two equations and two unknowns, namely the refractive index and the geometric wall thickness, can be established by comparison according to the present invention. This system is explicitly solvable, provided that the angles of the first and second main beam directions relative to the measuring plane or the respective surfaces of the article toward the first or second transmitter are numerically different. Based on this, according to the present invention, it is possible to determine the refractive index and geometric wall thickness of the article solely by measuring the propagation path or interval between the transmitter and receiver, and especially, in the absence of direct measurement of the wall thickness, based on the terahertz radiation reflected at the boundary surface of the article. The propagation path or interval measurement according to the present invention is possible using a narrow-band terahertz transmitter and receiver, which would otherwise be impossible to determine using direct, simultaneous wall thickness and refractive index. Thus, the bandwidth of the terahertz radiation applied according to the present invention only needs to be sufficient for the propagation path measurement, which, as explained, can be achieved through propagation time measurement. This allows for the application of simpler and more cost-effective transmitters and receivers. The analysis and processing of the measurement signal is also relatively simple.

[0021] Provided the terahertz radiation emitted by the first or second transmitter has an opening angle, the direction of the main beam corresponds to the middle or central radiation or central direction of the emitted radiation angle or radiation cone. For particularly simple analytical processing, the first transmitter can emit terahertz radiation perpendicularly onto the surface of an object or the measurement plane.

[0022] The analysis and processing mechanism of the device according to the invention can be configured separately from the first and / or second transmitter and / or receiver. Alternatively, the analysis and processing mechanism can be partially or completely integrated into the first and / or second transmitter and / or receiver. The analysis and processing mechanism can also be configured as a single piece or multiple pieces accordingly. The device according to the invention may also include an article to be measured.

[0023] According to one design, the analysis and processing mechanism can further be configured to determine a first reference propagation path by measurement signals received by a first receiver when no object is placed in the beam path of terahertz radiation emitted by a first transmitter, and / or to determine a second reference propagation path by measurement signals received by a second receiver when no object is placed in the beam path of terahertz radiation emitted by a second transmitter. Therefore, in this case, the measurement technique for the reference propagation path is achieved by performing terahertz measurement in the absence of an object in the measurement area of ​​the first or second transmitter and receiver. This measurement of the reference propagation path can be performed accordingly before or after the measurement of the object. The reference propagation path thus exists with particular precision. The measurement of the reference propagation path can also be achieved by propagation time measurement.

[0024] According to another design, a first transmitter and a first receiver may be positioned on the same side of the measurement area containing the article during measurement. A first reflector is positioned on the opposite side of the measurement area, configured to reflect the terahertz radiation emitted by the first transmitter back to the first receiver after passing through the article. Alternatively, a second transmitter and a second receiver may be positioned on the same side of the measurement area containing the article during measurement. A second reflector is positioned on the opposite side of the measurement area, configured to reflect the terahertz radiation emitted by the second transmitter back to the second receiver after passing through the article. Therefore, in this design, the terahertz radiation emitted by the first or second transmitter is reflected back to the first or second receiver positioned on the same side as the first or second transmitter after the first passage through the article. Thus, the terahertz radiation passes through the article twice before being received by the corresponding receiver. As mentioned, this can be taken into account in the calculation of the determination of the corresponding propagation path. As described above, it is also possible alternatively that the first or second transmitter and the first or second receiver are positioned on opposite sides of the article, so that the terahertz radiation is received by the first or second receiver after passing through the article once.

[0025] The first transmitter and the first receiver can be located substantially in the same position. Correspondingly, the second transmitter and the second receiver can be located substantially in the same position. According to one design, 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.

[0026] According to another design, the analysis and processing mechanism can further be configured to determine a first propagation path difference between the propagation path of the first measurement and a first reference propagation path, and a second propagation path difference between the propagation path of the second measurement and a second reference propagation path. The analysis and processing mechanism can further be configured to consider the angle between the first main beam direction and the second main beam direction when determining the refractive index and / or wall thickness. Furthermore, the analysis and processing mechanism can be configured to consider the angle of the first main beam direction relative to the surface of the measuring layer or article opposite the first transmitter and the angle of the second main beam direction relative to the surface of the measuring layer or article opposite the second transmitter when determining the refractive index and / or wall thickness.

[0027] These reflectors can be constructed in a planar manner, provided that the first or second reflector is configured to reflect terahertz radiation emitted by the first or second transmitter after passing through an object.

[0028] The first propagation path difference Δ between the first measured propagation path and the first reference propagation path α This produces the following equation:

[0029]

[0030] The second propagation path difference Δ between the second measured propagation path and the second reference propagation path β This results in the following equation:

[0031]

[0032] Where n is the refractive index of the material of the object, d is the geometric wall thickness of the object, α is the angle of the first main beam direction relative to the normal on the surface of the object facing the first transmitter, and β is the angle of the second main beam direction relative to the normal on the surface of the object facing the second transmitter. The system of equations consisting of two equations and two unknowns is definitely solvable with respect to the refractive index n and the geometric wall thickness d, and with known angles α and β.

[0033] As already explained, the present invention allows for the application of simple and cost-effective narrowband terahertz transmitters and receivers while simultaneously and accurately determining the refractive index and geometric wall thickness of an article. Therefore, according to the invention, it is possible for a first and a second transmitter to emit terahertz radiation with a bandwidth of less than 10 GHz, preferably less than 5 GHz, more preferably less than 1 GHz. The first or second receiver can also have a correspondingly small bandwidth. Thus, according to the invention, for example, it can be applied to the easily implementable ISM band between 122 and 123 GHz.

[0034] The method according to the invention can be implemented using the apparatus according to the invention. Accordingly, the apparatus according to the invention can be configured to implement the method according to the invention. Attached Figure Description

[0035] Embodiments of the invention are further illustrated below with reference to the accompanying drawings. (Unique) Figure 1 The apparatus according to the invention is shown schematically. Detailed Implementation

[0036] The apparatus according to the invention includes 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 by the first transmitter of the first transceiver 10 along a first main beam direction 14. Terahertz radiation is emitted by the second transmitter of the second transceiver 12 along a second main beam direction 16. The second main beam direction 16 extends at an angle β to the first main beam direction 14. The terahertz radiation emitted by the first transmitter, in this example, reaches along the normal direction onto the surface 18 of the plate-shaped article 20 to be measured, which faces the first transceiver 10, in the illustrated example. Surface 18, in the illustrated example, simultaneously forms a measuring layer (18) that accommodates the article during measurement. The article 20 is at least partially transparent to the terahertz radiation emitted by the first and second transmitters and may be made of, for example, plastic or glass. Terahertz radiation emitted by the first transmitter of the first transceiver 10, after passing through the article 20, reaches the first reflector 22 on the plane and is reflected back to the transceiver 10 and thus the first receiver, which receives the terahertz radiation as a measurement signal after passing through the article 20 twice. Terahertz radiation emitted by the second transmitter of the second transceiver 12 reaches the surface 18 of the article 20, also facing the second transceiver 12, at approximately the same location as the terahertz radiation emitted by the first transmitter, at an angle β with respect to the normal on the surface 18. The terahertz radiation emitted by the second transmitter also passes through the article 20 and undergoes refraction upon entering and exiting the article 20 according to the refractive index of the material of the article 20. After passing through the article 20, the terahertz radiation from the second transmitter also reaches the reflector 24 on the plane and is reflected back to the second transceiver 12 and thus the second receiver, which receives the terahertz radiation as a measurement signal.

[0037] Measurement signals received by the first and second receivers are applied to an analysis and processing unit 26, which determines a propagation path for a first measurement between the first transmitter and the first receiver based on the received measurement signals. This first propagation path is determined by the propagation time of the terahertz radiation from transmission through the first transmitter to reception through the first receiver. Therefore, a dual propagation path is involved between the first transceiver 10 and the first reflector 22. Correspondingly, the analysis and processing unit 26 determines a propagation path between the second transmitter and the second receiver based on the propagation time measurement of the measurement signal received through the second receiver, which in turn corresponds to a dual propagation path between the second transceiver 12 and the second reflector 24. Based on the fact that the refractive index of the material of the article 20 is greater than the density of the ambient medium, the first and second propagation times for the measurements of terahertz radiation emitted and received by the first transmitter or terahertz radiation emitted and received by the second transmitter are longer when the article 20 is present in the measurement area compared to when the article 20 is not present in the measurement area between the transceivers 10, 12 and the reflectors 22, 24. Based on previously implemented reference measurements, a first reference propagation path can be measured between the first transmitter and the first receiver, i.e., a dual propagation path between the first transceiver 10 and the first reflector 22. Specifically, this is achieved by measuring the propagation time of terahertz radiation emitted by the first transmitter until it is received by the first receiver, without the object 20 being placed in the measurement area. Correspondingly, a second reference propagation path can be measured between the second transmitter and the second receiver, i.e., a dual propagation path between the second transceiver 12 and the second reflector 24, again without the object 20 being placed in the measurement area.

[0038] Based on this, the analysis and processing mechanism 26 forms a first propagation path difference Δ between the propagation path of the first measurement and the first reference propagation path. α and the second propagation path difference Δ between the second measured propagation path and the second reference propagation path β As mentioned above, for the propagation path difference Δ α and Δ β The following equations apply:

[0039]

[0040]

[0041] The angle α between the direction 14 of the first main beam and the surface normal 18 of the article 20 is known and is 0° in this case. Figure 1 Correspondingly, this is not depicted. In Figure 1The angle β depicted between the first and second main beam directions 14, 16 is correspondingly equal to the angle between the second main beam direction 16 and the surface normal on the surface 18 of the article 20. This angle is also known and is, for example, 45°. Thus, the refractive index n and geometric thickness d of the article 20 can be definitively determined using the above set of equations.

[0042] Since only propagation path or propagation time measurements are required here, the first transceiver 10, including the first transmitter and the first receiver, and the second transceiver 12, including the second transmitter and the second receiver, can be designed to be narrowband. For example, they can have bandwidths of less than 10 GHz, preferably less than 5 GHz, and more preferably less than 1 GHz, respectively.

[0043] List of reference numerals in the attached diagram:

[0044] 10 First Transceiver

[0045] 12 Second transceiver

[0046] 14 First main beam direction

[0047] 16 Second main beam direction

[0048] 18 Surfaces / Measurement Levels

[0049] 20 items

[0050] 22 First reflector

[0051] 24 Second reflector

[0052] 26 Analysis and Processing Units

Claims

1. A device for determining the refractive index and / or wall thickness of an article (20), preferably a plate-shaped article (20), said device comprising a first transmitter (10) for terahertz radiation and a first receiver (10) for terahertz radiation, wherein, The first transmitter (10) is configured to emit terahertz radiation along the first main beam direction (14) onto the article (20), and the first receiver (10) is configured to receive the terahertz radiation emitted by the first transmitter (10) after penetrating the article (20), characterized in that, - A second transmitter (12) for terahertz radiation and a second receiver (12) for terahertz radiation are provided, wherein the second transmitter (12) is configured to emit terahertz radiation onto the article (20) along a second main beam direction (16), wherein the first and second main beam directions (14, 16) extend at different angles relative to the measuring layer (18) that houses the article in the case of measurement, and the second receiver (12) is configured to receive the terahertz radiation emitted by the second transmitter (12) after penetrating the article (20). - An analysis and processing unit (26) is provided, which is configured to determine a first measured propagation path of terahertz radiation between the first transmitter (10) and the first receiver (10) using a measurement signal received by the first receiver (10) after penetrating the article (20), and compare it with a first reference propagation path of terahertz radiation between the first transmitter (10) and the first receiver (10) without penetrating the article (20); and to determine a second measured propagation path of terahertz radiation between the second transmitter (12) and the second receiver (12) using a measurement signal received by the second receiver (12) after penetrating the article (20), and compare it with a second reference propagation path of terahertz radiation between the second transmitter (12) and the second receiver (12) without penetrating the article (20), and The analysis and processing mechanism (26) is further configured to determine the refractive index and / or wall thickness of the article (20) based on the comparison.

2. The apparatus according to claim 1, characterized in that, The analysis and processing mechanism (26) is further configured to determine a first reference propagation path by a measurement signal received by a first receiver (10) in the absence of an article (20) placed in the beam path of terahertz radiation emitted by the first transmitter (10), and / or to determine a second reference propagation path by a measurement signal received by a second receiver (12) in the absence of an article (20) placed in the beam path of terahertz radiation emitted by the second transmitter (12).

3. The apparatus according to any one of the preceding claims, characterized in that, The first transmitter (10) and the first receiver (10) are disposed on the same side of the measurement area in which the article (20) is contained during the measurement, wherein a first reflector (22) is provided on the opposite side of the measurement area, the first reflector being configured to reflect the terahertz radiation emitted by the first transmitter (10) back to the first receiver (10) after passing through the article (20), and / or the second transmitter (12) and the second receiver (12) are disposed on the same side of the measurement area in which the article (20) is contained during the measurement, wherein a second reflector (24) is provided on the opposite side of the measurement area, the second reflector being configured to reflect the terahertz radiation emitted by the second transmitter (12) back to the second receiver (12) after passing through the article (20).

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

5. The apparatus according to any one of the preceding claims, characterized in that, The analysis and processing mechanism (26) is further configured to determine a first propagation path difference between the propagation path of the first measurement and the first reference propagation path and a second propagation path difference between the propagation path of the second measurement and the second reference propagation path.

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

7. The apparatus according to any one of the preceding claims, characterized in that, The analysis and processing mechanism (26) is further configured to take into account the angle of the first main beam direction (14) relative to the measurement layer (18) and the angle of the second main beam direction (16) relative to the measurement layer (18) when determining the refractive index and / or wall thickness.

8. The apparatus according to any 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, and more preferably less than 1 GHz.

9. A method for determining the refractive index and / or wall thickness of an article (20), preferably a plate-shaped article (20), wherein, The method involves emitting terahertz radiation from a first transmitter (10) along a first main beam direction (14) onto the article (20), and receiving the terahertz radiation emitted by the first transmitter (10) after penetrating the article (20) via a first receiver (10), characterized by the following steps: - A second transmitter (12) emits terahertz radiation along a second main beam direction (16) onto the article (20), wherein the first and second main beam directions (14, 16) extend at different angles relative to the surface (18) of the article (20) opposite to the first transmitter (10) and the second transmitter (12), and a second receiver (12) receives the terahertz radiation emitted by the second transmitter (12) after penetrating the article (20). - The propagation path of the terahertz radiation between the first transmitter (10) and the first receiver (10) is determined by the measurement signal received by the first receiver (10) after penetrating the article (20) and compared with a first reference propagation path of the terahertz radiation between the first transmitter (10) and the first receiver (10) without penetrating the article (20); the propagation path of the terahertz radiation between the second transmitter (12) and the second receiver (12) is determined by the measurement signal received by the second receiver (12) after penetrating the article (20) and compared with a second reference propagation path between the second transmitter (12) and the second receiver (12) without penetrating the article (20); and - Determine the refractive index and / or wall thickness of the article (20) based on the comparison.

10. The method according to claim 9, characterized in that, A first reference propagation path is determined by a measurement signal received by a first receiver (10) in the absence of an article (20) placed in the beam path of terahertz radiation emitted by a first transmitter (10), and / or a second reference propagation path is determined by a measurement signal received by a second receiver (12) in the absence of an article (20) placed in the beam path of terahertz radiation emitted by a second transmitter (12).

11. The method according to claim 9 or 10, characterized in that, The comparison determines the first propagation path difference between the first measured propagation path and the first reference propagation path, and the second propagation path difference between the second measured propagation path and the second reference propagation path.

12. The method according to any one of claims 9 to 11, characterized in that, Consider the angle between the first main beam direction (14) and the second main beam direction (16) when determining the refractive index and / or wall thickness.

13. The method according to any one of claims 9 to 12, characterized in that, When determining the refractive index and / or wall thickness, consider the angle of the first main beam direction (14) relative to the surface (18) of the article (20) opposite to the first transmitter (10) and the angle of the second main beam direction (16) relative to the surface (18) of the article (20) opposite to the second transmitter (12).

14. The method according to any 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, and more preferably less than 1 GHz.

15. The method according to any one of claims 9 to 14, characterized in that, The method is carried out using the apparatus according to any one of claims 1 to 8.

Images