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

Abstract

The present invention relates to an apparatus for determining the refractive index and / or thickness of an object, preferably a planar object, the apparatus comprising a first transmitter for terahertz radiation and a first receiver for terahertz radiation, wherein the first transmitter is configured to irradiate the object with terahertz radiation in a first main beam direction, and the first receiver is configured to receive the terahertz radiation irradiated by the first transmitter after it has passed through the object. The present invention also relates to a corresponding method.

Description

[Technical Field]

[0001] The present invention relates to an apparatus for determining the refractive index and / or thickness of an object, preferably a planar object, comprising a first transmitter for terahertz radiation and a first receiver for terahertz radiation, wherein the first transmitter is configured to irradiate the object with terahertz radiation in a first main beam direction, and the first receiver is configured to receive the terahertz radiation irradiated by the first transmitter after the radiation has passed through the object.

[0002] The present invention also relates to a method for determining the refractive index and / or thickness of an object, preferably a planar object, wherein terahertz radiation is irradiated onto the object in a first main beam direction by a first transmitter, and a first receiver receives the terahertz radiation irradiated by the first transmitter after the radiation has passed through the object. [Background Art]

[0003] By using terahertz radiation, it is possible to determine the optical thickness of an object at least partially transparent to terahertz radiation, for example based on measurement of propagation time. Terahertz radiation is reflected at the boundary surfaces of the object, and the reflected radiation is measured by a corresponding receiver. To determine the geometric thickness from the optical thickness, the refractive index of the material of the object must be known. In many cases, the refractive index is unknown or not known with sufficient accuracy. This applies, for example, to objects extruded in an extrusion apparatus. That is, prior to extrusion molding, various additives are mixed into the extruded material to optimize the properties of the manufactured object. In many cases, the exact composition of the extruded material, including the additives, is not known with sufficient accuracy. At the same time, the refractive index changes as the composition changes.

[0004] International Publication No. 2016 / 139155 (WO2016 / 139155A1) discloses an apparatus and method for determining the refractive index of an object to be measured for thickness using terahertz radiation. This makes it possible to precisely determine the geometric thickness of an object even when accurate knowledge of the refractive index is not available beforehand.

[0005] The direct simultaneous determination of refractive index and wall thickness described in prior art requires transmitters and receivers for terahertz radiation with relatively large bandwidths, especially when the wall thickness is thin. 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 consequently expensive. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2016 / 139155 [Overview of the project] [Problems that the invention aims to solve]

[0007] Starting from the prior art described above, the object of the present invention is therefore to provide the apparatus and method of the type described at the beginning that can determine the refractive index and / or geometric thickness of an object in a simple and cost-effective manner. [Means for solving the problem]

[0008] The present invention achieves this objective by independent claims 1 and 9. Advantageous embodiments are disclosed in the dependent claims, specification, and drawings.

[0009] Regarding the type of device mentioned at the beginning, the present invention achieves its objective through the following configuration. A second transmitter and a second receiver for terahertz radiation are provided, the second transmitter being configured to irradiate the object with terahertz radiation in a second main beam direction, the first and second main beam directions extending at different angles with respect to the measuring surface that receives the object during measurement, and the second receiver being configured to receive the terahertz radiation irradiated by the second transmitter after it has passed through the object. The evaluation device is provided, and the evaluation device is configured to determine a first measurement propagation path of terahertz radiation between the first transmitter and the first receiver from a measurement signal received by the first receiver after the radiation has passed through the object, and to compare this with a first reference propagation path of terahertz radiation between the first transmitter and the first receiver when the radiation has not passed through the object, and to determine a second measurement propagation path of terahertz radiation between the second transmitter and the second receiver from a measurement signal received by the second receiver after the radiation has passed through the object, and to compare this with a second reference propagation path of terahertz radiation between the second transmitter and the second receiver when the radiation has not passed through the object. The evaluation device is further configured to determine the refractive index and / or thickness of the object based on the comparison.

[0010] With regard to the type of law mentioned at the beginning, the present invention achieves its objective through the following steps. The first and second main beam directions are irradiated onto the object by a second transmitter in a second main beam direction, wherein the first and second main beam directions extend at different angles to the surface of the object opposite the first transmitter and the surface of the object opposite the second transmitter, and the terahertz radiation irradiated by the second transmitter is received by a second receiver after the radiation has passed through the object. - A step in which a first measurement propagation path of terahertz radiation between the first transmitter and the first receiver is determined from a measurement signal received by the first receiver after the radiation has passed through the object, and this is compared with a first reference propagation path of terahertz radiation between the first transmitter and the first receiver when the radiation has not passed through the object, and a second measurement propagation path between the second transmitter and the second receiver is determined from a measurement signal received by the second receiver after the radiation has passed through the object, and this is compared with a second reference propagation path between the second transmitter and the second receiver when the radiation has not passed through the object. The step of determining the refractive index and / or thickness of the object based on the above comparison. [Effects of the Invention]

[0011] The object to be measured according to the present invention may be, for example, a plastic product or a glass product. The object may have a planar configuration having, for example, two surfaces parallel to each other. The object is at least partially transparent to the terahertz radiation of the first or second transmitter, and therefore the terahertz radiation can pass through it. The object may be manufactured, for example, in an extrusion apparatus. During measurement, the object may be transported by a transport device (which may be part of the apparatus according to the present invention) to pass through the measurement area of ​​the apparatus. The terahertz radiation irradiated by the first and second transmitters may have frequencies in the range of, for example, 1 GHz to 10 THz. This is the fundamental frequency at the center when the terahertz radiation has a particular bandwidth. According to the present invention, this bandwidth can be made smaller compared to known measuring devices, as will be described in more detail below.

[0012] According to the present invention, terahertz radiation is irradiated onto an object from a first transmitter and a second transmitter, and in particular, onto substantially the same location on the surface of the object. The angles of the first and second main beam directions of the terahertz radiation from the first and second transmitters are different from each other with respect to the measuring surface that receives the object during measurement, or rather, the surface of the object facing the first or second transmitter. During measurement, the object is positioned in the apparatus such that its surface facing the first or second transmitter coincides with the measuring surface. When the apparatus according to the present invention includes an object, the measuring surface is formed by the surface facing the first or second transmitter. Therefore, the measuring surface can be a two-dimensional plane, for example, if the object is planar with a planar surface facing the first or second transmitter, or it can be a curved surface, for example, if the object has a cylindrical outer surface. In this case, the measuring surface corresponds to the cylindrical outer surface of the object. These angles can be measured in particular with respect to the normal to the measuring surface or the surface of the object. In this case, the angle difference may be, for example, in the range of 10° to 80°, preferably in the range of 30° to 60°. The terahertz radiation emitted by the first transmitter and the second transmitter passes through the object in each case and, after passing through, is received as a measurement signal by the first receiver and the second receiver, respectively. As will be described in more detail below, the first transmitter and the first receiver may be located on opposite sides of the object, and / or the second transmitter and the second receiver may be located on opposite sides of the object. However, it is also possible that the first transmitter and the first receiver, and / or the second transmitter and the second receiver, are located on the same side of the object. In this case, the first or second reflector is located on the opposite side and reflects the emitted terahertz radiation back to the corresponding receiver after it has passed through the object. Thus, the terahertz radiation received as a measurement signal has passed through the object twice, which can be taken into consideration in calculations.

[0013] According to the present invention, a first measurement propagation path between a first transmitter and a first receiver is determined, in particular, by an evaluation device of the apparatus according to the present invention, from a measurement signal received by the first receiver after the terahertz radiation irradiated from the first transmitter has passed through the object (at least once). Similarly, a second measurement propagation path between a second transmitter and a second receiver is determined from a measurement signal received by the second receiver after the terahertz radiation irradiated from the second transmitter has passed through the object (at least once). The first or second propagation path can be determined, for example, by measuring the propagation time of the terahertz radiation from irradiation by the first or second transmitter to reception by the first or second receiver. The first measurement propagation path is compared to a first reference propagation path between the first transmitter and the first receiver, i.e., a path in which the radiation does not pass through the object (i.e., the object is not placed within the beam path of the terahertz radiation). The determined second measurement propagation path is similarly compared to a second reference propagation path between the second transmitter and the second receiver, i.e., a path in which the radiation does not pass through the object (i.e., without the object being placed within the beam path of the terahertz radiation). Thus, these reference propagation paths correspond to the propagation paths between the first transmitter and the first receiver, and between the second transmitter and the second receiver, respectively, when no object is placed within the measurement area. From these comparisons of the first and second measurement propagation paths with the first and second reference propagation paths, the refractive index and / or thickness of the object are determined, in particular, by the evaluation apparatus of the apparatus according to the present invention. The thickness to be determined is, in particular, the geometric thickness.

[0014] The first and second measurement propagation paths are larger than the first and second reference propagation paths. This is because the propagation speed of terahertz radiation decreases in a medium denser than the surrounding medium (e.g., air). As a result, when an object is placed within the measurement area, the propagation time of terahertz radiation passing through the object is longer than when it does not. Therefore, the propagation paths between the first transmitter and the first receiver, and between the second transmitter and the second receiver, appear longer when an object is placed within the measurement area than when it is not.

[0015] According to the present invention, the difference between the first and second measurement propagation paths and the first and second reference propagation paths is proportional to the thickness of the object, regardless of the angle of the main beam direction of the terahertz radiation irradiated onto the object, while the effect of 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 is a function of the refractive index and the angle of the incident terahertz radiation. Therefore, by using the comparison performed according to the present invention, a system of two equations can be set up with two unknowns, namely the refractive index and the geometric thickness, and this equation can be uniquely solved if the magnitudes of the angles of the first and second main beam directions with respect to the surface of the object facing the first or second transmitter in each case are different. Based on this, according to the present invention, it is possible to determine the refractive index and geometric thickness of an object solely from the measurement of the propagation path, or rather distance, between the transmitter and receiver, and in particular, there is no need to directly measure the thickness based on terahertz radiation reflected from the boundary surface of the object. The measurement of the propagation path, or rather distance, according to the present invention is possible using a narrowband terahertz transmitter and receiver, and it is inherently impossible to directly and simultaneously determine the thickness and refractive index with these instruments. That is, the bandwidth of the terahertz radiation used according to the present invention only needs to be sufficient for measuring the propagation path, which can be achieved by measuring the propagation time as described. This makes it possible to use simpler and more cost-effective transmitters and receivers. The evaluation of the measured signal is also relatively simple.

[0016] If the terahertz radiation emitted by the first or second transmitter has an aperture angle, the main beam direction corresponds to the irradiated radiation angle or the center or central ray of the radiation cone, or rather, the direction of the center. For the sake of particularly simple evaluation, for example, the first transmitter can irradiate the terahertz radiation perpendicular to the measurement surface or rather the surface of the object.

[0017] The evaluation device of the apparatus according to the present invention may be configured separately from the first and / or second transmitter and / or receiver. Alternatively, the evaluation device may be partially or completely integrated with the first and / or second transmitter and / or receiver. Therefore, the evaluation device may consist of a single component or multiple components. The apparatus according to the present invention may further include an object to be measured.

[0018] According to one embodiment, the evaluation device may be further configured to determine a first reference propagation path from a measurement signal received by a first receiver when no object is positioned within the beam path of terahertz radiation emitted from a first transmitter, and / or to determine a second reference propagation path from a measurement signal received by a second receiver when no object is positioned within the beam path of terahertz radiation emitted from a second transmitter. In this case, the reference propagation path is determined based on measurements by performing terahertz measurements when no object is positioned within the measurement areas of the first or second transmitter and receiver. Therefore, the measurement of this reference propagation path can be performed before or after the measurement of the object. This makes the reference propagation path particularly precise. The measurement of the reference propagation path can similarly be performed by measuring the propagation time.

[0019] In a further embodiment, a first transmitter and a first receiver may be located on the same side of the measurement area that accepts the object during measurement, and a first reflector may be located on the opposite side of the measurement area to reflect back the terahertz radiation emitted from the first transmitter and passing through the object to the first receiver; and / or a second transmitter and a second receiver may be located on the same side of the measurement area that accepts the object during measurement, and a second reflector may be located on the opposite side of the measurement area to reflect back the terahertz radiation emitted from the second transmitter and passing through the object to the second receiver. In this embodiment, the terahertz radiation emitted from the first or second transmitter first passes through the object and is then reflected back to the first or second receiver 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 associated receiver. As described, this may be taken into consideration in calculations when determining the associated propagation path. As mentioned above, alternatively, the first or second transmitter and the first or second receiver can be placed on opposite sides of the object so that the terahertz radiation passes through the object once before being received by the first or second receiver.

[0020] The first transmitter and the first receiver may be located in substantially the same location. Similarly, the second transmitter and the second receiver may be located in substantially the same location. According to one embodiment, the first transmitter and the first receiver may be formed by a first transceiver, and / or the second transmitter and the second receiver may be formed by a second transceiver.

[0021] According to a further embodiment, the evaluation device may be further configured to determine a first propagation path difference between a first measurement propagation path and a first reference propagation path, and a second propagation path difference between a second measurement propagation path and a second reference propagation path. The evaluation device may be further configured to take into account different angles of the first main beam direction and the second main beam direction when determining the refractive index and / or the wall thickness. Furthermore, the evaluation device may be configured to take into account, when determining the refractive index and / or the wall thickness, the angle of the first main beam direction relative to the measurement surface, or rather the surface of the object on the opposite side of the first transmitter, and the angle of the second main beam direction relative to the measurement surface, or rather the surface of the object on the opposite side of the second transmitter.

[0022] Where a first or second reflector is provided for reflecting the terahertz radiation emitted from the first or second transmitter and passing through the object, these reflectors may be configured in a planar shape.

[0023] For the first propagation path difference Δα between the first measurement propagation path and the first reference propagation path, the following formula is derived.

[0024] [Formula]

[0025] For the second propagation path difference Δβ between the second measurement propagation path and the second reference propagation path, the following formula is similarly derived.

[0026] [Formula]

[0027] In these equations, n is the refractive index of the object's material, d is the geometric thickness of the object, α is the angle of the first main beam direction with respect to the normal to the surface of the object facing the first transmitter, and β is the angle of the second main beam direction with respect to the normal to 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 geometric thickness d, given that angles α and β are known.

[0028] As already described, the present invention enables the use of simple and cost-effective narrowband terahertz transmitters and receivers, while simultaneously allowing for precise determination of the refractive index and geometric thickness of an object. Accordingly, according to the present invention, the first and second transmitters can emit terahertz radiation having a bandwidth of less than 10 GHz, preferably less than 5 GHz, and more preferably less than 1 GHz. The first or second receiver may also have a correspondingly low bandwidth. Accordingly, according to the present invention, for example, the easily implementable 122-123 GHz ISM band (Industrial, Scientific, and Medical Frequency Band) can be utilized.

[0029] The method according to the present invention can be carried out by the apparatus according to the present invention. Accordingly, the apparatus according to the present invention can be configured to carry out the method according to the present invention. [Brief explanation of the drawing]

[0030] [Figure 1] Schematic diagram of the apparatus according to the present invention [Modes for carrying out the invention]

[0031] Illustrative embodiments of the present invention will be described in more detail below with reference to the drawings. The only drawing schematically illustrates the apparatus according to the present invention.

[0032] The apparatus according to the present invention comprises a first transceiver 10 equipped with a first transmitter and a first receiver, and a second transceiver 12 equipped with a second transmitter and a second receiver. Terahertz radiation is irradiated in a first main beam direction 14 by the first transmitter of the first transceiver 10. Terahertz radiation is also irradiated in a second main beam direction 16 by the second transmitter of the second transceiver 12. The second main beam direction 16 extends at an angle β with respect to the first main beam direction 14. In this example, the terahertz radiation irradiated from the first transmitter is emitted in a direction normal to the surface 18 of the object to be measured 20 (planar in the illustrated example) that faces the first transceiver 10. In the illustrated example, this surface 18 simultaneously forms a measuring surface 18 that receives the object during measurement. The object 20 is at least partially transparent to the terahertz radiation emitted from the first and second transmitters, and may be made of, for example, plastic or glass. The terahertz radiation emitted from the first transmitter of the first transceiver 10 passes through the object 20, then enters a planar first reflector 22, and is reflected back to the transceiver 10 (i.e., the first receiver) by the reflector. The first receiver receives the terahertz radiation after it has passed through the object 20 twice as a measurement signal. The terahertz radiation emitted from the second transmitter of the second transceiver 12 similarly enters approximately the same location on the surface 18 of the object 20 facing the second transceiver 12 as the terahertz radiation from the first transmitter. In this example, the radiation enters at an angle β with respect to the normal to the surface 18. The terahertz radiation emitted from the second transmitter also passes through the object 20, and refraction occurs during its entry into and exit from the object 20, depending on the refractive index of the material of the object 20. After passing through the object 20, the terahertz radiation from the second transmitter also reaches the planar reflector 24, where it is reflected back to the second transceiver 12 (i.e., the second receiver). The second receiver receives this radiation as a measurement signal.

[0033] The measurement signals received by the first and second receivers are sent to the evaluation device 26, which determines a first measurement propagation path between the first transmitter and the first receiver from the received measurement signals. In this case, the first propagation path is determined from the propagation time of the terahertz radiation from irradiation by the first transmitter to reception by the first receiver. Therefore, this corresponds to twice the propagation path between the first transceiver 10 and the first reflector 22. Similarly, the evaluation device 26 determines a propagation path between the second transmitter and the second receiver from the measurement signal received by the second receiver, based on the propagation time measurement, and this also corresponds to twice the propagation path between the second transceiver 12 and the second reflector 24. When the object 20 is positioned within the measurement area due to a density greater than that of the surrounding medium, corresponding to the refractive index of the object 20's material, the first and second propagation times measured for terahertz radiation irradiated from the first transmitter and received by the first receiver, or terahertz radiation irradiated from the second transmitter and received by the second receiver, will be longer than when the object 20 is not in the measurement area between the transceivers 10, 12 and the reflectors 22, 24. Based on a previously performed reference measurement, a first reference propagation path between the first transmitter and the first receiver can be measured, which in this example is twice the propagation path between the first transceiver 10 and the first reflector 22. This is done, in particular, by measuring the propagation time from when the terahertz radiation irradiated by the first transmitter is received by the first receiver, without placing the object 20 within the measurement area. Similarly, the second reference propagation path between the second transmitter and the second receiver can also be measured without placing the object 20 within the measurement area, and in this example, this is twice the propagation path between the second transceiver 12 and the second reflector 24.

[0034] Based on this, the evaluation device 26 calculates the first propagation path difference Δα between the first measurement propagation path and the first reference propagation path, and the second propagation path difference Δβ between the second measurement propagation path and the second reference propagation path. As described above, the following formulas are applied to the propagation path differences Δα and Δβ.

[0035]

number

[0036] The angle α between the first main beam direction 14 and the normal to the surface 18 of the object 20 is known and is 0° in this example. Therefore, it is not shown in the figure. The angle β between the first and second main beam directions 14 and 16 shown in the figure is therefore equal to the angle between the second main beam direction 16 and the normal to the surface 18 of the object 20. This is also known and is, for example, 45° in this example. In this way, by using the above system of equations, the refractive index n and geometric thickness d of the object 20 can be uniquely determined.

[0037] In this example, since only the propagation path or, rather, the propagation time needs to be measured, the first transceiver 10 having a first transmitter and a first receiver, and the second transceiver 12 having a second transmitter and a second receiver, can be configured in a narrow bandwidth. For example, each of them can have a bandwidth of less than 10 GHz, preferably less than 5 GHz, and more preferably less than 1 GHz. [Explanation of Symbols]

[0038] 10 First transceiver 12. Second transceiver 14. Direction of the first main beam 16. Second main beam direction 18 Surface / Measurement surface 20. Object (object to be measured) 22 First reflector 24. Second reflector 26 Evaluation device

Claims

1. An apparatus for determining the refractive index and / or thickness of an object (20), preferably a planar object (20), The system comprises a first transmitter (10) for terahertz radiation and a first receiver (10) for terahertz radiation, wherein the first transmitter (10) is configured to irradiate the object (20) with terahertz radiation in a first main beam direction (14), and the first receiver (10) is configured to receive the terahertz radiation irradiated by the first transmitter (10) after it has passed through the object (20). A second transmitter (12) and a second receiver (12) for terahertz radiation are provided, wherein the second transmitter (12) is configured to irradiate the object (20) with terahertz radiation in a second main beam direction (16), the first and second main beam directions (14, 16) extend at different angles with respect to the measuring surface (18) that receives the object during measurement, and the second receiver (12) is configured to receive the terahertz radiation irradiated by the second transmitter (12) after it has passed through the object (20). - An evaluation device (26) is provided, which is configured to determine a first measurement propagation path of terahertz radiation between the first transmitter (10) and the first receiver (10) from the measurement signal received by the first receiver (10) after the radiation has passed through the object (20), and to compare this with a first reference propagation path of terahertz radiation between the first transmitter (10) and the first receiver (10) when the radiation has not passed through the object (20), and to determine a second measurement propagation path of terahertz radiation between the second transmitter (12) and the second receiver (12) from the measurement signal received by the second receiver (12) after the radiation has passed through the object (20), and to compare this with a second reference propagation path of terahertz radiation between the second transmitter (12) and the second receiver (12) when the radiation has not passed through the object (20), - The evaluation device (26) is further configured to determine the refractive index and / or thickness of the object (20) based on the comparison. A device characterized by the following.

2. The apparatus according to claim 1, wherein the evaluation apparatus (26) is further configured to determine a first reference propagation path from a measurement signal received by the first receiver (10) when the object (20) is not positioned in the beam path of terahertz radiation irradiated by the first transmitter (10), and / or to determine a second reference propagation path from a measurement signal received by the second receiver (12) when the object (20) is not positioned in the beam path of terahertz radiation irradiated by the second transmitter (12).

3. The apparatus according to claim 1 or 2, characterized in that the first transmitter (10) and the first receiver (10) are located on the same side of the measurement area that accepts the object (20) during measurement, the first reflector (22) is located on the opposite side of the measurement area, and the terahertz radiation irradiated by the first transmitter (10) is reflected back to the first receiver (10) after passing through the object (20), and / or the second transmitter (12) and the second receiver (12) are located on the same side of the measurement area that accepts the object (20) during measurement, the second reflector (24) is located on the opposite side of the measurement area, and the terahertz radiation irradiated by the second transmitter (12) is reflected back to the second receiver (12) after passing through the object (20).

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

5. The apparatus according to any one of claims 1 to 4, characterized in that the evaluation device (26) is further configured to determine a first propagation path difference between the first measurement propagation path and the first reference propagation path, and a second propagation path difference between the second measurement propagation path and the second reference propagation path.

6. The apparatus according to any one of claims 1 to 5, characterized in that the evaluation apparatus (26) is further configured to take into account different angles of the first main beam direction (14) and the second main beam direction (16) when determining the refractive index and / or the wall thickness.

7. The apparatus according to any one of claims 1 to 6, wherein the evaluation apparatus (26) is further configured to take into consideration the angle of the first main beam direction (14) with respect to the measuring surface (18) and the angle of the second main beam direction (16) with respect to the measuring surface (18) when determining the refractive index and / or the wall thickness.

8. The apparatus according to any one of claims 1 to 7, characterized in that the first transmitter (10) and the second transmitter (12) irradiate terahertz radiation having 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 thickness of an object (20), preferably a planar object (20), Terahertz radiation is irradiated onto the object (20) from a first transmitter (10) in the direction of a first main beam (14), and the terahertz radiation irradiated by the first transmitter (10) is received by a first receiver (10) after the radiation has passed through the object (20). - Terahertz radiation is irradiated onto the object (20) by a second transmitter (12) in a second main beam direction (16), where the first and second main beam directions (14, 16) extend at different angles to the surface (18) of the object (20) on the opposite side of the first transmitter (10) and the surface of the object on the opposite side of the second transmitter (12), and the terahertz radiation irradiated by the second transmitter (12) is received by a second receiver (12) after the radiation has passed through the object (20), - A step in which a first measurement propagation path of terahertz radiation between the first transmitter (10) and the first receiver (10) is determined from the measurement signal received by the first receiver (10) after the radiation has passed through the object (20), and this is compared with a first reference propagation path of terahertz radiation between the first transmitter (10) and the first receiver (10) when the radiation has not passed through the object (20), and a second measurement propagation path between the second transmitter (12) and the second receiver (12) is determined from the measurement signal received by the second receiver (12) after the radiation has passed through the object (20), and this is compared with a second reference propagation path between the second transmitter (12) and the second receiver (12) when the radiation has not passed through the object (20), - A step in which the refractive index and / or thickness of the object (20) is determined based on the above comparison, A method characterized by the following.

10. The method according to claim 9, characterized in that the first reference propagation path is determined from a measurement signal received by the first receiver (10) when the object (20) is not positioned in the beam path of the terahertz radiation irradiated by the first transmitter (10), and / or the second reference propagation path is determined from a measurement signal received by the second receiver (12) when the object (20) is not positioned in the beam path of the terahertz radiation irradiated by the second transmitter (12).

11. The apparatus according to claim 9 or claim 10, characterized in that, from the above comparison, a first propagation path difference between the first measurement propagation path and the first reference propagation path, and a second propagation path difference between the second measurement propagation path and the second reference propagation path are determined.

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

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

14. The method according to any one of claims 9 to 13, characterized in that the first transmitter (10) and the second transmitter (12) irradiate terahertz radiation having 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 it is performed by the apparatus described in any one of claims 1 to 8.

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