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

Description

[0001] The invention relates to a device and a method for determining the refractive index and / or the layer thickness of a layer of an object.

[0002] For example, when working with plastic objects extruded in an extrusion die, such as plastic pipes or sheet-like plastic objects, there is a desire to perform precise layer thickness measurements as easily as possible. This is important, for instance, for correctly adjusting the extrusion die as early as possible to achieve the desired final geometry of the object. It is known to measure the layer thickness of, for example, plastic pipes using terahertz radiation. Plastics are at least partially transparent to terahertz radiation, so that terahertz radiation emitted onto a plastic object is at least partially reflected at the interfaces bounding the layer to be measured. Based on time-of-flight measurements, for example, the optical layer thickness can be measured precisely and reliably.

[0003] The optical layer thickness is first measured using established methods. Determining the geometric layer thickness from the optical layer thickness requires knowledge of the exact refractive index of the layer material. While it is possible to assume the refractive index of the extruded material is known, this is subject to uncertainty. Firstly, the refractive index is temperature-dependent, meaning it changes as an extruded object cools. Secondly, additives are frequently mixed into the material to be extruded, which alter the refractive index of the starting material. The exact quantity and sometimes even the type of additives are not always reliably known. Furthermore, the additive content within the object can vary.

[0004] WO 2016 / 139155 A1 discloses a method for determining the unknown refractive index of a plastic tube using terahertz radiation. Based on this, the optical layer thickness and the refractive index, and thus the geometric layer thickness, can be determined using a terahertz measuring device. In this process, terahertz radiation passes through the object and is reflected back to the terahertz receiver (which, for example, is combined with the transmitter to form a transceiver) by a reflector located on the opposite side of the terahertz transmitter. By comparing the transit time of the terahertz radiation with the object in the radiation path to the transit time without the object in the radiation path, the initially unknown refractive index of the material can be determined.

[0005] The described method allows for a reliable and rapid determination of an unknown refractive index of an object using only one measuring device. However, the known method requires access from both sides of the object to determine the time-of-flight difference caused by the object in the measurement path. This applies both to the described arrangement of a reflector on the side of the object opposite a transceiver and to an arrangement of the transmitter and receiver on opposite sides of the object. In practice, this accessibility from both sides is not always possible. For example, with pipes, this may require access to the interior of the pipe, which is difficult to achieve and is generally only possible at the ends of the pipe.Furthermore, the known method requires an object that can actually be completely penetrated by terahertz radiation. If the object is, for example, a multi-layered object in which the layer to be measured, which is transparent to terahertz radiation, is arranged on a substrate layer that is not transparent to terahertz radiation, for example made of metal, the aforementioned method for determining the refractive index is not applicable.

[0006] Based on the prior art described above, the invention aims to provide a device and a method of the type mentioned at the outset, with which the refractive index and / or the layer thickness of the object can be reliably and easily measured without the need for access from opposite sides of the object or without the need for an object suitable for complete irradiation with terahertz radiation.

[0007] From CN 108020165 A, a terahertz measuring device for measuring the thickness of non-metallic materials, even when the refractive index is unknown, is known. For this purpose, a positionally adjustable terahertz transmitter and terahertz receiver are proposed. In a first measurement step, these emit measuring radiation perpendicular to the surface of the material to be measured and receive terahertz radiation reflected from the interfaces of the layer. In a second measurement step, the terahertz transmitter and terahertz receiver are moved along a circular arc in opposite directions, and terahertz radiation is again emitted onto the surface of the material and terahertz radiation reflected from the interfaces of the layer is received. The refractive index and the thickness of the material are calculated from the time differences between the emission and reception of the terahertz radiation in the two measurement steps.

[0008] Further terahertz measuring devices for measuring the wall thickness of objects by determining the refractive index are known from DE 10 2020 116 810 A1 and DE 20 2018 006 759 U1.

[0009] The invention solves the problem according to independent claims 1 and 15. Advantageous embodiments are described in the dependent claims, the description, and the figures. The invention solves the problem for a device of the type mentioned above, among other things, by... that a first sensor pair consisting of a first transmitter and a first receiver for terahertz radiation is arranged on a first side of the object during operation, and a second sensor pair consisting of a second transmitter and a second receiver for terahertz radiation is also arranged on the first side of the object during operation, that the first transmitter emits terahertz radiation from the first side in a first principal beam direction onto the object during operation, and the first receiver receives terahertz radiation from the first transmitter reflected from the interfaces bounding the layer of the object, that the second transmitter emits terahertz radiation from the first side in a second principal beam direction, which is at an angle to the first principal beam direction,a device emits a signal onto the object, and a second receiver receives terahertz radiation from the second transmitter reflected from the interfaces bounding the object's layer, and an evaluation unit is provided to which the measurement signals received by the first receiver and the measurement signals received by the second receiver are connected, and which is designed to determine the refractive index and / or the layer thickness of the object's layer from the received measurement signals based on the different travel times of the terahertz radiation from the first transmitter reflected from the interfaces of the object's layer and the different travel times of the terahertz radiation from the second transmitter reflected from the interfaces of the object's layer.

[0010] For a method of the type mentioned above, the invention solves the problem, among other things, by the following steps: Terahertz radiation from a first transmitter located on a first side of the object is emitted onto the object in a first principal direction, and the terahertz radiation from the first transmitter reflected from the interfaces bounding the object's layer is received by a first receiver, also located on the first side of the object. Terahertz radiation from a second transmitter, also located on the first side of the object, is emitted onto the object in a second principal direction, which is at an angle to the first principal direction, and the terahertz radiation from the second transmitter reflected from the interfaces bounding the object's layer is received by a second receiver, also located on the first side of the object.From the measurement signals received by the first receiver and the measurement signals received by the second receiver, the refractive index and / or the layer thickness of the object are determined based on the different travel times of the terahertz radiation from the first transmitter reflected from the interfaces of the object's layer and the different travel times of the terahertz radiation from the second transmitter reflected from the interfaces of the object's layer.

[0011] The object measured according to the invention can be a tubular or plate-shaped object. It can be manufactured in an extrusion apparatus. The apparatus according to the invention can be used in an extrusion line for the manufacture of the object. Accordingly, the method according to the invention can be carried out in an extrusion line. The layer of the object to be measured is at least partially transparent to terahertz radiation. The object can, for example, consist of a plastic. The object can also have a multilayer structure. It is also possible that the object has a layer that is not transparent to terahertz radiation, for example, a metal layer, on which the layer to be measured, for example, a plastic layer, is applied. With the apparatus or the method according to the invention, the refractive index and / or the layer thickness of a layer of the object can be determined.The layer thickness refers to the geometric layer thickness. As is known, it is derived from the optical thickness, which can be measured directly using terahertz radiation, for example based on time-of-flight measurements, by dividing it by the refractive index of the layer of the object. The terahertz radiation used according to the invention can, for example, be in a frequency range from 1 GHz to 10 THz.

[0012] The first sensor pair comprises a first transmitter and a first receiver for terahertz radiation and is located on one side of the object. Also located on the first side of the object is the second sensor pair, with a second transmitter and a second receiver for terahertz radiation. Thus, the first and second transmitters irradiate the object from the same side. Also from this same side, the first and second receivers receive terahertz radiation reflected from interfaces of the object's layer. The second transmitter and second receiver of the second sensor pair are synchronized. The first transmitter and first receiver of the first sensor pair are also synchronized. This is particularly easy to implement if the first sensor pair is configured as a transceiver, as explained in more detail below. The first transmitter and first receiver can essentially be located at the same point.The terahertz radiation emitted by the first and second transmitters is reflected at interfaces of the layer of the object being measured. The interface of the layer facing the first side is correspondingly the interface facing the device, in particular the sensor pairs. It forms the front of the layer. The interface of the layer facing away from the first side is correspondingly the interface facing away from the device, in particular the sensor pairs. It forms the back of the layer being measured. At the interfaces, there is a transition between media with different optical densities. Accordingly, the terahertz radiation is at least partially reflected at these interfaces. The interfaces can be with air or another medium. For example, the interface facing the first side can be with air.The interface facing away from the first side can also be against air or, for example, against another medium, which may also be opaque to terahertz radiation. The interfaces can be flat, for example, if the object is plate-shaped, or curved, if the object is tubular. In the latter case, the terahertz radiation is directed onto the tube by the first and / or second transmitter, particularly transversely, for example, perpendicularly, to its longitudinal direction. The object can be conveyed through a measuring section of the device during the measurement; a tubular object along its longitudinal axis. This is particularly the case in extrusion lines, where the object exits the extrusion device and is conveyed further by means of a conveying system. The device can include a corresponding conveying system for this purpose.The object may still have an elevated temperature during the measurement according to the invention. In particular, the object may still contain flowable molten material.

[0013] The first transmitter emits terahertz radiation towards the object in a first principal beam direction. The second transmitter emits terahertz radiation towards the object in a second principal beam direction, which is at an angle to the first principal beam direction. As explained in more detail below, it is possible that the first principal beam direction is perpendicular to the interface facing the first side (and possibly also to the interface facing away from the first side) of the object's layer. The interfaces can be parallel. In particular, the terahertz radiation emitted by the second transmitter thus enters the layer of the object to be measured at an angle of incidence between 0° and 90°. At least the second transmitter and the second receiver have an opening angle of the emitted and received terahertz radiation, respectively. With an opening angle of the radiation, the principal beam direction corresponds to the center beam.the central direction of the emitted radiation angle or radiation cone. The terahertz radiation received by the first and second receivers is reflected at the same location on the first and second interfaces in the west. In particular, the terahertz radiation emitted by the second transmitter, entering the layer at an angle of incidence between 0° and 90°, undergoes refraction upon entering and exiting the object. The portion of the radiation reflected at the interface facing the first (front side) and the portion reflected at the interface facing away from the first (back side), and thus received by the second receiver, enter and exit the object at spaced-apart locations.

[0014] The evaluation unit receives the measurement signals from the first and second receivers. For the determination of the refractive index and / or the layer thickness according to the invention, the physical fact is utilized that the terahertz radiation from the first transmitter reflected at the interfaces of the layer exhibits a first time-of-flight difference, and that the terahertz radiation from the second transmitter reflected at the interfaces of the layer exhibits a second time-of-flight difference, wherein the first time-of-flight difference differs from the second time-of-flight difference, in particular due to the irradiation from different directions and the refraction, especially of the radiation from the second transmitter. For the evaluation according to the invention, the evaluation unit can determine the time-of-flights and / or time-of-flight differences of the radiation from the first transmitter reflected at the interfaces and the radiation from the second transmitter reflected at the interfaces.However, it is also possible that the time-of-flight differences are only indirectly incorporated into the evaluation, for example, by only evaluating the values ​​of the individual time-of-flight values ​​of the radiation components. An indirect consideration of the time-of-flights or time-of-flight differences, for example via frequency analysis, is of course also possible.

[0015] As explained in more detail below, the evaluation unit can, for example, determine a first time difference and / or first optical path difference between the terahertz radiation from the first transmitter reflected from the interfaces of the layer and the terahertz radiation from the second transmitter, based on the measurement signals received by the first receiver. This difference can be determined from time-of-flight measurements of the terahertz radiation from the first and second transmitters reflected from the interfaces of the layer. As also explained in more detail below, the evaluation unit can determine a first and second optical path difference from a first and second time difference, respectively.Because the terahertz radiation emitted by the first and second transmitters passes through the layer being measured twice, the time-of-flight measurement initially yields twice the optical thickness of the layer. Accordingly, the respective values ​​determined for calculating the time difference or optical path difference can be halved.

[0016] Based on the measurement signals from the first and second receivers and the time-of-flight differences between the terahertz radiation reflected from the interfaces of the object's layer by the first and second transmitters, the evaluation unit determines the refractive index and / or the (geometric) thickness of the object's layer. The first and second time-of-flight differences, and thus the first and second time differences or first and second optical path differences, depend on the geometric thickness and the refractive index of the layer. However, these dependencies differ because, as explained, the terahertz radiation emitted by the second transmitter, in particular, undergoes refraction by the layer.For example, if the first principal beam direction of the first transmitter is perpendicular to the interface of the layer facing the first side, the dependence of the first transit-time difference, and in particular the measured optical thickness, on the refractive index can be easily determined by multiplying the geometric layer thickness by the refractive index. The transit-time difference is then obtained by additionally dividing by the speed of light in a vacuum. The more complex dependence of the second transit-time difference of the radiation from the second transmitter on the refractive index and the geometric layer thickness can be calculated, especially when the distance between the second transmitter and the second receiver is known, taking into account the first transit-time difference of the radiation from the first transmitter, and in particular by means of numerical calculation methods, as will be explained in more detail below.By exploiting both relationships according to the invention—that is, the dependence of the first and second transit-time differences on the geometric layer thickness and the refractive index—a unique solution is obtained for a refractive index and a layer thickness that fulfills the dependence conditions for both the first and the second transit-time difference. The refractive index and (geometric) layer thickness pair thus determined is accordingly defined as the refractive index and / or layer thickness of the object.

[0017] With the device and method according to the invention, the refractive index of a layer, in addition to its thickness, can be reliably measured using only one terahertz measuring device. Access to the object from only one side of the layer to be measured is sufficient. It is not necessary to arrange components of the measuring device on opposite sides of the object. Accordingly, the device according to the invention is located only on one side of the object during operation. In particular, it is not necessary to arrange a reflector or receiver for terahertz radiation on the opposite side. Thus, the device and method according to the invention are particularly suitable for samples that are only accessible from one side.This also applies to the measurement of layers that are opaque to terahertz radiation, for example plastic coatings on metal pipes.

[0018] As already mentioned, the first principal beam direction can, during operation, run perpendicular to the interface of the object's layer facing the first side. In this case, the terahertz radiation emitted by the first transmitter, particularly the principal beam direction, is incident perpendicularly onto the object. This significantly simplifies the evaluation according to the invention, since there is a particularly simple relationship between the measured time-of-flight difference or the measured time or path difference and the geometric layer thickness via the refractive index.

[0019] According to a particularly practical design, as already mentioned, the first sensor pair can be formed by a transceiver comprising the first transmitter and the first receiver.

[0020] In a further embodiment, the second transmitter and receiver can be arranged on opposite sides of the first sensor pair, particularly the transceiver. The second transmitter and receiver can be equidistant from the first sensor pair, especially the transceiver. Due to this symmetrical arrangement of the second sensor pair relative to the first, both sensor pairs approximately measure the same location on the layer being measured, provided the sensors are parallel to the surface of the layer. In this configuration, the first sensor pair (or the transceiver), the second transmitter (or second receiver), and the common point of incidence of the terahertz radiation emitted by the first and second transmitters on the interface of the layer facing the first side each form a right-angled triangle.

[0021] In a further embodiment, the evaluation device may be configured to check, in order to verify the alignment of the device to the object during operation, whether the sum of the square of the distance between the transceiver and the second transmitter or receiver and the square of the distance of the transceiver from the interface of the object's layer facing the first side is equal to the square of the distance of the second transmitter or receiver from the point of impact of the terahertz radiation from the first transmitter on the interface of the object's layer facing the first side. In particular, the first transmitter, the first receiver, the second transmitter, and the second receiver may be arranged in a plane, especially along a straight line in the plane, which during operation is parallel to an interface of the object facing the first side.The straight line can, for example, run perpendicular to a longitudinal direction of a tubular object. As explained, it can be ensured that both sensor pairs observe the same location if the transceiver, the second transmitter or receiver, and the point of impact of the terahertz radiation emitted by the transceiver or second transmitter on the interface facing the first side form a right-angled triangle. This is verified in the aforementioned configuration. The verification is based on the assumption that the distances between the sensor pairs or the transmitters and receivers are known. The distance, in particular, between the transceiver and the interface of the object facing the first side can be measured by a simple time-of-flight measurement of the terahertz radiation.The orientation of the sample can then be checked by applying the Pythagorean theorem to a right-angled triangle in the manner explained above.

[0022] According to the invention, the evaluation device is configured to determine, from the measurement signals received by the first receiver, a specific first time difference and / or first optical path difference between the terahertz radiation of the first transmitter reflected from the interfaces of the layer of the object, and, from the measurement signals received by the second receiver, to determine a specific second time difference and / or second optical path difference between the terahertz radiation of the second transmitter reflected from the interfaces of the layer of the object. The determined or measured time difference, and thus also the determined or measured optical path difference, can of course also be determined indirectly or implicitly by simply determining and computationally processing the travel times of the terahertz radiation of the first or second transmitter reflected from the interfaces of the layer.An indirect determination of the time difference in Fourier space is also possible, of course. The optical path difference can, as is well known, be determined from the time difference, and vice versa, in particular by multiplication or division by the speed of light in a vacuum.

[0023] According to the invention, the evaluation device is further configured to determine combinations of possible refractive index values ​​and possible layer thickness values ​​of the object's layer that are suitable for the specific first time difference and / or first optical path difference, to determine from the determined combinations a combination suitable for the specific second time difference and / or specific second optical path difference, and to determine this specific suitable combination of refractive index value and layer thickness value as the refractive index and layer thickness of the object's layer, and / or that the evaluation device is further configured to determine combinations of possible refractive index values ​​and possible layer thickness values ​​of the object's layer that are suitable for the specific second time difference and / or second optical path difference.The evaluation device is designed to determine a combination suitable for the specified first time difference and / or first optical path difference from the determined combinations, and to define this specific suitable combination of refractive index value and layer thickness value as the refractive index and layer thickness of the object's layer. Furthermore, the evaluation device may be configured to calculate a second time difference and / or second optical path difference based on the combinations of possible refractive index values ​​and possible layer thickness values ​​determined for the specified first time difference and / or first optical path difference, to compare this with the determined second time difference and / or second optical path difference, and to define the combination of refractive index value and layer thickness value as the refractive index and layer thickness of the object's layer.where the determined and calculated second time difference and / or second optical path difference coincide, and / or that the evaluation device is designed to calculate a first time difference and / or first optical path difference based on the combinations of possible refractive index values ​​and possible layer thickness values ​​determined for the determined second time difference and / or second optical path difference, to compare this with the determined first time difference and / or determined first optical path difference, and to determine the combination of refractive index value and layer thickness value as the refractive index and layer thickness of the layer of the object where the determined and calculated first time difference and / or optical path difference coincide.

[0024] The aforementioned configurations are based on the understanding that for the specific first and second time or optical path differences, there is exactly one solution pair consisting of a refractive index value and a layer thickness value, from which both the first and second specific time or optical path differences can be calculated. According to the aforementioned configurations, this solution pair consisting of the refractive index value and the layer thickness value is identified and determined as the refractive index and / or layer thickness of the object.

[0025] The layer thickness values ​​determined from a time difference and / or optical path difference are possible values ​​for the layer thickness of the object. Similarly, the refractive index values ​​are possible values ​​for the refractive index of the object's layer. Due to the relatively simple computational relationship, a family of possible layer thickness values ​​for the determined first time difference or optical path difference can be easily calculated by varying the refractive index values. From this family of possible solution combinations of refractive index value and layer thickness value, the solution pair that leads to the determined second time difference or optical path difference can then be identified computationally, particularly using a numerical calculation method.Thus, for every possible solution pair of refractive index value and layer thickness value from the set of solution pairs, a calculated second time difference or optical path difference can be assigned, which can then be compared with the determined second time difference or optical path difference. By numerically varying, for example, the refractive index value, the solution pair can be found where the determined second time difference or optical path difference and the calculated second time difference or optical path difference coincide. It is not strictly necessary to calculate a corresponding second time difference or optical path difference for every pair from the determined set of refractive index values ​​and layer thickness values. Other computational approaches can also be used, for example, finding the zeros of the difference between the determined second time difference or optical path difference.The optical path difference and the calculated second time difference or optical path difference can be determined, for example, using Newton's method. Of course, a family of possible layer thickness values ​​can also be calculated for the determined second time difference or optical path difference by varying the refractive index values. From this family of possible solution combinations of refractive index value and layer thickness value, the solution pair that leads to the determined first time difference or optical path difference can then be identified computationally, particularly using a numerical calculation method.

[0026] With further refinement, the evaluation unit can be configured to calculate the possible layer thickness values ​​from the quotient of the determined first time difference multiplied by the speed of light in a vacuum or the determined first optical path difference, and a possible refractive index value. This relatively simple relationship between the first time difference or optical path difference and the layer thickness was explained earlier. The refractive index value can be varied so that a set of possible solution pairs for the refractive index value and the layer thickness value can be determined.

[0027] The evaluation unit can be further configured to numerically calculate the second and / or first time difference or optical path difference from a possible refractive index value and a possible layer thickness value, i.e., from a combination of the determined combinations of possible refractive index values ​​and possible layer thickness values, as already explained. The calculation of the second and / or first time difference or optical path difference can, for example, be implemented numerically as a minimum search.

[0028] Furthermore, the evaluation unit can be configured to take into account, particularly during calculations, especially numerical calculations, the distance of the transceiver from the interface of the object's layer facing the first side, and the distance of the second transmitter or receiver from the transceiver. The evaluation unit can also be configured to determine a minimum of a function for numerical calculation by varying a parameter that characterizes the angle of refraction of the terahertz radiation emitted by the second transmitter onto the object as it enters the object's layer. This utilizes the fact that refraction at planar interfaces always occurs in such a way that the optical path is minimal. The evaluation unit can also be configured to numerically determine the second optical path difference as the minimum of the following function: f n d B = t 1 2 + A − B 2 + n d 2 + B 2 − t 1 2 + A 2 with: t1 : Distance of the transceiver from the interface of the object's layer facing the first side, A : Distance of the second transmitter or receiver from the transceiver, n : Possible refractive index value of the object, d: Possible layer thickness value of the object's layer, B : Variable for finding the minimum, in particular a parameter characterizing the angle of refraction of the terahertz radiation emitted by the second transmitter upon entering the layer.

[0029] It should be noted that it may be advantageous to perform a subsequent correction of the evaluation system's results to compensate, for example, for the fact that the transmitters and receivers are not point-like or their arrangement is not perfect. This could be achieved through appropriate calibration.

[0030] In a further embodiment, the angle between the first and second main beam directions can be greater than 5°, preferably greater than 10°, and more preferably greater than 30°. A larger angle between the main beam directions facilitates evaluation, as the time-of-flight differences of the radiation from the first and second transmitters then vary more significantly.

[0031] According to a further embodiment, the device can be a portable handheld device. It can therefore be easily held by an operator and used at the measurement site. The invention is particularly suitable for a compact design, so that the device can be easily used as a handheld device. As explained, unlike in the prior art, a reflector behind the object to be measured is not required.

[0032] The device may further comprise the object. The invention also relates to a system comprising the device according to the invention and the object. The system may also include an extrusion device for producing the object and, optionally, a conveying device for conveying the object.

[0033] The method according to the invention can be carried out with a device according to the invention. In particular, the steps for which the evaluation unit of the device according to the invention is designed can be performed in the method. Conversely, the device according to the invention can also be configured to carry out the method according to the invention.

[0034] An embodiment of the invention is explained in more detail below with reference to the figures. It schematically shows: Figure 1 shows a device according to the invention in a side view, Figure 2 shows a diagram illustrating the determination of a solution pair from refractive index value and layer thickness value, and Figure 3 shows a further diagram illustrating the determination of a solution pair from refractive index value and layer thickness value.

[0035] Unless otherwise stated, the same reference symbols in the figures denote the same objects.

[0036] In Figure 1The figure shows a very schematic representation of an object 10 whose layer thickness is to be measured. The object 10 can, for example, be a plastic object, which may be tubular or plate-shaped. It can be manufactured in an extrusion apparatus and conveyed through a measuring area of ​​the device according to the invention by means of a conveying device. It is also possible that the object 10 still has an elevated temperature at the time of measurement, for example, if it has parts that have not yet fully solidified. For measuring the layer thickness of the object 10, the device according to the invention has a transceiver 12 forming a first sensor pair with a first transmitter and a first receiver for terahertz radiation, as well as a second sensor pair comprising a second transmitter 14 and a second receiver 16 for terahertz radiation.In the illustrated example, the second transmitter 14 and the second receiver 16 are arranged on opposite sides and at the same distance A from the transceiver 12. The transceiver 12, as well as the second transmitter 14 and the second receiver 16, are also arranged along a straight line in a plane on a first side of the object 10, which is parallel to the interface 18 of the layer of the object 10 facing the first side. During operation, the first transmitter of the transceiver 12 emits terahertz radiation 20 onto the object 10, which is at least partially transparent to this radiation. The terahertz radiation 20 is reflected accordingly at the interface 18 facing the first side and at the interface 22 of the layer of the object 10 facing away from the first side and returns to the transceiver 12, where it is measured by the first receiver as measurement signals. As in . Figure 1To be recognized, the first transmitter of the transceiver 12 emits terahertz radiation in a first principal beam direction towards the object 10, which is perpendicular to the interface 18 of the layer of the object 10 and to the interface 22 parallel to it. The second transmitter 14 also emits terahertz radiation 24 towards the object 10 at an opening angle and along a second principal beam direction, which is at an angle to the first principal beam direction of the transceiver 12. The in Figure 1The partial beams of the terahertz radiation 24 shown here strike the interface 18 at a distance from each other. One of the partial beams strikes the interface 18 at essentially the same location as the terahertz radiation 20 emitted by the transceiver 12. After reflection at the interface 18, this portion of the terahertz radiation 24 reaches the second receiver 16 at the same angle of incidence as the angle of reflection, where it is received as measurement signals. The second in Figure 1 The partial beam of terahertz radiation 24 shown is reflected at the interface 22 after undergoing refraction upon entering the layer of the object 10, namely at essentially the same location as the terahertz radiation 20 of the transceiver 12. After reflection and refraction again upon exiting the layer of the object 10, this component of the terahertz radiation 24 also reaches the second receiver 16 and is received by it as measurement signals.

[0037] The measurement signals from the first receiver and the second receiver 16 are forwarded to an evaluation unit 26 of the device according to the invention. The evaluation unit determines, from the measurement signals received by the first receiver, a specific first optical path difference between the terahertz radiation of the first transmitter reflected from the interfaces 18, 22 of the layer of the object 10, and, from the measurement signals received by the second receiver 16, a specific second optical path difference between the terahertz radiation of the second transmitter reflected from the interfaces 18, 22 of the layer of the object 10. With reference to the terahertz radiation 20 of the first transmitter, the optical path difference between the terahertz radiation reflected at interface 18 and the terahertz radiation reflected at interface 22, and thus the optical thickness of the layer of the object 10, is determined.With regard to the terahertz radiation 24 of the second transmitter 14, the optical path difference between the in . Figure 1 The partial rays of terahertz radiation 24 shown were determined.

[0038] As in Figure 1As can be seen, the transceiver 12, together with the second transmitter 14 and the point of impact of the terahertz radiation 20 on the interface 18, forms a right-angled triangle. Similarly, the second receiver 16, together with the transceiver 12 and the point of impact of the terahertz radiation 20 on the interface 18, also forms a right-angled triangle. This alignment can be verified by the evaluation unit by checking whether the sum of the square of the distance A between the transceiver 12 and the second transmitter 14 or the second receiver 16, and the square of the distance of the transceiver 12 from the interface 18, is equal to the square of the distance of the second transmitter 14 or the second receiver 16 from the point of impact of the terahertz radiation 20 from the first transmitter on the interface 18.

[0039] As explained, both the first optical path difference and the second optical path difference depend on the (geometric) layer thickness. dthe layer of object 10 and the refractive index n of the layer of object 10, the dependencies being different because refraction occurs in the terahertz radiation 24 of the second transmitter 14, as in Figure 1 to recognize. The first optical path difference can be simply determined by the layer thickness. d multiplied by the refractive index n yields the result. The dependence of the second optical path difference can be calculated numerically if the distance A and the distance of the transceiver 12 from the interface 18 are known. Utilizing both relationships results in a unique solution for the refractive index. n and the layer thickness d, which leads to both the first optical path difference and the second optical path difference.

[0040] In Figure 2This is illustrated by an example. For the sake of argument, a value of 50 mm was assumed for the distance A. The distance between the transceiver 12 and the interface 18 was also assumed to be 50 mm. The first optical path difference was assumed to be 4 mm and the second optical path difference 3.5 mm. From the fact that the first optical path difference is the product of the layer thickness... d with the refractive index n This results in a range of possible refractive index values, which are in Figure 2 plotted on the x-axis, which is in Figure 2 Solid line curve 28. The Y-axis shows the calculated layer thickness values ​​derived from the refractive index values. Curve 28 thus shows possible solution combinations for the layer thickness. d and the refractive index n. From the more complex relationship for the second optical path difference, it follows that Figure 2The calculation corresponds to the dashed curve 30. The intersection of curves 28 and 30 yields the uniquely correct solution for the refractive index. n and the layer thickness d layer of the object 10.

[0041] In principle, each of the possible solution pairs shown according to curve 28 can be determined from the refractive index n and layer thickness d Assign a calculated second optical path difference that can be compared with the determined second optical path difference. The refractive index n can be varied numerically accordingly to find the refractive index n at which the determined and the calculated second optical path difference coincide.

[0042] In Figure 3 This relationship is illustrated for the example mentioned above. The same situation is described as in... Figure 2It is represented only in a different form, with possible refractive index values ​​n plotted on the x-axis. The second optical path difference is shown on the y-axis. ds applied, whereby the in Figure 3 solid curve 32 represents the specific second optical path difference ds The dashed curve 34 shows the second optical path differences calculated for each of the set of possible refractive index values ​​n. The intersection of curves 32 and 34 indicates the correct refractive index. n and the layer thickness d This results from the quotient of the first optical path difference and the applicable refractive index.

[0043] Calculating the second optical path difference ds This can be implemented numerically, for example, as a minimum search. Thus, the second optical path difference ds the minimum of the following function: f n d B = t 1 2 + A − B 2 + n d 2 + B 2 − t 1 2 + A 2 , with: t1: Distance of the transceiver from the interface of the object's layer facing the first side, A :Distance of the second transmitter or receiver from the transceiver, n:Possible refractive index value of the object, d: Possible layer thickness value of the object's layer, B : Variable for finding the minimum, in particular a parameter characterizing the angle of refraction of the terahertz radiation emitted by the second transmitter upon entering the layer.

[0044] B is varied to find the minimum of the function. This minimum yields the calculated second optical path difference, which can then be compared with the measured optical path difference as described. Reference symbol list

[0045] 10 Object 12 Transceiver 14 Second transmitter 16 Second receiver 18 Interface 20 Terahertz radiation 22 Interface 24 Terahertz radiation 26 Evaluation unit 28 Curve 30 Curve 32 Curve 34 Curve

Claims

1. A device for determining the refractive index and / or the layer thickness of a layer of an object (10), characterized in that • a first sensor pair arranged on a first side of the object (10) during operation and consisting of a first transmitter and a first receiver for terahertz radiation (20), and a second sensor pair also arranged on the first side of the object (10) during operation and consisting of a second transmitter (14) and a second receiver (16) for terahertz radiation (24) is provided, • the first transmitter emits terahertz radiation (20) during operation onto the object (10) from the first side in a first main beam direction and the first receiver receives terahertz radiation (20) of the first transmitter reflected by the interfaces delimiting the layer of the object (10), • the second transmitter (14) emits terahertz radiation (24) during operation onto the object (10) from the first side in a second main beam direction, which extends at an angle to the first main beam direction, and the second receiver (16) receives terahertz radiation (24) of the second transmitter (14) reflected by the interfaces delimiting the layer of the object (10), and • an evaluation apparatus (26) is provided, to which the measurement signals received by the first receiver and the measurement signals received by the second receiver (16) are applied, and which is designed to determine the refractive index and / or the layer thickness of the layer of the object (10) from the applied measurement signals on the basis of the different times of flight of the terahertz radiation (20) of the first transmitter reflected by the interfaces (18, 22) of the layer of the object (10) and the different times of flight of the terahertz radiation (20) of the second transmitter (14) reflected by the interfaces (18, 22) of the layer of the object (10), • wherein the evaluation apparatus (26) is designed to determine a first time difference and / or first optical path difference between the terahertz radiation (20) of the first transmitter reflected by the interfaces (18, 22) of the layer of the object (10) from the measurement signals received by the first receiver, and to determine a second time difference and / or second optical path difference between the terahertz radiation (24) of the second transmitter (14) reflected by the interfaces (18, 22) of the layer of the object (10) from the measurement signals received by the second receiver (16), and • wherein the evaluation apparatus (26) is further designed to ascertain combinations of possible refractive index values and possible layer thickness values of the layer of the object (10) that correspond to the determined first time difference and / or first optical path difference, to determine a combination suitable for the determined second time difference and / or second optical path difference from the ascertained combinations, and to determine this determined suitable combination of the refractive index value and layer thickness value as a refractive index and layer thickness of the layer of the object (10), and / or in that the evaluation apparatus (26) is further designed to ascertain combinations of possible refractive index values and possible layer thickness values of the layer of the object (10) that correspond to the determined second time difference and / or second optical path difference, to determine a combination that corresponds to the determined first time difference and / or first optical path difference from the ascertained combinations, and to determine this determined suitable combination of the refractive index value and layer thickness value as a refractive index and layer thickness of the layer of the object (10).

2. The device according to claim 1, characterized in that the first main beam direction, during operation, extends perpendicularly to the interface (18) of the layer of the object (10) facing the first side.

3. The device according to one of the preceding claims, characterized in that the first sensor pair is formed of a transceiver (12), comprising the first transmitter and the first receiver.

4. The device according to one of the preceding claims, characterized in that the second transmitter (14) and the second receiver (16) are arranged on opposite sides of the first sensor pair, in particular of the transceiver (12).

5. The device according to claim 4, characterized in that the second transmitter (14) and the second receiver (16) are at the same distance from the first sensor pair, in particular the transceiver (12).

6. The device according to claims 3 and 5, characterized in that the evaluation apparatus (26) is designed, during operation, in order to check the orientation of the device relative to the object (10), to check whether the sum of the square of the distance between the transceiver (12) and second transmitter (14) or receiver (16) and the square of the distance between the transceiver (12) and the interface (18) of the layer of the object (10) facing the first side is equal to the square of the distance between the second transmitter (14) or receiver (16) and the point of incidence of the terahertz radiation (20) of the first transmitter on the interface (18) of the layer of the object (10) facing the first side.

7. The device according to one of the preceding claims, characterized in that the first transmitter, the first receiver, the second transmitter (14), and the second receiver (16) are arranged in a plane, in particular along a line in the plane that, during operation, is arranged parallel to an interface (18) of the object (10) facing the first side.

8. The device according to one of the preceding claims, characterized in that the evaluation apparatus (26) is designed to calculate a calculated second time difference and / or second optical path difference based on the combinations of possible refractive index values and possible layer thickness values ascertained for the determined first time difference and / or first optical path difference, to compare this calculated second time difference and / or second optical path difference with the determined second time difference and / or second optical path difference and to determine, as a refractive index and layer thickness of the layer of the object (10), the combination of refractive index value and layer thickness value with which the determined and calculated second time difference and / or second optical path difference match, and / or in that the evaluation apparatus (26) is designed to calculate a calculated first time difference and / or first optical path difference based on the combinations of possible refractive index values and possible layer thickness values ascertained for the determined second time difference and / or second optical path difference, to compare this calculated first time difference and / or first optical path difference with the determined first time difference and / or first optical path difference and to determine, as a refractive index and layer thickness of the layer of the object (10), the combination of refractive index value and layer thickness value with which the determined and calculated first time difference and / or first optical path difference match.

9. The device according to claim 3 or according to claims 3 and 8, characterized in that the evaluation apparatus (26) is designed to calculate the possible layer thickness values in each case from the quotient of the determined first time difference multiplied by the speed of light in a vacuum and / or first optical path difference and a possible refractive index value.

10. The device according to one of claims 8 or 9, characterized in that the evaluation apparatus (26) is designed to calculate the calculated second and / or first time difference and / or optical path difference numerically from a possible refractive index value and a possible layer thickness value.

11. The device according to claim 10, characterized in that the evaluation apparatus (26), for the numerical calculation, is designed to determine a minimum of a function by varying a parameter that characterizes the angle of refraction of the terahertz radiation (24) emitted by the second transmitter (14) onto the object (10) when said terahertz radiation enters the layer of the object (10).

12. The device according to claims 3 and 5 or according to claims 3 and 5 and one of the other preceding claims, characterized in that the evaluation apparatus (26) is designed to further take into account the distance between the transceiver (12) and the interface (18) of the layer of the object (10) facing the first side and the distance between the second transmitter (14) or receiver (16) and the transceiver (12).

13. The device according to one of the preceding claims, characterized in that the angle of the second main beam direction relative to the first main beam direction is more than 5°, preferably more than 10°, further preferably more than 30°.

14. The device according to one of the preceding claims, characterized in that it is a portable handheld device.

15. A method for determining the refractive index and / or layer thickness (d) of a layer of an object (10), characterized by the following steps: • terahertz radiation (20) is emitted by a first transmitter arranged on a first side of the object (10) onto the object (10) from the first side in a first main beam direction, and terahertz radiation (20) of the first transmitter reflected by the interfaces delimiting the layer of the object (10) is received by a first receiver also arranged on the first side of the object (10), • terahertz radiation (24) is emitted by a second transmitter (14) also arranged on the first side of the object (10) onto the object (10) from the first side in a second main beam direction, which extends at an angle to the first main beam direction, and terahertz radiation (24) of the second transmitter reflected by the interfaces delimiting the layer of the object (10) is received by a second receiver (16) also arranged on the first side of the object (10), • the refractive index and / or the layer thickness of the layer of the object (10) is determined by an evaluation apparatus (26) from the measurement signals received by the first receiver and the measurement signals received by the second receiver (16) on the basis of the different times of flight of the terahertz radiation (20) of the first transmitter reflected by the interfaces (18, 22) of the layer of the object (10) and the different times of flight of the terahertz radiation (20) of the second transmitter (14) reflected by the interfaces (18, 22) of the layer of the object (10), • wherein the evaluation apparatus (26) determines a first time difference and / or first optical path difference between the terahertz radiation (20) of the first transmitter reflected by the interfaces (18, 22) of the layer of the object (10) from the measurement signals received by the first receiver, and determines a second time difference and / or second optical path difference between the terahertz radiation (24) of the second transmitter (14) reflected by the interfaces (18, 22) of the layer of the object (10) from the measurement signals received by the second receiver (16), and • wherein the evaluation apparatus (26) further ascertains combinations of possible refractive index values and possible layer thickness values of the layer of the object (10) that correspond to the determined first time difference and / or first optical path difference, determines a combination suitable for the determined second time difference and / or second optical path difference from the ascertained combinations, and determines this determined suitable combination of the refractive index value and layer thickness value as a refractive index and layer thickness of the layer of the object (10), and / or in that the evaluation apparatus (26) further ascertains combinations of possible refractive index values and possible layer thickness values of the layer of the object (10) that correspond to the determined second time difference and / or second optical path difference, determines a combination that corresponds to the determined first time difference and / or first optical path difference from the ascertained combinations, and determines this determined suitable combination of the refractive index value and layer thickness value as a refractive index and layer thickness of the layer of the object (10).

16. The method according to claim 15, characterized in that it is carried out with a device according to one of claims 1 to 14.