Terahertz measuring device and method for measuring test objects

Abstract

Measuring device for measuring test objects includes a transmitter-receiver unit and a corresponding mirror arrangement. Mirror arrangement has a first mirror forming a first focal point and a second focal point in an x-y plane, and first mirror is, at least sectionally, elliptically curved for deflecting radiation between the focal points. Mirror arrangement includes a second mirror arranged in region of first focal point, second mirror is used for deflecting radiation between a z-direction extending transversely to the x-y plane and between the x-y plane. Test object to be measured is arranged in region of second focal point by a test object holder. Radiation reflected on test object is evaluated by a control unit. Measuring device allows measuring of test object in simple and flexible way, in particular measurement over the entire circumference of test objects in form of plastic pipes having circular cross sections.

Claims

1. A terahertz measuring device for measuring test objects, comprising: a) at least one transmitter-receiver unit including: i) a transmitter for emitting Terahertz radiation in the Terahertz frequency band between 0.01 THz and 50 THz; ii) an associated receiver for detecting a radiation reflected on the test object in the Terahertz frequency band between 0.01 THz and 50 THz; b) at least one mirror array including i) a first mirror, which forms in an x-y-plane a first focal point and a second focal point, and which is, at least partially, curved elliptically for deflecting the radiation between the first and second focal points; and ii) a second mirror, disposed in the area of the first focal point, for deflecting the radiation between a direction z running perpendicular to the x-y plane and the x-y plane; and c) a test object holder for arranging the test object in the area of the second focal point; and d) a control unit for determining a wall thickness or layer thickness of the test object, the control unit being connected to the transmitter and the receiver for controlling the transmitter and evaluating the reflected radiation detected by the receiver; and e) at least two mirror arrays are disposed displaced relative to each other in the direction z and their respective first focal points are spaced apart perpendicular to the direction z, and the at least two mirror arrays are constructed identically.

2. The terahertz measuring device according to claim 1, wherein: a) the at least one transmitter-receiver unit is disposed along an axis z running parallel to direction z through the first focal point.

3. The terahertz measuring device according to claim 1, wherein: a) the respective second focal points lie on a line which runs parallel to the direction z.

4. The terahertz measuring device according to claim 1, wherein: a) the respective first focal points of the exactly two mirror arrays lie on different sides of a y-z plane running through the second focal points.

5. The terahertz measuring device according to claim 1, wherein: a) each mirror array is associated with a transmitter-receiver unit.

6. The terahertz measuring device according to claim 1, wherein: a) the second mirror is pivotable about an axis z running parallel to the direction z through the first focal point.

7. The terahertz measuring device according to claim 1, wherein: a) the test object holder configured in such a way that the test object is pivotable about an axis of rotation running through the second focal point.

8. The terahertz measuring device according to claim 1, wherein: a) the respective first mirror is curved along an ellipse, the ellipse being defined by a first semiaxis of a length a and a second semiaxis of a length b shorter in comparison with the first semiaxis, whereby at least one of the following is true for a relationship of the lengths: a/b 1.3, a/b 1.2, and a/b 1.1.

9. The terahertz measuring device according to claim 1, wherein: a) the respective first mirror exhibits a concave curvature in the direction z, the curvature being selected, in particular, from the group of parabolic, elliptical and spherical.

10. The terahertz measuring device according to claim 1, wherein: a) the respective first mirror is planar in the direction z and at least one focusing element is provided for focusing the terahertz radiation in the direction z.

11. A method for measuring test objects, and for determining a wall thickness or layer thickness of the test objects, comprising the following steps: a) providing the terahertz measuring device according to claim 1; b) placing the test object such that its central longitudinal axis runs through the second focal point; c) emitting terahertz radiation, in a band between 0.01 THz and 50 THz, by use of the transmitter; d) deflecting the emitted terahertz radiation by use of the second mirror and the first mirror in the direction of the second focal point; e) reflecting the terahertz radiation on the test object; f) deflecting the reflected terahertz radiation by use of the first mirror and the second mirror in the direction of the receiver; g) detecting the reflected terahertz radiation by use of the receiver; h) evaluating the detected radiation; and i) determining the wall thickness or layer thickness from the detected reflected terahertz radiation.

12. The method according to claim 11, wherein: a) the test object and the second mirror are pivoted relative to each other, and the second mirror being pivoted about the axis z running parallel to the direction z through the first focal point.

13. The method according to claim 11, wherein: a) the test object at least partially includes a layer of material in the shape of a hollow cylinder.

14. The method according to claim 11, wherein: a) the at least one transmitter-receiver unit is configured in such a way that electromagnetic terahertz radiation at a frequency in the band between at least one of 0.01 THz and 50 THz, 0.05 THz to 20 THz, and 0.1 THz to 10 THz, is emitted.

15. The method according to claim 11, wherein: a) the measuring and determining of a wall thickness is carried out with respect to at least one layer of material across the entire circumference of the test object, the at least one layer of material is in the shape of a hollow cylinder.

16. The method according to claim 11, wherein: a) the wall thickness or the layer thickness of the test objects is made of plastic.

17. The terahertz measuring device according to claim 1, wherein: a) the test object holder for arranging the test object is configured for arranging a test object made of plastic.

18. The terahertz measuring device according to claim 17, wherein: a) the test object holder for arranging the test object made of plastic is configured for arranging a test object including plastic pipes having a circular cross section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 a side view of a terahertz measuring device for measuring a test object according to a first embodiment example,

(2) FIG. 2 a top view of the terahertz measuring device of FIG. 1,

(3) FIG. 3 a side view of a terahertz measuring device for measuring a test object according to a second embodiment example,

(4) FIG. 4 a top view of the terahertz measuring device of FIG. 3,

(5) FIG. 5 a side view of a terahertz measuring device for measuring a test object according to a third embodiment example,

(6) FIG. 6 a first side view of a terahertz measuring device for measuring a test object according to a fourth embodiment example,

(7) FIG. 7 a second side view, rotated by 90, of the terahertz measuring device of FIG. 6,

(8) FIG. 8 a top view of a terahertz measuring device for measuring a test object with two mirror arrays and two associated transmitter-receiver units according to a fifth embodiment example, and

(9) FIG. 9 a chronological sequence of a radiation emitted as a THz pulse.

DETAILED DESCRIPTION OF TEE INVENTION

(10) A first embodiment example of the invention is described below be means of the FIGS. 1 and 2. For measuring a test object 2 a measuring device 1 comprises a transmitter-receiver unit 3, an associated mirror array 4, a test object holder 5 and a control unit 6.

(11) The mirror array 4 includes a first mirror 7 which is configured and disposed symmetrically in relation to a plane x-y E.sub.xy. Said plane x-y E.sub.xy is defined by a direction x and a direction y extending perpendicular thereto. The first mirror 7 is curved elliptically in the plane x-y E.sub.xy and in parallel thereto. Thus the first mirror 7 forms, in the plane x-y E.sub.xy, a mirror surface S.sub.1 in the shape of an ellipse. Owing to the elliptical curvature the first mirror 7 has two focal points B.sub.1 and B.sub.2 in the plane x-y E.sub.xy. Said focal points B.sub.1 and B.sub.2 each have a distance e from the center point M of the ellipse in the direction x.

(12) The ellipse or the elliptical shape respectively of the first mirror 7 is defined by a first semiaxis A with an associated length a and a second semiaxis B with a length b shorter than that of the first semiaxis A. For the relationship of the lengths a/b the following applies: a/b1.3, in particular a/b1.2, and in particular a/b1.1.

(13) The mirror array 4 further includes further a second mirror 8 located in the area of the first focal point B.sub.1. The second mirror 8 is configured planar, i.e. it has a planar mirror surface S.sub.2. The second mirror 8 encloses with the plane x-y E.sub.xy an angle , whereby the following applies to depending on the disposition of the transmitter-receiver unit 3: 3060, in particular 3555, and in particular 4050. Preferably, the angle =45. Preferably, the second mirror 8 is disposed in such a way the first focal point B.sub.1 lies essentially centrally on the mirror surface S.sub.2.

(14) The transmitter-receiver unit 3 comprises a transmitter 9 for emitting radiation S. Hereafter, the radiation emitted, from the transmitter 9 up to the test object 2, is referred to as S. Hereafter, the radiation reflected from the test object 2, from the test object 2 up to a receiver 10, is referred to as R. The receiver 10 serves to detect the radiation R reflected on the test object 2. For measuring the test object 2 the detected radiation R is evaluated by way of the control unit 6.

(15) Owing to the elliptical curvature of the first mirror 7 the radiation S, R is deflected between the focal points B.sub.1 and B.sub.2. In contrast thereto, the second mirror 8 serves to deflect the radiation S, R between a direction z, running transversely or perpendicularly respectively to the plane x-y E.sub.xy, and the plane x-y E.sub.xy. The direction z-direction extends perpendicularly to the direction x and the direction y so that the directions x, y and z form a Cartesian coordinate system.

(16) The transmitter-receiver unit 3 is disposed in the direction z spaced apart from the plane x-y E.sub.xy. The transmitter-receiver unit 3 is disposed along a first axis z Z.sub.1 running parallel to the direction z through the first focal point B.sub.1.

(17) The transmitter-receiver unit 3, the mirror array 4, the test object holder 5 and the control unit 6 are affixed to a base frame 11 of the measuring device 1. The test object holder 5 is configured such that the test object 2 is rotatable about a second axis z Z.sub.2. The second axis z Z.sub.2 extends parallel to the direction z through the second focal point B.sub.2. To that end the test object holder 5 is provided with, for example, two holding receptacles 12, 13 which are disposed at both sides of the plane x-y E.sub.xy and concentrically to the second axis z Z.sub.2. The holding receptacles 12, 13 can be rotated in relation to the base frame 11 about the second axis z Z.sub.2. To that end the test object holder 5 is provided with a first electric drive unit 14 which rotary drives the holding receptacles 12, 13 synchronously. The test object holder 5 can be linearly displaced in the direction z by means of a second electric drive unit 15.

(18) The test object 2 is configured in the shape of a hollow cylinder and exhibits a circular or, respectively, ring-shaped cross-section. The test object 2 is disposed or arranged by use of the test object holder 5 such that a central axis L is congruent with the second axis z Z.sub.2. The test object 2 is configured in two layers and comprises two layers of material K.sub.1 and K.sub.2 in the shapes of hollow cylinders. The test object 2 is made of plastics whereby, in particular, the two material layers K.sub.1 and K.sub.2 are made from differing plastic materials. For measuring the test object 2 the transmitter-receiver unit 3 is configured in such a way that the electromagnetic radiation S, R can be emitted or detected respectively at a frequency in the range between 0.01 THz and 50 THz, in particular from 0.05 THz to 20 THz, and in particular from 0.1 THz to 10 THz. Preferably, the radiation S is emitted in pulse, i.e. THz pulses are generated.

(19) For focusing the radiation S, R in the direction z the first mirror 7 is curved concavely in the direction z. As illustrated in FIG. 1, the mirror surface S.sub.1 of the first mirror 7 exhibits a concave curvature in the direction z.

(20) The measuring device 1 operates as follows:

(21) The transmitter 9 emits radiation S in the form of THz pulses. It is known as such how to generate THz pulses. For example, THz pulses are generated optically by use of femtosecond laser pulses and photoconductive switches. The radiation S is emitted essentially in the direction z and focused onto the first focal point B.sub.1.

(22) By use of the second mirror 8 the radiation S is deflected from the direction z into the plane x-y E.sub.xy and hits the mirror surface S.sub.1 of the first mirror 7. Owing to the elliptical curvature the radiation S coming from the direction of the first focal point B.sub.1 is reflected on the mirror surface S.sub.1 in the direction of the second focal point B.sub.2. Because the text object 2 is located in the beam path between the mirror surface S.sub.1 and the second focal point B.sub.2, the radiation hits the test object 2 radially and is reflected on the various boundary layers of the test object 2. The individual boundary layers are the outer surface and the inner surface of the test object 2 as well as the boundary layers of the material layers K.sub.1 and K.sub.2 lying between them. Thus, the THz pulses are irradiated by the transmitter-receiver unit 3 radially onto the test object 2 or the pipe 2 respectively.

(23) The radiation R reflected or, respectively, the THz pulses reflected run back along the same beam path to the transmitter-receiver unit 3 where they are detected by the receiver 10. The construction of the receiver 10 is known as such. THz pulses are detected, for example, by use of optical scanning (sampling) using femtosecond laser pulses.

(24) When measuring the test object 2, in particular, a wall thickness d.sub.W of the test object 2 as well as layer thicknesses d.sub.1 and d.sub.2 of the material layers K.sub.1 and K.sub.2 are determined. The measurement of the wall thickness d.sub.W as well as of the layer thicknesses d.sub.1 and d.sub.2 are based on the measurement of delay times of the THz pulses reflected on the individual boundary layers. The delay times are evaluated and the thicknesses d.sub.W, d.sub.1 and d.sub.2 are determined by the control unit 6.

(25) In the FIGS. 1 and 2 an ideal beam is shown in a continuous line. As an ideal beam the radiation S hits the text object 2 in a punctiform manner so that the text object 2 is measured in one measuring point. By rotating the text object 2, using the test object holder 5, about the second z axis Z.sub.2 forming an axis of rotation, the test object 2 can be measured completely along a circumferential line. To that end the holder receptacles 12, 13 are rotated about the second axis z Z.sub.2 using the drive unit 14. Moreover, by use of the drive unit 15 the test object holder 5 can be linearly displaced along the direction z so that the test object 2 is measured completely also along its length.

(26) Further, in the FIGS. 1 and 2 a real beam proliferation of the radiation S, R is shown in dotted lines. The radiation S emitted is focused by use of the transmitter-receiver unit 3 firstly into the first focal point B.sub.1. To that end the transmitter-receiver unit 3 comprises e.g. a lens. The radiation S reflected on the second mirror 8 exhibits a beam divergence. The radiation S, R has, on the one hand, a divergence angle .sub.R in the plane x-y E.sub.xy and a divergence angle .sub.Z in the direction z. Due to the elliptic curvature of the first mirror 7 in the plane x-y E.sub.xy the radiation S, R is focused onto the respective focal point B.sub.1 and B.sub.2. In the case of a real beam proliferation the radiation S hits the test object 2 not in a punctiform manner but in a measuring area whereby the individual beams or beam parts respectively each hit the test object 2 radially. The size of the measuring area depends on the divergence angle .sub.R and a radius r of the test object 2.

(27) The second mirror 8 is disposed in such a way that the radiation S ideally is reflected to the first mirror 7 at an angle .sub.R to the direction x whereby the radiation S, due to the real beam proliferation, exhibits a divergence angle .sub.R. Due to the fixed arrangement of the second mirror 8 the following applies to the angle .sub.R: .sub.R=90 . In the case of ideal beam proliferation the radiation S hits the test object 2 at an angle .sub.L, relative to the direction whereby the radiation S exhibits a divergence angle .sub.L, when hitting the test object 2. The following applies to the divergence angle or, respectively, opening angle .sub.L:

(28) $\begin{array}{cc}{}_L\left({}_R,{}_R,a,b\right)=\mathrm{arc}\cos \left(\frac{2a^2-b^2-\frac{{\mathrm{ab}}^2}{a+\sqrt{a^2-b^2}\cos \left({}_R+{}_R/2\right)}}{\sqrt{a^2-b^2}\left(2a-\frac{b^2}{a+\sqrt{a^2-b^2}\cos \left({}_R+{}_R/2\right)}\right)}\right)-\mathrm{arc}\cos \left(\frac{2a^2-b^2-\frac{{\mathrm{ab}}^2}{a+\sqrt{a^2-b^2}\cos \left({}_R-{}_R/2\right)}}{\sqrt{a^2-b^2}\left(2a-\frac{b^2}{a+\sqrt{a^2-b^2}\cos \left({}_R-{}_R/2\right)}\right)}\right)& \left(1\right)\end{array}$

(29) For a size d of the measuring area in the plane x-E.sub.xy the following applies by approximation:
d(.sub.L,r)=.sub.L.Math.r (2)

(30) Thus, the size d of the measuring area under a specific angle .sub.L is directly proportional to the radius r of the test object 2. In order to attain a measuring area as small as possible .sub.L must be as small as possible. The opening angle .sub.L depends on the angle .sub.R. The smaller .sub.R the smaller the angle .sub.L.

(31) Moreover, the angle .sub.L depends on the lengths a and b of the semiaxes A and B.

(32) Thus, the test object 2 can be completely measured using one single transmitter-receiver unit 3 as well as the associated mirror array 4. Despite the diverging beam dispersion the radiation S or the respective THz pulses hit the boundary layers of the test object 2 radially, i.e. perpendicularly and, moreover, at identical points in time. This guarantees that the reflected radiation R exhibits a high signal quality whereby, in particular, the reflected THz pulses are not washed out and attenuated in their amplitude.

(33) Owing to the fact that the mirror surface S.sub.1 is elliptically curved also in the direction z the radiation S, R is focused also in the direction z. The first mirror 7 is elliptically curved in the direction z preferably in such a way that a first focal point coincides with the first focal point B.sub.1 and a second focal point lies on the outer surface of the test object 2. This is illustrated in FIG. 1. Thus, the radiation S exhibits an optimized, i.e. as small as possible, measuring area also in the direction z.

(34) In the following a second embodiment example of the invention is described by way of FIGS. 3 and 4. In contrast to the first embodiment example the second mirror 8 is pivotable about the first z axis Z.sub.1, in particular, rotatable by 360, by use of a third drive unit 16. Thus, the angle .sub.R is changeable by such rotation. Hereby, the test object 2, even when fixed about the second z axis Z.sub.2, can be measured across a wide circumferential area. FIG. 4 shows additional, in relation to FIG. 2, beam paths of the radiation S illustrating a measuring of the test object 2. Independent of the angle .sub.R the radiation S is deflected by means of the first mirror 7 between the focal points B.sub.1 and B.sub.2. Upon measuring of the test object 2 the radiation S always hits the test object 2 always radially or perpendicularly, independent of the angle .sub.R.

(35) At an angle .sub.R,max the radiation S running towards the test object 2 is tangent to the test object 2 in such a way that the test object 2 at angles larger than .sub.R,max is shaded on a side facing away from the second mirror 8. The shaded area of the test object 2 is shown as a dotted line in FIG. 4 and designated as D. For the angle .sub.R,max the following is true:

(36) $\begin{array}{cc}{}_{R,\max }=-\mathrm{arc}\sin \left(\frac{r}{2e}\right)& \left(3\right)\end{array}$

(37) At an angle .sub.R,max the radiation S hits the test object 2 at an angle .sub.L,max so that the maximum measurable angular range of the test object 2, if this is fixed about the second z axis Z.sub.2, equals 2.sub.L,max.

(38) The shaded area D is measured in that the test object 2 is pivoted, corresponding to the first embodiment example, about the second z axis Z.sub.2. In contrast to the first embodiment example, however, the test object 2 does not have to be rotatable by 360 about the second z axis Z.sub.2, rather, it requires pivoting such that the shaded area D in a pivoted position lies within the measurable angular range 2.sub.L,max.

(39) According to equation (1) the angle of divergence .sub.L depends on the angle .sub.R. The alteration of .sub.L as a function of the angle .sub.R, however, is the smaller the closer the lengths a of the semimajor A and b of the semiminor B are together. Thus, the measuring accuracy of the measuring device 1 is the less dependent on the angle .sub.R the closer the relationship a/b is to 1.

(40) A further difference in comparison to the first is that the first mirror 7 is parabolically curved in the direction z, whereby the first focal point B.sub.1 coincides with the focal point of the parable at all angles .sub.R. By reflection of the radiation S the radiation S is collimated in the direction z. This is illustrated in FIG. 3. By virtue of the parabolic curvature of the first mirror 7 the radiation S is collimated in the direction z so that all entire beams or, respectively, partial beams independently of the radius r of the test object 2 always hit the test object 2 perpendicularly. The spread of the measuring area in the direction z is the diameter of the collimated radiation S which is defined by or may be adjusted respectively by the angle of divergence .sub.z.

(41) As regards the construction otherwise and the operation otherwise reference is made to the first embodiment example.

(42) In the following a third embodiment example of the invention is described by way of FIG. 5. In contrast to the afore-mentioned embodiment examples the first mirror 7 is spherically curved along the direction z. Thus, the first mirror 7 is an equipotential ellipsoid forming upon rotation about an axis y. Thus, the radiation S is focused in the plane x-y E.sub.xy and along the direction z into the second focal point B.sub.2. This is advantageous, in particular, when the radius r of the test object 2 is substantially smaller than the lengths a or b respectively. As regards the construction otherwise and the operation otherwise reference is made to the above embodiment examples.

(43) In the following a fourth embodiment example of the invention is described by way of FIGS. 6 and 7. In contrast to the afore-mentioned embodiment examples the first mirror 7 is designed to be planar in the direction z. For focusing the radiation S two focusing elements 17, 18 are disposed in the beam path between the transmitter-receiver unit 3 and the second mirror 8. The focusing elements 17, 18 rotate, together with the second mirror 8, about the first z axis Z.sub.1. The incident collimated radiation S is focused by way of the first focusing element 17 in the direction x onto the first focal point B.sub.1. To that end the first focusing element 17 is designed as a cylindrical lens. The second focusing element 18 focuses the radiation in the direction y whereby, after being reflected on the second mirror 8, the radiation S is focused in the direction z. This is illustrated in FIG. 6. The second focusing element 18 is also designed as a cylindrical lens, the cylinder of which is aligned in the direction x and runs perpendicular to the cylinder of the lens 17 which is aligned in the direction y. The spacing of the lens 18 up to the surface of the test object 2 corresponds to the der focal length of the lens 18. Thus, the radiation S only diverges in the plane x-y E.sub.xy. Focusing of the radiation S in the direction z onto test objects of differing radii r happens by changing the position of the second lens 18 along the z axis Z.sub.1. The means that the focusing in the direction z can be adjusted to test objects of differing radii r by way of changing the position of the second lens 18 along the z axis Z.sub.1. Alternatively, the second mirror 8 and the second focusing element 18 may be designed in an integrated manner so that the second mirror 8 itself causes focusing in the direction z by virtue of a curvature. As regards the construction otherwise and the operation otherwise reference is made to the above embodiment examples.

(44) In the following a fifth embodiment example of the invention is described by way of FIG. 8. In contrast to the afore-mentioned embodiment examples the measuring device 1 comprises exactly two mirror arrays 4 and two associated transmitter-receiver units 3. In the following, for purposes of differentiation the second mirror array 4 as well as the die associated components are designed by an . The mirror arrays 4 and 4 are designed identical, however, spaced apart from each other along the direction z and rotated in relation to each other such that the second focal points B.sub.2 and B.sub.2 lie spaced apart from each on the second axis z Z.sub.2 and the first focal points B.sub.1 and B.sub.1 in a plane x-y E.sub.xz running through the second focal points B.sub.2 and B.sub.2. Thus, the focal points B.sub.1 and B.sub.1 have a maximum distance from a plane y-z E.sub.yz running through the second focal points B.sub.2 and B.sub.2. The second mirrors 8, 8 are rotatable about their associated first z axes Z.sub.1 and Z.sub.1 running through the respective first focal point B.sub.1, B.sub.1. The test object 2 is measured inline within the manufacturing process and thus cannot be rotated about its own central longitudinal axis L, i.e. the second z axis Z.sub.2 or Z.sub.2 respectively. As explained already in the context of the second embodiment example, the test object 2, when measured by the respective mirror array 4, 4 and the associated transmitter-receiver unit 3, 3, exhibits a shaded area D, D. By virtue of the positioning of the mirror arrays 4, 4, however, the mirror array 4 can measure the shaded area D of the mirror array 4 and, correspondingly, the mirror array 4 the shaded are D of the mirror array 4. Thus, the test object 2 can be measured entirely, although it is not pivotable or rotatable. Owing to the manufacturing process, the test object 2 exhibits an extrusion direction running in the direction z so that the test object 2 can also be or will be respectively measured along its length. There is no need for the test object holder 5 to either actively pivot or to linearly displace the test object 2 and, therefore, the test object holder may be simplified in construction such that merely a guide for the test object 2 is guaranteed.

(45) Thus, one area of application of the measuring device 1 is the inline full examination of the wall thickness d.sub.W and the layer thicknesses d.sub.1 and d.sub.2 of the test object 2 being a plastic tube in the extrusion process. Measuring the test object 2 happens in accordance with the above embodiment examples contact-less and without any coupling medium. Owing to the fact that merely two transmitter-receiver units 3, 3 are required for the full or fully circumferential measuring of the test object 2, the construction remains comparatively simple thereby guaranteeing an acceptable cost-value ratio of the measuring device 1. Of course, it is possible to provide more than two mirror arrays 4 and corresponding transmitter-receiver units 3 for the measuring device 1 if they should be required. Moreover, the transmitter-receiver units 3, 3 are mounted in a fixed position relative to the associated first mirrors 7, 7 also allowing for a simple construction.

(46) As regards the construction otherwise and the operation otherwise reference is made to the above embodiment examples.

(47) The features of the described embodiment examples may be combined with each other at well. In particular, the focusing or collimation of the radiation S in the direction z may happen as desired and combined at will with other features of the measuring device 1. In addition, the mirror surface S.sub.1 of the respective first mirror 7 or 7 in the direction z may be optimized in such a manner that for a pre-defined radius area of the test object 2 a measuring are or measuring point respectively with an acceptable focus dimension is attained. To that end the mirror surface S.sub.1 may be designed as a free-form surface in the direction z.

(48) The preferred area of application of the measuring device 1 according to the invention is the full examination or inline full examination respectively of wall and/or layer thicknesses of plastic tubes, in particular, in the manufacturing or extrusion process respectively.

(49) The emitted radiation is provided in particular, in the form of pulsed THz radiation, of CW THz radiation (CW: continuous wave) and/or of FMCW THz radiation (FMCW: frequency modulated continuous wave). A temporal sequence of a pulsed THz radiation is shown in FIG. 9. The successive THz pulses T.sub.1 and T.sub.2 as well as corresponding further THz pulses each exhibit a frequency spectrum within the afore-mentioned THz range. By way of the der THz radiation the measurements happen contact-free and without coupling medium.

(50) Moreover, further evaluations or measurements can be carried out by way of measuring device 1 according to the invention. For example, the position of the central longitudinal axis L of the test object 2 relative to the second focal point B.sub.2 may be determined. Owing to the elliptical curvature of the first mirror 7 or 7 respectively the distance of a beam path migrating from one focal point, by way of reflection on the mirror surface S.sub.1 to the other focal point is always constant. Correspondingly, in the case of a test object 2 disposed concentrically to the second focal point B.sub.2 al beam paths exhibit the exact same distance. Thus, the transit time of the reflected radiation R or THz pulse respectively does not change, and the detected temporal position stay constant when scanning the test object 2. In the event that the central longitudinal axis L and the second focal point B.sub.2 do not coincide, the temporal position of the THz pulses changes upon scanning the test object 2. The THz pulses that incline along the lines defined by the central longitudinal axis L and the second focal point B.sub.2 will exhibit the maximum pulse shift. Therefore, the direction of the shift as well as the dimension of the shift results from the maximum transit time difference and may be determined on. The position of the central longitudinal axis L relative to the second focal point B.sub.2 may thereby be determined. Such information may be used, for example, for the automatic adjustment of the test object 2 upon starting up the extrusion process or for a later adjustment of the measuring device 1 which may become necessary. A reference measurement is required.

(51) Moreover, the transit time of the THz pulses can be utilized to determine the diameter or radius r of the test object 2 as well as possible deviations from the circular shape such as eccentricity or ovality. The diameter of the test object 2 follows, with know parameters of the elliptical mirror 7 or 7 respectively, directly from the transit time of the respective THz pulse. Form parameters, such as eccentricity or ovality, may be calculated from the deviations of the transit time of individual THz pulses.

(52) The measuring device 1 may also be operated by way of electromagnetic waves or electromagnetic radiation respectively in other Frequency bands that the afore-mentioned THz frequency band or by way of other types of waves, for example, by way of ultrasound waves. For example, a test object 2 can be measured by way of visible or infrared radiation. Prerequisite for the applicability of the method or the operability of the measuring device 1 is the waves or particles respectively used disperse radially and that the material of the mirrors and the test object 2 reflects the waves or particles respectively.

(53) While this invention has been described as having a preferred design, it is understood that it is capable of further modifications, and uses and/or adaptations of the invention and following in general the principle of the invention and including such departures from the present disclosure as come within the known or customary practice in the art to which the invention pertains, and as may be applied to the central features hereinbefore set forth, and fall within the scope of the invention.