Measuring device for measuring test objects having - a transmitter-receiver unit (3; 3a) having - a transmitter for emitting terahertz radiation in an x-y plane (E xy) starting from a beam point and - an associated receiver for detecting terahertz radiation reflected at a test object (2; 2a), - a first collimator-focussing arrangement - having collimator-focussing elements (14, 15, 16, 17) for transforming the emitted terahertz radiation into collimated radiation and for focussing the reflected terahertz radiation onto the beam point, - wherein the collimator-focussing elements (14, 15, 16, 17) are arranged in such a way that two adjacent collimator-focussing elements (14, 15, 16, 17) enclose an angle (α KF) of 90° in the x-y plane (E xy), so that the collimator-focussing elements (14, 15, 16, 17) form a collimator-focussing square, - wherein a common focal point (B 1) of the collimator-focussing elements (14, 15, 16, 17) in the x-y plane (E xy) lies in the beam point, - a test object receiving area lying in the x-y plane (E xy) for positioning the test object (2; 2a), - at least one mirror arrangement (5; 39) - having mirror elements (19, 20, 21, 22; 41, 42, 43, 44) for deflecting the collimated terahertz radiation and the reflected terahertz radiation between the collimator-focussing arrangement and the test object receiving area, - wherein the mirror elements (19, 20, 21, 22; 41, 42, 43, 44) are arranged in such a way that in each case two mirror elements (19, 20, 21, 22; 41, 42, 43, 44) enclose an angle (α SE) of 90° in the x-y plane (E xy), so that the mirror elements (19, 20, 21, 22; 41, 42, 43, 44) form a mirror rectangle (23; 40), - wherein mirror surfaces (24, 25, 26, 27) of the mirror rectangle (23; 40) are parallel to one of the collimator-focussing diagonals (28, 29) of the collimator-focussing square, - wherein the collimator-focussing elements (14, 15, 16, 17) lie within the mirror rectangle (23; 40), and - a control unit for evaluating the detected terahertz radiation. - a transmitter-receiver unit (3; 3a) having - a transmitter for emitting terahertz radiation in an x-y plane (E xy) starting from a beam point and - an associated receiver for detecting terahertz radiation reflected at a test object (2; 2a), - a transmitter for emitting terahertz radiation in an x-y plane (E xy) starting from a beam point and - an associated receiver for detecting terahertz radiation reflected at a test object (2; 2a), - a first collimator-focussing arrangement - having collimator-focussing elements (14, 15, 16, 17) for transforming the emitted terahertz radiation into collimated radiation and for focussing the reflected terahertz radiation onto the beam point, - wherein the collimator-focussing elements (14, 15, 16, 17) are arranged in such a way that two adjacent collimator-focussing elements (14, 15, 16, 17) enclose an angle (α KF) of 90° in the x-y plane (E xy), so that the collimator-focussing elements (14, 15, 16, 17) form a collimator-focussing square, - wherein a common focal point (B 1) of the collimator-focussing elements (14, 15, 16, 17) in the x-y plane (E xy) lies in the beam point, - having collimator-focussing elements (14, 15, 16, 17) for transforming the emitted terahertz radiation into collimated radiation and for focussing the reflected terahertz radiation onto the beam point, - wherein the collimator-focussing elements (14, 15, 16, 17) are arranged in such a way that two adjacent collimator-focussing elements (14, 15, 16, 17) enclose an angle (α KF) of 90° in the x-y plane (E xy), so that the collimator-focussing elements (14, 15, 16, 17) form a collimator-focussing square, - wherein a common focal point (B 1) of the collimator-focussing elements (14, 15, 16, 17) in the x-y plane (E xy) lies in the beam point, - a test object receiving area lying in the x-y plane (E xy) for positioning the test object (2; 2a), - at least one mirror arrangement (5; 39) - having mirror elements (19, 20, 21, 22; 41, 42, 43, 44) for deflecting the collimated terahertz radiation and the reflected terahertz radiation between the collimator-focussing arrangement and the test object receiving area, - wherein the mirror elements (19, 20, 21, 22; 41, 42, 43, 44) are arranged in such a way that in each case two mirror elements (19, 20, 21, 22; 41, 42, 43, 44) enclose an angle (α SE) of 90° in the x-y plane (E xy), so that the mirror elements (19, 20, 21, 22; 41, 42, 43, 44) form a mirror rectangle (23; 40), - wherein mirror surfaces (24, 25, 26, 27) of the mirror rectangle (23; 40) are parallel to one of the collimator-focussing diagonals (28, 29) of the collimator-focussing square, - wherein the collimator-focussing elements (14, 15, 16, 17) lie within the mirror rectangle (23; 40), and - having mirror elements (19, 20, 21, 22; 41, 42, 43, 44) for deflecting the collimated terahertz radiation and the reflected terahertz radiation between the collimator-focussing arrangement and the test object receiving area, - wherein the mirror elements (19, 20, 21, 22; 41, 42, 43, 44) are arranged in such a way that in each case two mirror elements (19, 20, 21, 22; 41, 42, 43, 44) enclose an angle (α SE) of 90° in the x-y plane (E xy), so that the mirror elements (19, 20, 21, 22; 41, 42, 43, 44) form a mirror rectangle (23; 40), - wherein mirror surfaces (24, 25, 26, 27) of the mirror rectangle (23; 40) are parallel to one of the collimator-focussing diagonals (28, 29) of the collimator-focussing square, - wherein the collimator-focussing elements (14, 15, 16, 17) lie within the mirror rectangle (23; 40), and - a control unit for evaluating the detected terahertz radiation.