Device for measuring the oscillation amplitude of a capillary of a wire bonder, wherein the capillary is positioned between a light source and a detector arrangement, and the oscillation amplitude is determined from blocking a beam with the capillary, wherein - the beam emitted by the light source (1; 101) is divided into a measurement beam (M) and a reference beam (R), wherein an edge of the capillary (K) in the oscillating state at least partially blocks the measurement beam (M), while the reference beam (R) experiences no blocking and - the detector arrangement comprises a measurement detector (8.1; 108.1) assigned to the measurement beam (M) and at least, one reference detector (8.2; 108.2, 108.3) assigned to the reference beam (R) and the oscillation amplitude of the capillary (K) is determined from the output signals (S M , S R ) of the measurement detector (8.1; 108.1) and of the reference detector (8.2; 08.2, 133.3), said signals being interconnected with each other, characterized in that - a first imaging optical unit is arranged downstream of the light source (1; 101) in a beam propagation direction (Z), by means of which first imaging optical unit the light source (1; 101) is imaged into an image plane (B), in which the capillary (K) oscillates, in the measurement beam (M) and in the reference beam (R) and - a second imaging optical unit is arranged between the image plane (B) and the detector arrangement, by means of which second imaging optical unit the measurement beam (M) and the reference beam (R) are focused into a detection plane (D) or into a stop plane (E) and, - neighbouring the detection plane (D) or in the stop plane (E), - an aperture (7.1; 107.) is arranged in the measurement beam (M), the measurement detector (8.1 108.1) being arranged downstream of said aperture in the beam propagation direction (Z) and - an aperture (7.2; 107.2) is arranged in the reference beam (R), the at least one reference detector (8.2; 108.2, 108,3) being arranged downstream of said aperture in the beam ion direction (Z). - the beam emitted by the light source (1; 101) is divided into a measurement beam (M) and a reference beam (R), wherein an edge of the capillary (K) in the oscillating state at least partially blocks the measurement beam (M), while the reference beam (R) experiences no blocking and - the detector arrangement comprises a measurement detector (8.1; 108.1) assigned to the measurement beam (M) and at least, one reference detector (8.2; 108.2, 108.3) assigned to the reference beam (R) and the oscillation amplitude of the capillary (K) is determined from the output signals (S M , S R ) of the measurement detector (8.1; 108.1) and of the reference detector (8.2; 08.2, 133.3), said signals being interconnected with each other, - a first imaging optical unit is arranged downstream of the light source (1; 101) in a beam propagation direction (Z), by means of which first imaging optical unit the light source (1; 101) is imaged into an image plane (B), in which the capillary (K) oscillates, in the measurement beam (M) and in the reference beam (R) and - a second imaging optical unit is arranged between the image plane (B) and the detector arrangement, by means of which second imaging optical unit the measurement beam (M) and the reference beam (R) are focused into a detection plane (D) or into a stop plane (E) and, - neighbouring the detection plane (D) or in the stop plane (E), - an aperture (7.1; 107.) is arranged in the measurement beam (M), the measurement detector (8.1 108.1) being arranged downstream of said aperture in the beam propagation direction (Z) and - an aperture (7.2; 107.2) is arranged in the reference beam (R), the at least one reference detector (8.2; 108.2, 108,3) being arranged downstream of said aperture in the beam ion direction (Z). <EOS>