An apparatus (100a; 100b; 100c; 100d; 100e; 100f; 100i; 100j) for measuring a property of a transparent tubular object having a refractive index, n, and a longitudinal axis, the apparatus comprising: a laser beam scanner configured for scanning a laser beam in a scanning direction with a constant scanning speed, v s, along a scanning path at right angles to the longitudinal axis of the object, wherein the scanning laser beam is directed in a first laser beam direction to the object at right angles to the scanning direction and at right angles to the longitudinal axis of the object; a detector assembly configured for detecting an intensity of laser light originating from the object in a first detection direction parallel to the first laser beam direction, and for detecting an intensity of laser light originating from the object in a second detection direction at a detection angle, α, greater than zero to the first detection direction; and a processing system configured for: - determining a first time, τ, and a second time, τ (2 R D), from the intensity of the laser light detected in the first detection direction by the detector assembly, τ being representative of the laser beam being tangential to the outer surface of the object at a first point of the object, and τ (2 R D) being representative of the laser beam being tangential to the outer surface of the object at a second point of the object diametrically opposite the first point; - determining a third time, τ ( x D2), and a fourth time, τ ( x d2), from the intensity of the laser light detected in the second detection direction by the detector assembly, τ ( x D2) being representative of the laser beam being reflected at the outer surface of the object at a third point of the object, and τ ( x d2) being representative of the laser beam being refracted at the outer surface of the object and reflected at the inner surface of the object at a fourth point of the object; and - calculating an inner radius, R d, of the object based on n, v s, α, τ, τ (2 R D), τ ( x D2), and τ ( x d2), wherein calculating R d comprises calculating a difference between τ (2 R D) and τ, and a difference between τ ( x d2) and τ ( x D2). a laser beam scanner configured for scanning a laser beam in a scanning direction with a constant scanning speed, v s, along a scanning path at right angles to the longitudinal axis of the object, wherein the scanning laser beam is directed in a first laser beam direction to the object at right angles to the scanning direction and at right angles to the longitudinal axis of the object; a detector assembly configured for detecting an intensity of laser light originating from the object in a first detection direction parallel to the first laser beam direction, and for detecting an intensity of laser light originating from the object in a second detection direction at a detection angle, α, greater than zero to the first detection direction; and a processing system configured for: - determining a first time, τ, and a second time, τ (2 R D), from the intensity of the laser light detected in the first detection direction by the detector assembly, τ being representative of the laser beam being tangential to the outer surface of the object at a first point of the object, and τ (2 R D) being representative of the laser beam being tangential to the outer surface of the object at a second point of the object diametrically opposite the first point; - determining a third time, τ ( x D2), and a fourth time, τ ( x d2), from the intensity of the laser light detected in the second detection direction by the detector assembly, τ ( x D2) being representative of the laser beam being reflected at the outer surface of the object at a third point of the object, and τ ( x d2) being representative of the laser beam being refracted at the outer surface of the object and reflected at the inner surface of the object at a fourth point of the object; and - calculating an inner radius, R d, of the object based on n, v s, α, τ, τ (2 R D), τ ( x D2), and τ ( x d2), wherein calculating R d comprises calculating a difference between τ (2 R D) and τ, and a difference between τ ( x d2) and τ ( x D2). - determining a first time, τ, and a second time, τ (2 R D), from the intensity of the laser light detected in the first detection direction by the detector assembly, τ being representative of the laser beam being tangential to the outer surface of the object at a first point of the object, and τ (2 R D) being representative of the laser beam being tangential to the outer surface of the object at a second point of the object diametrically opposite the first point; - determining a third time, τ ( x D2), and a fourth time, τ ( x d2), from the intensity of the laser light detected in the second detection direction by the detector assembly, τ ( x D2) being representative of the laser beam being reflected at the outer surface of the object at a third point of the object, and τ ( x d2) being representative of the laser beam being refracted at the outer surface of the object and reflected at the inner surface of the object at a fourth point of the object; and - calculating an inner radius, R d, of the object based on n, v s, α, τ, τ (2 R D), τ ( x D2), and τ ( x d2), wherein calculating R d comprises calculating a difference between τ (2 R D) and τ, and a difference between τ ( x d2) and τ ( x D2).