The invention relates to a method for detecting structural defects of a wrapped
yarn comprising a core formed of parallel fibres, whereby the core is bound together by a fibre wrapping layer comprising wrapping fibres and wild fibres, wherein an image of the moving
yarn is captured by at least one optical
yarn sensor over at least a part of the yarn length, whereby the optical yarn sensor comprises at least one row of
radiation sensitive elements and at least one
radiation source, and the optical sensor is connected to an electronic device equipped with
software, whereby the captured yarn image is processed by a
digital image analysis method to obtain data about the yarn
surface structure, based on the obtained data, changes of the yarn
surface structure along the yarn length are evaluated, these changes of the yarn
surface structure are compared to preset criteria, and if these changes of the yarn surface structure exceed these preset criteria, the relevant position of the yarn is determined as the occurrence of a yarn structural defect. Furthermore, the invention relates to an apparatus, a
computer program and a computer readable medium for carrying out the method.