The invention relates to a device for detecting foreign substances in a thread using a detector which line-scans the thread, the latter being lit by an illumination element. The aim of the invention is to provide a means of continuously testing the thread with good resolution and at a high speed. To this end, the inventive device consists of a compact unit made up of a sensor or a detector, an objective and an illumination element. These elements have a common axis and the illumination element enables the thread to be illuminated with a very high light intensity. To this end, the illumination element (1) is hemispherical and has light sources which are distributed across the hemisphere (5).