Abstract: Apparatus for detecting irregularities in the diameter of a filament has a small light source placed at the focus of an objective lens system. A horizontal slot constitutes a diaphragm for the objective lens system. A first optical system forms an image of the slot in a plane in which the filament runs past. Detectors detect the beam intercepted by the filament. An optical splitting system (P1) is interposed in the path of the beam (A), downstream of the slot (3) and upstream of the filament (5). The system is designed to form two sharp images (3', 3") of the slot (3) in a plane (4). The detectors are two oppositely-connected detectors (D1, D2) each receiving a respective beam (C, C') intercepted by the filament (5).