Abstract: A method and a device for indicating the presence of an object in a measuring zone (E), and for objects having a measurable cross-sectional shape, also the extent of the object along a reference line (R). The method and device makes use of lines (A, B) of radiation emitters (11) and radiation receivers (12) located on opposite sides of the object, the receivers being fewer than the emitters. Created between the emitters and the receivers is a network of intersecting radiation paths in the measuring zone. An electronic unit (20) establishes which intersections (P.sub.1) of the radiation paths are screened-off by the object. The invention is useful for indicating the presence of objects with arbitrary cross-section when it is desired to determine a lower limit for the size an indicated object shall have, and to determine a diameter measurement, in particular in the saw mill industry.