The invention concerns a measurement device, intended to be mounted at a tree harvester (1) for the measurement of the diameters of tree trunks (3) fed forwards in their longitudinal direction (3) during trimming in the harvester. In order to achieve a reliable way of measuring a tree trunk that is fed forwards and processed in a harvester, the arrangement comprises a pair of optical sensors (20a, 20b), each one of which is integrated into a housing (30) with viewing ports (33, 35) for the passing of emitted and reflected light, respectively, that each sensor includes a source (22a, 22b) of light for the projection of a patch of light onto the tree trunk and an optical device (24a, 24b) for the imaging of the patch of light on a position-sensitive photodetector, a pair of brackets (23a, 23b) designed as supports for the sensors (20a, 20b), which brackets are so arranged at a frame (2) that is a component of the harvester (1) that the sensors (20a, 20b) are located on diametrically opposite sides of the tree trunk (3) that is fed forwards in the harvester in order to measure the diameter of this tree trunk.
