A measurement device, intended to be mounted at a tree harvester for the measurement of the diameters of tree trunks fed forwards in their longitudinal direction during processing in the harvester, comprising: a first and a second optical sensor (20a, 20b), each one of which is integrated into a housing provided with openings (41, 42; 33, 35) for the passing of emitted and reflected light, respectively, whereby each sensor includes a source (22a, 22b) of light arranged in the housing for the projection of a patch of light onto the tree trunk and an optical receiver (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 arrangeable on a frame, which is a component of the harvester, that the said housing and the sensors (20a, 20b) that are arranged in it are located on opposite sides of the tree trunk as it is fed forwards in the harvester, characterised in that each housing is provided with a subcompartment having the form of a slot and is limited between a removable protective cover or screen arranged on the housing and a wall of the housing located behind the removable protective cover or screen, wherein, the openings for the passing of emitted and reflected light for the optical sensors (20a, 20b) in the housing are arranged as two pairs of separate openings arranged at a distance in front of each other and being comprised of; a first pair of viewing ports (41, 42) arranged as openings in the wall of the housing and which openings are covered with a transparent optical protection glass, and a second pair of viewing ports (33, 35) which are arranged in the removable protective cover or screen. a first and a second optical sensor (20a, 20b), each one of which is integrated into a housing provided with openings (41, 42; 33, 35) for the passing of emitted and reflected light, respectively, whereby each sensor includes a source (22a, 22b) of light arranged in the housing for the projection of a patch of light onto the tree trunk and an optical receiver (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 arrangeable on a frame, which is a component of the harvester, that the said housing and the sensors (20a, 20b) that are arranged in it are located on opposite sides of the tree trunk as it is fed forwards in the harvester, characterised in that each housing is provided with a subcompartment having the form of a slot and is limited between a removable protective cover or screen arranged on the housing and a wall of the housing located behind the removable protective cover or screen, wherein, the openings for the passing of emitted and reflected light for the optical sensors (20a, 20b) in the housing are arranged as two pairs of separate openings arranged at a distance in front of each other and being comprised of; a first pair of viewing ports (41, 42) arranged as openings in the wall of the housing and which openings are covered with a transparent optical protection glass, and a second pair of viewing ports (33, 35) which are arranged in the removable protective cover or screen.