A method in an AVSD system that is configured to detect particles in a volume being monitored, the AVSD system comprising: at least one primary light source (1700,1800) configured to emit a beam of light across a volume being monitored; an image sensor having a field of view and arranged to capture images of a least part of the beam of light (1702, 1802); and means to analyse images captured by the image sensor to detect the presence of particles within the volume on the basis of light scattered from the beam and captured in the images; said method comprising: emitting the beam of light (1702, 1802) across the volume being monitored at a primary beam position to enable detection of particles within the volume based on the analysis of images captured by the image sensor on the basis of light scattered from the beam (1702, 1802) at the primary beam position and captured in the images; and determining an intrusion of an object into the field of view of the image sensor and obstructing the image sensor's view of part of the beam (1702, 1802) in the primary position, characterised in that the intrusion of the object is determined by: (a) scanning a beam of light (1702, 1802) across a part of the volume being monitored to at least temporarily illuminate a region within the volume that lies between a primary beam position and an image capture means comprising the image sensor; (b) analysing the output of the image sensor to determine whether an object has impinged upon the beam of light (1702, 1802) emitted in step (a), to identify an object that blocks a line of sight between the image capture means and the beam of light (1702, 1802) when it is in its primary beam position; wherein step (a) is performed using a primary light source of the AVSD system. at least one primary light source (1700,1800) configured to emit a beam of light across a volume being monitored; an image sensor having a field of view and arranged to capture images of a least part of the beam of light (1702, 1802); and means to analyse images captured by the image sensor to detect the presence of particles within the volume on the basis of light scattered from the beam and captured in the images; said method comprising: emitting the beam of light (1702, 1802) across the volume being monitored at a primary beam position to enable detection of particles within the volume based on the analysis of images captured by the image sensor on the basis of light scattered from the beam (1702, 1802) at the primary beam position and captured in the images; and determining an intrusion of an object into the field of view of the image sensor and obstructing the image sensor's view of part of the beam (1702, 1802) in the primary position, characterised in that the intrusion of the object is determined by: (a) scanning a beam of light (1702, 1802) across a part of the volume being monitored to at least temporarily illuminate a region within the volume that lies between a primary beam position and an image capture means comprising the image sensor; (b) analysing the output of the image sensor to determine whether an object has impinged upon the beam of light (1702, 1802) emitted in step (a), to identify an object that blocks a line of sight between the image capture means and the beam of light (1702, 1802) when it is in its primary beam position; wherein step (a) is performed using a primary light source of the AVSD system. emitting the beam of light (1702, 1802) across the volume being monitored at a primary beam position to enable detection of particles within the volume based on the analysis of images captured by the image sensor on the basis of light scattered from the beam (1702, 1802) at the primary beam position and captured in the images; and determining an intrusion of an object into the field of view of the image sensor and obstructing the image sensor's view of part of the beam (1702, 1802) in the primary position, characterised in that the intrusion of the object is determined by: (a) scanning a beam of light (1702, 1802) across a part of the volume being monitored to at least temporarily illuminate a region within the volume that lies between a primary beam position and an image capture means comprising the image sensor; (b) analysing the output of the image sensor to determine whether an object has impinged upon the beam of light (1702, 1802) emitted in step (a), to identify an object that blocks a line of sight between the image capture means and the beam of light (1702, 1802) when it is in its primary beam position; (a) scanning a beam of light (1702, 1802) across a part of the volume being monitored to at least temporarily illuminate a region within the volume that lies between a primary beam position and an image capture means comprising the image sensor; (b) analysing the output of the image sensor to determine whether an object has impinged upon the beam of light (1702, 1802) emitted in step (a), to identify an object that blocks a line of sight between the image capture means and the beam of light (1702, 1802) when it is in its primary beam position; wherein step (a) is performed using a primary light source of the AVSD system.