A method of detecting particles comprising the steps of: emitting a beam of radiation into a monitored region; capturing images of the region using an image capture device; determining a region of interest containing a path of a beam of radiation comprising: locating a first point, being the position of a source (16, 55, 54, 58, 59, 68, 65, 66, 84, 106) of the beam, using the image capturing device; locating a second point being the intersection of the beam of radiation with a field of view of the image capturing device (14, 50, 60, 67, 64, 82, 102, 104); determining the path of the beam in accordance with the first and second point; and calculating a region of interest containing the determined beam path; defining a background region (101, 103) on each side of the region of interest; modulating the beam of radiation; capturing a plurality of images including "on" frames, being images when the modulated beam of radiation is emitted, and "off" frames, being images when the modulated beam of radiation is not emitted; performing background cancellation to reduce interfering temporally variant and invariant information from the images, wherein the step of performing background cancellation includes: determining a brightness compensation factor for scaling the "off" frames, scaling the "off" frames by the brightness compensation factor to compensate for variance in illumination levels of the images, and subtracting the scaled "off" frames from the "on" frames; and detecting a variation in images of the region indicating the presence of particles, the detected variation being an increase in scattered radiation intensity. emitting a beam of radiation into a monitored region; capturing images of the region using an image capture device; determining a region of interest containing a path of a beam of radiation comprising: locating a first point, being the position of a source (16, 55, 54, 58, 59, 68, 65, 66, 84, 106) of the beam, using the image capturing device; locating a second point being the intersection of the beam of radiation with a field of view of the image capturing device (14, 50, 60, 67, 64, 82, 102, 104); determining the path of the beam in accordance with the first and second point; and calculating a region of interest containing the determined beam path; defining a background region (101, 103) on each side of the region of interest; modulating the beam of radiation; locating a first point, being the position of a source (16, 55, 54, 58, 59, 68, 65, 66, 84, 106) of the beam, using the image capturing device; locating a second point being the intersection of the beam of radiation with a field of view of the image capturing device (14, 50, 60, 67, 64, 82, 102, 104); determining the path of the beam in accordance with the first and second point; and calculating a region of interest containing the determined beam path; defining a background region (101, 103) on each side of the region of interest; modulating the beam of radiation; capturing a plurality of images including "on" frames, being images when the modulated beam of radiation is emitted, and "off" frames, being images when the modulated beam of radiation is not emitted; performing background cancellation to reduce interfering temporally variant and invariant information from the images, wherein the step of performing background cancellation includes: determining a brightness compensation factor for scaling the "off" frames, scaling the "off" frames by the brightness compensation factor to compensate for variance in illumination levels of the images, and subtracting the scaled "off" frames from the "on" frames; and determining a brightness compensation factor for scaling the "off" frames, scaling the "off" frames by the brightness compensation factor to compensate for variance in illumination levels of the images, and subtracting the scaled "off" frames from the "on" frames; and detecting a variation in images of the region indicating the presence of particles, the detected variation being an increase in scattered radiation intensity.