Patent ID: 8797531

Claim:
A beam detector system configured to detect the presence of particles in a monitored space, the beam detector including: a light source adapted to generate at least one light beam having components in at least a first wavelength band and a second wavelength band, wherein the light source is configured such that the at least one beam emitted by the light source having a first wavelength band has a spatial intensity light profile across the width of the beam that is different to the spatial intensity profile in the second wavelength band, and an image sensor having a field of view being arranged relative to the light source such that the at least one beam from the light source is received at the image sensor after the at least one beam traverses the monitored space, and to generate output signals representing a level of received light at each of the first and second wavelengths within a region or regions within the field of view of the image sensor that include the light source; a processor configured to receive the output signals from the image sensor and being configured to determine, at least partly on the basis of a relative reduction in the received light in the second wavelength band compared to the first wavelength band, from within the region or regions, that particles are impinging on the at least one beam; wherein the spatial intensity profiles for the components of the beam in the first wavelength band and the second wavelength band are such that, in the event the light source moves out of alignment with the sensor, the level of light from the region or regions that is received by the image sensor in the first wavelength band decreases before the level of light received by the image sensor in the second wavelength band, thereby causing a relative change in received light level that is distinguishable from said relative reduction in the received light intensity in the second wavelength band compared to the first wavelength band caused by particles in the monitored space impinging on the at least one beam.