PARTICLE DETECTING DEVICE

PURPOSE:To expand an area to be detected and to uniform the photon density of laser beam light by providing a projection optical system consisting of a lens system which projects sheet-shaped laser beam light on the area to be detected and a spatial filter in a specific shape. CONSTITUTION:Laser beam light l2 expanded by a beam expander 21 passes through the spatial filter 22 and is polarized in a sectional shape l3 to have a cone-shaped Gaussian distribution, and laser beam light l4 passed through a cylindrical lens 23 is sectioned in a fusiform shape at a focus distance (f) to have nearly uniform light distribution intensity. Further, the thickness W of laser beam light l4 on the plane of a focus (f) is lambdaf/(piW0), where W0 is the thickness of the laser beam l3; when the thickness of the laser beam light l3 incident on the cylindrical lens 23 is varied through the spatial filter 22, the thickness W of the laser beam light l4 on the plane of the focus (f) is controlled, so that the photon density in the area 16 to be detected becomes uniform.