Abstract: Apparatus for preventing the deposition of dust on a surface, for example, the transparent surface of an optical instrument, is provided. The apparatus includes a tube and a hollow porous frustrum. The surface is placed inside the tube. The small opening of the frustrum is placed adjacent the surface, and the large opening of the frustrum is sealed against the inner wall of the tube. Compressed gas is flooded into the tube, passes through the frustrum and sets up a laminar gas flow away from the surface, thus preventing the deposition of dust on the surface.

Abstract: A method and apparatus are provided for detecting and removing foreign material which may be found in a stream of particulate matter, such as tobacco. The tobacco is allowed to fall in a cascade past an optical detector. The turbulence of the falling motion brings a large proportion of the particles in the cascade into the field of view of the detector. When foreign material is detected, a signal is generated to activate a fluid blast directed at the portion of the cascade in which the foreign material is located.