The invention relates to monitoring equipment for smoke and/or fine particle emissions from fossil fuel systems, of the type where a beam of visible light is passed across a flue from a light source to a detector, a difference in the light intensity received by the detector from the known intensity of the source indicating absorption of light by smoke and/or fine particles in the flue, and the degree of difference indicating the level of smoke and/or fine particles. The objective of the invention is to provide monitoring equipment of the above type, of simpler, lower cost, more robust nature than has hitherto been provided. The objective is met by a construction comprising two units (1, 2) for location of opposite sides of a flue or chimney each unit comprising a light source, (L.sub.1, L.sub.2) a lens (4) and a light detector (D.sub.1, D.sub.2), the light from the light source of each said unit being directed by its lens across the flue or chimney in a divergence path, to be gathered by the lens of the opposite unit and directed to the detector in the opposite unit, the outputs from the detectors of each unit being combined to provide a reading of the degree of smoke and/or fine particulate material existing in the flue or chimney.