A portable particle analyser is compact and can determine the size, geometry and number of particles in a sample of fluid. A laser beam (15) intercepts the flow of fluid (17) at the first focal point of a parabolic mirror (11). Light is scattered and collected by radiation collectors (13) and low angle scattered radiation is detected in a second chamber (19) by reflection by an ellipsoid mirror (20) towards a radiation collector (21). Photomultiplier units (23) convert the radiation collected into electrical signals for analysis.