The dia. of small objects of round X-section, specifically textile fibres, is measured by analysing diffraction pattern (19) of an object generated by passing it across the path of a beam of monochromatic light, pref. a laser beam (11). Pref the diffraction pattern (19) is projected onto a screen (17), detected by an optoelectronic transducer (20) such as a vidicon camera, and analysed electronically pref. by digital computer. Specifically, the object is conveyed across the beam (11) in a flow of liq. (8) through a transparent conduit (10) of square or rectangular X-section, the liquid flow pref. comprising a stream constricted by an outer surrounding stream, itself surrounded by a further stream, for hydrodynamic focalisation of fibre snippets w.r.t. the beam (11) as they pass through it in rapid succession. Pref. pattern data is stored for analysis by computer.
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