This International Standard specifies the procedure and the conditions of measurement for the determination of wool fibre diameter by means of the projection microscope. The method is suitable for wool fibres in any form and also for other fibres of reasonably circular cross-section.( In the case of dyed, bleached or finished fibres, it should be noted that the diameter may be different from that of fibres not subjected to such treatments. The estimates of fibre diameter obtained at the various stages of processing one lot of wool will not necessarily be the same.)