Abstract: A device for indicating the fibre length distribution of a fibre sample for use with an apparatus which produces output signals representative of the number of fibres in a cross section of a fibre sample by slit-wise illumination of the sample, has a pulse generator to produce a pulse stream representative of the relative positions of the sample and illuminating beam, comparators to compare signals received by opto-electric converter with reference signals to stop counters driven by the pulse stream when predetermined values of the received signals are reached and an indicator for the contents of the counters. The reference signals are derived from the received light signals at predetermined relative positions of the sample and beams.