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The quality and stability of materials or products containing natural fibres (textiles, fleece, fibre-reinforced materials) is mainly determined by the length of the employed natural fibres. To this day, the only possibility for determining the length distribution of natural fibres in a fibre bundle consisting of irregularly arranged fibres is to either manually sort and classify the single fibres or use a measuring instrument called Almeter. The device has not been produced for 20 years and is getting outdated with respect to hardware-interface and operating system compatibility. However, more than 500 of these devices are still in use all over the world. In this paper we present a novel method and device for optically determining the length distribution of fibres and fibre bundles with lengths greater than 5 cm. A fibre or fibre bundle consisting of fibres with varying lengths is put on a needle bed where a gripper pulls the fibres at their one end. During this draw-out process a laser line is projected onto the fibres and the reflected light is detected by a line sensor taking grey-scale pictures with a frequency of (currently) 8 kHz, but this can be easily increased by a factor of 3 or 4 with current line sensor technology. The image stack can be analysed, and the lengths of the fibres may be determined. To date, owing to the dimensions of the pre-existing apparatus that has been modified, the maximum length of fibres to be measured is 350 mm. The resolution of the optical device has been proven to be 20 microns. In addition to that, the distribution of the width of single fibres can be determined, as well. The measuring method may be easily extended to other measurement parameters, such as fibre colour. An additional advantageous feature of the novel measuring method is the possibility to adjust the resolving powers in two perpendicular directions to given desired values independently.
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