(233c) High-Speed Image Analysis and Dispersion for Size and Shape Characterisation of Fibres

Image analysis is widely in use for the characterisation of shape and size of particles. A highly diluted particle flow is required for reliable particle size analysis, because overlapping particle images must be avoided. Separation of particles is often replaced by sophisticated image processing algorithms. Proper dry dispersion techniques, which usually accelerate the particles to high speeds (up to 100 m/s), are not applicable to standard imaging methods due to the motion blur of the particles. The limited amount of images leads to low particle counts and large statistical errors.

An innovative imaging sensor has been developed, which even allows for the direct use of well established dry and wet dispersers. It combines a new light source which provides an exposure time of less than 1 ns and an adaptable optical system for perfect illumination and imaging of the fast particles on a high speed camera, an integrated image pre-processor and a Gigabit digital transmission line to the computer. A first device has been exhibited on the ACHEMA 2003 and was presented on the Innovators forum on the PSA 2003, showing that particles can be clearly imaged and analysed at the output of a well established and proven dry dispersing injection system.

Meanwhile the device has proven its prospected performance in a plurality of applications. Up to 450 images with 1024 x 1024 square pixels of 10x10 µm² can be acquired, processed, compressed, transferred and stored in a powerful database per second. Different measuring ranges can be selected by software: 1 to 340 µm, 3.3 to 1,140 µm, 10 to 3,410 µm and 20 to 6,830 µm (according to ISO 13322-2) are currently implemented. Different dispersion units can be applied to an open measuring zone, e.g. a dry disperser with injection system for fine particles or a gravity disperser for coarser particles. A powerful selection and display facility of particles has been implemented in the software, which is fully compliant with CFR 21 Rule 11 and concurrently supports other particle sizing instruments using laser diffraction, ultrasonic extinction or photon-cross-correlation.

The combination of powerful dispersion and high frame rate allows for the acquisition and analysis of extreme numbers of even > 10⁷ randomly oriented particles in short times. This results in statistical errors far below 1%, showing that image analysis now even reaches the reproducibility of renowned laser diffraction with traceability to the individual particles and shape information in addition.

Now the field of applications has been extended to the analysis of fibres. The particlular possibilities of dry and wet dispersers for the dispersion of fibres are displayed. Different algorithms for the evaluation of the fibre length, their width and the curl index have been investigated on complex fibre structures. Examples are given, that today's algorithms are capable for precisely evaluation of fibres, even if they look like Chinese characters.