Focused Beam Techniques
are based on a focused light beam, typical a laser, with the focal point spinning on a circle parallel to the surface of a glass window. When the focal point passes a particle, the reflected and/or scattered light of the particle is detected. The focal point moves along the particle on circular segments, as displayed in FIG. 1. Sophisticated threshold algorithms are used to determine the start and endpoint of the chord, i.e. the edges of the particle. The chord length is calculated from the time interval and the track speed of the focal point. Focused beam techniques measure a chord length distribution, which corresponds to the size and shape information of the particles typically in a complicated way [J. Worlische, T. Hocker, M. Mazzoti: Restoration of PSD from chord length distribution data using the method of projections onto convex sets. Part. & Part. Sys. Char.: Wiley, Vol. 22, S. 81 ff.]. So often the chord length distribution is directly used as the finger print information of the size, shape and population status.
FIG. 1 Different chords measured on a constantly moving single spherical particle by focused beam techniques.
Instruments of this type are commercially available as robust finger probes with small probe diameters. They are used in on-line and preferably in in-line applications, monitoring the chord length distribution of suspensions and emulsions. Special flow conditions are used to reduce the sampling errors. Version with fixed focal distance (Focused Beam Reflectance Measurement (FBRM®)) and variable focal distance (3D ORM technology) are available. The latter improves this technique for high concentrations and widens the dynamic range, as the focal point moves horizontally and vertically with respect to the surface of the window.
