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The transmission fluctuation method of Gregory J 1985 has been enhanced by Breitenstein M et al 1999 to allow the determination of both the particle number concentration and particle size. This enhancement relied on analyzing the dependence of the average transmission of a light beam with the diameter much smaller than that of the particle on the extent of averaging (in time or space) rather than just averaging the fluctuating transmission.
The enhanced technique, referred to as the transmission fluctuation spectrometry (TFS), has been later extended to the case of the combined temporal and spatial averaging of transmission of a uniform light beam by a dispersion (Breitenstein M et al 2001). These latter authors point out that by using a focused beam, the particle size and concentration can be measured locally, as particles closest to the focus contribute most significantly to transmission fluctuations.
A significant limitation of the TFS theory remained the assumption of the light beam having a uniform irradiance profile. Shen J and Riebel 2003a extended that theory to the case of a Gaussian light beam and also used low-pass signal filtering instead of averaging. These improvements permitted the authors to obtain spectra of the transmission fluctuations and retrieve PSDs of sample dispersions by using a prototype instrument (Shen J and Riebel 2003b). The results were compared with those obtained with a laser diffractometer. To improve the measurement resolution of the PSD, the 2nd order low-pass filter and a band-pass filter have been used in the signal processing (Xu Y et al 2008, 2006).
More recently, the TFS method has been modified by measuring a decay of correlation between transmission of two narrow co-linear beams as a function of the beams' separation instead of analyzing temporal fluctuations of a single beam with a variable diameter (Shen J et al 2005a). This modification allowed the authors to dispense with the need for measuring the dispersion flow velocity. However, it introduced technical problems related to maintaining the co-linearity and varying the beam positions synchronously vs. each other and the respective light detectors used for the transmission measurements. These problems were corrected by replacing the spatial correlation approach to two-beam transmission fluctuation measurements with a temporal correlation approach utilizing just a single beam (Shen J et al 2005b).
The TFS theory has recently been extended to the high-concentrated particle suspensions by theoretical study and numerical simulation (Yu B and Shen 2007, Riebel U and Shen 2004, Shen J and Riebel 2004) and validated by measurements (Yu B et al 2008, Shen J and Riebel 2005).
| CITATION: Shen J., Jonasz M. 2008. Transmission fluctuation spectrometry (TFS) (www.tpdsci.com/Tpc/TFS.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 18-Jan-2006 Modified: 07-Nov-2008 Peer-reviewed: 03-Jul-2006 |
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