Signal acquisition techniques, laser

Local instantaneous liquid velocity measurements in bioreactors that can quantify turbulence statistics are challenging using conventional laser-based techniques because optical access is critical for effective signal acquisition. Laser Doppler anemometry (LDA) and PIV have been used to determine local liquid velocities within multiphase flows. Reviews of LDA and PIV with applications to multiphase flows have appeared in the literature (Boyer et al., 2002 Chaouki et al., 1997 Cheremisinoff, 1986). 

Laser microbeams offer several advantages over other fluorescence excitation techniques. In spectrofluorometry, observations are often made on a population of cells in a cuvette, resulting in a combined signal that lacks information about individual cellular responses. In flow cytometry, many individual cells are measured, but there is no temporal resolution since each cell is observed only once, and there is no spatial resolution since the entire cell is illuminated as it passes through the laser beam (see Chapter 30). In conventional fluorescence microscopy, individual cells can be monitored over time, and information about the two-dimensional spatial distribution of fluorescence can be obtained. However, some samples may be more susceptible to photobleaching by the arc lamps used for excitation, and the temporal resolution is limited to video-rate data acquisition (30 frames/s) (see Chapter 14). 

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