BSA Flow Software Installation and User's Guide - CSI

Choice of polarization
Choice of scattering angle
3.) Connect the electrical cables from the BSA P processor signal processor
channel U1 and U2 to the detector unit channels V1 and V2. Connect the
electrical cables from the BSA P processor signal processor channel V1 and
V2 to the detector unit channels U1 and U2. This is necessary because the
burst detector acts on channel U1 of the BSA P processor signal processor.
The planar PDA attributes its name not only to the fact that the detectors lie
in the same plane as the transmitting beams but also that the polarization lies
in this plane (perpendicular polarization). Very early calculations indicated
that visibility and the phase/diameter linearity was only acceptable for this
polarization. Since the DualPDA receiving unit uses a common polarization
filter for all channels, the polarization is chosen to be the same on all
transmitted beams, i.e. parallel for the U-channels (green) and perpendicular
for the V-channels (blue).
This also corresponds to the preferred polarization in the case of the CPDA,
i.e. the polarization at which a Brewster angle can be expected when
working for instance with droplets in air (see 7.3 Setting up a PDA system).
From section 7.3 Setting up a PDA system, we can expect scattering angles
in the range 25° < Φ < 75° to be acceptable for the conventional PDA in the
DualPDA. However further considerations must be given to the visibility
and the phase/diameter linearity of the planar PDA as well as to the overall
measurement size range. To investigate these influences, GLMT
computations are recommended, some of which will be illustrated below.
Prior to this however, some characteristic features of the DualPDA optical
arrangement will be discussed. The following examples are valid for the
standard DualPDA arrangement using either 160 mm or 400 mm lenses on
the transmitting and receiving probes. To begin, the phase/diameter
conversion factor for the CPDA and PPDA is computed using geometric
optics, using the aperture centroid as the scattering direction. The results for
the case of 160 mm focal length lenses is shown in Figure 7-52 an in Figure
7-53 for various refractive indexes and for scattering angles in the range 20°
to 70°. We can see that for scattering angle above 40-45deg, the V12 phase
factor is low and phase factor ratio increases. This means that the
measurement resolution will be very low for high scattering angles and
therefore it is not recommended to use such a configuration with DualPDA.
Note This figure indicates that scattering angles beyond 40° would severely limit the PPDA at
low relative refractive indexes. Values below 20° would lead to disturbances through
diffraction. Therefore the operating range of 20° < Φ < 40° is expected to be most useful.
BSA Flow Software:Reference guide 7-69