BSA Flow Software Installation and User's Guide - CSI

The DualPDA principle
Figure 7-27:Suppression of scattered light due to the slit aperture (slit effect).
For flux measurements a well-defined cross-section through which particles
are counted is required. In PDA the length dimension of this cross-section is
fixed by placing slit apertures in front of the photo detectors. The projection
of these slits onto the beam intersection volume is depicted in Figure 7-27 by
a lighter shade of grey.
For certain particle positions (trajectories) within the volume, certain
components of the scattered light can be suppressed because they fall outside
of the slit aperture. In particular, certain positions suppress refractively
scattered light, leaving only reflectively scattered light to be detected. As
with the trajectory effect, such signals will result in incorrect size
measurements, since refractively scattered light was presupposed in the
choice of β.
It is interesting to note that the slit effect occurs also for small particles, i.e.
small particles can appear as large particles, thus the potential error in the
mass flux is equally high as with the trajectory effect.
The DualPDA combines a conventional two-detector PDA and a planar twodetector
PDA in one single optical receiving probe, as shown in Figure 2-22,
and therefore requires at least a 2D transmitting optics.
Four receiving The 57X80 DualPDA Probe thus contains four receiving apertures. The
effective
apertures size and the shape of the apertures is defined by an interchangeable aperture
mask positioned directly in front of the segmented focussing lens. Together
with the optical configuration of the transmitting system and the focal length
of the DualPDA receiving lens, the mask determines the measurable size
range.
DualPDA concept The basic concept of the DualPDA is to make two independent size
measurements, using for one the conventional PDA and for the other the
planar PDA. Each system will yield the same result if only refractively
scattered light is received at the photodetector and the particle is spherical.
If however, reflectively scattered light dominates, due either to the slit effect
or the trajectory effect, each system will yield a different size and they will
no longer agree. This redundancy can therefore be used as a validation
criteria to avoid the slit effect and the trajectory effect.
7-32 BSA Flow Software: Reference guide