BSA Flow Software Installation and User's Guide - CSI

Measuring volume
proportional to particle velocity. If the beams are badly misaligned, you will
probably not be able to get results at all, but if the beams are just slightly
misaligned, the errors will be small, and you may not be aware of them.
Measurements take place in the intersection between the two incident laser
beams, and the measuring volume is defined as the volume within which the
modulation depth is higher than e -2 times the peak core value. Due to the
Gaussian intensity distribution in the beams the measuring volume is an
ellipsoid as indicated in Figure 7-5.
df
θ
δf
Figure 7-5 Measuring volume.
The size of the measuring volume can be calculated from the beam waist
diameter df of the focused laser beams and the angle θ between them:
d
x
d f
d f
= , d = d , d =
(7-9)
cos 2 2
y f z
( θ ) sin(
θ )
-where dx is the height, dy the width and dz the length of the measuring
volume.
Since beam intersection angles are usually small, dx and dy are often almost
equal, and are sometimes referred to as the diameter of the measuring
volume.
You should be aware that the measuring volume dimensions in (7-9) are
guidelines only. A very small seeding particle in the outskirts of the
measuring volume may not reflect sufficient light to be detected. On the
other hand a very large seeding particle may reflect so much light, that it is
detected even if technically it is slightly outside the measuring volume as
defined above.
Other parameters such as system gain and threshold level influence the
measurements similarly.
From the height dx of the measuring volume and the fringe spacing δf, the
total number of fringes can be calculated:
N
f
dxd λ 2 d
= = =
δ cos θ sin θ λ
f
( 2) 2 ( 2)
dz
f f
tan
( θ )
2 (7-10)
BSA Flow Software:Reference guide 7-7
dx