In vitro Biofluid Mechanics - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 362

documented with a digital camera that is focused at the region of interest. Based on the
wavelength of the incident beams and the angle between the two beams, fringe patterns
would be formed with a spacing of

λ
2sin 2

d 5 ð 14 : 1 Þ

where d is the fringe pattern spacing, is the

angle between the two laser beams ( Figure 14.2 ). As a particle moves through the laser
beam area of intersection, scattered light will have a different intensity and frequency than
the normal fringe patterns. This new frequency (

λ is the wavelength of the laser light, and θ

ω ) is defined by the velocity of the particle

and the fringe spacing as

2 v sin 2
λ

v
d 5

ω 5 ð 14 : 2 Þ

where v is the velocity of the particle in the direction perpendicular to the fringe patterns.
This frequency can be obtained by the Fourier transformation of the intensity signal
acquired by a camera focused at the region of interest. With a high frame rate camera,
multiple particle velocities can be obtained and correlated by measuring the distortion at
different subsequent fringes as the particle moves through the region of interest.
The only component of a laser Doppler system that may hinder the usage of this tech-
nique is the size of the particles within the fluid. Small air bubbles and dust (present
within the fluid) can be used to generate distortions within the fringe pattern, but they
might not be present in a large enough quantity to obtain meaningful data. By over-
seeding particles within the fluid, it is difficult for the computer program to determine
where the distortion is coming from because the particle distortions can be very large and

FIGURE 14.2 Schematic of a laser

Doppler system, with fringe patterns
that are distorted by particles within
the fluid. A digital camera (not
shown) would be focused at a region
that would contain the fringe patterns
so that as particles distort the fringes,
information can be obtained about the
particle and fluid velocity.

Fringe pattern
with spacing d

θ

v 1

v 2

Laser source 1 Laser source 2