BSA Flow Software Installation and User's Guide - CSI

7.1.4 Principles of LDA
Laser beam
• High spatial and temporal resolution.
The optics of the laser anemometer is able to define a very small
measuring volume and thus provides good spatial resolution and allows
local measurement of Eulerian velocity. The small measuring volume in
combination with fast signal processing electronics also permits high
bandwidth, time-resolved measurements of fluctuating velocities,
providing excellent temporal resolution. Usually the temporal resolution
is limited by the concentration of seeding rather than the measuring
equipment itself.
• Multi-component bi-directional measurements.
Combinations of laser anemometer systems with component separation
based on color, polarization or frequency shift allow one-, two- or threecomponent
LDA systems to be put together based on common optical
modules. Acousto-optical frequency shift allows measurement of
reversing flow velocities.
Together these properties of laser anemometers certainly constitute a very
attractive description of a measuring instrument. As is often the case
however, optimization of the performance of a system with respect to certain
parameters may influence other performance characteristics negatively. As a
matter of fact, some of the compromise decisions which have to be made
when selecting and setting up a laser anemometer system can be traced back
to the famous uncertainty principle of wave theory, which describes the
impossibility of attaining complete information of both spatial and temporal
location of a wave train simultaneously.
In the following we describe the principles of Laser Doppler measurements
and mention some of the consequences of the results of the theory on
practical measurements.
The special properties of the gas laser, making it so well suited for the
measurement of many mechanical properties, are the spatial and temporal
coherence. At all cross sections along the laser beam, the intensity has a
Gaussian distribution, and the width of the beam is usually defined by the
edge-intensity being 1/e 2 =13% of the core-intensity. At one point the cross
section attains its smallest value, and the laser beam is uniquely described by
the size and position of this so-called beam waist.
7-2 BSA Flow Software: Reference guide