Light Sheet Generation
Light sheets can be setup in a variety of ways. The most common one is the ordinary setup with a cylindrical lens.
The drawback of these light sheets is that the gaussian profile of the laser beam is simply extended, resulting in a loss of resulting dynamics in the receiving optics.
In QLS measurements two ways of generating light sheets have shown their aptitude for different purposes.
Galvanometer Scanners are especially useful for time averaged measurements , when the intensity homogeneity is of major interest. By optimizing the command signal for the scanner motion a top hat profile can be achieved. By changing the signal amplitude the light sheet can be adjusted to the needs in the test rig.
Polygon Mirrors are more advantegous in time resolved measurements. The light sheet is produced by rotating a set of reflecting mirror surfaces with high speed. In QLS measurements mirrors with 12 and 20 facets have been used, yielding a maximum beam frequency of aproximately 16 kHz.
QLS is a particle based measurement technique. The appropriate choice of particles and a stable production are of great importance.
In QLS there it is not requirement that the particle distribution is monodisperse. Although, the distribution of particle diameters must not change in space nor time. This is the most important exigation coupled to the seeding.
At DLR Cologne a specific 'hybrid' particle generator has been develped, using an atomized oil that undergoes a heating inside a rod. At the outlet the vapour condesates to a regelbare number of particles in a diameter range between 1 and 2 microns.
QLS measurements are amplitude measurements. Each parameter changing the intensity of scattered light will have an influence on the measurement value.
This is why the suppression of background radiation is of great importance. The following viewgraph shows a typical QLS application and the location of surface reflections.
A large part of the surface reflections can be rejected by laser line filtering. This means that the test rig must be coated with a thin film of fluorescent colour shifting the reflected light to higher wavelengths. If the receiving optics is equiped with a laser line filter according to the applied laser line, then a good signal to noise ratio becomes possible.