The significance of coherent flow structures for the turbulent mixing ...

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4 Multiplane Stereo Particle Image Velocimetry
The disadvantage of these films is their extreme sensitivity to both wavelength and angle of
incidence (the effect of increasing the angle of incidence equals to a shift to slightly shorter
wavelength and an increase of the long wavelength reflectance). Since multiple-plane stereo
PIV applications involve other than normal angles of incidence and high numerical aperture
(low f-number) optics, it is better to use a broadband anti-reflection coating. These are multilayer
films, comprising alternate layers of various index materials, which reduce the overall
reflectance to an extremely low level for the broad spectral range covered.
Polarisation effects are normally not considered for anti-reflection coatings as these are
nearly always used at normal incidence where the two polarisation components are equivalent.
High reflectance or partially reflecting coatings, which work on the same principles as
dielectric anti-reflection coatings, are frequently used away from normal incidence, particularly
at ü
for mirroring or beam-splitting purposes and the polarisation plane is arbitrarily
ø
orientated with respect to the plane of incidence. Under these conditions the maximum s-
polarisation reflectance is always greater than the maximum p-polarisation reflectance due to
the difference in effective refractive index of the coating for the s- and p-components of the
incident beam as can be seen from Fresnel equations or figure 4.8. Thus, the reflected (or
1
1
0.8
0.8
Reflection
0.6
š R
|
R
||
0.4
Transmission
0.6
0.4
T
|
T
||
0.2
0.2
0
30 60 90
0˜ α
0
30 60 90
0˜ α
FIGURE 4.8: Reflectance and transmission of a plane monochromatic wave with "œ›—ž& nm at a
planar glass interface as a function of the angle of incidence and state of polarisation.
transmitted) beam is still linearly polarised, but the oscillation plane of the electric field vector
has rotated relative to the plane of incidence as the magnitude of both reflected components
is different. By observing the illuminated flow under a certain angle of incidence through the
polarising beam-splitter cube, the intensity variation over the field of view (increasing from
the centre to the edge) can be easily observed for appropriate choice of polarised light and
beam-splitter orientation. These effects have to be taken into account before arranging the
multiple-plane stereo PIV, especially for the large field of view applications.
Since the polarisation of the scattered light, as a function of the observation angle, mainly
depends on the diameter of the particles (for spheres of finite conductivity and finite dielectric
constant), the separation of the light using a polarising beam-splitter cube might not be
accurate enough when the variance of the particle diameter is large, and the spacing between
the light-sheets is of the order of the depth-of-focus (for larger distances the weakly depolarised
particles are out of focus and thus far below the digital registration threshold). Weakly
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