  Every volumetric velocimetry measurements based on tracer (particles,
bubbles, etc.) detection can be strongly influenced by the optical screening
phenomenon. It has to be taken into account when the the statistical properties
associated to the performances of the particle detection and tracking
algorithms are significantly affected. It leads to a maximum concentration of
particles in the images thus limiting the final spatial resolution of the
instantaneous three-dimensional three-components (3D3C) velocity fields. A
volumetric velocimetry system based on Defocused Digital Particle Image
Velocimetry is used to show that above a critical visual concentration of
particles in the images, the concentration and accuracy of the final
instantaneous raw velocity vector field drop. The critical concentration
depends on physical parameters as well as on the processing algorithms. Three
distinct regimes are identified. In the first regime, the concentration is well
adapted to volumetric velocimetry, the largest concentration being optimal to
maximize the number of valid velocity vectors. In the second regime, the
performances of the detection and tracking algorithms are no longer optimal.
Finally, the third regime is strongly influenced by optical screening. A
rigorous methodology is proposed to optimize volumetric measurements taking
into account the system specifications (pixel size, focal length, system
magnification, etc.). An optimal particle concentration of 0.045
particles/pixel (ppp) is found. A relationship between the imaged concentration
and the optimal final spatial resolution for fully interpolated instantaneous
velocity field is obtained. The present study can be used as a guideline to
achieve the measurement of instantaneous three-dimensional velocity fields with
a good spatial resolution. It is finally successfully applied to a complex 3D
flow test case: a jet in crossflow.
