Defocusing μPTV With Increased Measurement Depth Using Shadowgraphy

In this study we can show that shadowgraphy is a feasible alternative to fluorescence for microscopic particle imaging. Furthermore, using the particle shadow image geometry (diameter for defocusing methods or axis lengths for astigmatism methods), the particle location along the optical axis can be determined, enabling a three-dimensional flow measurement with a single camera. However, the rate of change of the particle shadow image geometry as a function of the distance to the focal plane is one order of magnitude smaller as compared to fluorescence/scattering imaging approaches, such that the measurement depths is effectively increased. This increase is due to the fact that the signal to noise ratio of the relatively small particle shadow images is large, even if the particle is located far away from the focal plane, as the intensity is distributed over a small sensor area. In addition, due to their smaller dimensions, more particle shadow images can be captured on the camera sensor, yielding a larger information density for microscopic particle imaging. A measurement of a micro-channel (channel height: 650 µm) flow showed a good agreement with the ideal parabolic flow profile, proving the viability of this microscopic particle shadow imaging approach as a suitable addition to the existing fluorescence-based methods.