# Imaging Flow Velocimetry with Laser Mie Scattering
## 6 Conclusions  
Imaging flow velocity measurements with high measurement rates are challenging, because the available light energy needs to be distributed over space and time. Since Mie scattering on seeding/scattering particles provides a higher signal to noise ratio than Rayleigh scattering on the fluid molecules, flow velocity measurement principles based on Mie scattering are advantageous. The respective measurement principles can be subdivided into Doppler and time-of-flight principles. The measurement principles based on the Doppler effect can be further subdivided into principles with amplitude-based and frequency-based signal evaluation. Realizations of these principles with imaging capability are DGV and ILDV, respectively. The time-of-flight principles can be subdivided into principles with time and space measurements. Well-known examples with imaging capability are SFV and PIV/PTV, respectively. All measurement approaches are in principle suitable for imaging flow velocity measurements. However, the Doppler principles with frequency-based signal evaluation and the time-of-flight principles with space measurements require further work to be applicable for the measurement of meso-scale flows. The most advanced field measurement techniques especially regarding a high measurement rate are PIV/PTV and DGV, because a minimum of one or two camera frames are sufficient for one flow velocity measurement. Continuous measurements with up to 100 kHz have been demonstrated, which is sufficient for a wide area of flow applications. Higher measurement rates up to 1 MHz are possible, as well, but are restricted in the number of successive measurements.  
Fundamental measurement limits exist due to the measurement at seeding particles, which are the retroaction to the fluid flow, the flow sampling phenomenon, the particle slip, the Brownian particle motion, as well as the illumination-induced force on the particles. An important measurement limit for imaging flow velocity measurements with a high measurement rate occurs due to the quantum noise, i.e., the photon shot noise regarding the number of photons. The measurement uncertainty is indirectly proportional to the square root of the number of photons, which decreases when the available light energy needs to be distributed over space and time. A comparison of the minimal achievable measurement uncertainty shows that Doppler techniques with amplitude-based signal evaluation provide a lower precision than the other Mie-based flow measurement techniques for sub-sonic air flows. However, the image processing requirements are minimal, which is beneficial for real-time imaging flow velocity measurements.  
DGV and PIV/PTV are well-developed field measurement tools that have been validated, e.g., for aerodynamic flow studies, near-wall measurements, as well as for two-phase spray flows and flame flows. However, coping with optical flow measurements through scattering media and in fluids with a spatiotemporal variation of the refractive index field, as well as combined measurements of the flow velocity, the temperature and the pressure are topical challenges for Mie-based flow measurements. Furthermore, an accelerated image (pre-)processing either directly on the camera chip or externally with separate processing units allows one to further increase the data rate of the flow velocity imaging techniques in future.  
The authors declare no conflict of interest.