# Imaging Flow Velocimetry with Laser Mie Scattering
## 3 Developed Measurement Techniques and Their Imaging Evolution
### Measurement Techniques Using the Doppler Principle  
#### 3.2.1 Amplitude-Based Signal Evaluation  
In 1970, [PERSON] and [PERSON] described a 0d reference laser Doppler velocimeter, where the scattered light with the Doppler frequency shift is analyzed with a confocal piezo-electrically-scanned Fabry-Perot interferometer (FPI) [40]. The position of the maximal transmitted light signal through the FPI is a measure of the flow velocity. For 2d measurements, an FPI with planar mirrors can be used. Examples for such measurements are reported for Rayleigh scattering [12] and the fluid motion of the self-glowing Sun [41]. In 2013, [PERSON] et al. presented 1d flow measurements based on Mie scattering, but only for seven pixels [42]. The imaging to the FPI is performed with a self-made fiber-bundle with seven multi-mode fibers. For planar Doppler velocimetry with FPI (FPI-PDV), the crucial challenge is to achieve a high effective finesse of the interferometer. The finesse degrades with an increasing numerical aperture, which is required for maximizing the signal to noise ratio and for realizing an acceptable working distance. One solution to decouple the uncertainty between the FPI resolution and the numerical aperture was presented in 2003 by using single-mode fibers with an integrated FPI based on fiber Bragg grating technology [43]. A challenge for achieving a high signal to noise ratio is the low acceptance angle of single-mode fibers compared with multi-mode fibers. The fiber-integrated FPI approach was tested for 0d measurements on a rotating disc. The development of an FPI-PDV system for 2d flow measurements is an open task. In place of a multiple ray interferometer, a two-ray interferometer can also be used, which is easier to apply for 2d measurements. In 1978, [PERSON] and [PERSON] described a 0d reference laser Doppler velocimeter with a Michelson interferometer (MI) [44; 45]. The extension to planar Doppler velocimetry with MI (MI-PDV) followed in 1982 by [PERSON], [PERSON] and [PERSON] [46; 47], who named the measurement approach Doppler picture velocimetry (DPV). The mirror in the reference path of the interferometer is slightly tilted to generate a linear changing optical path difference and, thus, parallel interference fringes over the image. The velocity-dependent phase shift of these fringes is then determined with an image processing. The developed DPV system consists of a cwargon ion laser emitting at 514.5 nm and a charge coupled device (CCD) camera with 1374 \(\times\) 1024 pixels and a shutter time of 2 ms [48]. An image series was not acquired, but CCD cameras usually provide a maximal frame rate of \(<\)60 Hz. In order to increase the sensitivity of the MI, a dispersive element (iodine vapor cell) was inserted in the reference path by [PERSON] and [PERSON] in 2009 [49; 50]. Furthermore, a normal CCD and an intensified CCD camera with an image resolution of 1008 \(\times\) 1008 and 1280 \(\times\) 1024 pixels are applied, respectively. The light sheets are generated with a pulsed Nd:YAG laser emitting at 532 nm with 100 mJ/pulse and a pulse width of 40 ns. The pulse repetition rate amounts to 10 Hz. High-speed flow imaging with MI-PDV remains to be investigated. Almost in parallel, the usage of a Mach-Zehnder interferometer (MZI) was tested in 2007 by [PERSON] et al. [51], which is an alternative to the MI. In addition, imaging fiber bundles are used to perform 2d3c measurements with a single CCD camera [52]. Each fiber bundle consists of 500 \(\times\) 600 fibers and is 4 m long. An argon ion laser is used as the cw light source emitting at 514.5 nm. The available light power after the fiber that feeds the light sheet optics amounts to 200 mW. The capabilities of planar Doppler velocimetry with MZI (MZI-PDV) for time-resolved flow measurements with high-speed need further attention in future studies. In contrast to interferometric filters, molecular or atomic filters are more robust with respect to mechanical vibrations.