# Imaging Flow Velocimetry with Laser Mie Scattering
## 3 Developed Measurement Techniques and Their Imaging Evolution
### Measurement Techniques Using the Time-Of-Flight Principle
#### 3.3.2 Space Measurement  
The current standard field measurement technique is particle image velocimetry (PIV), where the seeded flow is illuminated by at least two consecutive light pulses and the light sheet is imaged with a camera. By cross-correlating sub-images (the so-called interrogation windows) that usually contain more than one particle and extracting the position of the correlation maximum, 2d2c flow velocity measurements are obtained. The name of PIV was introduced by [PERSON] in 1984 [111]. Reviews of the PIV development are available in [31; 112; 113]. Note that PIV requires short light pulses with a high energy per pulse and with a typical pulse width in the nanosecond range. Although light-emitting diodes can be used as the light source for PIV measurements, which was shown by [PERSON] et al. in 2010 [114], shorter pulses with higher energy are achievable with pulsed lasers. This is in particular crucial for high-speed PIV measurements. [PERSON] and [PERSON] demonstrated in 2004 PIV measurements with a high-speed PIV system, which is capable of MHz measurement rates [35]. However, four CCD cameras are operated together with a delayed acquisition to finally obtain seven velocity vector maps out of eight frames. With two CMOS high-speed cameras each operated with 10 kHz or 25 kHz, [PERSON] demonstrated in 2007 continuous, time-resolved PIV measurements (tr-PIV) [115]. The cameras are capable of acquiring a time series of more than 10,000 frames each consisting of 1024 \(\times\) 144 pixels or 640 \(\times\) 80 pixels, respectively. For the light sheet generation, a Q-switched laser is operated at 10 kHz, which provides 6 mJ/pulse, and at 25 kHz with 2.5 mJ/pulse, respectively. The pulse length amounts to 130 ns. With a light sheet illumination and a single camera, PIV enables 2d2c measurements. With a stereoscopic PIV setup, which means using two cameras with different viewing directions, 2d3c measurements are possible. Stereo-PIV was pioneered in 1991 by [PERSON] and [PERSON] [116]. High-speed stereo-PIV systems with 5 kHz and 10 kHz were developed for instance by [PERSON] et al., in 2009 and 2012, respectively [117; 118; 119]. A dual cavity, pulsed Nd:YAG laser emits at 532 nm pairs of light pulses with 2.6 mJ/pulse at repetition rates up to 10 kHz. The pulse duration is 14 ns. With a CMOS high-speed camera, 20 kfps are acquired with 512 \(\times\) 512 pixels. The size of the interrogation window is 16 \(\times\) 16 pixels. Extensions of the stereo-PIV technique to 3d3c exist. The tomo-PIV technique is based on a volumetric illumination, multiple cameras, a tomographic reconstruction and the cross-correlation of interrogation volumes [120]. Another technique is holo-PIV, which is based on a holographic volumetric reconstruction and only requires one camera [121]. A high-speed system that combines the tomographic and holographic approaches was reported by [PERSON] et al. in 2013 [122]. Furthermore, volumetric PIV measurements with a light-field camera form part of ongoing research [123; 124; 125; 126]. A high-speed system based on light-field PIV is a topic for future research. Note that standard PIV systems evaluate the particle positions in the images of two sequent light sheet pulses. It is mentioned for the sake of completeness that the accuracy in flows with strong acceleration can be increased by evaluating more than two image frames. The idea of such multi-frame PIV techniques goes back to [PERSON] in 1991 and was thoroughly investigated by [PERSON] and [PERSON] in 2007 [31; 32]. As an alternative to PIV, where image subsections are evaluated that usually contain several seeding particles, the technique to track the motion of each single particle, which is named particle tracking velocimetry (PTV), arose in 1991 [31; 127]. [PERSON] et al. demonstrated 3d3c measurements with multiple cameras in 1993 [128; 129]. The 3d3c tracking with a single camera was introduced by [PERSON] and [PERSON] in 1992 by using defocussing in connection with a coding aperture that is a mask with three pinholes [130].