# 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 addition, the transmission behavior does not depend on the light incidence direction, which simplifies the imaging. On the other hand, a suitable vapor material is required, which absorbs light at and near the laser wavelength. Observing the scattered light through the molecular filter, the transmitted light intensity is a measure of the Doppler frequency. The cross-sensitivity with respect to the scattered light intensity is corrected by applying a beam splitter and directly measuring the scattered light intensity. Hence, 2d measurements are achievable only by using two cameras for the photo detection, and each frame results in one flow velocity image. The measurement technique is named Doppler global velocimetry (DGV), later also planar Doppler velocimetry (PDV). [PERSON] described the technique first in 1990 in a patent [53]. There, he already mentioned resonance lines of atomic alkali metal vapors such as cesium (459 nm), rubidium (420 nm), potassium (404 nm), sodium (589 nm), lithium (671 nm), as well as molecular gases such as diatomic iodine and bromine with several resonance lines in the visible spectrum. Typically, the laser sources are combined with a molecular filter that contains iodine vapor. [PERSON] further mentioned the pulsed frequency doubled Nd:YAG laser (532 nm) and cw argon ion laser (514.4 nm) as possible light sources for instantaneous and time-averaged measurements, respectively. In 1991, such 2d3c flow velocity measurements were demonstrated with three observation units (each consisting of a camera pair, a beam splitter and a molecular filter) with different observation directions and a laser light sheet illumination [54]. [PERSON] et al. reviewed the progress in 1997 and described a method to reduce the required number of cameras by imaging both the signal image and the reference image onto a single camera [55]. Nobes introduced the fiber bundle technology in 2004 to measure all three velocity components on a single CCD camera with a single laser light sheet [56]. The fiber bundle consists of four arms each consisting of 500 \(\times\) 600 fibers, which enables the scattered light detection from different observation directions with a single camera. As an alternative approach, [PERSON] and [PERSON] presented in 2001 a 2d3c DGV system with a single observation unit and successively switching the light into one of the three light sheet optics with different illumination directions [57]. [PERSON] et al. extended this approach in 2014 for simultaneous measurements of all three velocity components by sinusoidally varying the intensity of each light sheet with a different frequency [58]. Analyzing the camera signals in the frequency domain allows one to separate the three channels. A performance test between two state-of-the-art DGV systems is described in [59]. With pulsed DGV systems, a temporal resolution of 10 ns is achieved. Due to the usage of CCD cameras and a low signal to noise ratio, the measurement rate of common DGV systems did not exceed a video rate of 25 fps. However, [PERSON] demonstrated in 2004 a measurement sequence of 28 images with 250 kHz [60]. The high repetition Nd:YAG laser system provided pulses with an average energy of 9 mJ/pulse at 532 nm, and the applied high-speed cameras consisted of at least 80 \(\times\) 160 pixels. Taking polarization effects into account and using a Faraday cell instead of a molecular absorption cell was considered by [PERSON] et al. in 1991 [61]. However, only 0d measurement systems have been developed with a Faraday cell to date [62]. In the classical DGV approach, two cameras (two measurements) are required to cope with two unknown quantities, the intensity and the frequency of the scattered light. In order to get rid of the second camera, [PERSON] et al. tried in 1998 the light sheet illumination with two different laser wavelengths and the simultaneous separate detection of the scattered light with a single polychrome camera [63].