# 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
However, the different scattering characteristics at two different wavelengths hinder measurements with polydisperse seeding particles. As a solution, the laser frequency can be modulated, and an image sequence is captured with a single camera. Note that the laser modulation and the image acquisition have to be faster than the movement of the seeding particles in the measurement volume, so that changes of the flow velocity and the scattered light intensity are negligible. This approach was developed by [PERSON] et al. in 2004 switching between two laser frequencies (2-\(\
u\)-DGV) [64; 65], by [PERSON] et al. in 2004 switching between three laser frequencies also known as the frequency shift keying (FSK) technique (3-\(\
u\)-FSK-DGV) [66; 67], by [PERSON] et al. in 2005 switching between four different laser frequencies that allows a self-calibration (4-\(\
u\)-FSK-DGV) [68; 69], by [PERSON] et al. in 1999 with a continuous sinusoidal laser frequency modulation (FM) and a signal evaluation in the frequency domain (FM-DGV) [70; 71], which was developed further by [PERSON] et al. in 2007 [72; 73; 74], and by [PERSON] and [PERSON] in 2015 with a linear scan of the laser frequency and a detection of the position of the maximal transmitted scattered light through the molecular cell using a cross-correlation (CC) algorithm (CC-DGV) [75; 76]. With the FM technique, a method for simultaneous 3c flow velocity measurements using three different illuminations with different incidence directions and different modulation frequencies was introduced by [PERSON] et al. in 2011 [77]. Later in 2015, the FM approach was combined with a light field camera to demonstrate simultaneous FM-DGV measurements at two parallel light sheets located at different depths [78]. For a topical model-based review of the different DGV variants with laser frequency modulation, which summarizes the laser sources and molecular filters and explains the different signal processing algorithms, we refer the reader to [79]. Except for FM-DGV, the developed measurement systems were not yet tested in combination with high-speed cameras, but with slower CCD cameras, which hinders continuous high-speed imaging of the flow velocity field. For FM-DGV, the developments started with a linear fiber array where each fiber is coupled with a photo detector resulting in 25 measurement channels. The applied modulation frequency of the laser frequency modulation amounts up to 100 kHz, and the measurement of steady flow velocity oscillations with frequencies up to 17 kHz was demonstrated [80; 81]. The FM-DGV experiments with a high-speed camera based on complementary metal oxide semiconductor (CMOS) technology began in 2014 by [PERSON] et al. [82; 83]. The developments culminated with a power amplified diode laser with fiber-coupling emitting 0.6 W at 895 nm, which is modulated with a maximal modulation frequency of 250 kHz. As a result, flow velocity images with 128 \(\times\) 16 pixels and 128 \(\times\) 64 pixels are resolved with a maximal measurement rate of 500 kHz and 250 kHz, respectively [84; 85; 86]. However, the effective bandwidth is usually smaller, because a sufficient signal to noise ratio is crucial to resolve unsteady flow phenomena with such high speeds. As a result, the existing Doppler techniques with amplitude-based signal evaluation enable flow field measurements with a high data rate. The evaluation of superposed scattered light from multiple particles is possible with no additional signal processing, which is in contrast to Doppler techniques with frequency-based signal evaluation. However, the typically lower sensitivity and the cross-sensitivity with respect to scattered light intensity variations (over time or space) are critical issues. Comparing the usage of interferometric and atomic/molecular filters, atomic/molecular filters work with light absorption, while interferometric filters work with light reflection. Hence, atomic/molecular filters waste light that contains information.