# Imaging Flow Velocimetry with Laser Mie Scattering
## 3 Developed Measurement Techniques and Their Imaging Evolution
### Measurement Techniques Using the Doppler Principle
#### 3.2.2 Frequency-Based Signal Evaluation  
In 1964, only four years after the laser principle was demonstrated, [PERSON] and [PERSON] described a (reference) laser Doppler anemometer (R-LDA) with a He-Ne laser [87]. It is described as a reference method, i.e., the scattered light with the Doppler frequency shift is superposed with the non-shifted light, and the superposed light is measured with a photomultiplier. The frequency of the resulting beat signal is proportional to the desired flow velocity. In principle, the Doppler frequency can also be resolved with a laser frequency comb [88]. However, the capabilities of this approach for flow velocity measurements are an open question. The R-LDA approach is capable of 3c measurements when using three measurement systems with three different illumination directions. Due to the usage of a point detector, 0d measurements are possible. Combining the LDA principle with the ranging principle known from radar allows one to perform 1d Doppler LiDAR measurements. The LDA principle was also extended to 2d by [PERSON] in 2000 [89] and also by [PERSON] and [PERSON] in 2009 [90]. The resulting technique is named heterodyne Doppler global velocimetry (HDGV). The major new component is a camera instead of point detector, which provides planar flow measurements. For this purpose, the flow is illuminated with a set of parallel laser beams or a light sheet, respectively. For instance, [PERSON] and [PERSON] used a cw laser (532 nm) with a power of 0.5 W and a smart pixel detector array with 144 \(\times\) 90 pixels performing dual phase lock-in detection. The maximal tested image rate for a flow velocity field was 4.35 kHz, but only time-averaged results were presented. An alternative approach to the reference LDA method is the difference LDA method by superposing the Doppler shifted light with light with a different Doppler frequency shift. The common measurement arrangement is the intersection of two laser beams, where the intersection region formsthe measurement spot that allows 0d measurements. This technique is now known as (difference) laser Doppler anemometry/velocimetry (LDA/LDV), and an early review is presented in [91]. Further details regarding the LDA measurement technique and experimental setups can be found in the review [92] and the book [26]. The laser Doppler velocity profile sensor (LDV-PS) invented by [PERSON] in 2001 consists of two pairs of intersecting laser beams that allow one to resolve the 1d position of the crossing scattering particle within the intersection volume [93; 94; 95]. Note that the photo detection is still accomplished with a point detector, which hinders high data rates, which means multiple seeding particles in the intersection volume. In order to overcome this drawback and to yield 2d flow measurements, an imaging optic with a high-speed camera or a smart pixel detector array for the photo detection can be applied together with intersecting light sheets for the illumination. This technique was introduced as imaging laser Doppler velocimetry (ILDV) by [PERSON] and [PERSON] in 2012 [96]. The experimental setup is similar to the HDGV experiments. However, the maximal laser power was increased to \(5.5\,\mathrm{W}\). Using the high-speed camera, the beat signal at 512 \(\times\) 64 pixels was directly acquired with 16 kfps. Again, the capabilities for time-resolved imaging measurements was estimated, but not yet tested in flow experiments. To summarize, the development of imaging Doppler techniques with a frequency-based signal evaluation has been successful due to the progress of powerful lasers and high-speed cameras or smart pixel detector arrays. One key challenge is the increasing frequency of the beat signal with an increasing flow velocity, which currently limits the measurement range to about \(\pm 1\,\mathrm{m/s}\). Regarding this issue, the approach of a smart pixel detector array is promising, because it allows a fast, parallelized signal (pre-)processing on-chip.