## 2 Pulp Suspensions Imaging Overview
Laser Doppler Anemometry (LDA) is an optical technique used to investigate velocity and turbulence in gas, liquid, and mixed fluids, among other applications [22]. The basic idea underlying LDA is to measure the velocity of tiny particles transported by the flow. If these particles are small enough, their velocity is assumed to be that of the liquid stream, and LDA thus provides a measure of the local instantaneous velocity, the mean velocity, and the turbulent quantities. Laser anemometers offer unique advantages in comparison with other fluid flow instrumentation [22]: (1) Non-contact optical measurement--LDA probes the flow with focused laser beams and can determine the velocity without disturbing the flow; (2) Well-defined directional response--the quantity measured by LDA is the projection of the velocity vector in the measuring direction defined by the optical system; (3) High spatial and temporal resolution--the optics of the laser anemometer are able to define a very small measuring volume, thus providing good spatial resolution and allowing for local measurement of velocity; (4) Multi-component and multi-directional measurements--combinations of laser anemometer systems with component separation based on colour, polarisation or frequency shift allow one-, two-, or three-component LDA systems to be put together based on common optical modules. So indeed, LDA has a lot of advantages, but some compromise has to be made when selecting and setting up laser anemometer systems; indeed, among the disadvantages of LDA are the high cost of the equipment, the need for a transparent flow through which the light beams can pass (usually corresponding to a low concentration of the disperse phase), and the fact that they do not give continuous velocity signals. Nevertheless, the special properties of the gas lasers (such as high energy, spatial and temporal coherence, and stability) make this method applicable for solving a large number of problems. LDA has been used to examine low concentrated pulp suspensions containing air bulbs by [PERSON] et al. [23]; carry out a study of 0.5% fibre suspensions in water (w/w) by [PERSON] et al. [24]; study pulp suspension flow at concentrations between 1.2 and 12 g per litre of water, using transparent model systems, based on the refractive index matching of the liquid and the fibres by [PERSON] [25; 26]; and, using the same model system presented by [PERSON], performing measurements in a stirred tank with fibre concentrations in the range 3-20% (w/w) by both [PERSON] et al. [27] and by [PERSON] and [PERSON] [28; 29]. Since Acoustic Doppler devices achieved easy handling and low cost, their use has experienced a high rate of growth in recent years. Doppler effect-based echography equipment allows the fabrication of systems that are able to measure velocity profiles nearly instantaneously [30]. The Ultrasonic Velocity Profile (UVP) technique was initially limited to the inspection of non-transparent fluids [31], and to measurements through pipe walls in small-scale systems [32]. In the UVP technique, a probe, acting firstly as an emitter, produces an ultrasound signal that travels along a certain direction, usually the suspension direction, and later, acting as a receiver, senses the reflexions originated by particles present in the suspension; the time, as well as the Doppler frequency shift between emission and reflections, is measured. Using the value of sound velocity in the fluid, the distance and velocity of the particles originating the echoes (the reflexions), along the suspension direction, are calculated. UVP was originally established for 1D measurements; however, promising 2D velocity profiles have recently been published [30]. The UVP technique has been used extensively in recent years to obtain instantaneous velocity profiles, but very few studies on pulp suspensions can be found in the literature. [PERSON] [33] used an early ultrasonic profiling technique to obtain radial velocity profiles in pulp suspensions with concentrations up to 1% (w/w).