Molecular tagging velocimetry (MTV) is a specific form of flow velocimetry, a technique for determining the velocity of currents in fluids such as air and water. In its simplest form, a single "write" laser beam is shot once through the sample space. Along its path an optically induced chemical process is initiated, resulting in the creation of a new chemical species or in changing the internal energy state of an existing one, so that the molecules struck by the laser beam can be distinguished from the rest of the fluid. Such molecules are said to be "tagged". This line of tagged molecules is now transported by the fluid flow. To obtain velocity information, images at two instances in time are obtained and analyzed (often by correlation of the image intensities) to determine the displacement. If the flow is three-dimensional or turbulent the line will not only be displaced, it will also be deformed. Description There are three optical ways via which these tagged molecules can be visualized: fluorescence, phosphorescence and laser-induced fluorescence (LIF). In all three cases molecules relax to a lower state and their excess energy is released as photons. In fluorescence this energy decay occurs rapidly (within s to s at atmospheric pressure), thus making "direct" fluorescence impractical for tagging. In phosphorescence the decay is slower, because the transition is quantum-mechanically forbidden. In some "writing" schemes, the tagged molecule ends up in an excited state. If the molecule relaxes through phosphorescence, lasting long enough to see line displacement, this can be used to track the written line and no additional visualisation step is needed. If during tagging the molecule did not reach a phosphorescing state, or relaxed before the molecule was "read", a second step is needed. The tagged molecule is then excited using a second laser beam, employing a wavelength such that it specifically excites the tagged molecule. The molecule will fluoresce and this fluorescence is captured by means of a camera. This manner of visualisation is called laser induced fluorescence (LIF). Optical techniques are frequently used in modern fluid velocimetry but most are opto-mechanical in nature. Opto-mechanical techniques do not rely on photonics alone for flow measurements but require macro-size seeding. The best known and often used examples are particle image velocimetry (PIV) and laser Doppler velocimetry (LDV). Within the field of all-optical techniques we can distinguish analogous techniques but using molecular tracers. In Doppler schemes,