Editing Flow measurement (section)

==Laser Doppler flow measurement==
A beam of laser light impinging on a moving particle will be partially scattered with a change in wavelength proportional to the particle's speed (the [[Doppler effect]]). A [[Laser Doppler velocimetry|laser Doppler velocimeter]] (LDV), also called a [[anemometer|laser Doppler anemometer]] (LDA), focuses a laser beam into a small volume in a flowing fluid containing small particles (naturally occurring or induced). The particles scatter the light with a Doppler shift. Analysis of this shifted wavelength can be used to directly, and with great precision, determine the speed of the particle and thus a close approximation of the fluid velocity.

A number of different techniques and device configurations are available for determining the Doppler shift. All use a [[photodetector]] (typically an [[avalanche photodiode]]) to convert the light into an electrical waveform for analysis. In most devices, the original laser light is divided into two beams. In one general LDV class, the two beams are made to intersect at their focal points where they [[Wave interference|interfere]] and generate a set of straight fringes. The sensor is then aligned to the flow such that the fringes are perpendicular to the flow direction. As particles pass through the fringes, the Doppler-shifted light is collected into the photodetector. In another general LDV class, one beam is used as a reference and the other is Doppler-scattered. Both beams are then collected onto the photodetector where [[optical heterodyne detection]] is used to extract the Doppler signal.<ref>Adrian, R. J., editor (1993); ''Selected on Laser Doppler Velocimetry'', S.P.I.E. Milestone Series, {{ISBN|978-0-8194-1297-3}}</ref>