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Laser Doppler velocimetry (LDV, also known as laser Doppler anemometry, or LDA) is a technique for measuring the direction and speed of fluids like air and water. In its simplest form, LDV crosses two beams of collimated, monochromatic laser light in the flow of the fluid being measured. A microscopic pattern of bright and dark stripes forms in the intersection volume. Small particles in the flow pass through this pattern and reflect light towards a detector, with a characteristic frequency indicating, via the Doppler effect, the velocity of the particle passing through the probe volume. LDV may be unreliable near solid surfaces, where stray reflections corrupt the signal. NASA is working on a variation that uses two pairs of beams so that polarization can be used to improve noise rejection. LDV is chosen over other forms of fluid velocity measurement such as Pitot tube measurements because the measurement equipment can be outside of the flow being measured and therefore have minimal effect on the flow.
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