A Laser Doppler Velocimeter (LDV), also known as a Laser Doppler Anemometer (LDA), is a type of interferometer that measures the velocity of objects using laser light. The objects can be microscopic particles in a fluid or solid surfaces—the concept is even used in some high-end laser mice.
A laser beam is split and the two "arms" are made to cross outside the sensor. This crossing region is called the probe volume. Because the two arms came from the same beam, when they cross, an interference pattern is generated, and light and dark stripes form inside the probe volume. These stripes are called "fringes".
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