Vibrometer contain a laser , which on the surface to be measured focus is. Due to the Doppler effect shifts upon movement of the surface to be measured the frequency of the backscattered laser light. This frequency shift is in the vibrometer by means of an interferometer is evaluated and as a voltage signal or digital data stream output. A scanning vibrometer enables field measurement of vibrations.
A vibrometer is generally a two beam laser interferometer that measures the frequency (or phase) difference between an internal reference beam and a test beam. The most common type of laser in an LDV is the helium-neon laser, although laser diodes, fiber lasers, and Nd:YAG lasers are also used. The test beam is directed to the target, and scattered light from the target is collected and interfered with the reference beam on a photodetector, typically aphotodiode. Most commercial vibrometers work in a heterodyne regime by adding a known frequency shift (typically 30–40 MHz) to one of the beams. This frequency shift is usually generated by a Bragg cell, or acousto-optic modulator.