Laser Vibrometry

Unlike traditional contact vibration transducers, laser-based vibration transducers, or laser vibrometers, require no physical contact with the test object. Remote, mass-loading-free vibration measurements on targets that are difficult or impossible to access, are typical examples of applications where a laser-based vibration transducer would be the natural choice. Furthermore, the ability to incorporate advanced, miniaturised, optical mirror systems together with the laser source provides automated scanning measurements, where a high number of measurement points can be measured consecutively. Non-contact vibration measurements with very high spatial resolution are possible with such a scanning system and can lead to significant improvements in the accuracy and precision of experimental modal models.

The measurement principle of a laser vibrometer is based upon the Doppler Effect. When monochromatic laser light is scattered back from a vibrating target it undergoes a frequency shift proportional to the velocity of the target. This is known as the Doppler Effect. As the target moves towards the light source, the back-scattered light undergoes an increase in frequency. As the target moves away, the back-scattered light undergoes a lowering of frequency. If the target is vibrating, the frequency of the back-scattered beam will be frequency modulated at the so-called Doppler frequency. The Doppler frequency is directly proportional to the velocity of the target. Therefore, tracking this Doppler frequency provides a direct measurement of the target's velocity relative to the motion of the light source. The Doppler Effect can be utilised in systems measuring translational (linear) vibration as well as systems measuring torsional (angular) vibration.

The laser source in the Brüel & Kjær line of laser vibrometers is either a HeNe gas laser emitting visible red light, or a solid state laser diode emitting infrared light. In both cases, the emitted energy level is low, ensuring a safe instrument that, depending upon type, can be operated with no or minimum safety precautions. Using only the best and latest within optical technology, every Laser Doppler Vibrometer from Brüel & Kjær exhibits world class performance, accuracy and reliability.