Frequently Asked Questions about Laser Doppler Vibrometer
Question number FAQ-1358
Is it acceptable to install the sensor by visual estimation? How does the tilt of the sensor affect the measured value?
If the surface is not a mirror, visual estimation for placement is usually sufficient.
When the sensor (≒laser optical axis) is tilted relative to the detection surface, the following two things are affected:
The tilt of the sensor reduces the amount of reflected laser light, which lowers the resolution (SNR).
When the object is a mirror surface, depending on the degree of tilt, it may be possible for no specular reflection component to be detected at all, making detection difficult. In the case of mirror surface detection (especially when the detection distance is long), it is necessary to adjust the tilt of the sensor while checking the position of the reflected light returning to the sensor.
*In the case of objects that are not mirror surfaces (scattering surfaces), the decrease in resolution with respect to tilt is slower than with mirror surfaces, enabling non-contact three-dimensional measurement using vector calculations.
The amplitude error can be calculated as a cosine error based on the angle between the tilted laser beam and the amplitude axis. For example, with a tilt of 5 degrees, the error from the original amplitude value is approximately -0.4%, and with a tilt of 10 degrees, it's approximately -1.5%. Generally, the human eye can distinguish a tilt of 1 minute (1/60th of a degree), so visual estimation is usually sufficient for installation. However, if you want to standardize your work, we recommend placing a mirror on the irradiation surface and checking the reflected light during installation.
Influence of sensor tilt on measured values
Last updated: 2011-05-23