Uncertainty due to speckle noise in laser vibrometry
MartinPeter
2010
This thesis presents fundamental research in the field of laser vibrometry for the
application to vibration measurements. A key concern for laser vibrometry is the effect of
laser speckle which appears when a coherent laser beam scatters from an optically rough
surface. The laser vibrometer is sensitive to changes in laser speckle which result from
surface motions not in the direction of the incident beam. This adds speckle noise to the
vibrometer output which can be indistinguishable from the genuine surface vibrations.
This has been termed ‘pseudo-vibration’ and requires careful data interpretation by the
vibration engineer. This research has discovered that measurements from smooth
surfaces, even when no identifiable speckle pattern is generated, can produce noise and
therefore reference to speckle noise, in such circumstances, is inappropriate. This thesis
has, therefore, adopted the more general term of pseudo-vibration to include noise
generated from any surface roughness or treatment, i.e. including but not limited to
speckle noise.
This thesis develops and implements novel experimental methods to quantify pseudovibration
sensitivities (transverse, tilt and rotation sensitivity) with attention focussed on
commercially available laser vibrometers and consideration is given to a range of surface
roughnesses and treatments. It investigates, experimentally, the fundamental behaviour of
speckles and attempts to formulate, for the first time, a relationship between changes in
intensity to pseudo-vibration sensitivity levels. The thesis also develops and implements
models for computational simulation of pseudo-vibrations using the fundamental
behaviour of speckles. The combination of experimentation and simulation improves
current understanding of the pseudo-vibration mechanisms and provides the vibration
engineer with a valuable resource to improve data interpretation.
Two experimental methods of quantifying pseudo-vibration sensitivity are developed and
successfully applied in the evaluation of transverse, tilt and rotation sensitivity for two
models of commercial laser vibrometer. These evaluations cover both single beam
(translational vibration measurement) and parallel beam (for angular vibration
measurement) modes. The first method presented requires correction of the vibrometer
measurement with an independent measurement of genuine velocity to produce an
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apparent velocity dominated by the required noise components. The second method
requires a differential measurement using two vibrometers to cancel common components
such as genuine velocity, leaving only uncorrelated noise from each measurement in the
resulting apparent velocity. In each case, a third measurement is required of the surface
motion component causing pseudo-vibration and this is used to normalise the apparent
velocity. Pseudo-vibration sensitivity is then presented as a map showing the spectral
shape of the noise, as a mean and standard deviation of harmonic peaks in the map and as
a total rms level across a defined bandwidth.
The simulations employ a novel and effective approach to modelling speckle evolution.
Transverse and tilt sensitivity are predicted for the first time and are verified by the
experimental study. They provide the vibration engineer with the potential to estimate
pseudo-vibrations using a simple piece of software.
The laser beam spot diameter has a large influence on the pseudo-vibration sensitivity.
Transverse sensitivity has been quantified as around 0.03% and 0.01% (per order) of the
transverse velocity of the surface for beam spot diameters of 100 μm and 600 μm
respectively. Larger beam spots have been shown to significantly reduce transverse
sensitivity and measurements from smoother surfaces have also shown a reduced level of
transverse sensitivity. Tilt sensitivity has been quantified at about 0.1 μms-1/degs-1 and 0.3
μms-1/degs-1 (per order) of angular velocity of the surface for beam spot diameters of 100
μm and 600 μm respectively. Smaller beam spot diameters significantly reduce tilt
sensitivity. The surface roughness or treatment has been shown to have little effect on the
level of tilt sensitivity. Rotation sensitivity has been quantified at approximately 0.6 μms-
1/rads-1 and 1.9μms-1/rads-1 (per order) of rotation velocity of the rotor for 90 μm and 520
μm. Smaller beam spot diameters have shown a significant reduction in rotation
sensitivity and measurements on smoother surfaces have shown a reduced rotation
sensitivity. Focussing the laser beam approximately on the rotation axis has also shown a
significant reduction in rotation sensitivity. Parallel beam rotation sensitivity has been
quantified at 0.016 degs-1/rads-1 and it is demonstrated that this can adequately be
estimated using the single beam rotation sensitivity.