Introduction to Acoustics

Optical Methods for Acoustics and Vibration Measurements 27.2 Measurement Principles and Some Applications 1105
27.1.5 Some Pros and Cons
of Optical Metrology
In the following, measuring principles for holographic
interferometry, speckle metrology, vibrometry, pulsed
TV holography, Moiré methods and PIV are discussed
together with some applications. The main advantages
of these methods are:
• There are no contact probes. They are nondisturbing.
• Most of them are whole-field, all-electronic methods.
They give pictures.
• They give not only qualitative but also quantitative
measures.
• The (digital) processing is often very fast, sometimes
as if in real time.
27.2 Measurement Principles and Some Applications
27.2.1 Holographic Interferometry
for the Study of Vibrations
With the introduction of holographic interferometry in
1965, amplitude fields of vibrating, diffusely reflecting
surfaces could be mapped. Earlier only mirror-like,
polished objects were used in interferometry but now
suddenly all kinds of objects could be studied. The interest
in holographic methods increased dramatically.
The most often used technique for vibration studies
was time-average holographic interferometry [27.9,10].
A hologram of the vibrating object was recorded while
the object was vibrating. The exposure time of the photographic
plates was quite long, often about one second.
This time was long compared to the period time of the
object vibration so light fields from many cycles contributed
to the total exposure of the photographic plate.
After development this plate is called a hologram. In
the optical reconstruction of the hologram, these object
fields were reconstructed simultaneously. Now, a sinusoidally
vibrating object spends most of its time in each
cycle at the turning points, where the velocity is momentarily
zero. These parts (at a distance of twice the
amplitude of vibration) therefore contribute the most
(spend the longest time) to the total exposure and therefore
these parts will also dominate the reconstructed
fields. A time-average field is therefore reconstructed,
and essentially light beams from the turning points interfere
with each other. Iso-amplitude fringes that cover
• Pulsed lasers give very short light pulses (exposure
times ≈ 10 ns), that freezes propagating sound fields
and vibrations as if stationary.
Some disadvantages are:
• Optical equipment and lasers are, in many cases, still
quite expensive.
• Trained personnel are needed to get full use of hightechnology
equipment.
• Speckle interferometry methods such as TV holography
(DSPI, ESPI) most often need auxiliary
equipment such as vibration-isolated optical tables
placed in rooms that are not too dusty or noisy. Others,
such as pulsed TV holography and vibrometry
instruments, work well in more hostile environments
without vibration isolation.
• Lasers are used and must be handled in a safe way.
the object surface are mapped using this technique. The
intensity of the reconstructed image of a sinusoidally
vibrating surface can be described by
I(x, y) = I0(x, y)J 2 0 (Ω) , (27.1)
where I0(x, y) is the intensity of the object at rest and J0
is the zero-order Bessel function of the first kind. The
exposure does not have to span many cycles to get the J 2 0
function in time-average techniques. It is also possible
to record time-average interferograms of low-frequency
vibrations.
The argument
Ω = K · L , (27.2)
where K is a sensitivity vector defined as
K = k1 − k2 , (27.3)
and L(x, y) is the vibration amplitude field. k1 and k2 are
the illumination and observation directions, respectively,
measured relative to the normal of the object surface.
With normal illumination and observation directions the
maximum sensitivity becomes
|K| = 4π/λ , (27.4)
where λ is the laser wavelength used.
A deformation field or a vibration field L(x, y) is
however vectorial, that is, it has three components.
Part H 27.2