Introduction to Acoustics

1108 Part H Engineering Acoustics
Part H 27.2
and recorded by time-average hologram interferometry.
This picture was used as a Christmas card interferogram
and it appeared in a number of physics textbooks, see
also [27.56, 57]. It may be pointed out that the quality
of interferograms as well as the spatial resolution, obtained
in this way was high but it was a time-consuming
task to obtain them compared to today’s methods.
Practical resolution of vibration modes by visual observation
in real time is often better than 100 nm (λ/10)
– compare Fig. 27.2b. Practical maximum measurement
range for time-average recordings as in Fig. 27.2c is
about 4 µm, otherwise the fringe density might be too
high to be resolved. The maximum possible object size
in width and depth depends very much upon the laser
used, both its output energy and its coherence length.
The coherence length of the laser determines how big
a difference in path length between the reference wave
and the object beam is allowed if recording high-quality
holograms. The difference in path length between the
reference beam, from the laser via mirrors to the hologram,
and the object beam, from the laser via the object
to the hologram, should therefore not exceed the coherence
length. This is easily determined with a soft
string fastened at the laser and at the hologram. The
power of the laser and the object reflectivity determines
the width of the object and the coherence length, and
thus the depth of the object volume. Objects that absorb
light might be dusted with chalk or painted white.
In difficult cases retro-reflective paint or tape might be
PS
PM
Laser
BS
R
Optical fibre
BS
Reference beam
SAM
CCD
Illumination
Video lens
used. The guitar interferogram shown in Fig. 27.3 was
recorded using a 20 mW HeNe laser having a wavelength
of 633 nm and a coherence length of about 25 cm,
the guitar shown in Fig. 27.3 was recorded. Remember
that a holographic setup is sensitive to disturbances; an
optical vibration-isolated table is recommended. Powerful
lasers shorten the exposure time and lower the
requirements for vibration isolation.
Holographic interferometry has been further developed
using pulsed or stroboscopic techniques, temporal
phase modulation as well as for studies of transparent
objects such as flames, pressure waves in air and
water. The books by Vest [27.12], Kreis [27.23]andHariharan
[27.48] are recommended. In practice today, all
electronic, digital speckle interferometry methods have
more or less replaced wet-processing photographic holographic
interferometry, at least for industrial use. The
fundamental ideas are, however, still the same.
27.2.2 Speckle Interferometry –
TV Holography, DSPI and ESPI
for Vibration Analysis
and for Studies of Acoustic Waves
Speckles [27.32–35, 46, 47] are the granular, speckled
appearance that a laser-illuminated diffusely reflecting
object gets, when imaged by an optical system such as
the eye or a camera. The speckles can be viewed as a fingerprint
that is as a unique, random pattern generated by
Z
Object beam
Fig. 27.4 The optical unit of TV holography (electro-optic holography, DSPI). Abbreviations: beam splitter (BS), speckle
average mechanism (SAM), mirrors on piezoelectric devices used for phase stepping (PS) and sinusoidal phase modulation
(PM), relay lenses (R). The distance between the optical head to the left and the object to the right can vary considerably
depending of object size, magnification etc.
Object
X