Lasers and coherent illumination sources produce a grainy effect while shining
on optically rough surfaces. This effect can appear with or without lens imaging
the light of the object onto a photosensitive surface. It is called subjective
or objective SPECKLE, respectively.
Lasers have been used to record holograms since the 1960’s after Gabor’s
and Leith and Upatnieks’ experiments. Transmission or Reflection (Denisyuk)
Holograms can record phase modulation of light intensity from the shiny shell
of objects mixed with a reference beam thanks to the interference phenomenon.
Some people encountered fringes in the reconstructed hologram of a “resting
state” object, and have shown that these fringes were linked to the microdisplacement
or the deformation of the object during the recording process.
The quantitative interpretation of these fringes led to Holographic Interferometry
and Phase Computation. Quantitative phase evaluation was obtained through Phase
Shifting and Phase Unwrapping (both temporal and spatial).
With the use of electronic cameras, holographic interferometry fringes were
videorecorded and phase computation could be more easily applied. The poor spatial
resolution and the lack of camera sensitivity forced users to open wide the
lens apertures, and so speckle was not significant at this time. During the
1970’s, some clever users thought about removing the hologram plate to
directly image the object surface onto the camera detector. This was achieved
by the generation of “large” speckle grains while closing the aperture.
The gain was immediate: no more hologram recording (even in self-developing
media). This also meant increased stability facing environmental perturbations,
because camera resolution was still poorer than holographic recording media.
Speckle Interferometry or TV-Holography was born. In-plane and out-of-plane
set-ups have been designed, and also Shearographic devices. Low coherence sources
can also be used for dedicated applications in Low-Coherence Speckle Interferometry
and White-Light Speckle Interferometry.
Resolutions (spatial and dynamic) of digital cameras now enable the recording
of interferometric wave fronts almost like the holographic recording medium.
When the required computer processor power also became available, wave propagation
equations could be solved: Digital Holography was born. This allowed computer
processing of the hologram and also the possibility to send it through the web
via adapted compression for remote interferometric devices.
Nowadays, traditional holographic interferometry is still used by coupling a
CCD camera with a self-developing crystal (e.g. BSO) to record the hologram.
Evolution of optical fibers and lasers has even lead to powerful lighting systems
together with this real-time holographic camera.
On the other hand, Speckle Photography (double exposure of the speckle pattern)
leads to in-plane measurements through a correlation process (optical or computed).
Since the birth of Speckle Photography, cameras and computer technologies have
evolved. Acquisition, phase calculation and correlation can be done in real
time making the observation of static and dynamic speckles much simpler.
Sometimes laser speckle effect can be replaced by a paint tag of the object
for correlation purposes. This procedure is now commonly used and is also called
Digital Image Correlation. The speckle pattern is a kind of flyspeck applied
on the object surface, or it is just a representative optical pattern naturally
present. This technique is mainly used to measure in-plane displacements, but
also 3D displacements when coupled to stereovision devices.
The main difference between speckle interferometry and speckle photography is
sensitivity range. In interferometry, displacement range is related to the wavelength
of the light, and then varies from nanometers to micrometers between two states.
In speckle photography (or DIC), the displacement range is linked to the optical
magnification of the objective, from SEM lenses to telescope mirrors …
New developments now include the possibility of associating these sensitivities.
By correlating in-plane displacement fields to correct decorrelation of interferometric
phase maps, interferometric measurements can be taken even when large rigid
body motions occur.
Moreover we can now actively control the wavefront, the wavelength and the coherence
of light sources, providing brand-new techniques of strain or displacement measurements
The use of these techniques in the lab and also in industry to obtain better
quantitative and qualitative measurements is spreading rapidly. Some speckle
effects are also present in “non -visible optical” techniques like
Synthetic Aperture Radar, X-Ray Imaging and many others. A better understanding
of the whole process is encouraged and conferences dedicated to these subjects
are now widely attended…
WELCOME TO SPECKLE06
The
organization of the third edition comes from a round table of experts
and users
present in
Since
June2004, this new edition has widely been announced at different
National and
also International Conferences, as the wish of the Speckle users to
discuss theory
and applications of the Speckle effect.
The speckle
and phase measurements are now under study in several labs and
countries, and Speckle06
will enable all researchers (from Emeritus Professors
to Ph.D.
Students) to present their work. Participants range from experts
in techniques development to end-users interested by the application
fields (e.g. medical, forensic, fluid dynamics, optics or mechanics
(photomechanics)
…)
Speckle
techniques are also widely applied in industry, and special
attention
will be paid to industrial apparatus and off-the-shelf
components
characterization.
Speckle techniques
follow evolution
in computers and camera,
and
of course the availability of Application
Specific Integrated Circuits, lasers
and coherent diodes will be taken into account.
To enhance
result quality and reliability, speckle measurements must also go
through certification
evaluation. Measurement uncertainties must be determined as
quantitative
measurements are desired.
Papers will
be selected by the International Scientific Committee for oral
presentation
or poster sessions.
Each oral presentation will last
about 20 minutes including 5 minutes for discussion.
Industrial
exhibition will be
present in different showrooms near the conference auditorium
(lasers or
cameras manufacturers, editors, system developers …) to promote
applications and exchanges between users and manufacturers.