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Chapter 3.6 Fluorescence LIDAR Technique for Cultural Heritage Valentina
Raimondi, Giovanna Cecchi, David Longnoli, Lorenzo Palombi, Gaia Ballerini Institute for Applied Physics ‘Nello
Carrara’, CNR,
Contents 3.6.2.1
Fluorescence LIDAR Technique 3.6.2.3
Hyper-Spectral LIDAR Imaging and Data Analysis 3.6.3.1 Laser-Induced Fluorescence of Stones and Other Building Materials 3.6.3.2 Laser-Induced Fluorescence of Protective Treatments 3.6.3.3 Monitoring of Biological Growth on Monument Surfaces 3.6.3.4 Fluorescence LIDAR Measurements on
the Cathedral and Baptistery of Parma The
fluorescence lidar technique is a remote sensing technique that enables to analyse a remote object without direct contact
with the object itself. This technique is non-destructive and essentially
consists in analysing from a distance the light emitted (fluorescence) by the
object when the latter is illuminated with a light pulse of a proper colour
(i.e. a proper wavelength). The use of fluorescence to point out features
that are normally not visible under natural illumination is well known:
credit cards are exposed to UV illumination to check their authenticity and
paintings are analysed under UV lamps to detect, for example, remnants of
transparent varnish layers. Thus, the fluorescence lidar technique enables to
apply the fluorescence technique in the outdoor environment by using a pulsed
laser to illuminate the target and a telescope to collect the fluorescence
light emitted by the target itself. The
fluorescence lidar technique has been applied to the investigation of the
cultural heritage only quite recently. Several experiments conducted in the
last decade have proved this technique to be a useful tool for the remote
diagnostics of monuments, providing helpful information for the assessment of
the conservation status of monuments in the outdoor and for the
characterization of the materials employed in their construction. Main
applications include the detection and characterization of different stones,
mortars and other construction materials, of protective treatments, of
biodeteriogens and the study of the effects of biocide treatments. The
fluorescence lidar technique does not require sampling procedures and the
subsequent laboratory analysis, procedures which can require considerable
resources in terms of time, money and specialised personnel, especially in
the case of an extensive control of the historical building heritage.
Moreover, the acquisition of samples implies a violation of the integrity of
the monument and a manipulation of the original material that may lead to
artefacts in the results, e.g. stone samples with spores or quiescent forms
that become active only once isolated and cultivated in the laboratory. In
addition, it allows the control of areas that are difficult to reach without
scaffolding since fluorescence measurements are obtained from a remote
location. In
the following the fluorescence lidar measurement concept will be briefly
introduced together with main applications and some key experiments carried
out for the diagnostics of the monuments in the outdoor.
The
lidar, acronym of LIght Detection And Ranging, is an instrument that allows
the remote analysis of a target by means of a pulsed laser. The lidar can be
regarded as the optical counterpart of the radar: the radar operates in the
microwave-range, the lidar operates in the visible range of the
electromagnetic (e.m.) spectrum and uses a pulsed laser as an excitation
source. In general, the lidar allows obtaining information about the
geometrical or the chemical-physical characteristics of a target, depending
if either the temporal behaviour or the spectral composition (different
colours) of the signal is analysed, respectively. Fluorescence
lidar remote sensing is a lidar technique that enables to detect remotely the
light emitted (fluorescence, see next section) from the target when the latter
is illuminated with a laser of a proper wavelength. A fluorescence lidar
(Fig. 3.6.1) is essentially composed of a laser, a telescope, a dispersion
system and a detector. The laser beam, which can also be collimated by using
a beam expander, is sent to the target and interacts with its internal
electronic structure. The fluorescence emitted by the target, which contains
information about the chemical-physical characteristics of the target, is
collected by the telescope and then fed to the dispersion and detection
system, usually featuring high spectral resolution to obtain information,
similar to that obtained in the laboratory, directly in the field. The data
are finally stored in a PC for the analysis of the signal.
The
lidar technique was initially developed for the remote sensing of the
atmosphere at the beginning of the 1960s: first papers [1, 2] on this subject
were published in 1963, only one year after the invention of the giant pulse
laser [3]. In the following years the lidar technique was also applied to the
remote sensing of natural waters [4] leading to the first experiments with
fluorescence lidars [5, 6] that were initially used for oil spills detection
and characterization and then for the study of dissolved organic matter and
phytoplankton. In the 80’s fluorescence lidar remote sensing extended its
field of application to the investigation of vegetation fluorescence [7],
with an emphasis on the monitoring of physiological processes and the effect
of environmental stresses on plants [8]. At the same time, the development of
these applications and further technological advances made it possible to
implement more compact and versatile prototypes which could fit the
application of the remote sensing technique to the cultural heritage. It
was not until the mid 90s that the fluorescence lidar technique was applied
to the diagnostics of the surfaces of historical buildings. First field
experiments [9, 10] for the application of the fluorescence lidar technique
to the monitoring of the cultural heritage were conducted on the Baptistery
and Cathedral of In
the following years further experiments investigated the fluorescence lidar
remote sensing potentials for the characterisation of different types of
stones [11, 12] and for the detection and characterisation of
photoautotrophic biodeteriogens (green algae and cyanobacteria) on stone
materials [13]. Other studies were conducted in the laboratory for the
characterisation of protective treatments on stone samples [14] and for the
investigation of the effects of biocide treatments [15]. A
further step forward was taken in 1997 when the first experiment of lidar
multispectral imaging was performed [16]. The first experiment of
fluorescence imaging concerned the More
recently, a further work [17] has extended the results obtained previously
[16] with the specific purpose of enhancing the feasibility of identifying
different lithotypes on a composite surface and of exploiting the potential
of the lidar multispectral imaging technique for an extensive monitoring. The
experiment was conducted on the same monuments chosen as targets for the
first lidar experiment [10] held in 1994. During the experiment an extensive
fluorescence multispectral imaging of both the monuments was performed, with
an emphasis on the monitoring of protective treatments and decorative
pigments, features that were not investigated before on monuments with the
lidar technique.
Fluorescence
is the emission of electro magnetic radiation due to electric dipole transitions
between different energy states according to selection rules which reflect
the conservation of the angular momentum for the molecule system [18]. Fig.
3.6.2 schematically shows the fluorescence process for a molecule. The
molecule is initially in the electronic ground state E0 (at room
temperature most molecules are in the lowest vibrational sublevel due to the
Boltzmann energy distribution). On the absorption of a photon hnin the molecule is excited to a rotovibrational state E3 of
the first excited electronic state. Within this electronic level the molecule
decays quickly to the lowest rotovibrational sublevel E3 by means
of non-radiative processes. The lowest rotovibrational state has a lifetime
that is longer (10-9 s) than those of the upper levels. The
transition from the E2 level to the E1 level is
associated to the emission of a photon (hnout). Finally, the molecule relaxes in the ground state by non-radiative
processes.
Fluorescence
in natural stones is due to diadochic and adsorbed cations and to defects in
the mineralogical periodicity. The presence of different contributions to
fluorescence in the same sample and the effects of sensitisation and
quenching, due to other types of cations or different minerals, make the
investigation of the fluorescence features fairly difficult. Nevertheless,
these features constitute a good source of information about the material
under investigation, to be related to both its geochemical and petrographic
characteristics. Typical activators in natural samples are divalent or
trivalent transition-metal ions and trivalent rare-earth ions [19]. Mineral
defect fluorescence usually features a broad emission band in the blue [20].
The most important quenchers in carbonates and oxide minerals are Fe2+,
Fe3+, Co2+, and Ni2+, while, for example, Ce3+
has been found to act as a sensitiser, and also as an activator, in calcite
[21, 22]. Fluorescence
in biodeteriogens is mainly due to pigments and other compounds such as
waxes, ferulic acids, phenylpropanoids etc. [24]. Chlorophyll a, that
is present in all photoautotrophic organisms, has a typical fluorescence peak
around 680 nm. In addition to Chlorophyll a, several biodeteriogens
contain other fluorescent pigments, such as chlorophyll b, c
and d, and phycobilins. Thus their fluorescent features can be used as
a signature to identify the biodeteriogens [25]. For example, green algae and
cyanobacteria, which are two taxa of biodeteriogens, show significant
differences in their pigment content. The green algae contain Chlorophyll a
and small quantities of other pigments, but they do not have
phycobiliproteins; cyanobacteria contain Chlorophyll a and other fluorescent
pigments including phycobiliproteins, such as phycoerythrin and phycocyanin.
The latter ones show typical fluorescence features that can be exploited to
differentiate the two taxa on the basis of their Laser Induced Fluorescence (LIF) spectral signatures.
3.6.2.3
Hyper-Spectral LIDAR Imaging Technique and Data Analysis A
remarkable enhancement in the readability of the final data can be achieved
by using the hyper-spectral fluorescence lidar imaging technique. This
essentially consists in scanning the target area with the lidar system and
acquiring a complete spectrum for each position so that, at the end of the
scansion, a fluorescence hyper-spectral fluorescence image of the
investigated area can be retrieved. The measurement concept is synthetically
sketched in Fig. 3.6.3: the investigated area on the surface of the monument
is ideally divided into m x n 'squares' (Fig.
3.6.3A) and then scanned with the lidar system to obtain a fluorescence image
of the target. For each 'square' of the target image, or ‘image pixel’, a
full high resolution fluorescence spectrum is acquired (Fig. 3.6.3B). The m x
n set of fluorescence spectra is finally analysed to obtain thematic maps
that outline specific fluorescence features of the target (Fig. 3.6.3C).
Fluorescence-based
thematic maps are particularly attractive for the control of monuments:
firstly, they provide a comprehensive assessment on the status of the whole
monument and a spatial definition that cannot be obtained by means of mere
sampling. Moreover, the opportunity of recording time-dependent, repetitive
fluorescence images opens new prospects for reliable control, repeated in
time, of the status changes of the monument. Another important aspect of
thematic maps is that they make it easier to transfer the knowledge gained
with a sophisticated analysis of the fluorescence data, which necessarily
requires a specific scientific background, to the conservation specialist
and/or to the decision maker. The
hyper-spectral image analysis and then the realisation of thematic maps can
be achieved in different ways: one of the simplest ways is to operate a ratio
between the areas of two different spectral bands. The area ratio is a useful
technique to investigate well known characteristics: for example, the
determination of areas affected by biodeteriogens growth can be often
achieved by operating a ratio between the spectral range centred around 680
nm, corresponding to the chlorophyll a fluorescence signature, and another
suitable spectral range chosen as a background. A
more refined technique for the analysis of hyper-spectral fluorescence data
is the Principal Component Analysis (PCA) [23]. Differently from the
ratio-based technique, the PCA technique permits a classification without
preliminary information about spectral differences of the samples. PCA is a
multivariate technique that consists in projecting the spectral information
onto a lower dimensional subspace. The latter is built up by a set of
orthonormal eigenvectors of the covariance matrix (PCs), chosen so that the
first PC describes as much as possible of the covariance among the samples,
the second PC as much of the residual covariance and so on. New PCs are added
until the residual is considered as just noise. If a sufficient number of PCs
are used, each spectrum can be reconstructed by a linear combination of the
PCs. The expansion coefficients are called "scores". Thematic
maps can be obtained by plotting the scores of the relevant PC, or a ratio
between the scores of two PCs, in a false colour set so as to point out
specific features of the samples. It is possible to obtain a thematic map
that shows the scores on three different PC using a RGB colour map. The map
is composed by three independent monochromatic maps, the red map that shows
the scores on first chosen PC, the green one that shows the scores on second
chosen PC, the blue one that shows the scores on third chosen PC. The three
maps are then summed up in RGB false colour image.
3.6.3.1
Laser-Induced Fluorescence of Stone and Other Building Materials Figure
3.6.4 shows a photo of miscellaneous samples as they were arranged during a
joint experiment aimed at the investigation of the fluorescence features of
natural stones and other building materials [24]. The samples were arranged
on a special scaffold that was previously covered with non fluorescent
material to simulate an operative scenario. The samples included several
types of marbles, among which Thematic
maps obtained either operating ratios between proper spectral bands or
applying the PCA technique are presented in Fig. 3.6.4. The thematic map of
Fig. 3.6.4 was obtained by applying the PCA technique in the spectral range
355-387 nm to the spectra collected with the 300 nm excitation wavelength.
The spectra were normalized to the maximum value before the PCA. The colour
map was built by superimposing three independent channels Red, Green and Blue.
The Red channel was associated to the PC1 score, the Green channel to the PC2
score, the Blue channel to the PC3 score. The map permits to distinguish the
marbles (MS1 and MS2 in green colours and MCA in blue colours) and the
sandstones (red colours). Lime mortar as well is in red colour, but it can be
differentiated from sandstones with a PCA applied to different spectral
ranges. Similar thematic maps can be obtained by applying the PCA in
different spectral ranges allowing the differentiation of pozzolanic mortar
from the other materials, as well as for the differentiation among similar
limestone lithotypes (calcirudites and packstones).
3.6.3.2 Laser-Induced Fluorescence of Protective Treatments The
use of conservative treatments on stony materials has old origins: however, often
there is no documented trace of the type of treatment applied in the past
times on certain monuments. Sometimes even modern treatments can be
craft-made. Consequently, a method able to detect them on the monument’s
surface, and possibly to identify them, can be extremely helpful. Fig.
3.6.5 shows the comparison between the fluorescence spectra of a sample of
dolomitic marble and of samples of the same material with different
protective treatments as shown in the figure’s legend. Although the spectral
shapes are quite similar, a more refined analysis with the PCA technique
allowed to differentiate the samples treated with different treatments and
the control sample. Fig. 3.6.6 shows the results obtained processing the LIF
spectra with the PCA: a first analysis to the full data set enables to
distinguish the samples with the protective treatments from the control one.
Fig. 3.6.6a shows the PC4 score plotted against the PC1 one to differentiate
the Fomblin (FOM) and the Rinforzante OH (ROH) protective treatments.
Fig. 3.6.6b shows the PC2 score plotted against the PC1 one to distinguish
also the other two protective treatments, Akeogard (AKCO) and Paraloid
B72 (PB72), from the control sample.
3.6.3.3 Monitoring of the Bbiological Growth on Monument Surfaces The
characterization of photoautotrophic biodeteriogens with the fluorescence lidar
technique is based on the analysis of the fluorescence signatures of the
chlorophyll and the other fluorescent pigments. Fig. 3.6.7 shows as an
example the fluorescence spectra of several species belonging to different
classes. These spectra were obtained in the laboratory with excitation at 514
nm during a set of measurements aimed at the identification of phytoplankton
in the sea, but the method can be equally applied to the identification of
biodeteriogens on monuments.
Specific
LIF measurements were also conducted on some species of cyanobacteria and
green algae isolated directly on monuments [13]. The species Pleurococcus
sp. and Chroococcus sp. belonging to the green algae and
cyanobacteria, respectively, were cultivated in the laboratory and then
inoculated at different concentrations on marble (dolomitic marble) and
sandstone (Pietra di Lecce) samples. Fig.
3.6.8 shows the fluorescence spectra obtained on marble samples inoculated
with different biodeteriogens: Fig. 3.6.8a shows the fluorescence spectrum of
the sample inoculated with green algae (Pleurococcus sp.) and Fig.
3.6.8b the fluorescence spectrum of that inoculated with cyanobacteria (Chroococcus
sp.). The concentration was 2.000 cells/cm2. Excitation wavelength
was 488 nm. The comparison between the two spectra points out the remarkable
differences in the spectral shape: the green algae show the typical
Chlorophyll a fluorescence peak at 680 nm, whereas the cyanobacteria, in
addition to the chlorophyll fluorescence peak, have additional peaks at 660
nm and 570 nm which are typical of phycocyanin and phycoerithrin,
respectively. The minimum concentrations detected in the laboratory were 200
cells/cm2, while lidar measurements in the outdoor from a distance
of about 20 m detected only higher concentrations (20,000 cells/cm2)
that were anyway not detectable by the naked eye.
3.6.3.4
Fluorescence
LIDAR Measurements on the Cathedral and Baptistry of In
September 2000 a joint experiment was performed on the Cathedral and the
Baptistery of The
Cathedral and the Baptistery of During
the experiment, 12 areas of the two monuments were monitored: 8 areas on the
Cathedral and 4 areas on the Baptistery. The locations of the monitored areas
are shown in Fig. 3.6.9a and in Fig. 3.6.9b. The investigated areas were
scanned with the LTH lidar system to acquire hyper-spectral fluorescence
images of the target. In addition, several point fluorescence measurements
were acquired on spots selected for their special features with both the
lidar systems.
The
experiment allowed pointing out the presence of an unknown protective
treatment on various areas of the Cathedral façade, especially around the Protiro,
that were not visible by the naked eye. In addition, several thematic maps
referring to areas of both the Cathedral and the Baptistery showed the
presence of biological growth. Several pieces of information were instead
retrieved concerning the lithotypes and other materials employed for the
decoration of the monuments [17]. Here only a selection of the main results are
presented. Fig.
3.6.10 shows the fluorescence measurements taken in the area around the Protiro
with the FLIDAR system featuring an excimer laser at 308 nm as an excitation
source. Fig. 3.6.10a displays the spots where the point fluorescence
measurements were taken. The results obtained exciting at 308 nm are shown in
Fig. 3.6.10b: the fluorescence signature of residual protective treatment is
present in almost all the spectra (spot 3 and 4 mainly show the fluorescence
due to the stone). The 308-nm excitation in the areas affected by the protective
treatment yields a peak at about 380 nm. The presence of residual protective
treatment is more marked in the area around the Protiro of the
Cathedral, especially in those areas that are less subject to weathering.
A
picture of the balustrade (area L in Fig. 3.6.9b) on the roof of the
Baptistery is shown in Fig. 3.6.11a. Fig. 3.6.11b shows a thematic map
obtained with the 355 nm excitation: here the ratio between the PC2 and PC1
scores is plotted. The yellow-red pixels point out the areas characterised by
the presence of biodeteriogens. The shape of the PC2, actually, is associated
to the chlorophyll fluorescence spectral shape while the shape of PC1 is
clearly related to the fluorescence of the stone substrate (not shown here).
In this image the laser beam is not normal to the scanned surface due to the
large pointing angle. This is a clear example of an area difficult to be
reach without scaffolding for a monitoring of biological growth on a regular
basis.
The
fluorescence lidar technique has been applied successfully to the diagnostics
of monuments in the outdoor. The technique allows to investigate several
aspects concerning the conservation status of the monument and to obtain
information on the materials employed for its construction that can be used
for a correct arrangement of restoration intervention or for the
reconstruction of its history. In particular the technique was demonstrated
helpful for the detection and characterization of biodeteriogens and
protective treatments. Further experiments pointed out the feasibility of a
fluorescence-based characterization of different lithotypes and other
building materials such as mortars, cements. This information can be
exploited to identify areas of previous (unknown) restorations/interventions
or to rebuild the construction history of the monument. Several
experiments on monuments have also demonstrated the practicability of
extensive monitoring of historical buildings with thematic maps by using the
hyper-spectral fluorescence lidar imaging technique. In particular, thematic
maps can be used to outline the presence of the areas subject to protective
treatment and to biological growth, to identify areas constituted by
different lithotypes. Thematic maps can be seen as a helpful support for
decision-makers in view of a routinely planned monitoring of our stone cultural
heritage.
The authors wish to remember the
memory of Prof. Luca Pantani, prematurely died in 2001, who first believed in
this application of the fluorescence lidar technique to the investigation of
the cultural heritage and undertook this activity with great enthusiasm. The
authors are very grateful to Prof. Sune Svanberg and his group of the Lund
Institute of Technology,
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FLIDAR group Applied Physics Institute ‘Nello Carrara’ Italian National Research Council (CNR-IFAC) Via Panciatichi 64 I-50127 Firenze Italy Department of Physics Lund Institute of Technology S-221 00
V. Raimondi Institute for Applied Physics ‘Nello
Carrara’ CNR Via Panciatichi 64 I-50127 Firenze Italy Giovanna Cecchi Institute for Applied Physics ‘Nello Carrara’ CNR Via Panciatichi 64 I-50127 Firenze Italy |
