Optical micro-profilometry for archaeology Pierluigi Carcagnì a , Claudia Daffara b , Raffaella Fontana* b , Maria Chiara Gambino b , Maria Mastroianni b , Cinazia Mazzotta c , Enrico Pampaloni b , Luca Pezzati b a Istituto Nazionale di Ottica Applicata – Sez. di Lecce, via Barsanti, 73010 Arnesano (LE), Italy b Istituto Nazionale di Ottica Applicata - Largo E. Fermi 6 – 50125 Firenze, Italy c Dipartimento di Beni Culturali, Università di Lecce, via D. Birago 64, 73100 Lecce, Italy ABSTRACT A quantitative morphological analysis of archaeological objects represents an important element for historical evaluations, artistic studies and conservation projects. At present, a variety of contact instruments for high-resolution surface survey is available on the market, but because of their invasivity they are not well received in the field of artwork conservation. On the contrary, optical testing techniques have seen a successful growth in last few years due to their effectiveness and safety. In this work we present a few examples of application of high-resolution 3D techniques for the survey of archaeological objects. Measurements were carried out by means of an optical micro-profilometer composed of a commercial conoprobe mounted on a scanning device that allows a maximum sampled area of 280×280 mm 2 . Measurements as well as roughness calculations were carried out on selected areas, representative of the differently degraded surface, of an ellenestic bronze statue to document the surface corrosion before restoration intervention started. Two highly-corroded ancient coins and a limestone column were surveyed to enhance the relief of inscriptions and drawings for dating purposes. High-resolution 3D survey, beyond the faithful representation of objects, makes it possible to display the surface in an image format that can be processed by means of image processing software. The application of digital filters as well as rendering techniques easies the readability of the smallest details. Keywords: roughness, 3D survey, digital model, micro-profilometer, scanning device, conoprobe, archeometry. 1. INTRODUCTION Optical techniques are widely diffused and extremely well received in the field of conservation because of their effectiveness and safety [1-5]. The characteristics of being non-invasive make them particularly suitable for measuring frail (and precious!) objects. Many optical devices for three-dimensional survey are derived from industrial metrology, but the peculiarity of each artwork does not allow for a straightforward application. Challenges in artwork diagnostics are mainly due to shape irregularity and polychromy as well as the high-accuracy required to catch even the smallest details of a work of art. Surface cleaning is one of the most important and sometimes controversial stages of the conservation process: decisions have to be made regarding partial or complete removal of the outer patina, and restorers and conservators work would be considerably helped by the knowledge of surface morphology. A process of surface examination and analysis is, thus, critical to the definition and interpretation of corrosion or degradation in order to plan the restoration intervention. Moreover, a statistical analysis based on roughness calculation, can assess the condition of the object surface to monitor changes due to restoration intervention, surface decay due to wearing agents, or the evolution with time in terms of shape variations. When describing the surface integrity of an artwork, in fact, an important parameter to deal with it is roughness [6]. *[email protected]; phone: + 39 055 23.08.313; fax: +39 055 233.77.55; www.ino.it ; http://arte.ino.it
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Optical micro-profilometry for archaeology
Pierluigi Carcagnì
a, Claudia Daffara
b, Raffaella Fontana*
b, Maria Chiara Gambino
b,
Maria Mastroiannib, Cinazia Mazzotta
c, Enrico Pampaloni
b, Luca Pezzati
b
aIstituto Nazionale di Ottica Applicata – Sez. di Lecce, via Barsanti, 73010 Arnesano (LE), Italy
bIstituto Nazionale di Ottica Applicata - Largo E. Fermi 6 – 50125 Firenze, Italy
cDipartimento di Beni Culturali, Università di Lecce, via D. Birago 64, 73100 Lecce, Italy
ABSTRACT
A quantitative morphological analysis of archaeological objects represents an important element for historical
evaluations, artistic studies and conservation projects.
At present, a variety of contact instruments for high-resolution surface survey is available on the market, but because of
their invasivity they are not well received in the field of artwork conservation. On the contrary, optical testing techniques
have seen a successful growth in last few years due to their effectiveness and safety.
In this work we present a few examples of application of high-resolution 3D techniques for the survey of archaeological
objects.
Measurements were carried out by means of an optical micro-profilometer composed of a commercial conoprobe
mounted on a scanning device that allows a maximum sampled area of 280×280 mm2.
Measurements as well as roughness calculations were carried out on selected areas, representative of the differently
degraded surface, of an ellenestic bronze statue to document the surface corrosion before restoration intervention started.
Two highly-corroded ancient coins and a limestone column were surveyed to enhance the relief of inscriptions and
drawings for dating purposes.
High-resolution 3D survey, beyond the faithful representation of objects, makes it possible to display the surface in an
image format that can be processed by means of image processing software. The application of digital filters as well as
rendering techniques easies the readability of the smallest details.
Keywords: roughness, 3D survey, digital model, micro-profilometer, scanning device, conoprobe, archeometry.
1. INTRODUCTION
Optical techniques are widely diffused and extremely well received in the field of conservation because of their
effectiveness and safety [1-5]. The characteristics of being non-invasive make them particularly suitable for measuring
frail (and precious!) objects. Many optical devices for three-dimensional survey are derived from industrial metrology,
but the peculiarity of each artwork does not allow for a straightforward application. Challenges in artwork diagnostics
are mainly due to shape irregularity and polychromy as well as the high-accuracy required to catch even the smallest
details of a work of art.
Surface cleaning is one of the most important and sometimes controversial stages of the conservation process: decisions
have to be made regarding partial or complete removal of the outer patina, and restorers and conservators work would be
considerably helped by the knowledge of surface morphology. A process of surface examination and analysis is, thus,
critical to the definition and interpretation of corrosion or degradation in order to plan the restoration intervention.
Moreover, a statistical analysis based on roughness calculation, can assess the condition of the object surface to monitor
changes due to restoration intervention, surface decay due to wearing agents, or the evolution with time in terms of shape
variations. When describing the surface integrity of an artwork, in fact, an important parameter to deal with it is
Every surface has a certain amount of microscopic roughness, even if only at a molecular level, and the defects or
features which contribute to it may be either random or regular (periodic). For instance, roughness measurements are the
basis of many industrial quality controls: in this case, the roughness arises from the working process.
Generally speaking, the surface of an object can be described by three parameters, according to the spatial frequencies
considered: form (low frequencies), waviness (mid frequencies) and roughness (high frequencies or short wavelengths).
The three frequency ranges depend on the object dimensions and manufacturing: for instance, for an optical component,
shape is related to the designed form, waviness is related to the deviation between projected and manufactured form, and
roughness is the surface irregularity that causes light scattering. The UNI ISO 468 [7] norm reports on a relation between
an object manufacturing method and the roughness parameters. However there are no case studies that conclusively
prove this relationship, and, furthermore, artworks cannot be classified in any of the standard engineering surface
categories, neither for materials, nor for manufacture.
Roughness computation deriving from a shape survey is quite a new application in the Cultural Heritage field [8]. The
main problem when dealing with artworks is the lack of rules defining both a measurement and an analysis protocol for
cultural heritage applications.
In this work we present the preliminary results concerning statistical analysis carried out on one area, representative of
the corroded surface, of an Hellenistic bronze statue. Determination of the roughness and its relative characteristic
wavelength was carried out before the cleaning process started, and measurements will be repeated after the restoration
intervention.
Image processing was applied to a few images of archaeological findings in order to make out inscriptions and to
enhance their readability. Snapshots were obtained from the high-resolution digital model of the surface.
All the three-dimensional surveys were carried out by means of a high-resolution laser scanning micro-profilometer.
2. THREE-DIMENSIONAL SURVEY OF ARCHAEOLOGICAL FINDINGS
The archaeological survey differs from any other kind of survey mainly because of two reasons: the first one is the huge
variety of the objects of interest, ranging from buildings to skeletons, from wreckages to statues, etc. The second one is
the lack of generally accepted measurement standards. For example, the archaeological drawing, which is presented as a
scientific document that is supposed to reproduce the reality through the graphic interpretation following given criteria
and rules, remains, to a certain extent, a subjective interpretation of reality. Therefore the application of a method clearly
expressed in its rules is foreboded.
In each single case, researchers have the opportunity/freedom to choose the best suited technique, the graphic code for
representation and the set of objects that is representative of the whole site to be documented.
For this purpose, both a deep knowledge of the above mentioned objects and the possibility of measuring different
objects in different contexts is necessary.
The final goal of an extensive archaeological survey is the acquisition of meaningful and intrinsically correct data so to
compare the results from different techniques for the documentation and historical interpretation of an archaeological
finding.
Up to now, archaeological findings were recorded by means of photographs, relying, thus, on the photographer and
drawer skill. This kind of information is only qualitative, lacking of any quota information that is usually obtained by
means of contact techniques (stylus profilometer) or, otherwise, by means of optical instruments based on light scattering
that, by integrating over a macroscopic area (in the cm2
range) are not well suited for a proper detailed reproduction. Alternatively, a variety of microscopes is available, but only for laboratory applications and all of them entail micro-
sampling.
In this work we present an application of the optical micro-profilometry to some archaeological findings. Besides a high-
resolution three-dimensional documentation, not achievable with any photographic image, this technique allows a data
post-processing that can lead to either an increase in inscription readability (imaging analysis) or roughness computation
(statistical analysis).
2.1. The optical micro-profilometer
The optical micro-profilometer realized at INOA (National Institute for Applied Optics, Florence) is composed of a
commercial conoscopic probe mounted on two motorized high-precision linear stages. The probe (Conoprobe 1000 by
Optimet) working principle is as follows: a light beam projected by a diode laser on the sample is both reflected and back
scattered, and it impinges on a uniaxial birefringent crystal placed between two circular polarizers (see Fig. 1). The
ordinary and the extraordinary beams are then generated inside the crystal and produce an interference pattern [9, 10].
Surface height is computed from intensity and phase information on the interference pattern: quota differences result in
optical path differences that are seen as light and dark fringes on the CCD camera. The probe we used is equipped with a
50 mm lens which sets a quota resolution of nearly 1 µm and a dynamic range of 8 mm at a stand-off distance about 40
mm. The overall accuracy is better than 6 µm.
Surface shape is obtainable by mechanical, non-contact scanning. The scanning device is composed of two motorized
high-precision (0.1 µm) linear stages, that are perpendicularly assembled. The system allows measurement on a
maximum area of about 300×300 mm2. The instrument has a maximum transversal resolution of 20 µm, with an
acquisition speed ranging from 100 to 400 point/s depending on the set spatial sampling frequency. The whole system is
computer controlled.
The micro-profilometer allows measurements on surfaces with almost any reflectivity, with an incident angle up to 85°,
that is to say, scanning at very close to grazing incidence is possible. Therefore, the survey of very small details, like
holes of < 1 mm diameter and 25:1 ratio between quota and diameter, is possible [11]. Besides, the instrument is not
sensitive to color gradients being, therefore, also suitable for the survey of surfaces characterized by high chromatic
contrast [12]. These characteristics, combined with co-linearity, allow the measuring of thin grooves and deep holes.
Figure 1: Conoscopic probe working principle.
3. APPLICATIONS
3.1. The Minerva of Arezzo
The Minerva of Arezzo is a bronze statue discovered in Arezzo in 1541 (see Fig. 2) whose origin is still uncertain: it
could be either an original Hellenistic bronze dating back to third century B.C., or a variant produced in the Roman
Imperial period (first century A.C.). The statue, usually located at the Archaeological Museum in Florence, is currently
under repair at the Archaeological Restoration Centre of the Tuscany Region in Florence. Restoration was considered
very urgent because of the highly endangered status of the statue (precarious conditions of the structural wooden
elements inside the statue and extensive corrosion of the bronze layer). Besides that, the statue was extensively restored
in the past (the main restorations date back to the sixteenth and eighteenth centuries), and these actions modified in a
significant manner the aspect and integrity of the artwork. As an example, a missing arm (the right one) was replaced in
eighteenth century restoration; plaster was used to join disconnected bronze sections and to fill gaps; finally a dark
greenish paint covers most of the statue surface, giving a uniform aspect to plaster and bronze sections, but actually
covering the original patina.
The statue was completely demounted in its parts, and, in order to keep trace of the surface corrosion before restoration
started, a three-dimensional survey was carried out on a few differently corroded areas, representative of the differently
degraded statue surface (pony-tail, helmet, shoulder, dress, armour). “Bronze disease” is one of the most serious hazards
of bronze. This disease takes the form of a sudden outbreak of small patches of corrosion and is distinguished by rough,
light green spots.
Each investigated area is 4×5 cm2 and was sampled with a 50 µm step, corresponding to a range map of 801×1001
points.
Figure 2: The Minerva of Arezzo.
Analysis of surface features was carried out by using the three-dimensional data set. Actual surface data is made of a set
of 2D heights data acquired in parallel trace lengths to form the final 3D data set. In order to compute the roughness, a
suitable evaluation surface was taken consisting in a selected region of interest (ROI) of 201×201 sampled points, i.e. a 1
cm squared sized area. In other words, the ROI is representative of the surface pattern superimposed to a form as regular
as possible, e.g. plane or conic.
The first step was removing the mean form by least squares fit. The regression 2nd order polynomial (Zform) was
subtracted from the 3D surface data (Z) to give the conditioned surface Zcond = Z - Zform (see Fig. 3a and 3b).
This resulting conditioned surface represents a surface texture including both the spatial features at short wavelength
(roughness) and the components at longer wavelength (waviness). Roughness is related to the microstructure of the
surface material whereas waviness is the surface deviation from the projected form. The spatial length at which
roughness becomes waviness is not a set point for Cultural Heritage applications, especially in case of archaeological
surfaces whose conservation state depends on many different factors. For this reason we investigated the roughness
texture at different spatial lengths.
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Figure 3a: ROI on the Minerva shoulder Figure 3b: Corresponding conditioned ROI
As a first step, a number of surface parameters were calculated for the unfiltered 3D data, i.e. data including waviness
and roughness, and the results are reported in table 1. Amplitude statistical parameters include the root-mean-square
deviation of the surface Sq (i.e. the RMS roughness), the average roughness Sa, the skewness Ss and the kurtosis Sk of
topography heights distribution. The maximum peak-valley z height St is also reported. The hybrid parameters ∆q and ∆a
(i.e. the RMS slope and average slope of the surface within the sampling ROI) have been calculated, as well as the RMS
wavelength λq and the average wavelength λa of the surface.