The core of the paper is focused on the experimental characterization of four different 3D laser scanners based on Time
of Flight principle, through the extraction of resolution, accuracy and uncertainty parameters from specifically designed
3D test objects. The testing process leads to four results: z-uncertainty, xy-resolution z-resolution and z-accuracy. The
first is obtained by the evaluation of random residuals from the 3D capture of a planar target, the second from the
scanner response to an abrupt z-jump, and the last two from direct evaluation of the image extracted by different
geometric features progressively closer each other. The aim of this research is to suggest a low cost characterization
process, mainly based on calibrated test object easy to duplicate, that allow an objective and reliable comparison between
3D TOF scanner performances.
While 3D imaging systems are widely available and used, clear statements about the possible influence of material
properties over the acquired geometrical data are still rather few. In particular a material very often used in Cultural
Heritage is marble, known to give geometrical errors with range sensor technologies and whose entity reported in the
literature seems to vary considerably in the different works. In this article a deep investigation with different types of
active range sensors used on four types of marble surfaces, has been performed. Two triangulation-based active sensors
employing laser stripe and white light pattern projection respectively, and one PW-TOF laser scanner have been used in
the experimentation. The analysis gave rather different results for the two categories of instruments. A negligible light
penetration came out from the triangulation-based equipment (below 50 microns with the laser stripe and even less with
the pattern projection device), while with the TOF system this came out to be two orders of magnitude larger,
quantitatively evidencing a source of systematic errors that any surveyor engaged in 3D scanning of Cultural Heritage
sites and objects should take into account and correct.
Resolution analysis represents a 2D imaging topic for the use of particular targets for equipment characterization. These
concepts can be extended in 3D imaging through the use of specific tridimensional target object. The core of this paper is
focused on experimental characterization of seven different 3D laser scanner through the extraction of resolution,
accuracy and uncertainly parameters from 3D target object. The process of every single range map defined by the same
resolution leads to different results as z-resolution, optical resolution, linear and angular accuracy. The aim of this
research is to suggest a characterization process mainly based on resolution and accuracy parameters that allow a reliable
comparison between 3D scanner performances.
The calibration of a range camera greatly influences the whole 3D acquisition and modeling process, allowing to
minimize the equipment inaccuracy. However, depending on the range camera "openness", we might have systems precalibrated
only once by the industrial manufacturer or systems requiring a regular (and mandatory) end-user calibration
before any scan session. Independently of the calibration approach, the metrological system characterization represents a
point of paramount importance for making the user aware of the actual performances of his equipment. This permits the
choice of appropriate resolution in 3D scan planning and allows to properly interpret the feedback indices during the
alignment of several range maps trough Iterative Closest Point (ICP). Finally, in polygonal model editing, the
modification of geometrical features is greatly helped by the awareness about the 3D capturing device performances.
These remarks are effective for both triangulation based instruments, like Minolta Vivid 910, ShapeGrabber SG100 and
SG1000 evaluated in this paper, or TOF based instruments. The proposed experimental method is based on post
processing of the range data produced by acquiring the surface of a precise test object with a 3D laser scanner. In this
procedure resolution, accuracy, and precision parameters are obtained sequentially, through the application of a set of
simple geometric processing steps. Such operating easiness make this approach a possible candidate as a mandatory step
in any 3D acquisition and modeling project.
Computer modeling through digital range images has been used for many applications, including 3D modeling of objects belonging to our cultural heritage. The scales involved range from small objects (e.g. pottery), to middle-sized works of art (statues, architectural decorations), up to very large structures (architectural and archaeological monuments). For any of these applications, suitable sensors and methodologies have been explored by different authors. The object to be modeled within this project is the "Plastico di Roma antica," a large plaster-of-Paris model of imperial Rome (16x17 meters) created in the last century. Its overall size therefore demands an acquisition approach typical of large structures, but it also is characterized extremely tiny details typical of small objects (houses are a few centimeters high; their doors, windows, etc. are smaller than 1 centimeter). This paper gives an account of the procedures followed for solving this "contradiction" and describes how a huge 3D model was acquired and generated by using a special
metrology Laser Radar. The procedures for reorienting in a single reference system the huge point clouds obtained after each acquisition phase, thanks to the measurement of fixed redundant references, are described. The data set was split in smaller sub-areas 2 x 2 meters each for purposes of mesh editing. This subdivision was necessary owing to the huge number of points in each individual scan (50-60 millions). The final merge of the edited parts made it possible to create a single mesh. All these processes were made with software specifically designed for this project since no commercial package could be found that was suitable for managing such a large number of points. Preliminary models are presented. Finally, the significance of the project is discussed in terms of the overall project known as "Rome Reborn," of which the present acquisition is an important component.
In a 3D acquisition project range maps collected around the object to be modeled, need to be integrated. With portable range cameras these range maps are taken from unknown positions and their coordinate systems are local to the sensor. The problem of unifying all the measurements in a single reference system is solved by taking contiguous range maps with a suitable overlap level; taking one map as reference and doing a rototranslation of the adjacent ones by using an "Iterative Closest Point" (ICP) method. Depending on the 3D features over the acquired surface and on the amount of overlapping, the ICP algorithm convergence can be more or less satisfactory. Anyway it always has a random component depending on measurement uncertainty. Therefore, although each individual scan has a very good accuracy, the error's propagation may produce deviations in the aligned set respect to real surface points. In this paper a systematic study of the different alignment modality and the consequent total metric distortions on the final model, is shown. In order to experiment these techniques a case-study of industrial interest was chosen: the 3D modeling of a boat's hull mold. The experiments involved a triangulation based laser scanner integrated with a digital photogrammetry system. In order to check different alignment procedures, a Laser Radar capable to scan all the object surface with a single highly accurate scan, was used to create a "gold-standard" data set. All the experiments were compared with this reference and from the comparison several interesting methodological conclusions have been obtained.