A mobile hand held battery powered sensor based on fringe projection technique for preservation of fossil traces and archaeological excavations was developed. It consists of a projector and two cameras and covers a measuring field of about 240 mm x 175 mm x 160 mm. The core time for data acquisition is 0.34 s and the final result of a 3D point cloud is obtained in less than five seconds. Errors due to movements of the sensor are detected and can be swept out. The sensor allows the capturing of 3D data of the observed surface together with colour information. It was successfully applied at fossil find of traces of a dinosaur at rock layers from Triassic. 3D reconstruction of a part of the excavation was realized including the determination of the depth of traces. Keywords: mobile 3D measurement, fringe projection, 3D surface model production, colour representation, fossil and
archaeological excavations
1. INTRODUCTION Sensors for contactless acquisition of the 3D surface of measuring objects are increasingly used in applications of industry and quality inspection, medicine and scientific research, as well as architecture and cultural heritage preservation. Such systems provide increasing accuracy, growing data volume treatment, shorter measurement times, and a higher degree of flexibility. Flexibility mainly means mobility, light weight, and easy handling for the user.
The recording and digitalization of cultural heritage objects as well as archaeological and fossil excavations will be supported more and more by laser scanning and photogrammetric methods (see [1]). Coded structured light is often used in 3D scanning devices for 3D surface acquisition. Recently, at our institute we developed a number of optical 3D sensors for various applications [2, 3] and measuring objects of different size up to 1500 mm x 1000 mm x 200 mm. Usual, whole body measurement is possible, and self-calibration [4] was used. In 2007 a mobile, hand held, cordless sensor for acquisition of 3D surfaces was developed [5]. For documentation of archaeological and fossil objects, however, colour information considerably enhances the information of a 3D model of a captured object.
Lambers and Remondino [6] give a survey over recent developments and applications of optical 3D measurement techniques in archaeology. They describe three categories of optical data acquisition: image based methods, range based methods, and combination of both. Optical measurement techniques for cultural heritage documentation are introduced e.g. by Bitelli et al. [7] and Rizzi et al. [8].
The construction of 3D surface models of cultural heritage objects has been established in the last few decades (see e.g. [9, 10]). However, the effort for model production is still considerable. This holds for both effort in equipment and time. The documentation usually includes photographs and drawings. However, using a 3D model of the object, photographs and drawings may be produced from the model.
Laser scanners and fringe projection based techniques for cultural heritage recording have been compared and discussed by Kadobayashi et al. [1]. Akca et al. show the modelling of ancient statues by means of a fringe projection system and 3D modelling software [11].
In this work the realization of a fringe projection based optical 3D scanner for real-time measurements of archaeological and fossil objects is introduced. It is shown, how the documentation of an excavation can be strongly supported by means of the scanned 3D data.
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2. MEASURING PRINCIPLES 2.1. Phasogrammetric 3D measurement Using structured light for finding point correspondences leads to the principle of phasogrammetry which is the combination of photogrammetry and fringe projection in closed mathematical form (see [12]). A projection unit produces sequences of fringe patterns (Gray code and sinusoidal sequences) in order to produce phase values.
Phase values may be produced in order to identify projector image coordinates or to produce virtual landmarks (see e.g. [3]) which may be used both for calibration of the system and for the calculation of the 3D measurement data.
A well-defined sequence of fringe images is projected onto the measuring object which is observed by one or more cameras Ci. One sequence of fringe images usually consists of a Gray-code sequence (see e.g. [13]) which realizes the uniqueness of the fringes and a sequence of sinusoidal fringe patterns.
One sequence of fringe images is processed resulting in phase images Φi,x for each measuring position and camera. After rotation of the fringe pattern by 90°, the sequence may be projected and observed again resulting in a second phase image Φi,y for each camera Ci. The phase values ξ=φx and η=φy (see fig. 1) correspond to image coordinates in the projector plane. The resulting 3D points are obtained by triangulation between the coordinates of the camera and the projector, or between corresponding points of two cameras. This can be regarded as standard procedure in photogrammetry (see [14]).
Fig. 1. Principle of phasogrammetry
2.2 3D model production The developed sensor produces 3D point clouds of the captured surface. However, finally a 3D model of the whole considered area should be available. For the insertion of a dataset into an existing model different methods are possible. First, a common world coordinate system may be defined and each measurement is performed using this coordinate system. This may be realized e.g. by using a connecting observation camera (see [3]). However, this technique provides some disadvantages for excavations. The camera must be fixed in relation to the measuring object. Hence it should be
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placed on a tripod and it should observe the whole scene. At every measurement a synchronized image must be recorded. If the object is large or the area to be documented is very extended the spatial resolution of the connection camera images becomes low.
The next possibility to realize a fusion of the singe measurement datasets is the use of special merging software. This software must detect the identity of overlapping model parts, merge the datasets and suppress or correct deviations in the overlapping regions. For the surface production from the point clouds and the merging of different parts of the object model the Software “geomagic” was used.
The production of the 3D model was performed by the following steps. First, a schedule is made which parts of the object should be fused. Then all parts are recorded such that sufficient overlapping to the data captured so far is realized. The measurement results are stored as independent 3D point clouds. If all data are captured, 3D point clouds are converted to surface models using “geomagic” software. Then the obtained surfaces are merged into a model using also “geomagic” software. 2.3 Insertion of colour information In order to store maximal data in the model, the colour of the object surface will be captured using an additional colour camera in the sensor (see fig. 2). In principle, there would be the possibility to replace one of the black and white cameras or both by a colour camera. However, this would decrease the accuracy of the coordinate measurement because the colour cameras use only a third of the intensity compared to the black and white cameras due to the Bayer-pattern (see [15]).
With the help of the additional colour camera a pixel synchronous assignment of the colour information to the 3D measurement data is obtained. The method of phase correlation (see [3]) is used as follows. The projected fringe pattern is recorded by the colour camera and phase values are produced in the same matter as by the two monochrome cameras. The correlation of the phase values lead to finding correct point correspondences with sub-pixel accuracy. Every point in the 3D model corresponds to a pixel coordinate in the first measuring camera and to a sub-pixel exact coordinate on the colour camera sensor. Hence a corresponding colour value for the 3D model point can be obtained by e.g. bilinear interpolation in the colour camera image.
This method provides a number of advantages over the technique proposed by Dellipane et al. (see [16]). First the calculation effort is low, because for the mapping no complete model is necessary. Next, distortion of the recording lenses has no influence on the measuring accuracy, because phase and colour values suffer from exactly the same distortion error, if present.
Automatic colour calibration is not performed before every measurement. However, this can be obtained manually by an off-line white balance. The parameters of the white balance can be stored and used for subsequent measurements.
Fig. 2. Ultra mobile hand held 3D colour sensor
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3. REALIZATION OF THE SENSOR
3.1. Sensor design Regarding to the application field the sensor which had to be developed was designed according to the following requirements. It should be a truly portable handheld 3D scanning system using fringe projection technique which allows the mobile on-site, non-contact scanning and measuring of objects of any size, in various environments. The system should light weight, battery powered and a truly cordless one. This should be achieved using innovative high-speed LED-based projection technique and WLAN data communication.
The following diagram shows the principle measurement and data transfer process, see fig. 3.
synchronisierte Bildaufnahme
3D-sensor
Start with release button
Quick data aquisition
Laptop
Control panelVisualization, Live View
Host computer3D-data treatment
Data archivingiPod touch at the arm
System control Power supply (battery)
3D-coordinate calculationVisualization
3D- data treatment
Mobile control device
synchronisierte Bildaufnahme
3D-sensor
Start with release button
Quick data aquisition
Laptop
Control panelVisualization, Live View
Host computer3D-data treatment
Data archivingiPod touch at the arm
System control Power supply (battery)
3D-coordinate calculationVisualization
3D- data treatment
Mobile control device
Fig. 3. Measurement process and data transfer of the 3D sensor.
3.2. Sensor features and technical specifications The sensor was realized accomplishing the requirements self-positioning, cordless, hand-held, user-friendly, and mobile. In particular, the specific features make the system very convenient to use. Self-positioning means that no external tracking devices needed (no CMM, portable arms, etc.), no positioning targets are necessary, and a complete freedom of movement (virtually no limitations on scan orientation or accessibility in confined spaces due to the scanner’s truly portable design) is realized.
The cordless feature means that it has a WLAN data connection and it is battery powered. Because it is hand-held it makes a quick data acquisition possible. It is light weight and ergonomic. Because of the very simple handling (no adjustment or accurate positioning), the easy user interface via iPod touch, the shortest learning curve, and live view of the scanning result at the iPod it is very user friendly. It is easy to set up and scan. Users are up and ready to start scanning in under two minutes.
The scanner is very suitable for mobile use. The transport is realized within a case. It can be used in-house or outdoor, and it works in confined places.
Projection unit and cameras realize a frame rate of 60 Hz. Thus projection and image recording cycle is 16.7 ms. Further technical specifications are the following:
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• Measurement field: 240 mm x 175 mm (single shot) • Working distance: typical about 40 cm • Sensor weight: 1,9 kg • Dimension: 215 x 220 x 165 mm3 • Weight of the mobile control device: 10 kg • Accuracy: up to 50 µm • Lateral resolution / point density: 250 – 450 µm • Scanning time: 340 ms • 3D calculation time: < 5 s • Number of measurement points: 310.000 • Battery lifetime: ≈ 1,5 h • Mounting screw for a possible mount on a unipod or other handling system
4. DOCUMENTATION OF FOSSIL EXCAVATION 4.1 The excavation The sensor was used at fossil find of traces of dinosaur at rock layers from Triassic near Jena. The documentation task consists in the capturing of imprints of dinosaur feet in the rocks. The rock mainly consists of sandstone. The feed imprint is located on a slab of sandstone of a size of about 800 mm x 600 mm x 60 mm. 4.2 Measurements The stone slab (see a photograph in fig. 4) was the only measuring object of this excavation so far. Measurements were performed using the described sensor. A series of eleven single measurements were performed in order to cover the whole surface of the stone slab. Figure 5 shows the arrangement of the measurements at a view from above.
For all measurements the resulting 3D point clouds together with the images of the colour camera were stored. Figure 6 shows the 3D model representation of two selected measurements. See fig. 7 (right) for colour mapping onto the 3D model representation.
After storage of all measurements, the 3D point clouds were merged using the “geomagic” software. Figure 8 shows a representation of the whole model obtained from the eleven single measurements. An improvement of the visual impression of the representation is tried in the right photograph of fig. 8.
Mapping the colour images of the different scans onto the model representation a varying brightness must be determined. This is due to the changing illumination conditions. However, this was corrected by a special algorithm.
Fig. 4. Photograph of the stone slab with imprints of dinosaur feet (left) and with highlighted imprints by flash illumination (right)
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Fig. 5. Arrangement of the eleven measurements to be merged
Fig. 6. Two of eleven single patches (no.2 – left, and no. 11 - right) of the 3D model of the stone slab
Fig. 7. Single patch of the 3D model in monochrome rendering (left) and with mapped color texture (right)
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Fig. 8. Complete monochrome 3D model (left) and floating of the 3D model for better visualization of different imprint depths (right)
5. SUMMARY, DISCUSSION, AND OUTLOOK A hand held 3D sensor for was introduced which is suitable for production of 3D surface models of fossil and archaeological objects and hence also for the documentation of fossil and archaeological excavations. The sensor is a battery powered cordless 3D scanner and allows an object surface acquisition with a measuring field of 240 mm x 175 mm. The data acquisition time is 350 ms and the complete measurement time including data storing and display representation is less than five seconds. Because of the relative small observed area of 0.042 m² per scan it is mainly convenient for small measuring objects. However, the opportunity of data fusion makes a 3D model production of quite larger objects possible.
In the case of excavations the use of the scanner is meaningful if small regions are of interest. In our example, we digitized a stone slab with a size of about 800 mm x 600 mm with dinosaur feet impression from Triassic age with eleven scans. The resulting model contains all metric information of the observed surface as well as the pixel synchronized colour information obtained by an integrated color camera.
The main advantages over previous systems are the extreme mobility, the light weight and ergonomic shape. Because of the very simple handling it is easy to use.
The main problem of the actual measurements was the inhomogeneous brightness of the different scans to be merged to the resulting colour extended 3D surface model. Here, additional software had to be used. Thus, one task for future work should be to realize an automatically working algorithm which corrects the colour difference that a homogeneous texture mapping is realized. This may be reached either by a-posteriori local scaling of the brightness of the colour images or by realization of a better adaptation of the colour image recording on difficult illumination situations.
The stone slab with the dinosaur feet trace is the only part of the excavation which is scanned so far. However, it is planned to capture the whole excavation. With an extension of about 10 m x 2 m this dig is one of the biggest of its kind in Europe. This requires of course considerable effort because more than 500 scans have to be made and merged into the 3D surface model.
The sensor should also be used at other excavations in order to obtain more experiences. In the case of larger objects to be scanned, an extension of the measuring field should be useful. Here, new concepts for more flexibility should be developed.
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