Haijun Niu
email: hjniu@buaa.edu.cn

Fan Fan

Rui Wang

Qiang Zhang

Fei Shen

Pengling Ren

Tao Liu

Yubo Fan

Pascal Laugier

Elastic properties measurement of human enamel based on resonant ultrasound spectroscopy

Keywords:

Objectives: To investigate the elastic properties of human enamel using resonant ultrasound spectroscopy (RUS). Methods: Six rectangular parallelepiped specimens were prepared from six human third molars. For all specimens, the theoretical resonant frequencies were calculated using the Rayleigh-Ritz method, knowing the specimen mass density and dimensions, and using a priori stiffness constants. The experimental resonant frequencies were measured and extracted by RUS. Then, the optimal stiffness constants were retrieved by adjustment of the theoretical resonant frequencies to the measured ones based on the Levenberg-Marquardt method. The engineering elastic moduli, including Young's moduli, shear moduli, and Poisson's ratios, were also calculated based on the optimal stiffness constants. Results: The five independent stiffness constants C 11 , C 12 , C 13 , C 33 , and C 44 were 90.2 ± 6.65 GPa, 34.7 ± 6.90 GPa, 29.5 ± 4.82 GPa, 83.5 ± 8.93 GPa, and 37.0 ± 10.9 GPa, respectively. Young's moduli E 11 and E 33 , shear moduli G 13 and G 12 , and Poisson's ratios υ 12 and υ 13 were 71.7 ± 7.34 GPa, 69.2 ± 7.32 GPa, 37.0 ± 10.9 GPa, 28.1 ± 4.35 GPa, 0.303 ± 0.098, and 0.248 ± 0.060, respectively. Significance: Elastic properties are critical for developing dental materials and designing dental prostheses. The RUS method may provide more precise measurement of elastic properties of dental materials.

Introduction

Human teeth are primarily composed of enamel, dentin, and dental pulp. Enamel is the hardest and stiffest structure of the human tooth, covering the crown surface and protecting the inner tissue [START_REF] He | Understanding the mechanical behaviour of human enamel from its structural and compositional characteristics[END_REF]. Enamel has a multi-level composition of inorganics (mainly hydroxyapatite, about 96% in mass), organics, and water. The basic constituent elements of enamel are enamel prisms, the nano-sized fibril-like hexagonal hydroxyapatite crystals. The shape and arrangement of enamel prisms vary with depth [START_REF] Cui | New observations of the hierarchical structure of human enamel, from nanoscale to microscale[END_REF]. Enamel is not repairable once it is abraded or damaged. Therefore, alternative materials must be used to repair or rebuild the tooth [START_REF] Cui | New observations of the hierarchical structure of human enamel, from nanoscale to microscale[END_REF][START_REF] Cuy | Nanoindentation mapping of the mechanical properties of human molar tooth enamel[END_REF]. Understanding the elastic properties and the mechanisms in relation to the compositional and hierarchical microstructure of enamel is critical for developing new approaches to conservative and restorative dentistry.

The methods of measuring enamel elastic properties mainly include conventional macroscopic mechanical testing methods [START_REF] Bowen | Tensile strength and modulus of elasticity of tooth structure and several restorative materials[END_REF][START_REF] Ye | Measurement of Poisson's ratio of human enamel[END_REF][START_REF] Stanford | Determination of some compressive properties of human enamel and dentin[END_REF][START_REF] Craig | Compressive properties of enamel, dental cements, and gold[END_REF]) (e.g., stretching and compression), microscopic mechanical testing methods [START_REF] He | Understanding the mechanical behaviour of human enamel from its structural and compositional characteristics[END_REF][START_REF] Cui | New observations of the hierarchical structure of human enamel, from nanoscale to microscale[END_REF][START_REF] Zhang | Review of research on the mechanical properties of the human tooth[END_REF][START_REF] Habelitz | Mechanical properties of human dental enamel on the nanometre scale[END_REF][START_REF] Zhou | Depth dependence of the mechanical properties of human enamel by nanoindentation[END_REF][START_REF] He | Elastic modulus and stress-strain response of human enamel by nano-indentation[END_REF]) (e.g., nano-indentation), and acoustic methods (e.g., pulse-echo method [START_REF] Watanabe | Determination of elastic modulus of the components at dentin-resin interface using the ultrasonic device[END_REF][START_REF] Ng | Determination of ultrasonic velocity in human enamel and dentine[END_REF], ultrasonic interferometry [START_REF] Gilmore | Elastic properties of bovine dentine and enamel[END_REF], and critical angle reflection method [START_REF] Lees | Anisotropy in hard dental tissues[END_REF]). While these methods contribute to the understanding of macro-and micro-level elastic properties of enamel, they also have certain drawbacks. For example, macroscopic stretching and compression methods are destructive, and require a relatively large specimen volume, which is difficult to acquire for human enamel [START_REF] He | Understanding the mechanical behaviour of human enamel from its structural and compositional characteristics[END_REF]. Nano-indentation is a good method for calculating the microscale elastic moduli in different directions for a small-sized specimen, but the results mainly reflect the local elastic properties of the microstructure, which are not necessarily representative of the elastic properties at the macro scale [START_REF] He | Understanding the mechanical behaviour of human enamel from its structural and compositional characteristics[END_REF]. Elastic properties can be obtained in a non-destructive manner using ultrasonic methods by measuring the speed of sound, but these approaches require a minimum volume of material with dimensions larger than the wavelength, which can be challenging given the small dimensions of teeth. In addition, obtaining the anisotropic elastic properties requires multiple timeconsuming measurements along the main axes of symmetry using both shear and compression sensors [START_REF] Bernard | Accurate measurement of cortical bone elasticity tensor with resonant ultrasound spectroscopy[END_REF].

As Table 1 shows, there is no general agreement on the measurement results of enamel elastic properties reported in the literature, particularly for the elastic modulus. Ye et al. measured the elastic modulus of human enamel in the range of 17.29-36.13 GPa by using the stretching method [START_REF] Ye | Measurement of Poisson's ratio of human enamel[END_REF]. Stanford et al. and Craig et al. compressed human enamel specimens and determined their elastic moduli, which ranged from 62.74 to 95.84 GPa and from 8.27 to 37.23 GPa, respectively [START_REF] Stanford | Determination of some compressive properties of human enamel and dentin[END_REF][START_REF] Craig | Compressive properties of enamel, dental cements, and gold[END_REF]. Cuy et al. measured the elastic moduli of the human enamel surface to be greater than 115 GPa using the nano-indentation method [START_REF] Cuy | Nanoindentation mapping of the mechanical properties of human molar tooth enamel[END_REF]. Gilmore et al. measured the elastic modulus of bovine enamel in the range of 32.4-131.0 GPa using ultrasonic interferometry [START_REF] Gilmore | Elastic properties of bovine dentine and enamel[END_REF]. Even the same macro-compression methods on the same site did not yield consistent values for the elastic modulus, suggesting that mechanical testing methods are not that accurate [START_REF] Stanford | Determination of some compressive properties of human enamel and dentin[END_REF][START_REF] Craig | Compressive properties of enamel, dental cements, and gold[END_REF]. These methods have significant limitations for small-sized brittle materials. Therefore, there is a critical need to develop an accurate method that can be used for these small-sized biomaterials.

Resonant ultrasound spectroscopy (RUS) has been used as an accurate and efficient method to characterize material elastic properties since the 1990s [START_REF] Migliori | Implementation of a modern resonant ultrasound spectroscopy system for the measurement of the elastic moduli of small solid specimens[END_REF][START_REF] Migliori | Resonant Ultrasound Spectroscopy: Applications to Physics, Materials Measurements, and Nondestructive Evaluation[END_REF][START_REF] Migliori | Resonant ultrasound spectroscopic techniques for measurement of the elastic moduli of solids[END_REF]. The basic principle of an RUS experiment is (1) to measure a series of mechanical resonant frequencies of an object by generating free vibrations with ultrasound excitations, (2) to predict the theoretical resonant frequencies of the object with the Lagrangian variational method using guessed stiffness constants (the forward problem), and (3) to determine the full stiffness tensor of the object by adjusting the stiffness constants until the measured frequencies fit the predicted ones (the inverse problem). RUS is considered as the most accurate method to measure the stiffness constants of high Q (i.e., quality factor) solid materials, such as crystals or rocks. RUS allows all stiffness constants of a small-sized specimen (typically between 1 mm and 1 cm in size, but even less than 1 mm in size) to be measured in a single rapid nondestructive experiment with high reproducibility [START_REF] Migliori | Implementation of a modern resonant ultrasound spectroscopy system for the measurement of the elastic moduli of small solid specimens[END_REF]. Recently, Bernard et al. applied RUS with enhanced signal processing methods to highly dissipative materials, such as bone, which broke the limit of the RUS method for measuring stiffness constants of low Q materials [START_REF] Bernard | Accurate measurement of cortical bone elasticity tensor with resonant ultrasound spectroscopy[END_REF][START_REF] Bernard | Resonant ultrasound spectroscopy for viscoelastic characterization of anisotropic attenuative solid materials[END_REF][START_REF] Bernard | Bayesian normal modes identification and estimation of elastic coefficients in resonant ultrasound spectroscopy[END_REF]. Kinney et al. measured the elastic properties of human dentin in both the dry and humid states using RUS [START_REF] Kinney | The mechanical properties of human dentin: a critical review and re-evaluation of the dental literature[END_REF][START_REF] Kinney | Resonant ultrasound spectroscopy measurements of the elastic constants of human dentin[END_REF]. Fan et al. used RUS combined with an improved resonant frequency extraction method to estimate the full stiffness constants, as well as engineering elastic moduli, including Young's moduli, shear moduli, and Poisson's ratios, of human dentin specimens [START_REF] Fan | The elasticity coefficients measurement of human dentin based on RUS[END_REF].

RUS has proven to be an effective method for measuring the elastic properties of biological hard tissues, but it has not been applied to measure enamel to date. Our goal was to primarily extend our previous RUS investigation of the elastic properties of human dentin to those of human enamel at the macroscopic level. The results can provide references to understand the structure and function of enamel, to compare the elastic properties of different human dental tissues (e.g., dentin and enamel), and to assist in the design, development, and evaluation of clinical dental restorative materials.

Materials and methods

We will not detail here the entire procedure implemented to retrieve the stiffness constants from RUS measurements. It has been described extensively in reports to which the reader may refer [START_REF] Bernard | Accurate measurement of cortical bone elasticity tensor with resonant ultrasound spectroscopy[END_REF][START_REF] Bernard | Resonant ultrasound spectroscopy for viscoelastic characterization of anisotropic attenuative solid materials[END_REF][START_REF] Bernard | Bayesian normal modes identification and estimation of elastic coefficients in resonant ultrasound spectroscopy[END_REF][START_REF] Migliori | Implementation of a modern resonant ultrasound spectroscopy system for the measurement of the elastic moduli of small solid specimens[END_REF][START_REF] Migliori | Resonant Ultrasound Spectroscopy: Applications to Physics, Materials Measurements, and Nondestructive Evaluation[END_REF][START_REF] Migliori | Resonant ultrasound spectroscopic techniques for measurement of the elastic moduli of solids[END_REF][START_REF] Fan | The elasticity coefficients measurement of human dentin based on RUS[END_REF]. For the sake of clarity, however, we briefly describe hereinafter the main steps. Fig. 1 shows a diagram illustrating the three steps of an RUS experiment. The theoretical resonant frequencies were calculated using the Rayleigh-Ritz method, knowing the specimen mass, its dimensions, and a set of a priori stiffness constants. Then, the resonant vibrational modes were ultrasonically excited and the resonant frequencies were extracted from the measured vibrational spectrum. Finally, an iterative numerical procedure, the Levenberg-Marquardt method, was used to adjust the stiffness constants of the model and find the optimal set of stiffness constants for which the theoretical resonant frequencies match the measured ones. Finally, the engineering elastic moduli, including Young's moduli, shear moduli, and Poisson's ratios, were calculated for all specimens based on the optimal stiffness constants.

Experimental system

Fig. 2(a) contains a schematic of the RUS experimental system. The enamel specimen was mounted on opposing corners between two shear wave transducers (V154RM, Panametrics Inc., U.S.). A network analyzer (Bode 100, OMICRON electronics GmbH, Austria) was used to output a swept-frequency signal as the excitation from the emitter. The resonant signals were amplified by a broadband amplifier (HQA-15M-10T, Femto Messtechnik GmbH, Germany), recorded by the vector network analyzer, and imported into a computer for processing.

Specimen preparations

The specimens used in this study, collected from the Peking University Hospital of Stomatology, consisted of six complete, fresh, and non-carious third molars, removed based on the orthodontic needs of six women aged 25-27 years. Before conducting the experiment, approval was obtained from the Medical Ethics Committee of the administering institution.

After tooth removal, the tartar and granulations were immediately removed from the tooth surface. To roughly cut the occlusal region of the enamel, a low-speed diamond saw (SYJ-150, Shenyang Kejing Auto Instrument Co., Ltd., China) with 0.01-mm positioning accuracy was used, as shown in Fig. 2(b). In order to prepare all specimens with the same orientation and to solve the inverse problem accurately, axis 1 and axis 2 correspond to the two orthogonal axes perpendicular to the main orientation of the enamel prisms and axis 3 corresponds to the axis of symmetry with respect to the main orientation of the enamel prisms, and perpendicular to the tangent direction of the enamel surface. After being polished with 500-grade, 800-grade, and 2000-grade abrasive papers, the standard rectangular parallelepiped specimen was complete. The specimens were stored in saline to maintain a hydrated state prior to experiment. All measurements were made on a fully hydrated specimen.

Mass densities of the enamel specimens were deduced from mass measurements using an electric analytical balance (ME204E, Mettler Toledo, Swiss). They were then scanned by micro computed tomography (micro-CT, SkyScan1076, Bruker Micro-CT N.V., Belgium) at 70 kV, 142 μA, 10 W, Al 1.0 mm filtering, and a voxel size of 9 µm. The images acquired from micro-CT were reconstructed with NRecon software (Bruker Micro-CT N.V., Belgium). The dimensions of the specimens were calculated by Mimics 17.0 (Materialise, Belgium) and are presented in Table 2. Fig. 3(a) shows a photo of a typical enamel specimen and Fig. 3(b) shows the reconstructed model.

Experimental resonant frequency measurements

The frequency response of the specimen, mounted between two transducers, as shown in Fig. 2(a), was measured by the frequency-sweeping mode with 100 Hz resolution. The frequency-sweeping range was set to include at least the first 30 resonant frequencies. Six independent measurements were performed on each enamel specimen with intermediate repositioning and six groups of experimental resonant frequencies were obtained by seeking local peak amplitude points in the spectra. A typical series of six resonance spectra of one enamel specimen is shown in Fig. 4. The frequencies that occurred at least two times out of six were selected, and their mean values, standard deviations (SD), and coefficients of variation (CV) were calculated. The mean values of the selected frequencies with standard deviations less than 2 kHz and CV less than 0.3% were retained as the experimental resonant frequencies [START_REF] Bernard | Accurate measurement of cortical bone elasticity tensor with resonant ultrasound spectroscopy[END_REF].

Theoretical resonant frequency calculations

According to the previous calculations using finite element analysis (FEA) [START_REF] Wen | Finite element analysis on effect of organic phase on mechanical properties of human tooth enamel[END_REF], human enamel is assumed to be a transversely isotropic and inviscid material, with five independent stiffness constants, as shown in (1), where C is the fourth order stiffness tensor. (1)

Based on the findings of a previous study [START_REF] Wen | Finite element analysis on effect of organic phase on mechanical properties of human tooth enamel[END_REF], the initial values of the elastic coefficients adopted for this study were deducted to be: C 11 = 82.96 GPa, C 12 = 36.04 GPa, C 13 = 34.66 GPa, C 33 = 92.89 GPa, and C 44 = 22.90 GPa.

The theoretical resonant frequencies of an enamel specimen can be obtained by finding the stationary points of the Lagrangian L given by

= L f u C u x u x dV 1 2 (2 ) , i i i j k l i j k l 2 2 , , , (2)
where ρ is the specimen apparent tissue density, V is the specimen volume, C are initial stiffness constants, f is the resonant frequency, and u i is the displacement field. The equation was solved by the Rayleigh-Ritz method [START_REF] Migliori | Resonant Ultrasound Spectroscopy: Applications to Physics, Materials Measurements, and Nondestructive Evaluation[END_REF].

Stiffness constants

To obtain the final stiffness constants based on the experimental and theoretical resonant frequencies, a cost function F(C) was defined as

= = F C w f f C ( ) ( ( )) , i N i i exp i cal 1 2 (3)
where f i cal is the i-th calculated theoretical resonant frequency, f i exp is the i-th experimental resonant frequency, N is the number of theoretical resonant frequencies, C is the stiffness tensor of enamel and, w i is the weight, given by

= w f f does not match f f matches f 0 1/( ) . i i exp i cal i exp i cal i exp 2 (4)
Under the convergent condition, the optimal stiffness tensor C was the one that minimized F(C), using the Levenberg-Marquardt method [START_REF] Bernard | Accurate measurement of cortical bone elasticity tensor with resonant ultrasound spectroscopy[END_REF].

According to the generalized Hooke's law, the 6 × 6 stiffness tensor was constructed and numerically inverted to obtain the compliance matrix C 1 , from which the engineering elastic moduli were calculated. (5)

= C E E E E E E E E E G G G 1/ / / 0 0 0 / 1/ / 0 0 0 / / 1/ 0 0 0 0 0 0 1/ 0 0 0 0 0 0 1/ 0 0 0 0 0 0 1/ , 1 1
where E i are the Young's moduli (GPa), G ij the shear moduli (GPa), and υ ij the Poisson's ratios [START_REF] Migliori | Resonant ultrasound spectroscopic techniques for measurement of the elastic moduli of solids[END_REF][START_REF] Bernard | Resonant ultrasound spectroscopy for viscoelastic characterization of anisotropic attenuative solid materials[END_REF][START_REF] Bernard | Bayesian normal modes identification and estimation of elastic coefficients in resonant ultrasound spectroscopy[END_REF].

Results

Individual values of the stiffness tensor for all measured specimens are shown in Table 3. The mean values ± SD for C 11 , C 12 , C 13 , C 33 , and C 44 are 90.2 ± 6.65 GPa, 34.7 ± 6.90 GPa, 29.5 ± 4.82 GPa, 83.5 ± 8.93 GPa, and 37.0 ± 10.9 GPa, respectively. Table 4 shows the values for the engineering elastic moduli of the enamel specimens. The mean values ± SD for Young's moduli E 1 and E 3 are 71.7 ± 7.34 GPa and 69.2 ± 7.32 GPa, respectively; for Poisson's ratios υ 12 and υ 13 are 0.303 ± 0.098 and 0.248 ± 0.060, respectively; and for shear moduli G 13 and G 12 are 37.0 ± 10.9 GPa and 28.1 ± 4.35 GPa, respectively.

Discussion

Researchers have used a variety of measurement techniques to obtain elastic properties of dental materials [START_REF] Bowen | Tensile strength and modulus of elasticity of tooth structure and several restorative materials[END_REF][START_REF] Ye | Measurement of Poisson's ratio of human enamel[END_REF][START_REF] Stanford | Determination of some compressive properties of human enamel and dentin[END_REF][START_REF] Craig | Compressive properties of enamel, dental cements, and gold[END_REF][START_REF] Zhang | Review of research on the mechanical properties of the human tooth[END_REF][START_REF] Habelitz | Mechanical properties of human dental enamel on the nanometre scale[END_REF][START_REF] Zhou | Depth dependence of the mechanical properties of human enamel by nanoindentation[END_REF][START_REF] He | Elastic modulus and stress-strain response of human enamel by nano-indentation[END_REF][START_REF] Watanabe | Determination of elastic modulus of the components at dentin-resin interface using the ultrasonic device[END_REF][START_REF] Ng | Determination of ultrasonic velocity in human enamel and dentine[END_REF][START_REF] Gilmore | Elastic properties of bovine dentine and enamel[END_REF][START_REF] Lees | Anisotropy in hard dental tissues[END_REF]. However, due to the small size of human enamel or dentin specimens, accurate measurement of their elastic properties remains challenging. The limitations of the measurement techniques have led to large differences in the measurement results. As one of the most accurate methods to measure the stiffness of high Q solid materials [START_REF] Bernard | Accurate measurement of cortical bone elasticity tensor with resonant ultrasound spectroscopy[END_REF][START_REF] Bernard | Resonant ultrasound spectroscopy for viscoelastic characterization of anisotropic attenuative solid materials[END_REF][START_REF] Bernard | Bayesian normal modes identification and estimation of elastic coefficients in resonant ultrasound spectroscopy[END_REF][START_REF] Migliori | Implementation of a modern resonant ultrasound spectroscopy system for the measurement of the elastic moduli of small solid specimens[END_REF][START_REF] Migliori | Resonant Ultrasound Spectroscopy: Applications to Physics, Materials Measurements, and Nondestructive Evaluation[END_REF][START_REF] Migliori | Resonant ultrasound spectroscopic techniques for measurement of the elastic moduli of solids[END_REF][START_REF] Kinney | The mechanical properties of human dentin: a critical review and re-evaluation of the dental literature[END_REF][START_REF] Kinney | Resonant ultrasound spectroscopy measurements of the elastic constants of human dentin[END_REF][START_REF] Fan | The elasticity coefficients measurement of human dentin based on RUS[END_REF] for small-sized specimens, RUS was applied to measure the stiffness of six small-sized human enamel specimens in this study, which is believed to be the first study to investigate the anisotropic elastic properties of enamel using this method.

Stiffness constants and engineering elastic moduli (Young's moduli, shear moduli, and Poisson's ratios), are the primary parameters for characterizing the elastic properties of enamel. Due to the difference in microstructures and symmetries of different materials, the number of independent stiffness constants for each material also varies. For example, an isotropic material has two independent stiffness constants, a transverse isotropic material has five independent stiffness constants, and an orthotropic material has nine independent stiffness constants. Similarly, the elastic properties of enamel are closely related to its microstructure and symmetry. For simplification, it has been generally assumed that enamel is isotropic [START_REF] Bowen | Tensile strength and modulus of elasticity of tooth structure and several restorative materials[END_REF][START_REF] Ye | Measurement of Poisson's ratio of human enamel[END_REF][START_REF] Stanford | Determination of some compressive properties of human enamel and dentin[END_REF][START_REF] Craig | Compressive properties of enamel, dental cements, and gold[END_REF][START_REF] Zhang | Review of research on the mechanical properties of the human tooth[END_REF][START_REF] Habelitz | Mechanical properties of human dental enamel on the nanometre scale[END_REF]. Considering the hierarchical assembled structure of human enamel proposed by Cui et al., the enamel prisms on the enamel surface are radially arranged in parallel, which presents a higher hardness and elastic modulus [START_REF] Cui | New observations of the hierarchical structure of human enamel, from nanoscale to microscale[END_REF]. As the depth increases, the orientation and alignment of enamel prisms change to curved crosses, which leads to reduced values of the mechanical properties. To describe the enamel materials more accurately, Wen et al. first assumed that enamel specimens are transversely isotropic based on the hierarchical structure and have five independent stiffness constants C 11 , C 12 , C 13 , C 33 , and C 44 [START_REF] Wen | Finite element analysis on effect of organic phase on mechanical properties of human tooth enamel[END_REF]. This study also assumed that the specimens are transversely isotropic so that RUS is highly appropriate to estimate the five constants of the enamel specimens.

Comparing the resonance ultrasound spectra of the enamel specimens from this study (Fig. 4) with the resonance ultrasound spectra recorded for a dentin specimen in a previous study [START_REF] Fan | The elasticity coefficients measurement of human dentin based on RUS[END_REF], Fig. 2], the resonant peaks of enamel were found to be sharper and the overlaps between the peaks were found to be smaller, reflecting a higher Q value of enamel compared to dentin. Consequently, experimental resonant frequencies can be extracted with less ambiguity in the case of enamel.

Compared to the stiffness constants of dentin similarly calculated by the RUS method [START_REF] Kinney | The mechanical properties of human dentin: a critical review and re-evaluation of the dental literature[END_REF][START_REF] Kinney | Resonant ultrasound spectroscopy measurements of the elastic constants of human dentin[END_REF][START_REF] Fan | The elasticity coefficients measurement of human dentin based on RUS[END_REF], the stiffness constants of enamel were higher, which might be attributed to the higher mineralization of enamel and a denser and less porous material structure of enamel material. This result was consistent with previous researches [START_REF] Ye | Measurement of Poisson's ratio of human enamel[END_REF][START_REF] Stanford | Determination of some compressive properties of human enamel and dentin[END_REF].

Despite an extensive literature review, there is no data found where experimental measurement of the actual stiffness constants of pure enamel material occurred under transversely isotropic conditions. Wen et al. used FEA to estimate the five stiffness constants of enamel, yielding values of 82.96 GPa, 36.04 GPa, 34.66 GPa, 92.89 GPa, and 22.90 GPa, respectively (Wen et al., 2014). All of Wen's stiffness constants were consistent with the measurement results in this study, except for C 44 , which was higher in this study.

As discussed, the values of Young's moduli of human enamel obtained in previous studies vary widely. The diversity of the results may come from the use of different measurement methods. Young's modulus measurement results in this study were in the range of 60.7-80.5 GPa, which was consistent with the results of [START_REF] Stanford | Determination of some compressive properties of human enamel and dentin[END_REF], but with less measurement deviation. In addition, Poisson's ratios υ 12 (0.33) and υ 13 (0.25) and shear modulus G 12 (24.8 GPa) measured by [START_REF] Wen | Finite element analysis on effect of organic phase on mechanical properties of human tooth enamel[END_REF] with FEA were consistent with the measurement results of this study. The reason for the smaller G 13 measured by Wen et al. compared to this study might be from their hypothesis of a simple enamel model, which did not capture the complexity of the actual enamel structure.

For Poisson's ratio υ 12 and the shear modulus G 13 , the differences among the specimens were about 30%. The main reason might be the difference in characteristics between each specimen, the different cutting sites among the specimens, as well as the preparation of the enamel specimens.

Due to the brittleness of enamel, it is difficult to make specimens that are pure enamel. The enamel specimens could contain some dentin or small defects [START_REF] Cuy | Nanoindentation mapping of the mechanical properties of human molar tooth enamel[END_REF][START_REF] Stanford | Determination of some compressive properties of human enamel and dentin[END_REF]. Moreover, due to the small size of the specimen, a very high geometrical quality of the enamel specimen is not practicable, as shown in Fig. 3. These shape imperfections would result in errors in the forward model for calculating the theoretical value of the resonant frequencies [START_REF] Cai | Quantification of stiffness measurement errors in resonant ultrasound spectroscopy of human cortical bone[END_REF].

To achieve more accurate measurements of elastic properties of human enamel, future work is needed by using more accurate processing equipment to prepare more regular and pure enamel specimens to reduce the theoretical calculation error, and using another theoretical frequency estimation theory, such as FEA, to relax the size and shape requirements of the enamel specimens.

It should be noted that only a limited number of specimens were measured, so only preliminary results are shown. More studies are required to strengthen the conclusions.

In conclusion, RUS is an effective method to measure the engineering elastic moduli of small-sized enamel specimens. The results of this study pertaining to the stiffness constants and elastic moduli of enamel can be used to understand the elastic properties of dental materials and provide valuable information for dental material development and oral mechanics research.

Fig. 1 .

 1 Fig. 1. Flow chart of RUS.

Fig. 2 .

 2 Fig. 2. Platform of the RUS experiment (a) and sampling of specimen (b).

Fig. 3 .

 3 Fig. 3. Photo of a typical enamel specimen (a) and the reconstructed volume (b).

Fig. 4 .

 4 Fig. 4. Typical resonance spectra of the enamel specimen.

Table 1

 1 Summary of previous research results regarding enamel specimens.

	Group	Method	Scale	Source	Shape	Symmetry	Number	Young's Moduli
	Ye et al. (1994)	Stretching	Macro-	Human enamel	Rectangular parallelepiped	Isotropic	18	17.29-36.13 GPa
	Stanford et al. (1958)	Compression	Macro-	Human enamel	Cylinder	Isotropic	14	62.74-95.84 GPa
	Craig et al. (1961)	Compression	Macro-	Human enamel	Cylinder	Isotropic	12	8.27-37.23 GPa
	Cuy et al. (2002)	Nano-indentation	Micro-	Human enamel	Surface	Isotropic	3	> 115 GPa
	Gilmore et al. (1970)	Ultrasonic interferometry	Macro-	Bovine enamel	Parallel slice	Isotropic	4	32.4-131.0 GPa

Table 2

 2 Mass and dimensions of the enamel specimens.

	No.	1	2	3	4	5	6
	Mass (mg)	51.0	31.0	24.0	30.1	21.1	40.0
	Length (mm)	3.68	3.07	2.96	3.24	2.50	3.19
	Width (mm)	3.04	1.98	1.88	2.12	1.48	2.86
	Height (mm)	1.66	1.87	1.67	1.65	2.24	1.65
	Density (mg/mm 3 )	2.746	2.727	2.583	2.656	2.546	2.657

Table 3

 3 Stiffness constants of the enamel specimens.

	No.	C 11 (GPa)	C 12 (GPa)	C 13 (GPa)	C 33 (GPa)	C 44 (GPa)
	1	92.0	26.3	32.7	94.1	29.7
	2	96.7	43.5	23.4	72.3	51.9
	3	84.8	41.8	29.6	93.8	27.3
	4	96.6	30.5	31.9	83.6	32.6
	5	80.0	30.1	24.0	76.8	30.7
	6	91.3	36.2	35.3	80.3	50.0
	Mean	90.2	34.7	29.5	83.5	37.0
	SD	6.65	6.90	4.82	8.93	10.9

Table 4

 4 Engineering elastic moduli of the enamel specimens.

	No.	E 1 (GPa)	E 3 (GPa)	υ 12	υ 13	G 13 (GPa)	G 12 (GPa)
	1	77.9	76.1	0.185	0.283	29.6	32.9
	2	74.7	64.5	0.402	0.193	51.9	26.6
	3	61.5	80.0	0.430	0.180	27.3	21.5
	4	80.5	67.6	0.217	0.299	32.6	33.0
	5	65.4	66.3	0.312	0.215	30.7	26.9
	6	70.1	60.7	0.273	0.320	50.0	27.5
	Mean	71.7	69.2	0.303	0.248	37.0	28.1
	SD	7.34	7.32	0.098	0.060	10.9	4.35

Acknowledgements

This work was supported by the National Natural Science Foundation of China [Grant nos. 31570945 and 11772037].

Declaration of interest

None.


est destinée au dépôt

Varying access and a myriad of experiences for audience members, toward a reconfiguration of "cultural" practices

Madeleine Planeix-Crocker, Sylvia Girel

To cite this version:

Madeleine Planeix-Crocker, Sylvia Girel. Varying access and a myriad of experiences for audience members, toward a reconfiguration of "cultural" practices. 2016. halshs-01971881

 Seminar

Varying access and a myriad of experiences for audience members, toward a reconfiguration of "cultural" practices In order to understand the significance of artistic products, we must forget about them for a while, we must turn away from them and take recourse to the ordinary forces and conditions of the experiences that are generally considered unaesthetic. -John Dewey, "Art as experience"

Professor Girel defends the importance of studying the establishment of the Ministry of Culture in France in 1959 as well as the institutions evolution to this day. This transformation is marked by increased accessibility to works of art at once considered reserved to an elite. Such an evolution is inscribed in the greater scheme of contemporary societal transformations, especially with regard to the ways in which key players relate to cultural objects, which in turn never cease to evolve. However, Girel notes that this transformation is not only seen in a positive light; indeed, certain researchers observe a regression, a lack of accessibility in certain cultural sectors. As such, cultural democratization as a whole has been unsuccessful.

Girel notes the discrepancy between a contemporary sociological understanding of culture, and that of André Malraux, based on equal access to arts and culture, the diversification of audience members, the augmentation of attendees at cultural events, the increase in opportunities to access art, the heightened participation of audience members generally held at a distance from "high art", the intensification of elite cultural practices by all, etc. Finally, to this conception of democratization, we must add the social and civic missions of art and culture.

Furthermore, Girel contends that this conception of democratization contains the underlying notion of accession, of a "successful experience" that often refers to an idea type of understanding by the viewer (cultured audience member). Consequentially, there would be an ideal manner of viewing and listening.

There are implicit expectations as to how an audience should react to a work of art, so much so that they have been interiorized by cultured audiences and non-cultured audiences alike.

In addition, democratization has been founded on certain categories of art as well as the conception that these categories should be made accessible to populations that are generally distanced from culture.

As such, a hierarchy amongst art forms is established, and rarely is the question asked if "cultured" audiences should receive access to mass culture. Finally, Girel observes that this traditional conception of democratization is being revisited and questioned by contemporary researchers, rendering it difficult to find a harmonized definition of the concept. She proposes to examine two examples of cultural events that embody the contradictions and paradoxes held within this divergent conception of democratization: the establishment of the maison de la Culture in Le Havre (Normandy) in 1961, and the creation of the Nuit Blanche in 2002, a nocturnal celebration of the arts. These two events exemplify the evolution of creation, diffusion and reception of the arts in France. Cultural democratization is an informal process, a reflection of socio-cultural dynamics that exhibit varying forms of participation and appropriation of cultural practices and practitioners: the artistic experience is multifarious and multiform, which makes it impossible to take into account solely the traditional conception of democratization.

Girel makes the case for an attentive study of the population generally considered "non-audiences", separated from the high-cultured elites. Indeed, this population often adheres to the original and complex offerings of contemporary art. History shows that several efforts to render contemporary art accessible to all have been made: maisons de la culture in the 1960s, the Pompidou Center in the 1970s, the FRAC in the 1980s, squats in the 1990s, and public art spaces in the early 2000s. Cultural events such as the Ruée vers l'art (month dedicated to museums and the visual arts) in 1985, and the 10 days of contemporary art in 1997, are other manifestations of this initiative. Decentralized events also occurred in Rouen, Marseille for example.

The Nuit Blanche is yet another example of these cultural events geared towards all. This event, which takes place for one evening every fall, is a celebration of contemporary art, and attracts a diverse audience. This event shows that some progress has been made with regard to the democratization of contemporary culture, leaving aside an elitist understanding of art. The question remains however as to the transformation of the esthetic experience, a notion that escapes all quantitative analyses and surveys of audience members.

Indeed, these surveys do not take into account audience members who attend cultural events or exhibits in alternative or regional art centers (FRAC), and do not have the same social experiences as "cultured viewers". Girel notes that these methodological inconsistencies in cultural research explain the nonnuanced understanding of democratization we have to this day. A punctual study of these populations leads to a skewed understanding of their involvement in cultural practices: on paper, it would appear that such underrepresented audience members attend only festive cultural events, deemed "low culture". How can we qualify their cultural experience that exceeds a mere "social experience", yet is not necessarily a "cultural practice"?

Girel concludes that it is important to observe that the diversification of audience members in France is not automatically a sign of democratization in contemporary artistic practices. However, that is not to say that festive cultural events are not exempt of artistic excellence and are capable of soliciting an esthetic response from the viewer. As such, the term "cultural practice" needs to be redefined, in order to emphasize the esthetic dimension of the artistic experience. Girel proposes a qualitative and ethnographic study of these practices, which are diverse. Today, artists and audience members meet outside the boundaries of cultural institutions, and communicate in new ways than the traditional experience would have it. Sociologists must take into account these new circumstances, which are the basis of a new theoretical understanding od culture and society.


Mike Sharples

Paul Rudman

Peter Lonsdale

Julia Meek

Learning Bridges: a role for mobile technologies in education Giasemi Vavoula

In 1963 the science fiction writer Brian Aldiss wrote a short story for a children's science annual 1 about a world, thirty years in the future, where children learn through guided project work rather than formal schooling. The fictional London Educational Authority funds a trip for Jed, a 13 year old boy, and his father to Antarctica to survey the melting glaciers: "The sad masses of rock were heavily scarred where the ice flow had once rubbed them, for in this year of 1994, the glacier was smaller than it had been even a century ago."

Looking out over the ice floes, Jed adjusts a little apparatus behind his right ear that offers him immediately-relevant information about the world as he explores:

"It was a simple thing to do. Many of the parts of the miniputer were synthetic bio-chemical units, their 'controls' built into Jed's aural cavity; he 'switched on' by simple neural impulse. At once the mighty resources of the machine, equal to the libraries of the world, billowed like a curtain on the fringes of his brain…Its 'voice' came into his mind, filling it with relevant words, figures, and pictures. … 'Of all continents, the Antarctic has been hardest hit by ice.' As it spoke, it flashed one of its staggeringly vivid pictures into Jed's mind. Howling through great forests, slicing through grasslands, came cold winds. The landscape grew darker, more barren; snow fell."

Although the story is fanciful, its basic premise is sound. Children learn more effectively when they are in a more challenging environment than a school classroom, when they are investigating an open question of real interest (for Jed, the consequences of global warming 2 ), when they are accompanied by an adult guide, where mobile technology gives them rich and relevant information in context, and where they can make connections between formal knowledge and personal experience.

Mobile computers

are not yet controlled by neural impulse (though labs are working on it 3 ) and the funds of UK education authorities can only stretch to school trips to the local museum, not Antarctica. But a project called MyArtSpace, funded by the UK Department for Culture Media and Sport, is today exploring how children can engage in similar enquiry-led learning supported by mobile technology and how this can link to school and home learning. Using MyArtSpace as an example, we discuss the possibilities for mobile technology to form bridges between formal and informal learning. We also offer guidelines, drawn from our experience with MyArtSpace, for designing such bridges.

MyArtSpace

MyArtSpace is a service for children to spread their learning between schools and museums using mobile phones linked to a personal web space. It currently runs in three UK museums: the Urbis museum of urban life in Manchester, the D-Day museum in Portsmouth, and the Study Gallery in Poole. It can be used for informal learning, but is best suited to school field trips. The aim is to make a day out at the museum part of a sequence that includes setting a big question in the classroom, exploring it through a museum visit, reflecting on the visit back in the classroom or at home, and lastly presenting the results. The technology provides the essential link across the different settings.

The teacher starts by planning a class visit to the museum, consulting the MyArtSpace Teacher's Pack to prepare the trip. Typically, the teacher sets an open-ended question that the students can answer by gathering and selecting evidence during the museum visit. For example, Key Stage 3 (US Grade 6-8) students from a history class visited the D-Day Museum which interprets the Allied landings during World War II. Their task was to collect evidence on whether D-Day was a triumph or a disaster for Britain.

At the museum, the students are given multimedia mobile phones and each student keys in a personal identifier. Then they can explore the museum in any way they choose. They can 'collect' an exhibit by typing a two-letter code (shown on a printed label beside it) into the handset. This then shows a multimedia presentation on the phone and also automatically sends an image and description of that exhibit back to the online store on their personal web space (see Figure 1a and1b). The students are prompted to type in their reasons for collecting, encouraging them to reflect on what they see in the museum in relation to the big question they are trying to answer. After collecting an object, the students are shown a list of who else has collected it and prompted to find and talk with them face-to-face. In addition, they can use the mobile phones to create their own interpretation of the visit, by taking photos, recording sounds, or writing text comments (see Figure 1c and1d). After each action the phone sends the picture, sound or note to their online store. Back at school, or at home, the students can view their personal collections. Each student's web space shows a record of the visit: the exhibits they collected, the pictures they took, sounds they recorded, notes they wrote. They can also see the collections of other students in the class, and can add items from them to their collection as well as items from an online store provided by the museum.

The students can organise their collections into personal galleries (like simple webbased presentations), to present in the classroom or to share with their family (see Figure 2). Access to the web space is password protected, and the content published by the students is moderated to ensure privacy protection and appropriate use.

Figure 2. An example student gallery

MyArtSpace is now a fully working service that has been used by over 1500 students on visits to the three museums. It is also being trialled in other museums and outdoor sites. Our early evaluations of the service have collected positive feedback from students, teachers, Local Education Authority representatives and museum educators. This indicates the value of the service as a way to provide children with meaningful, engaging and enjoyable experiences of museum visits, complete with tangible outcomes that they can take away with them and work with after the visit:

"The day was of tremendous benefit to the pupils and their history studies. The mobile phones were easy to use and the children were quickly off exploring the museum and making their own collections. I have not seen pupils so engaged or enthusiastic on a museum visit before." (teacher, D-Day museum)

The way the 'collection' of museum items takes place encourages students to stop and think about each exhibit: what is the exhibit about, and how does it relate to their learning task?

"Made me look at artwork more ... Most people think going to galleries is boring, but when you put ideas on a web site and use the phones it's much more fun." (student, The Study Gallery)

By following up the visit online, the students' interest in the museum topic is increased as is their motivation for related learning. This is what makes MyArtSpace different from other multimedia museum guides: that it connects the museum visit to the classroom and to the children's home, so that the visit becomes part of a sequence of planning, engagement and reflection, rather than just a fun day out.

In this way, the mobile technology of MyArtSpace is used to bridge the children's experiences of different contexts, media and content, leading to an integrated learning experience across formal and informal settings. The various bridging roles of mobile learning -at least as important as the delivery of teaching content onto small screens -will be examined in the following paragraphs along with suggestions about how to design for them.

Designing technology bridges: divide and conquer

A successful learning activity should be integrated with other learning events, building on them and contributing to their outcomes. Likewise, successful mobile learning activities should be seamlessly integrated with other types of learning activity. Systems like MyArtSpace do not confine the learning experience just to interactions with a mobile device. Rather, they make use of highly mobile devices for the part of the experience where they bring the most value (e.g. for data collection in the museum, where the use of fixed technologies is impracticable and the use of traditional media such as pen and paper is cumbersome). The mobile device is then used as a bridge to technologies used in other parts of the learning experience (e.g. the exhibits, installations and printed media available in the museum that trigger reflection and inform data collection, or the Web-enabled ICT suite at school used for data analysis).

This is a wiser use of mobile technology than an indiscriminate digitisation and 'mobilisation' of all learning activities. For example, it would in principle be possible for students to use a web browser on a mobile phone to organize their personal MyArtSpace collections. However, a desktop PC is a better medium for students to manage their large collections of multimedia objects, collected through the mobile phone in the museum. Moreover, most schools in the UK are already equipped with desktop PCs in ICT suites, so taking advantage of existing technology and infrastructure is more cost-effective.

Deciding which type of technology to use should be done by 'divide and conquer'. The learning experience needs to be broken down into a sequence of activities and the following questions answered for each activity:

1. What will be the location of the activity? Will it be taking place in the field, in the classroom, or in the lab? Will it be indoors or outdoors? 2. What are the human factors of the learning activity? Does it involve physical movement and interaction? Will the learner's hands be otherwise occupied? Will the learner be standing, sitting, moving? 3. What technology is already available? Are there PCs, laptops, PDAs or tablet PCs, available? Do the learners bring their own devices that they are willing to use? Is there a network infrastructure already in place? 4. What are the technical requirements for the user interface? Will the user need to manage complex collections of data that require a large amount of display estate to be represented properly? Are there specific requirements for input/output form?

5. What is the cost of transition from this activity to the next one? Will the learner do another activity immediately after that will require them to switch to another device? How smooth can the switch between the two be?

Designing technology-activity bridges: put mobility in the learning experience, not only the technology

The design of educational technology of any kind needs a good balance between the technology and the education, and the same is true for the design of mobile learning systems. Although it is relatively straightforward to design a piece of technology that is usable, robust and delivers impressive functionality, the experience should go beyond the technology, with clear purpose for the teaching and learning.

In the case of MyArtSpace, the design of the mobile phone service and the web portal that hosts the students' collections went hand in hand with the design of the threestage learning experience (in the classroom, in the museum, and back in the classroom). Teachers, educational consultants, museum educators and Local Education Authority representatives were involved throughout the design of the system, providing expert advice on the kind of functionality that would be useful and shaping the template for the learning experience that would make use of that functionality.

Moreover, it is important that teachers are given the chance not only to adopt the designed system, but also to customise it for their classrooms. MyArtSpace implemented this through Teacher's Packs, a set of materials that describe the potential of the technology and make suggestions for activities. The teacher can then use these as a starting point to plan in detail a learning experience for their class that matches its teaching style, objectives, and background.

Designing learning space bridges: acknowledge and respect all learning spaces

MyArtSpace allows visitors to interact in three spaces: the physical space of the museum which they explore; the personal space on the mobile technologies that they use to collect and create items of personal interest; and the virtual space provided by a web portal that stores their collected items and additional resources for them to organise, share and present.

In designing the system, debates over how much of the experience should take place in each of the spaces had to be resolved. For example, when collecting an item in the museum, the student receives a list of other students who have also collected that item. Although it would be possible for the system to also display the other students' reasons for collecting that item, it was decided instead that it would only display a suggestion that the user might want to talk to them face-to-face -the rationale being that if face-to-face interaction is possible, then encourage it rather than replace it.

Decisions need to be made about when and where to make interactions in each space possible. For example, should the students be able to access their online collections whilst in the museum? What would be the price for an additional interaction space? As trials of mobile museum guides often show, there is a danger that the visitor's attention is drawn to the mobile device completely, at the expense of the rich museum environment. MyArtSpace therefore limits interactions in the virtual space outside the physical context of the museum.

Such design decisions discourage immersion in one learning space at the expense of the others. The technology that enables interactions in the personal and virtual spaces should be there to augment the experience in the physical space of the museum, not to 'swallow' or replace it without good reason. A good reason for technology to replace the physical experience might be when the student involved has particular special needs.

Designing context bridges: fill in the gaps between museum, classroom, and home

Many visits to museums and other similar school trips involve giving out numerous pieces of paper to children who will inevitably deface, tear, and possibly lose them before bringing the tattered remains into the classroom. MyArtSpace provides the means for children to end up with something less fragile and more engaging than a sheet of scribbled notes. Everything they collect in the museum automatically ends up being part of a meaningful artefact that they take away from the museum and then put to good use in later classroom sessions. The best thing is that neither the children nor the teachers need to put in any effort to make sure that this happens; the system just does it by default. One of the teachers at the D-Day museum in Portsmouth enthused about how MyArtSpace meant that the children's work wouldn't be lost on the bus on the way back to school -a real tangible benefit over visits without MyArtSpace. In this way, mobile learning technologies can help us build much needed bridges between different contexts and different learning spaces. It's hard to move museum experiences back into the classroom, but MyArtSpace shows us one way to do just that. The children ended up with something real to work with back at school. More than that, they then worked to produce something lasting that could be shown to their friends and family. The benefits go beyond simple mobility of artefacts -learners are able to continue their learning experiences across different locations and contexts.

Conclusions

In conclusion, the image of mobile learning in education is slowly crystallising into a picture of a learner enabled to not only use new technologies, but also to perform new activities with them; and of an educator who can not only put lots of learning 'stuff' in a mobile gadget and hand it to their students, but also to plan new learning experiences for them. Mobile devices can form steady bridges between technologies, contexts, experiences and learning spaces.

As Brian Aldiss might have written:

"It took just a moment. The mobiphone seemed made for him. Jeff tapped in the two-letter code written beside the exhibit and waited just a moment. Somewhere, far away, a massive electric-library sprang into life, fetching just those pictures and words that would give meaning and context to the battered soldiers' boots in the museum display. ...The machine let him explore on his own, or with his friends. It never disapproved or got cross, but was always ready with the most helpful facts and pictures at that moment for his age group. When he needed to ask questions, the teacher was there to help. Best of all, when he got home he could show everyone what he'd seen-he was curator for a day!"

Figure 1 .

 1 Figure 1. MyArtSpace mobile phone interface: (a) collecting an object, (b) multimedia presentation about a collected object, (c) main menu options, (d) taking photos

1

  Aldiss, B. (1963) The thing under the glacier. C. Pincher (ed.) Daily Express Science Annual No. 2, Norwich: Beaverbrook Newspapers Ltd.

In this story, Aldiss not only predicts the concern about global warming, but also personal computing in the 1970s ("it wasn't until the great developments in microtechnology in the seventies that portable computers were made") and mobile computing in the 1990s.

See e.g. http://en.wikipedia.org/wiki/Brain-computer_interface


Matjaž Debevc

Primož Kosec

Equal study: Lecture based automated video recording system for all

Keywords: webcast systems, webcasting, video recording systems

This paper presents a system, which (in one piece) captures the necessary video and sound equipment, interactive streaming video technology and virtual hypermedia environment into a new web based learning environment. In several steps a lecturer can prepare a video supported lecture, which is automatically synchronized with additional media (material, subtitles). Students can watch these lectures live or on demand. Later on additional form of subtitles can be simply added into the video and sound -for deaf and hard of hearing persons and for blind and weak sighted (audio subtitles). The paper also answers whether the lecture based automated video recording system evaluation is suitable for the needs of teachers and students with special needs.

Introduction

The information and communication technology (ICT) has been gaining popularity on both sides -providers and users. The popularity is a result of rapid development of broadband connections. It is well known in e-learning that participants need beside passive way of learning, which is typical for traditional learning, a possibility to get additional information. For better comprehension of the content which they were getting in traditional face-to-face class in the past, students were receiving this additional information by reading literature and attending extra live lectures, if they were available.

A question arises now, how can we provide additional information for immersing deeper knowledge with the help of ICT to students? One possibility is to give students an online video supported educational system, with feasibility of live streaming video and video on demand. Although the automated idea is fascinating, it is not easy to implement. First we have to record a lecture, next handle and synchronize it with the presentation material, and finally stream it on a website. Therefore, there is a strong need to accomplish fast and simple video streaming lectures either live or on demand with all additional information (presentation material, documents and transcriptions eventually).

Currently there are some webcast systems available, which enable video streaming together with a presentation material, for example the Microsoft Producer, Virage and GoodMoode, but these products require additional equipment and personnel for direct video streaming and later also the streaming of the materials to server. Moreover, these equipments do not consider entirely requirements for persons with special needs; only in rare cases (Horizon Live) they at least provide subtitles, which are suitable for deaf and hard of hearing persons. For blind or weak sighted persons a proper solution does not exist yet.

For this reason, we designed a lecture based automated video recording system. With its simple usage we tried to eliminate the necessity for extra personnel, which would have to be present during the lecture.

In this article we will focus on the following points:

• A detailed illustration of lecture based automated video recording system. As an example, the hardware and proper implemented software will be presented, which enables fast recording of lectures and transmission into a database of lectures.

• An explanation how we have taken into consideration the needs and demands of people with special needs (deaf, partially deaf and blind or weak-sighted) and hereby enabled them accessibility to these video e-materials.

• A presentation of possible evaluation tools, which are of vital importance for examination of the user-friendliness of video based educational system. Since these evaluations are of online courses, we have used evaluation methods on the basis of other studies, e.g. Squires and Price [START_REF] Squires | Predicting quality in educational software: Evaluating for learning, usability and the synergy between them[END_REF], Ardito et al [START_REF] Ardito | An approach to usability evaluation of e-learning applications[END_REF], Dringus [START_REF] Dringus | An adaptable Usability Heuristic Checklist for Online Courses[END_REF] and Holzinger [START_REF] Holzinger | Usability Engineering Methods for Software Developers[END_REF] and adjusted them considering the evaluated object. This includes the evaluation of: the user-friendliness of the e material, the usefulness of the whole system for people with special needs, accessibility and the effectiveness of the entire system.

Our innovative solution

All today's webcast systems provided by different manufacturers (for instance, Virage, Noterik, HorizonLive and GoodMood) offer mainly the same functionalities; that is a streaming video and sound, presentation slides, table of contents (TOC) and a system with interactive questions. The realization of additional services, such as simultaneously streaming the video interpreter and vocal subtitles (for blind and weak-sighted people) has not been provided yet. Additionally, these systems are expensive.

The idea of extended streaming video has been developed under the scope of project EQUAL which besides classical streaming medium enables additional functionalities (Figure 1). Our innovative solution includes the following:

audio subtitles for blind people with additional information about visual perceptions (for example, what holds a lecturer in his hands); video of sign language interpreter; subtitles for deaf and heard of hearing people (additional sound information, for example »peaceful music« or »menacing music«). This idea is substantiated by the proposal of European Broadcasting Union (EBU), which precisely defines recommendation for persons with special needs [1]. The European Broadcasting Union is the largest association of national broadcasters in the world. However, the EBU recommendations are designed for television. We have taken them into consideration and implemented several functionalities in our webcast system.

In the Faculty of Electrical Engineering and Computer Science, University of Maribor we have developed the automated video recording system for recording, webcasting and receiving lectures. We had two aspects in mind: simple usage with the possibility of eliminating any additional personnel and special dedication to people with special needs.

System's infrastructure

The infrastructure of the automated video recording system includes a mobile interactive system, with excellent video and audio equipment, an interactive streaming video technology and a virtual hypermedia study. All these technologies are integrated into a new learning environment. Besides the necessary video and audio equipment, the system uses an appropriate hardware and software. A student can attend live lectures or one can listen and observe lectures later, on demand. Additionally, subtitles can be added to the video and audio.

A trolley (Figure 2) is the core element of the system's infrastructure. It is composed of high speed computer (Figure 2 -1), LCD monitor (Figure 2 -2), keyboard and mouse (Figure 2 -3), wireless router (Figure 2 -4), wireless microphone (Figure 2345) and a digital video camera (Figure 23456) mounted on a robotic PTZ (pan/tilt/zoom) holder (Figure 234567). Listed parts are built into a solid aluminum framework placed on four wheels (Figure 2345678), which enable portability and can easily be shifted from one room to another.

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Figure 2: Portable video recording system -trolley A goal of the trolley is to stream lectures. In general the lecturer comes to a lecture room and turns on the trolley with one press on the power switch. The system automatically initializes itself and the application pops up on the monitor. Lecturer decides between two modes of streaming: live video streaming and video streaming on demand. He enters data and optionally loads additional materials. This is the first part of the procedure and can be done by one person.

In the second part, he walks to the presentation computer and loads another application. After that the lecturer selects initialized lecture and picks out the presentation slides, adjusts the digital camera and if everything is set, recording starts. PowerPoint presentation opens automatically and the lecturer starts to lecture. Recording of the lecture ends at the end of the slide show or with an ESC key. By watching the monitor the lecturer is checking the centric position.

As above mentioned, there are two modes of streaming. In live streaming mode, users can watch transmitted lectures on the web portal. If they miss a live lecture or do not have time for it, they can watch it later, on demand. These functionalities are feasible on the web portal. A snapshot of recorded lecture is illustrated on Figure 3. On the top we have lecture's title and

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lecturer's name. Video and sound is transmitted with Windows Media Player. Subtitles can be seen in live streaming mode with the help of typewriter, or they can be added additionally in on-demand mode of recording lectures. Table of contents is generated automatically from the presentation slides.

Figure 3: Viewing lecture on the web portal 4 Proposal of system's usability evaluation

Usability in general

Design methodologies in human computer interaction (HCI) aim to create functional user interfaces (e.g. computer applications). Therefore usability is an important factor for the evaluation of our webcast system. Despite recent advances of electronic technologies in elearning, a consolidated evaluation methodology for e-learning applications is not available [START_REF] Ardito | An approach to usability evaluation of e-learning applications[END_REF]. The question what usability is has several answers as the term itself has many meanings and definitions.

According to ISO 9241-11 [START_REF]ISO 9241: Software Ergonomics Requirements for office work with visual display terminal (VDT)[END_REF] usability may be defined as a measure to which a product (e.g. software) can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use. Therefore in computer science there is a strong relationship between quality and usability.

Usability is a quality attribute that assesses how user interfaces are usable. The word "usability" also refers to methods for improving the ease-of-use during the design process.

Nielsen [17] defined usability with five quality components:

• Learnability: How easy is it for users to accomplish basic tasks the first time they encounter the design? • Efficiency: Once users have learned the design, how quickly can they perform tasks? • Memorability: When users return to the design after a period of not using it, how easily can they re-establish proficiency? • Errors: How many errors do users make, how severe are these errors and how easily can they recover from the errors? • Satisfaction: How pleasant is it to use the design? There are many other important quality attributes. One of the most important is "utility" which refers to the design's functionality: Does it do what users need? Usability and utility are equally important since it matters little if something is easy to work on but does not do what we want. Neither does it help if the system has the potential to perform a task but the user is unable to reach it due to an inappropriate interface. To study design utility the same user research methods can be used as for improving usability [START_REF] Nielsen | Usability 101: Introduction to Usability, Jakob Nielse's Alertbox[END_REF]. The term "effectiveness" can be used instead of "utility" as the Oxford English Dictionary [START_REF]Oxford English Dictionary[END_REF] defines effective as "producing a desired or intended result". The other quality components, e.g. learnability, memorability and errors, have great impact on user satisfaction.

The need for usability has been recognized in web site design literature as a crucial quality when determining user satisfaction in such systems. Therefore it can be argued that the usability of e-learning application can significantly affect learning [START_REF] Costabile | On the usability evaluation of e-learning applications[END_REF].

Selected usability methodologies

There are several methods for studying usability that are suitable for testing on-line courses for students. Table 1 presents selected methods that we are going to conduct: heuristic evaluation [START_REF] Nielsen | Usability Engineering[END_REF] [10], SUMI questionnaire [13] and interviews [START_REF] Nielsen | Usability Engineering[END_REF]. These methods are in the process of evaluation, therefore we will only describe each method, but the results will not be presented yet.

Heuristic evaluation

Heuristic evaluation is the most popular informal inspection method in usability evaluation. For a specific area experts define if the elements of the user interface are according to heuristic rules -heuristics. They seek for errors and problems. Discovered problems are noted down with appropriate heuristic and the severity rating. To eliminate biases, it is important that experts identify problems (descriptive or visual) in isolation. In the end of the evaluation a group discussion follows. Experts collectively report about detected problems. The result of heuristic evaluation is the final heuristic report, which also contains (besides determined problems) recommendations for improvement of the user interface.

SUMI questionnaire

SUMI evaluation is a consistent method for assessing the quality of use of software products.

It is backed by an extensive reference database embedded in an effective analysis and report generation tool. SUMI has been hailed as the de facto industry standard questionnaire for analyzing users' responses to software. It is a commercially available questionnaire for the assessment of the usability of software which has been developed, validated, and standardized on an international basis. SUMI consists of 50 statements to which the user has to reply with either "Agree", "Don't Know", or "Disagree". SUMI gives reliable results with at least 10 users [START_REF] Kirakowski | SUMI: The Software Usability Measurement Inventory[END_REF].

The usability scales assessed by SUMI are:

• Efficiency (user's feeling that the software is quick and economic)

• Affect (user's emotional feeling that the software is stimulated and pleasant)

• Helpfulness (user's perception that the software communicates in a helpful way)

• Control (user's feeling that the software is responding in a normal and consistent way and assists him when errors occur) • Learnability (user's feeling to become familiar with the software, has a tutorial, handbooks etc.).

Semi-structured interview

Interviews can be seen as a "conversation with purpose". As in conversations the questions in interviews should be answered. Four main types of interviews exist: structured, unstructured, semi-structured and group interviews. Semi-structured interviews combine characteristics of structured and unstructured interviews and have open-ended and close-ended questions. In comparison to questionnaires, both give participants a set of questions and record their answers. Interviews involve an examiner who reads the questions to participant and then records the answers. In questionnaire participants fill out the answers by themselves. Interviews are flexible, since the examiner can explain or rephrase difficult questions. The struggling part is the analysis of results, which requires a lot of time. During evaluation examiners have to be cautious, not to give any help or hints. For this reason they have to remain neutral.

Present situation

Regarding system's usability evaluation we have proposed three methodologies (heuristic evaluation, SUMI questionnaire and semi-structured interviews), which we are momentarily conducting and are not finish yet. Because the heuristic evaluation was iteratively performed from the first phase of the system's development life-cycle, we can state something here. Five experts were included in the process. According to Nielsen [START_REF] Nielsen | How to Conduct a Heuristic Evaluation[END_REF] about 75% of all problems will be (in our case) identified. For noting down the problems we used Google Docs & Spreadsheets, a centralized Web-based application. Other two methodologies, SUMI questionnaires and semi-structured interviews, are going to be conducted additionally in the future.

Conclusion

The presented automated video recording system incorporates effective usage of technologies for video, audio and presentation material. For successful webcasting the appropriate software and hardware equipment is needed and also a proper infrastructure with qualified personnel.

The advantage of our system lies in its simplicity and swift usage. The webcasting lectures can easily be done only with one person only. Moreover, the system enables new possibilities in education. It offers a way to study equally for all participants, including people with special needs. For this reason, our system is designed and built in an innovative way, enabling elements for persons with special needs (visual subtitles for deaf people, audio subtitles for blind people and the video interpreter for sign language). This system will also bring new troubles and questions regarding authors' rights. Because of the psychological factor of lecturers, some authors do not want to be recorded, but we think this obstacle will disappear over the time, when they become more use to it.

Figure 1 :

 1 Figure 1: Extended webcast system

Table 1 :

 1 Proposed evaluation methods for usability testing of e-learning content.

		Heuristic	SUMI	Semi-structured
		evaluation	questionnaire	interview
	Purpose	Usability testing	Usability testing	Usability testing
	Project	Design	Final testing	Final testing
	Phase
	Object	E-learning	E-learning contents	E-learning contents
		contents
	Population	Experts	Users	Users

September 26 -28, 2007 Villach, Austria 7[START_REF] Nielsen | Usability Engineering[END_REF]

Primož, Kosec Universitiy of Maribor, Faculty of Electrical Engineering and Computer Science Smetanova 17, 2000 Maribor, Slovenia pkosec@uni-mb.si