# The Study of Tooth Erosion Tested by the Color Digital Holography (CDH) Detection System
###### Abstract  
This study presents a color digital holography (CDH) detection system that includes the optical path and a numerical calculation program to detect tooth erosion under different acid-etching conditions. Etched cattle enamel samples were pressurized within the elastic interval, and the CDH detection system was used to rebuild the three-dimensional deformation field of these samples. [PERSON]'s modulus of these samples was employed to evaluate the mechanical properties of eannels by statistical methods. The results show that Young's modulus of enamel decreases with the increase in etching time, while it decreases slightly with the increase in the etching temperature, and the etching temperature has no effect on the non-etched enamels. Moreover, the detection process proves the CDH detection system has the advantages of high precision, full-field detection, and being non-destructive, fast, and stable. It is expected to have broad application potential for CDH detection systems in the detection of tooth erosion.  
tooth erosion; color digital holography; [PERSON]'s modulus +
Footnote †: journal: Applied  
evaluation methods include scanning electron microscopy (SEM) [16; 19; 20], atomic force microscopy (AFM) [21], confocal laser scanning microscopy (CLSM) [20; 22], ordinary optical microscopy [13], and surface 3D focus variation scanning microscopy [16]. These methods have the disadvantages of complex equipment and high cost, and apply only to qualitative research. In addition, the detection accuracy of ordinary optical microscopy is low, the single detection area of AFM is only 250 \(\upmu\)m \(\times\) 250 \(\upmu\)m [13], and surface 3D focus variation scanning microscopy is time-consuming and suffers from low repeatability [13; 16]. Therefore, there is an urgent need to study new detection methods of tooth erosion.  
Digital holography (DH) is a new optical detection method. DH generally uses a single wavelength laser to irradiate objects. A digital image sensor is used to collect holograms, and to reconstruct the wavefront, before and after the object changes [23]. The results of the two reconstructions are subtracted to calculate the phase difference. The change information of the object deformation can be obtained by numerical calculation based on the phase difference. DH has been widely used in three-dimensional shape detection [24], temperature field detection [25; 26], the measurement of small vibrations of the strain of transparent objects [27], and other fields [28]. DH has high detection accuracy and a straightforward system structure, and is a non-destructive and non-contact method.  
Color digital holography (CDH) uses three different wavelengths of laser to illuminate the object to form three non-coplanar change vectors, and then uses a color digital image sensor to collect the three-channel color hologram [29; 30]. The three-dimensional change information of the object is obtained by processing the hologram [30; 31; 32]. CDH can directly reconstruct the three-dimensional deformation field of the object compared with ordinary DH, and has greater acquisition efficiency and real-time reproduction, and a wider application range [33; 34; 35]. However, there is no literature report on the application of DH to the study of tooth etching, and no relevant report on CDH.  
In this study, a CDH detection system was built, and bovine enamel was acid etched by carbonated drinks. The deformation field of the bovine enamel samples under different acid-etching conditions was detected by the CDH detection system, and [PERSON]'s modulus of these samples was obtained. The influences of different acid-etching conditions on the mechanical properties of bovine enamel were quantitatively analyzed.