## 2 Materials and Methods
### Preparation of the Sample
A dental handheld spectrophotometer (Spectro Shade Micro; MHT Optic Research, Niederhashi, Switzerland) was used for all color measurements according to the CIELab system. It emits light with a range of 410 to 680 nm; data are calibrated between 400 nm and 720 nm with a 10 nm pass for the captured image. It measures light at 2 \(\times\) 45\({}^{\circ}\), polarization at 0\({}^{\circ}\), and telecentric polarization. It has an 18 mm \(\times\) 14 mm reading area on 640 \(\times\) 480 points. The digital resolution is 307,200 \(\times\) 640 (n. of spectral curves), and in terms of optical resolution, each point is approximately 0.03 mm \(\times\) 0.03 mm. Repeatability for teeth is \(\Delta\)E \(<\) 0.5. The inter-instrument agreement for the teeth is \(\Delta\)E \(<\) 1.0. It features a black-and-white CCD sensor for reading spectral data. This handheld spectrophotometer has a built-in computer with analytical software and internal memory for the storage and the transfer of images and spectral data. Additionally, dental shades can be read in any lighting situation (natural or artificial light, fluorescent light, etc.). The CIELab system is a uniform color scale, where the L * value measures the lightness or luminosity of an object; a * is a measure of redness (positive) or greenness (negative); and b * is a measure of yellowish (positive) or blueness (negative). The color difference (\(\Delta\)E) of a test sample is reported by delta values, \(\Delta\)L *, \(\Delta\)a *, and \(\Delta\)b *, compared with standard conditions. These differences are calculated according to the following formula: [\(\Delta\)E = [(\(\Delta\)L *)2 + (\(\Delta\)a *)2 + (\(\Delta\)b *)2 ] /2] [28]. The tooth positioning angle control guidance system of the Spectro Shade Micro (MHT Optic Research, Niederhasli, Switzerland) device was used to ensure the standardization and repeatability of the measurements. This system was displayed on the LCD touchscreen (Figure 1). By using the angle control system, the colorimetric and morphological properties of the oral cavity can be considered. Calorimetrically, the gums are red, the teeth are white, and the oral cavity is black. Therefore, the angle control will not work if an image of a crown inserted into a white stone model is acquired, since the SpectroShade Micro will not recognize the components. To obtain the benefit from the angle control guidance system of the Spectro Shade Micro device, the crowns were fitted into a typodont model to simulate the oral cavity's morphology, as shown in Figure 2.  
Figure 1: Standardization of the measurement method through the tooth positioning guidance system of the Spectro Shade Micro device (Spectro Shade Micro, MHT). Figure 2: The crowns fit into the typodont model to obtain the benefit from the guidance system of the Spectro Shade Micro device for the standardization of shade measurements. The color of crowns was determined following the same steps used for shade selection in clinical situations, as follows: The device was calibrated on the white and green backgrounds provided on the device base. The Optic Handpiece was positioned at a 90-degree angle to the target tooth and flushed against the gum line. Using the tooth positioning guidance system, which displayed two yellow bars and a yellow frame overlaying the live image, the tooth under measurement was centered in the image. As the two bars crossed, they formed a yellow cross at the center of the acquisition window and the frame. There was no movement of the yellow frame or yellow cross. With the target tooth framed within the yellow frame as much as possible, some graduated references appeared on the yellow vertical bar. A vertically moving horizontal line indicates the acquisition angle by varying the angle-shot between the Optic Handpiece and the target tooth. The closer the horizontal line to the horizontal bar, the closer to the optimal angle-shot the angle is. The shot was taken when the line was green and superimposed on the yellow horizontal bar.