## 2 Materials and Methods
### Color Measurement  
An FR-\(\upmu\) Probe instrument was used in conjunction with a trinocular optical microscope (Figure 1). This setup incorporated two optical components: a spectrometer, which utilizes the microscope's light source and operates in the spectral range the microscope supports, and a module that is mounted on the microscope's C-port. Typically, for a 50\(\times\) objective lens, the monitored area size is a \(\sim\)5 \(\upmu\) diameter circle. Higher magnification or smaller aperture settings allow further sampling area reduction. Real-time specialized software (FR-Monitor\({}^{\otimes}\), ThetaMetrisis, Athens, Greece) processing is implemented in Visual C++ to allow film thickness (<10 nm up to 100 \(\upmu\)m) and optical constant (n & k) calculations of the areas of interest [23; 24; 25]. Very fast computations of reflectance (absorbance, transmittance, and fluorescence are also available) spectral measurements of free-standing and supported (over transparent or partially/fully reflective substrates) stacks of (<10 layers) films are accurately calculated utilizing the White Light Reflectance Spectroscopy (WLRS) algorithm (ThetaMetrisis\({}^{\text{TM}}\)) [26; 27; 28], which were employed for the measurements in this study.  
The definition of absorbance (A(\(\lambda\))) is  
\[\text{A}(\lambda)=-\text{log10}(\text{Is/Io}) \tag{1}\]  
where Io is the reference light intensity, typically the emission light, e.g., with an empty cuvette, and Is is the light intensity through the sample of interest. For precise measurements, the dark spectrum should be subtracted from both sample and reference spectra; thus, Equation (1) becomes  
\[\text{A}(\lambda)=-\text{log10}((\text{Isam-dark})/(\text{Iref-Idark})) \tag{2}\]  
where Iref is the reference light intensity, i.e., the light intensity of the light source used, as it is recorded by the spectrometer without passing through the sample of interest, e.g., with  
Figure 1: FR-\(\upmu\) probe installed on an optical microscope using an internal light source.  
an empty cuvette for the liquid sample. Isam is the light intensity after passing through the sample, i.e., the light intensity recorded by the spectrometer when the light from the light source passes through the sample of interest.  
Idark stands for the dark intensity, i.e., the light intensity recorded by the spectrometer with the light source switched off or blocked and without a sample. Usually, the absorbance value is in the [0, 2] range, reflecting the [0-99]% absorbance range. On the other hand, the definition of transmittance (T(\(\lambda\))) is  
\[\text{T}(\lambda)=\text{Is/Io}\times 100\% \tag{3}\]  
and by considering the dark spectrum, as in the absorbance case  
\[\text{T}(\lambda)=(\text{Is/Io}\times 100\%) \tag{4}\]  
Figure 2 depicts the typical equipment configuration for optical measurements.  
The workflow sequence consists of (a) the spectra recording, (b) specific wavelength(s) monitoring, and (c) the integration of a specific part of the spectrum. The FR-\(\upmu\)Probe and FR-Monitor ver.1.3 software were used following the above description. The experimental average depth in 5 different positions of each denture resin disc sample was calculated [25]. Then, in order to define the real difference in color measurements, the values of \(\Delta\)E1_0, \(\Delta\)E2_0, and \(\Delta\)E3_0 were presented in Arbitrary Units (U).