## 2 Measurement Setup and Method
All the samples were disks with a diameter of 45 mm and a thickness of 2-2.6 mm. One of the phantoms (A) was a reference sample with known parameters. Various amounts of India ink were added to the two remaining samples (B, C) to pre-evaluate the possibility of increased absorption of these samples, as the estimated absorption of the reference sample is small--close to zero. All the samples are shown in Figure 3.  
The measurements were executed using one integrating sphere in two modes of operation: reflective and transmissive. The principle of operation in both modes is presented in Figure 1. In each configuration, the detector head is firmly installed on the 90\({}^{\circ}\) port, while the north pole port is plugged to maintain the repeatability of measurements and to eliminate the external influence of light. At the beginning of each measurement, the CS-A21 white calibration plate was mounted to the sample port to provide a reference. The luxmeter indicator is proportional to the total power of the source \(P_{\text{reference}}\). In the reflectance measurement mode, the white calibration plate is replaced with the investigated sample. In this configuration the light propagates through the reduced 180\({}^{\circ}\) port and the sphere, illuminating the sample. Part of the radiation, reflected to the sphere (\(P_{\text{R sample}}\)), is measured by the  
Figure 2: Setup for the measurement of diffuse reflectance \(R_{\text{d}}\). 1—integrating sphere Labsphere, 2—luxmeter measuring head, 3—light source, 4—luxmeter, 5—rotation stoppers, 6—optical breadboard, 7—prepared phantoms, 8—white calibration plate CS-A21 by Konica Minolta. luxmeter. Next, the sphere is rotated 180\({}^{\circ}\) around its axis, so that the baffle changes its position for the measurement of transmittance. In this configuration, the 180\({}^{\circ}\) port is plugged. The light directly illuminates the sample. Similarly, the luxmeter measured part of the radiation that was transmitted into the sphere (\(P_{\text{T sample}}\)). Figure 4 presents a block diagram of the data analysis algorithm. The obtained measurement values were used to calculate the diffuse reflectance \(R_{\text{d}}\) and the diffuse transmittance \(T_{\text{d}}\) according to Formulas (1) and (2):  
\[R_{\text{d}}=\frac{P_{\text{R sample}}}{P_{\text{reference}}} \tag{1}\]  
\[T_{\text{d}}=\frac{P_{\text{T sample}}}{P_{\text{reference}}} \tag{2}\]  
where \(P_{\text{R sample}}\) and \(P_{\text{R reference}}\) are measurements in reflectance mode obtained from the investigated phantom and of the CS-A21 white calibration plate, respectively. Similarly, \(P_{\text{T sample}}\) is a measurement obtained by the investigation in transmittance mode. The calculated values of diffuse reflectance \(R_{\text{d}}\) and diffuse transmittance \(T_{\text{d}}\) were used as a basis for determination of absorption coefficient \(\mu_{\text{a}}\) and reduced scattering coefficient \(\mu_{\text{s}}\)\({}^{\prime}\) according to the Kubelka-Munk model using the following formulas [26]:  
\[S_{\text{KM}}=\frac{1}{b\cdot d}ln\left[\frac{1-R_{\text{d}}\cdot(a-b)}{T_{ \text{d}}}\right] \tag{3}\]  
\[K_{\text{KM}}=(a-1)*S_{\text{KM}} \tag{4}\]  
where \(S_{\text{KM}}\)--Kubelka-Munk scattering coefficient, \(K_{\text{KM}}\)--Kubelka-Munk is the absorption coefficient, \(d\)--sample thickness, and:  
\[a=\frac{1+R_{\text{d}}^{2}+T_{\text{d}}^{2}}{2R_{\text{d}}}(a-1)\cdot S_{ \text{KM}} \tag{5}\]  
Figure 1:0  
Figure 1:1 Block diagram of the data analysis algorithm, where \(P_{\text{R}}\)—reference measurement using the CS-A21 white calibration plate in reflectance mode. \(P_{\text{R sample}}\), \(P_{\text{T sample}}\)—measurements of the investigated phantom in reflectance and transmittance modes, respectively. \(T_{\text{d}}\)—diffuse transmittance. \(R_{\text{d}}\)—diffuse reflectance. \(\mu_{\text{a}}\)—absorption coefficient. \(\mu_{\text{s}}\)’—reduced scattering coefficient. Figure 1:2 Produced phantoms.