## 2 Measurement Setup and Method  
The measurement system was constructed using a 4P-GPS-053-SL (Labsphere Inc., North Sutton, NH, USA) integrating sphere. The internal diameter of the sphere is 5.3 in (13.46 cm), while the sample port diameter is 1 in (2.54 cm) [27]. The sphere utilizes a Spectralon(r) coating (Labsphere Inc., North Sutton, NH, USA), which provides a highly reflective surface--a diffuse reflection coefficient of above 99% for wavelengths from 400 to 1500 nm. Laser modules (Roithner Lasertechnik, Vienna, Austria) of red, green and blue radiation were used as the light sources during this investigation, i.e., a laser diode module with a wavelength of 635 nm (LDM635-03-08X25, red), a diode pumped solid state (DPSS) laser module operating on the second-harmonic with a wavelength of 532 nm (CW532-5-831, green) and a laser pointer with a wavelength of 447 nm (GLP-447-20, blue). These laser sources were supplied by voltages of 3 V (red), 3.3 V (green) and 4.7 V (blue). To ensure power stability for the standard GLP-447-20 laser pointer, the \(2\times 3\) V CR2 battery power supply was replaced by a laboratory power supply and the voltage was reduced to 4.7 V.  
All light sources utilized during the examination are of wavelengths within the visible spectral range [28]. They are also used for animal treatment in veterinary medicine. To ensure the accuracy and repeatability of the performed measurements, an L-100 luxmeter with a dedicated measuring head (Sonopan, Bialystok, Poland) was used as a detector. The measuring head of a luxmeter has a Lambertian input characteristic. The white calibration plate CS-A21 (Konica Minolta, Tokyo, Japan) was used as a reflectance standard (reflection coefficient--96.9%) [29]. The measurement system scheme is presented in Figure 1.  
The integrating sphere, the light source and the luxmeter measuring head were precisely mounted on a \(15\times 60\) cm optical breadboard (Thorlabs, Newton, NJ, USA) to provide acceptable system stability, which allows for the acquisition of accurate results. Furthermore, the integrating sphere can be rotated around its own axis, which helps to quickly change between configurations for transmittance and reflectance measurement. The stoppers, properly set on either side of an integrating sphere, guarantee that the sphere rotates exactly 180\({}^{\circ}\). The determination of scattering and absorption coefficients of a sample are based on relative measurements (absolute output is not very significant, provided it is in the sensitivity range of the  
Figure 1: Measurement system. (**a**) Mode for the measurement of diffuse reflectance \(R_{\mathrm{d}}\), (**b**) mode for the measurement of diffuse transmittance \(T_{\mathrm{d}}\). 1—Light source, 2—detector, 3—baffle, 4—sample port, 5—examined sample, 6—unused plugged port. detector). However, before performing measurements, each source was turned on for approximately 30 min (45 min in case of the blue source) for stabilization of the thermal condition. Room temperature was stabilized to 23 \({}^{\circ}\)C with an accuracy of 1 \({}^{\circ}\)C. Additionally, the aforementioned quick change between configurations allowed us to take measurements within a short amount of time (1-2 min), which is a key factor for relative measurements. The system operator aimed to eliminate all possible external light sources to maintain un-biased results. Therefore, all lighting was obscured during measurements. Figure 2 presents the physical implementation of the measurement system. To verify the correct operation of the system, measurements of diffuse reflectance and transmittance were executed. Furthermore, the coefficients of reduced scattering and absorption in a few scattering media (phantoms) were calculated [30]. Examined phantoms were prepared from the silicon-based organic polymer polydimethylsiloxane (PDMS) (Sylgard(r) 184, Dow Corning, Midland, MI, USA) and high clarity glycerol (Sigma-Aldrich, St. Louis, MO, USA). The details were described by [PERSON] et al. [30].