## Materials and Methods
### Spectral Optical Measurements  
Solar reflectance measurements were based on measurement of the fraction of the incident beam light of the wavelength reflected by a surface. A Lambda 1050 UV/VIS/NIR spectrophotometer (PerkinElmer, Waltham, MA, USA) equipped with a 150 mm Spectralon integrating sphere was used for this purpose. This device can register spectral reflectance properties from 200 to 3300 nm. Spectral curves and integrated total solar reflectance (TSR) values from 280 to 2500 nm are provided. The solar reflectance was calculated using the method of (ASTM) G173 standard [38] for hemispherical global tilt irradiance for air mass 1.5, and each specific region is demonstrated. This is described in \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Material** & **Surface Shade** & **Color** \\ \hline S1 & Blacksmith refractory paint & semi-gloss & black \\ S2 & Synthetic paint & mat & black \\ S3 & Synthetic paint & gloss & black \\ R1 & Spectrally selective & mat & black \\  & material—TiNOx-Nano & & \\ R2 & Spectrally selective & gloss & dark blue \\  & material—TiNOx-Cu & & \\ R3 & Spectrally selective & gloss & dark blue \\  & material—TiNOx-Al & & \\ P1 & Polymer based solar absorber & gloss & black \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of measured solar absorber samples. Figure 1: Solar absorber development: (**a**) scheme of the developed absorber: 1—polyethylene absorbent layer with 5% pigment, thickness (th.). 20 μm; 2—adhesive polyethylene bridge, th. 2 μm; 3—vapor-coated aluminum, th. 0.04 μm; 4—structural layer of MPET, th. 23 μm; (**b**) industrial roll-to-roll production line; (**c**) sample of a prototype with biaxially oriented polypropylene fabric. Equation (1), where \(R\) is the percent reflectance, \(I\) is the solar irradiance, and \(d\lambda\) is the wavelength interval of integration:  
\[\%TSR=\frac{\int\left(\%R\times Id\lambda\right)}{\int\ Id\lambda}\times 100 \tag{1}\]  
The available principles for the determination of spectral emissivity (reflectance) are based on spectral analysis with infrared spectroscopy in the longwave region. Reflective measurements utilizing a Michaelson wave interferometer and Fourier transform (FT-IR) were selected for this purpose. Therefore, a Nicolet 380 infrared spectrometer from the Thermo Electron Corporation (Waltham, MA, USA) equipped with a Mid-IRtm IntegratIR integrating sphere from PIKE Technologies (Madison, WI, USA) was used. This device can eliminate any reflected radiation loss by integrating a sphere from the whole measured surface area. The principle of the test is to measure the specimen on the reflection of the beam (angle of incidence of 8\({}^{\circ}\)) of infrared radiation ranging from 2.5 to 18 \(\upmu\)m of wavelength. The weighted average reflectance was calculated for 80 \({}^{\circ}\)C. Subsequently Equation (2) based on the law of the conservation of energy and [PERSON]'s laws was used for determination of the measurement results in terms of emissivity values:  
\[\varepsilon=\alpha=1-\rho,\text{ or }\varepsilon_{\lambda}=\alpha_{\lambda}=1- \rho_{\lambda} \tag{2}\]  
Consequently, [PERSON]'s formula (Equation (3)) of spectral radiance intensity of 80 \({}^{\circ}\)C (353 K) black body \(M_{0,\,\lambda}\) as the weighted function for determination of emissivity values was applied:  
\[M_{0,\lambda}(\lambda,T)=C_{1}\times\lambda^{-5}\times\ (\varepsilon^{\frac{C_{2}}{ kT}}-1)^{-1}\ \left[\text{W-}\text{m}^{-2}\text{:}\mu\text{m}^{-1}\right] \tag{3}\]  
Finally, the simplified calculation of the optical efficiency was performed using two parameters: selectivity (\(f\)) and figure of merit (_FM_, e.g., in [39; 40]) according to Equation (4). The first parameter can practically and effectively characterize performance as the selectivity parameter [41] with higher \(f\) indicating more selective material. However, it should be noted that the optical efficiency is affected by other parameters of a particular absorber application.