Let us further explore which transparent substrates are most promising for control of & at the visible and MIR spectral window. The transmission optical window of sapphire is down to 5 m in IR, while the absorbance of soda lime glass starts at a shorter that 2.8 m wavelength. This absorption limitation appeared in the transmission spectra in MIR. If there is no absorption by the substrate, it would be expected to observe the optical window effect for radiative cooling at the MIR wavelengths. Also, sapphire substrate can be useful to control the optical absorption at MIR, i.e, if we need a transparency window extending to the longer wavelengths, CaF 2 , MgF 2 , LaF 3 can be utilised as they reach the 7.5-to-10 m cut off wavelengths. Quartz or fused silica are not transparent at the MIR range and strongly absorbs in that spectral range. For the propose of this study, the application of window for buildings, we need to block the near-IR-to-MIR light that is the heating source for the room. Hence, soda lime glass is better for that purpose. However due to reciprocity, the heat generated inside room is not leaving it by radiation through the window (it is absorbed and reflected back causing a greenhouse effect).