Rosseland optical depth where monochromatic optical depth is unity – which reflects the strength of opacity – as a function of wavelength computed based on the (3D) sunA09 model (cf. Table 4 and Sect. 3 for basic information about the model and how it is constructed) and Blue opacities assuming the AGSS09 solar abundance. One out of ten points in our wavelength sampling is shown to avoid overcrowding the figure. Grey boxes numbered from 1 to 12 in red define the opacity bins used for the sunA09 simulation. Bins 1–3 divide the ultraviolet wavelength region according to the location where flux emerges; Bins 4, 5, and 9 mainly consist of wavelength points distributed near the continuum-forming layers; Bins 8 and 12 include only wavelengths with strong opacity, which typically correspond to lines.