Top left: gray atmosphere pressure-temperature profiles for T_eff = { 400,800,1200 } K (solid color lines, see Eq. (20)). The condensation curve of enstatite (black dashed line) is shown for solar metallicity (Burrows et al. 1997). Top right: mixing ratios of the most abundant gas molecules and atoms for the T_eff = 800 K model (Mollière et al. 2017). Bottom left: absorption cross sections of the most abundant gas molecules (solid color lines), calculated for T = 1000 K and P = 1 bar, as well as the Rayleigh scattering cross section of H₂ (black dashed line). Bottom right: emission spectra of cloudless gas giant atmospheres at a distance of 10 pc with effective temperatures of 400, 800, and 1200 K (solid color lines) together with the Planck functions for the same temperatures (dashed color lines). A comparison spectrum for which scattering is neglected is shown for the 400 K atmosphere (black dashed line). The width of the SPHERE/IRDIS broadband filters are displayed on the bottom of the figure. Vertically dotted lines correspond to the central wavelengths of the IRDIS dual-band filters (from left to right: Y2, Y3, J2, J3, H2, H3, H4, K1, and K2) which have a typical width of 50 nm in the Y, J, and H bands, and 100 nm in the K band.