The z-grid of optical properties is determined by the atmosphere_file, and, if specified, by other profile files like mol_file, rh_file, or refractive_index_file. Additional levels might be introduced by the zout option and the second argument of the altitude option. By default (if zout_interpolate is not specified) levels introduced by the zout option will not affect the optical property profiles, that is, the optical properties are constant within the layers specified by the atmosphere_file and profile files. If zout_interpolate is specified, the atmospheric profiles (tracegases, temperature …) are interpolated to the levels introduced by zout, and optical properties are determined from the interpolated atmospheric properties. If heating_rate, rte_solver polradtran, rte_solver rodents, or rte_solver twostrebe is specified, zout_interpolate will also be automatically activated. zout_interpolate generally causes smoother variation of the optical properties.