Radiation over a range of wavelengths for which the Planck function does not change significantly, but for which the spectral absorption coefficient may be highly variable

Transmission functions for narrowband radiation, unlike the exponential function for monochromatic radiation (see Bouguer's law), require the application of band models or numerical techniques such as the correlated-k method. Spectral intervals for narrowband radiation have widths that are typically 100 cm⁻¹.