For this model, the spectrum is sub-divided into spectral bands of finite width where radiative quantities are nearly uniform or can be averaged without losing accuracy. These bands should span the thermal radiation section of the spectrum. It is assumed that the value at a given spectral band is represented by the spectral band midpoint value in frequency domain.

CFX assumes that the main spectral variable is frequency because it is independent of the material refractive index and it will facilitate the setup of multidomain problems. Other spectral variables, such wavelength and wavenumber would be available for vacuum only.

Then, the radiative transfer equation is integrated within is spectral band and a modified RTE is obtained:

(8–37)

for , where the emission within the spectral band is weighted by:

(8–38)

After solving one RTE per spectral band, total radiation intensity can be computed as:

(8–39)

This immediately suggests that for an -band model, times as much work is required as for a gray, -band model. In the case of the Discrete Transfer model, for small this turns out not to be true because the tracking of the rays through the geometry is a major one-off overhead.

This model can be used in conjunction with all available radiation models.