sentences:  The radiative transfer equation is a monochromatic equation to calculate radiance in a single layer of the Earth's atmosphere.To calculate the radiance for a spectral region with a finite width (e.g., to estimate the Earth's energy budget or simulate an instrument response), one has to integrate this over a band of frequencies (or wavelengths).The most exact way to do this is to loop through the frequencies of interest, and for each frequency, calculate the radiance at this frequency.