We explore algorithms of estimating spectral flux over the entire longwave spectrum solely from hyperspectral radiance observations using AIRS data as an example. This is different from the traditional approach of estimating broadband flux from satellite observations in two ways: (1) no other remote sensing data sets are needed, and (2) the spectral details of the broadband flux can be derived. This study shows that the hyperspectral radiances can be used to directly obtain spectral flux.