First, you want to do spectral math not band math. 1. Create a spectral library containing the solar spectrum and plot it (solar spectrum is ESUN in their equation). 2. In spectral math: (double(s1)*!dpi*d2)/(s2*cosi) where d2 is the earth-sun distance squared that you have, and cosi is the cosine of incidence angle (convert to radians if you have it in degrees) for s1, select "map variable to input file", and select your radiance cube. For s2, select the solar spectrum.