I'm trying to reproduce a plot of Sun's black-body behavior like this one: http://en.wikipedia.org/wiki/File:Solar_Spectrum.png Problem is, after I convert the black-body radiance to irradiance, its curve is way too high as compared with measurement. The measurement data is taken from: http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.html The top of atmosphere (TOA) irradiance at Earth's distance is obtained in the following way: radiance (W/m^2/nm/Sr) L=2*h*c^2/(lamda^5*exp(h*c/(kB*lamda*T)-1)) where: c=3e8 m/s (speed of light) h=6.625e-34 Joul Second (Planck's) kB=1.38e-23 Joul/Kelvin (Boltzman's) omega=pi*r_sun^2/D_sun_earth^2 (Sun disk solid angle as seen from Earth) r_sun=6.96e8 m (Sun's radius) D_sun_earth=1.496e11 m (1AU) Finally irradiance is E=L*omega (W/m^2/nm) (and one needs to multiply 1e9 to be in nm) My curve is roughly twice above the measurement, so if I do: E=L*omege*cos(67-deg) I can get something close to the picture in the wiki link. This 67-deg is roughly Earth's spin inclination. However I really doubt multiplying cos(67-deg) makes sense, as we are talking about TOA irradiance, not anywhere on Earth surface. What I'm missing here? Thanks!