where we have substituted E3 (0) = 0.5. The total spectral power emitted by a planet is then 4-KR2FVi, where R is the radius of the planet and hence the measured spectral flux or irradiance of the disk-averaged spectrum seen at a distance D is given by
The spectral irradiance may be measured in units such as Wcm 2(cm 1) 1, Wm-2 p.m-1, or sometimes in Janskys (for Earth-based observations), which have units of 1 Jy = 10-26W m 2 Hz 1. To obtain the disk-averaged spectral radiance of a planet, we then divide this spectral irradiance by the solid angle projected by the planet which is equal to -k(R/D)2. Hence, the disk-averaged spectral radiance is given by
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