Radiative Intensity is the radiative energy flow per unit solid angle and unit area normal to the rays (as opposed to the emitting surface) - **spectral intensity** $I_\lambda$ = radiative energy flow/time/area normal to rays/solid angle/ wavelength - **total intensity** $I_\lambda$ = radiative energy flow/time/area normal to rays/solid angle [[Emissive Power]] an be obtained from intensity by integrating it across all directions $E_{b\lambda}(r,\lambda)=\pi I_{b\lambda}(r,\lambda)$ A black body is diffuse and so emits the same intensity in all direction. The spectral blackbody intensity is called *Planck function* Directionally the relation between intensity *normal to rays* and flux per unit *surface area of emitting surface* has a cosine relationship (because of how the areas are related) O This is called *Lambert's cosine law* - radiant intensity or luminous intensity observed from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle θ between the observer's line of sight and the surface normal [Lambert's cosine law - Wikipedia](https://en.wikipedia.org/wiki/Lambert%27s_cosine_law)