For a black body surface, the spectral radiance density (defined per unit of area normal to the propagation) is independent of the angle θ θ of emission with respect to the normal. However, this means that, following Lambert's cosine law, B ν ν ( T ) cos ⁡ ⁡ θ θ is the radiance density per unit area of emitting surface as the surface area involved in generating the radiance is increased by a factor 1 / cos ⁡ ⁡ θ θ with respect to an area normal to the propagation direction. At oblique angles, the solid angle spans involved do get smaller, resulting in lower aggregate intensities.