This is expressed by saying that radiation from the surface of a black body in thermodynamic equilibrium obeys Lambert's cosine law. This means that the spectral flux d Φ(dA, θ, d Ω, dν) from a given infinitesimal element of area dA of the actual emitting surface of the black body, detected from a given direction that makes an angle θ with the normal to the actual emitting surface at dA, into an element of solid angle of detection d Ω centred on the direction indicated by θ, in an element of frequency bandwidth dν, can be represented as d Φ Φ ( d A , θ θ , d Ω Ω , d ν ν ) d Ω Ω = L 0 ( d A , d ν ν ) d A d ν ν cos ⁡ ⁡ θ θ where L (dA, dν) denotes the flux, per unit area per unit frequency per unit solid angle, that area dA would show if it were measured in its normal direction θ = 0.