The quantity B ν (ν, T) is the spectral radiance as a function of temperature and frequency. It has units of W · m · sr · Hz in the SI system. An infinitesimal amount of power B ν (ν, T) cos θ dA d Ω dν is radiated in the direction described by the angle θ from the surface normal from infinitesimal surface area dA into infinitesimal solid angle d Ω in an infinitesimal frequency band of width dν centered on frequency ν. The total power radiated into any solid angle is the integral of B ν (ν, T) over those three quantities, and is given by the Stefan–Boltzmann law. The spectral radiance of Planckian radiation from a black body has the same value for every direction and angle of polarization, and so the black body is said to be a Lambertian radiator.