^Herrmann, F.; Würfel, P. (2005). "Light with nonzero chemical potential". Am. J. Phys. 78 (3). American Association of Physics Teachers: 717–721. doi:10.1119/1.1904623. Retrieved 2012-12-20. A necessary condition for Planck's law to hold is that the photon number is not conserved, implying that the chemical potential of the photons is zero. While this may be unavoidably true on very long timescales, there are many practical cases that are dealt with by assuming a nonzero chemical potential, which yields an equilibrium distribution which is not Planckian.