Thermodynamic Entropy of Partially Coherent Light Beams

The thermodynamic entropy of partially coherent light beams, as shown by M. von Laue, is given by the sum of the entropy of the principal waves, which represent the given light beams and are statistically independent of each other. The principal waves and their intensities are respectively the eigenfunctions and eigenvalues of the mutual intensity, cross-correlation function of waveamplitudes, which is positive semidefinite Hermitian operator. The well-known entropy of thermal radiation is applied to principal waves obtained after appropriate reversible optical transmission.