With a detailed series of spontaneous optical pattern formations, we reveal the existence of thermodynamical properties in nonlinear optical systems by defining the configurational entropy from the measured patterns. A phase diagram for pattern transitions in the form of stripes, reoriented stripes, hexagons, and spots is reported experimentally and theoretically for incoherent beams in non-instantaneous anisotropic photorefractive crystals, with demonstrations in the boundary of mixed-phase states. Emergence of optical pattern transitions is shown to occur at the local minimum values of effective optical temperature, and accompanied with the change of chemical potentials.
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