Does anyone know a good reference which discusses the thermodynamics of electromagnetic radiation? Not blackbody radiation, or radiative transfer, but assigning thermodynamic quantities (temperature, entropy, free energy, etc.) to the field itself? The initial question is simple enough: What's the temperature of a laser beam? The only decent references I could find are: Madel & Wolf, "Optical Coherence and Quantum Optics" , chapter 13 Essex, Kennedy and Berry,"How Hot is Radiation?", Am. J. Phys 71, 969-978 (2003) Essex's derivation requires a quantum description of the field- and since photon number is not conserved, it's not clear how applicable their result is. Mandel and Wolf discuss "thermal light", but I couldn't make heads or tails of their results. I'm thinking about simple things: even blackbody radiation that has passed through a (passive) spectral filter- what's the thermodynamics? Any help is appreciated.