Thermal radiation and amplified spontaneous emission from a random medium

We compute the statistics of thermal emission from systems in which the radiation is scattered chaotically, by relating the photocount distribution to the scattering matrix - whose statistical properties are known from random-matrix theory. We find that the super-Poissonian noise is that of a black body with a reduced number of degrees of freedom. The general theory is applied to a disordered slab and to a chaotic cavity, and is extended to include amplifying as well as absorbing systems. We predict an excess noise of amplified spontaneous emission in a random laser below the laser threshold.