Abstract: | Only within the coherence time of a light source are the emitted photons indistinguishable. For solid state light sources the coherence time may be as short as a few picoseconds. We will present a method by which photon indistinguishability can be studied on such short time scales in terms of photon-photon correlation functions. A few examples shall be given where this method has been applied. Namely the transition of a semiconductor microcavity from a thermal, chaotic to a coherent light emitter will be addressed and also the emission from polariton condensates will be investigated, for which full coherence will be demonstrated |