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measurements we have as many timestamps arrays as the number of excitation  spots.  Each arrayof timestamps  contains timestamps from both the donor (D) and the acceptor channel (A). (A) channels.  In ALEX measurements~\cite{Lee_2005}, we can further differentiate between  photons emitted during D and A excitation periods. In FRETBursts the different  selections of photons/timestamps are called "photon streams" and they are