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\subsection{Photon streams}  \label{sec:ph_streams}  The fundamental data at the core of smFRET experiments is the array of photon timestamps. In single-spot measurements, all the timestamps are stored in a single array. In multi-spot measurements we have as many timestamps array arrays  as the number of excitation spots. Each timestamps array contains timestamps from both the donor (D) and the acceptor channel (A). In ALEX measurement, measurements,  we can further differentiate between photons emitted during D and A excitation periods. In FRETBursts the different selection selections  of photons/timestamps are called "photon streams" and they are specified with a \href{http://fretbursts.readthedocs.org/en/latest/data_class.html#module-fretbursts.ph_sel}{\texttt{Ph\_sel} object} . In non-ALEX smFRET data we have 3 base photon streams (table~\ref{tab:ph_sel_smfret}), while in ALEX data we have 5 base photon streams (table~\ref{tab:ph_sel_alex}).  The \href{http://fretbursts.readthedocs.org/en/latest/data_class.html#module-fretbursts.ph_sel}{\texttt{Ph\_sel} class} allows to express any combination of photon streams.