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Identifying single-molecule bursts in the stream of photons is
one of the most crucial steps in the analysis of freely-diffusing single-molecule FRET data.
The widely used
"sliding window" ``sliding window'' algorithm, introduced by the Seidel group in 1998
(\cite{Eggeling_1998}, \cite{Fries_1998}), involves searching for
$m$ consecutive photons detected during a period shorter than
$\Delta t$. In other words, bursts are regions of the photon stream where the
local rate (computed using $m$ photons) is above a
minimal rate chosen as a
threshold. minimum threshold rate.
Eggeling did not provide
any criteria a criterion on how to choose the rate
threshold and the number of photons $m$
and as therefore and, therefore, it has become a common
practice to manually adjust those parameters for each specific measurement.
A more general approach consists in taking into account the background rate of
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during acceptor
excitation (\verb|Ph_sel(Aex='Aem')|).
After each burst search, it is
useful necessary to select
bursts having a minimum number of photons (burst size). In the most
basic form, this selection can be performed during burst search by discarding
bursts with size smaller than a threshold $L$, as originally proposed by
Eggeling~\textit{et al.}~\cite{Eggeling_1998}.
This method, however, neglects the effect
of background and
gamma γ factor on the burst size and can lead to a selection
bias of certain channels and/or sub-populations.
For this reason we encourage performing a burst size selection after background
correction, possibly taking into account the
gamma γ factor, as discussed in
sections~\ref{sec:burstsizeweights} and~\ref{sec:burstsel}.
\subsection{γ-corrected burst sizes and weights}
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The number of photon detected during a burst is commonly called the ``burst size''.
Bursts sizes are usually computed using either all photons, or photons detected
during donor excitation period. To compute
the burst
size, size $n_t$, FRETBursts uses
one of the following formulas:
\begin{equation}
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n_t = n_a + \gamma\,n_d + n_{aa}
\end{equation}
The former \noindent where $n_d$, $n_a$ and $n_aa$ have are the background-corrected
burst counts in different channels and excitation periods (section~\ref{sec:data_intro})
Eq.~\ref{eq:burstsize_dex} includes only photons during donor excitation periods,
while
the latter eq.~\ref{eq:burstsize_allph} includes all photons.
The inclusion of
$\gamma$ γ factor allows to obtain a ``fair threshold''
and to correct bias when comparing the number of bursts across several sub-populations.
The results of burst search is a distribution of burst sizes that approximately
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different photon detection efficiencies of donor and acceptor emission.
A simple way to mitigate the dependence on the burst size threshold is
weight weighting bursts according to their size (i.e. their information content)
so that the biggest bursts will have the highest weights.
The weighting can be used to build weighted histograms and Kernel Density
Estimation (KDE) plots. When using weights, the choice of a particular