Figure 5. Cross correlation of FLIC signals for single filament detection.

a) Time series trace of fluorescence recorded in detector region D₁ as filaments move over the Au line. Red arrows: peaks identified using the same peak-finding algorithm as in (e), resulting in many false positives: according to visual inspection, five filaments passed by the detector. A constant background was subtracted from the data. b) Background-subtracted fluorescence recorded in detector regions D₁ and D₂, each consisting of one Au line (w = 2 μm), with d = 7 μm (see Fig. 1). Inset: Schematic of detector regions. c) Cross correlation signal I_corr (see equation (1)) of the D₁(t) and D₂(t) signals from (b), as a function of t_s, indicating that I_corr is maximal for t_s,max = 0.8 s. d) The multiplied signal D₁(t)*D₂(t+t_s,max) after normalizing. The dashed line shows the threshold level T = 0.01 used to produce the signal in (e). e) Black curve: Same data as in (d) after subtraction of 0.01 of the maximum. Red arrows: extracted peaks using a peak-finding algorithm (see text). f) Number of peaks identified as a function of the threshold level T applied to the cross correlated data set from (d), spanning from 0.001–0.1 of the maximum. Red bar: a threshold between 0.004 and 0.01 produces the correct result of five filaments.