Figure 2 Determination of phase values Figure 2 | Determination of phase values. a | Action potential fluorescence recorded by an optical mapping system from one pixel, shown as arbitrary fluorescence units as a function of time. The fluorescence values (representing transmembrane potential, Vm) at each time point are colour-coded for the acquisition time, as indicated by the colour scale at the right. b | The same data as in panel a, shown on an expanded timescale. In both panels a and b, in addition to the colour-coded action potential, which shows fluorescence at any time t (F(t)), we also display (as a series of black dots) the fluorescence at t + tau (F(t+tau)), where in this case a tau value of 3 ms is used. Note that the F(t+tau) plot is simply the F(t) plot left-shifted by 3 ms. To help in tracking the phase transformation, we have followed a series of nine time points (numbered 1–9 and indicated by pink circles) in all panels. c | Calculation of the phase value. F(t+tau) is plotted on the y-axis versus F(t) on the x-axis. An average (mean) fluorescence value is calculated from all the fluorescence values on the fluorescence versus time curve (shown at point X). The angle calculated from each point to that mean value in radians is the phase value for that point. d | Phase values (θ) as a function of time. e | Radian values corresponding to different angles. See text for more detailed discussion. Nattel, S. et al. (2017) Demystifying rotors and their place in clinical translation of atrial fibrillation mechanisms Nat. Rev. Cardiol. doi:10.1038/nrcardio.2017.37