Single acquisition simulation of TCSPC.

This applet contains the same single photon simulation as the previous one, but now run several thousand times to build up a reasonable graph (transient).

The black line is the result of accumulating detected photons as a result of all those excitation pulses. The blue line represents a perfect exponentially decaying signal, of the correct lifetime, for comparison.

The "No. Excitation Pulses" controls how many times the molecules are illuminated. Lower this number if the simulation is too slow on your computer, or buy a faster computer, whichever is easier ;-)

The key number is the number of photons detected per illumination pulse: "Photons per pulse". If this number is too high them something called pulse pile-up can occur. Using the default values you should find that this number comes out at just under 0.1 and that the recorded transient (black line) is very similar to the theoretical exponential curve (blue line). Now lets turn up the laser power, increase the Excitation Probability to 0.1. We find the recorded transient starts to move away from the theoretical exponential and the photons per pulse has increased to around 0.6. We are now detecting many more photons (compare the total photon counts from the vertical scale on the graph) but because of the way TCSPC works photons preferentially build up in the early part of the transient, this is called pulse pile-up. To avoid it in any TCSPC system, real or simulated, you should keep the detected photons per pulse below 0.1.

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