Nivedya Prabhakar , you should first convert channel values to time (ns). Each fluorimeter has its specification of how many nanoseconds each channel corresponds to. Then, find the channel value at which intensity starts increasing (x0). Make it zero (by subtracting the whole channel column from that x0). Then multiply the new shifted channel column by the conversion value (i.e., 0.027 ns / channel). That'll be your time column.

Having that ready, from the plot of counts vs time (ns), an exponential fit (y = A exp (- t / T); the simplest form) is performed, and the time at which the intensity reaches 1/e of the initial value (T), is called the decay's lifetime.

The use of multiple exponential fit (y = A1*exp(-t/T1) + A2*exp(-t/T2) + ...) is often necessary for good fit convergence, especially when your system contains multiple pathways of excited state relaxation, e.g., when you have a heterogeneous molecular system.

The number of exponentials used to fit your experimental decay data should be physically motivated. Knowing when to truncate the number of free parameters to fit can be tricky, but previous knowledge of your system makes it quite feasible.

I recommend you to read Joseph R. Lakowicz's book "Principles of Fluorescence Spectroscopy" (https://doi.org/10.1007/978-0-387-46312-4) if you need more material.

Dear Nivedya,

Ruan gave already a great explanation. However, you can have a look to my lecture slides "Optical characterisation of materials", where you find typical graphs for luminescent lifetime measurements, which might help you!

All the best for your research,

Thomas