When fitting TCSPC data measured with a MicroTime 200 (MT200), SymPhoTime 64 (SPT) offers two options for defining the Instrument Response Function (IRF):
Automatically estimated IRF: SymPhoTime estimates the IRF directly from the rising edge of the decay curve.
Manually measured IRF: A separately recorded scatter measurement is used as the IRF.
Automatically estimated IRF
When to use it
The sample lifetime is significantly longer than the timing resolution of the system — roughly >700 ps as a guideline.
You are performing a quick, exploratory analysis and do not need maximum accuracy.
No scatter measurement was recorded during the experiment.
Limitations
Not suitable for very short lifetimes (below ~700 ps) where the IRF significantly overlaps with the decay.
Less reliable for multi-exponential decays with a fast component.
Not suitable for anisotropy measurements.
Manually measured IRF
When to use it
The sample has a very short lifetime (<700 ps), where the IRF has a significant contribution.
You need accurate results for multi-exponential decays with a fast component.
You are performing anisotropy measurements.
Highest accuracy is required.
Requirements
A scatter measurement must have been recorded during the experiment (e.g., using a scattering solution such as Ludox).
The scatter measurement must use the same optical path (objective, filters, pinhole settings).
Practical recommendation
Use the manually measured IRF whenever maximum accuracy is needed: short lifetimes (<700 ps), multi-exponential decays with fast components, or anisotropy measurements. In all other cases, the automatically estimated IRF is a convenient and reliable choice.