Why does the calculated IRF in Luminosa depend on the decay being fitted?
In Luminosa, the IRF is not determined independently of the decay. Instead, it is parameterized as a sum of asymmetric Gaussian functions, and these parameters are co-fitted together with the decay model during the initial fit.
As a result, the determined IRF depends on the selected lifetime model:
Fast decays tend to produce a narrower IRF
Slower or multi-exponential decays tend to produce a broader IRF
This behavior is directly related to the iterative reconvolution and fitting process. While this dependency may seem unusual, it is intentional: accurately separating short lifetimes from IRF effects requires a good model of the lifetime behavior itself.
Why does the broader Luminosa IRF agree better with the experimentally measured IRF?
For decays that contain slower or multiple lifetime components, Luminosa’s fitting approach can recover an IRF that more closely reflects the true instrument response. This is because:
The IRF parameters are optimized together with the decay model
Multi-exponential decay behavior is handled more robustly than with heuristic methods
In contrast, methods that estimate the IRF independently of the fit may struggle in the presence of short lifetimes or noise.
Can the calculated IRF from Luminosa be exported and used in SymphoTime?
At present, Luminosa does not support exporting the internally calculated IRF for external use. The IRF is tightly coupled to the fitting procedure and selected model, and is not intended as a standalone, transferable result.
If a common IRF is required across different software tools, it is generally recommended to:
Use an experimentally measured IRF, or
Ensure consistent fitting strategies and assumptions within each software environment
Why do the calculated IRFs from Luminosa and SymphoTime look so different?
The difference arises from fundamentally different IRF determination strategies:
SymphoTime (SPT):
Derives the IRF directly from the decay curve using heuristic methods
The IRF is determined prior to fitting
Advantages: model-free, fast
Disadvantages: highly sensitive to noise, limited accuracy for short or complex lifetimes
Luminosa:
Approximates the IRF using asymmetric Gaussian functions
IRF parameters are co-fitted with the decay model
Advantages: more robust for multi-exponential and short-lifetime decays
Disadvantages: depends on fitting quality, convergence behavior, and model selection
Is one IRF determination approach more accurate or preferable?
There is no universally “better” approach—each has trade-offs:
SymphoTime’s method is simpler and model-free but can produce unreliable IRFs in the presence of noise or short lifetimes.
Luminosa’s approach is more sophisticated and can better distinguish between IRF effects and true short lifetimes, provided the decay model is chosen carefully.
As a general rule, one should select the simplest model (including IRF determination) that yields reasonable residuals with the fewest parameters.
Where is this documented?
The SymphoTime IRF determination method is documented in the SPT help system.
Documentation for Luminosa is currently limited, but a new help system is planned, where these details are expected to be documented more thoroughly.