An ab initio calculation of Ω 0 might in principle be possible but would at the very least be prone to errors. But even if there was a non-controversial way, to determine Ω 0 within the framework of quasi normal modes, it would be very sensitive to the exact material parameters, the geometry and emitter position. To allow for an easy comparison with real measured data, we strive for a more practical way to retrieve the collective coupling strength by making use of the fact that Ω 0 is directly reflected in the splitting ∆ of the polariton peaks.