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Radiation trapping of photons emitted from Ar radio frequency plasma was studied. The escape factors were theoretically calculated in dependence on metastable and resonant densities, and utilized within the framework of effective lifetime concept. Resonant and metastable densities were then calculated from the measured escape factors of photons emitted from the plasma and compared with densities measured by means of tunable diode laser absorption spectroscopy. The results give a good agreement between the two measuring methods for the case of resonant density. There is, however, a large discrepancy for the latter metastable case. A rearrangement of measuring procedure helps to reduce this difference suggesting an improvement of tunable diode laser absorption spectroscopy.