Fluorescence parameters obtained during steady-state electron transport are frequently used to evaluate photosynthetic efficiency of plants. We studied the behaviour of those parameters as a function of irradiance-adapted fluorescence yields F_S and F'_M. Applied simulations showed that photochemical quenching evaluated by q_P is greatly influenced by the steady-state fluorescence level (F_S), and that its evolution is not complementary to non-photochemical quenching (q_N). On the other hand, the relative photochemical and non-photochemical quenching coefficients (q_P(rel) and q_N(rel)) proposed by Buschmann (1995) represent better the balance between the energy dissipation pathways. However, these relative parameters are also non-linearly related when the F_S level is varied. We investigated the application of a new parameter, the relative unquenched fluorescence (UQF_(rel)) which takes into account the fraction of non-quenched fluorescence yield (F_S), which is related to closed photosystem 2 reaction centres not participating in electron transport. By using computer simulations and real in vivo measurements, we found that this new parameter is complementary to q_P(rel) and q_N(rel), which may facilitate the use of PAM fluorescence as diagnostic tool in environmental studies.