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In this work, we propose an extended Boltzmann plot method to determine the usefulness of spectral lines for plasmaparameter calculations. Based on the assumption that transient plasmas are under ideal conditions duringan specific interval of time Δt, (i.e. thin, homogeneous and in local thermodynamic equilibrium (LTE)), the associated Boltzmann plots describe a surface in the space defined by the coordinates X=Energy, Y = Time and Z=ln (λjlIj/gjAjl), where Ij is the integrated intensity of the spectral line, gj is the statistical weight of the level j, λjl is the wavelength of the considered line and Ajl is its transition rate. In order to express the Boltzmann plot surface in terms of a reduced set of constants Bi, and δi, we developed as a power series of time, the logarithm of In(t)/In(t0), where In(t) is the integrated intensity of any spectral line at time t, and In(t0) at initial time. Moreover, the temporal evolution of the intensity of any spectral line and consequently the temperature of the plasma can be also expressedwith these constants. The comparison of the temporal evolution of the line intensity calculated using these constants with their experimental values, can be used as a criterion for selecting useful lines in plasma analysis. Furthermore, this method can also be applied to determine self-absorption or enhancement of the spectral lines, to evaluate a possible departure of LTE, and to check or estimate the upper level energy value of any spectral line. An advantage of this method is that the value of these constants does not depend onthe spectral response of the detection system, the uncertainty of the transition rates belonging to the analyzed spectral lines or any other time-independent parameters. In order to prove our method,we determined the constantsBi and δi and therefore the Boltzmann plot surface from the temporal evolution of carbon lines obtained from a plasma generated by a Nd:YAG laser. The plasma was produced in vacuum and was observed at differentdistances from the target. A good agreement between the temperature calculated by the traditional Boltzmann plot and by this method was obtained.©