Starting from the general expressions describing the line emission by a homogeneous plasma in local thermal equilibrium (LTE), in the approximation of Lorentzian shape of the line profile, a universal expression can be obtained relating the optical depth of the line to measurable quantities such as integrated line intensity, peak value, and full width at half-maximum (FWHM). This universal curve could be used for calculating the optical depth from experimental spectra without knowledge of the spectroscopic parameters of the emission line. Examples are given using experimental and computer-generated synthetic spectra.