Experimental studies of microdosimetry in therapeutic ion beams have been performed using several detectors. The differences among the microdosimeters lie on the shapes, the site sizes, and the material. The present study proposes a method to extract from the heterogeneous information collected with the different microdosimeters a univocal specification of the radiation quality of the radiation field. Historically the specification of the radiation quality is provided either in terms of linear energy transfer (LET) or in terms of lineal energy, y. The first part of this study focuses on identifying the correlation between the distributions of LET and the lineal energy spectra as well as the correspondence between their mean values calculated from the frequency and dose distributions. The evaluation is inspired by the method of LET analysis described by Kellerer (1972 Phys. Med. Biol. 17 232-40) with adaptation to the peculiarities of ion-beam therapy where the pristine irradiation is unidirectional and made of a single species of essentially mono-energetic ions. The second objective of this study is to interpret the spectra collected by a slab and perform the necessary conversion to estimate what the spectrum would be if it was collected by a detector different in shape, material, or size. An example is provided using as starting point the simulated lineal energy spectrum of carbon ions impinging a slab detector of graphite and applying the method to convert it to the spectra that would be obtained in the same radiation field with spherical, cylindrical, and slab detectors made of water.