We report nanosecond time-resolved photoluminescence measurements on the isoelectronic series of oxygen deficient centers in amorphous silica, Si-ODC(II), Ge-ODC(II) and Sn-ODC(II) which are responsible of fluorescence activities at $sim$4 eV under excitation at $sim$5 eV. The dependence of the first moment of their emission band on time, and that of the radiative decay lifetime on emission energy are analyzed within a theoretical model able to describe the effects introduced by disorder on the optical properties of the defects. We obtain separate estimates of the homogeneous and inhomogeneous contributions to the measured emission linewidth and we derive homogeneous spectroscopic features of the investigated point defects (Huangh-Rhys factor, homogeneous width, oscillator strength, vibrational frequency). The results point to a picture in which an oxygen deficient center localized on a heavier atom features a higher degree of inhomogeneity due to stronger local distortion of the surrounding matrix. For Si, Ge, Sn related defects the parameter $lambda$, able to quantify inhomogeneity, results to be 65, 78 and 90$%, respectively.
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