We discuss a method to extract the K\"all\'{e}n-Lehmann spectral density of a
particle (be it elementary or bound state) propagator by means of 4d lattice
data. We employ a linear regularization strategy, commonly known as the
Tikhonov method with Morozov discrepancy principle. An important virtue over
the popular maximum entropy method is the possibility to also probe unphysical
spectral densities, as, for example, of a confined gluon. We apply our proposal
to the SU(3) glue sector.Comment: 7 pages, 9 figures, talk given at the 31st International Symposium on
Lattice Field Theory (LATTICE 2013), July 29-August 3 2013, Mainz, German