The ionization energies of donor and acceptor centers measured by thermal stimulation methods are systematically lower than the energies deduced from optical measurements. This result can be explained by the perturbations of the emission probability due to tunneling, Poole-Frenkel effect and coulombic interaction between impurities. The effect of the last two terms is calculated in realistic situations corresponding to DLTS or admittance spectroscopy experiments and compared with experimental results obtained for three impurities (Au, Cu, Ag) in ZnTe. After correction the thermal ionization energies are well identified with the optical ones if the deep acceptor concentration does not exceed too much that of the shallow ones.