Top panel: the bolometric luminosity (represented by the sum of synchrotron and inverse Compton peak luminosities) is plotted against . The non-thermal optical light of blazars with no strong emission lines above the dashed line would be bright enough to swamp the emission from the host galaxy making the source appear featureless thus hampering any redshift measurement. The lower limits, representing BL Lacs with no known redshift, are estimated assuming that the non-thermal light is ten times brighter than that of a giant elliptical host galaxy (see text for details). The bottom panel shows the same plot for the subsample of sources satisfying the selection criteria of Fossati et al. (1998).