[O III] (in erg s⁻¹ units) emission line versus radio luminosity (in erg s⁻¹ Hz⁻¹ units) for the 3C sources, separated into the various spectroscopic classes. The data for the objects at z < 0.3 are from Buttiglione et al. (2010, 2011), those at higher redshift are from our analysis. The two green lines represent the linear correlation between radio power and line luminosity derived by Buttiglione et al. (2010) for HEGs (dashed) and LEGs (solid) separately. The cyan symbols represent the izRGs from Tadhunter et al. (1998) added to our sample to improve the statistical significance of our analysis.