Representation of our NB excess detection method. Grey lines in both panels show the observed spectra of typical z​ ∼ ​2 Lyα-emitting sources (from the publicly available VUDS DR1 spectroscopic dataset, see e.g. Le Fèvre et al. 2015; Tasca et al. 2017). Upper panel: SF LAE spectrum showing a single prominent Lyα line (here redshifted at λ_obs ∼ 3900 Å) and no other significant features. Bottom panel: QSO spectrum with evident CIV and CIII] lines in addition to Lyα (at λ_obs ∼ 4000 Å). We show the transmission curves and associated synthetic photometry of four J-PLUS bands as coloured lines and squares. From left to right: u (purple), J0395 NB (violet), g (green), and r (red). The additional lines of this caption were moved to Sect. 3.1