(B^cont_W–NB403) color–magnitude diagram for all sources detected in either the B_W or NB image within the Boötes survey field. Right and top axes show the corresponding equivalent widths in the observed frame and NB fluxes, respectively. The sharp boundary at NB403 24.0 is due to sources detected only in the B_W band. We select line-emission objects with the criteria NB  < 23.5 (dot-dashed line) and B^cont_W–NB > 1.5 (dashed line; EW_obs > 140 Å). Squares represent the final Lyα blob candidates (see Figure 3).