U − G vs. G − R diagram that was used for the selection of the LBG candidates in the field Q1623, that is the deepest of the three. Red lines indicate the selection color locus. Green symbols indicate sources with spectroscopic redshift z_sp ≤ 2.7, and blue symbols show LBGs with z_sp > 2.7 (Reddy, priv. comm.). Vertical arrows indicate upper limits in the U band. Black points indicate all sources detected in the field with R ≤ 25.0. Sources with spectroscopic redshift have magnitudes R ≤ 25.5.