Example of a detected strong excess in the filter 7.0 (J0395), which corresponds to Ca ii H and K emission lines. The figure shows the J-PLUS photometry of the first object in Table 3, with error bars representing the error in the magnitude. The algorithm detects that the magnitude in the filter J0395 is 0.8 times the magnitude in the filter 9.0 (J0430), and recognises this as a possible emission line. In this case, the threshold value for the excess detection was 0.96.