Photometric criteria applied for GC identification when aiming for high completeness. The three rows demonstrate these criteria for compactness index (C_2 − 4), ellipticity (ε), and I_E−Y_E colour (as an example). In each row, we show the real sky sources detected in Euclid I_Eimages (grey points), the detected artificial GCs (black points), and the real sky sources selected as GC candidates (yellow points). The dashed red curves represent the selection boundaries adopted based on the distribution of artificial GCs in each parameter space. In the top row, we see that the artificial GCs follow the vertical sequence of point sources (with an average C_2 − 4 = 0.7), and the scatter becomes larger with fainter magnitude due to an increase in photometric uncertainties. In the bottom row, we see that the selection boundaries reject the majority of objects with I_E < 24 and I_E−Y_E > 1, which are foreground stars. However, a fraction of those stars end up in the final catalogue. This contamination is unavoidable considering the expected colour range of GCs. Tightening the selection criteria removes these contaminant stars, but leads to lower completeness.