We present the first results of the SUMO project, aimed at studying multiple populations in globular clusters. For this purpose we obtained wide-field photometry in a large sample of clusters using both archival and proprietary U, B, V, and I data from ground-based telescopes. We have defined a new photometric index c_U,B,I=(U-B)-(B-I), that we show is very effective to split multiple sequences along the red giant branch (RGB). We found a direct connection with the chemical properties of different sequences, that display different abundances of light elements.