Arches luminosity functions of all filters (left: JHK_s, right: L′). The K_s luminosity functions are shown for both proper motion epochs used for membership derivation. The detection limits in the WFC3 JH observations are comparable to those obtained in the Quintuplet, yet the absolute number of sources matched with K_s is limited by the shallower Arches NACO photometry caused by the fainter guide stars and required adaptive optics observing mode. The dashed lines in the L′ luminosity functions indicate the faintest L′-excess source in each field. In Arches Field 2, no cluster member with circumstellar disc emission is detected. All L′ data sets show a sensitivity limit close to L′ = 15.0 mag, fainter than the faintest detected excess source in each field.