The samples were excited by a laser through a 60x objective (Nikon 60x Plan Fluor ELWD, N.A. = 0.75) and the emission was collected in a reflection geometry through the same objective. Excitation and emission were separated by a filter assembly (dichroic beamsplitter, plus emission and excitation filters). The characteristics of the filter were chosen according to the excitation wavelength. We used filter sets with cut-off wavelengths of 550 nm and 650 nm. The collected luminescence was detected by an EMCCD camera (Andor Newton) behind a grating spectrograph (Andor Kymera 328i). For tunable visible excitation we utilized a supercontinuum laser (NKT SuperK EXTREME) coupled to a monochromator (NKT SuperK Varia). The shortest wavelength achievable with this setup was 425 nm. The excitation power was fixed to 16 kW cm -2 (0.5 mW @ 2 µm spot size) for all measurements.