In this work we study the emittance of electron beams generated by three photocathodes (PCs), one based on Cu sample and two on different nanodiamond (ND) layers. The active layers of the ND-based PCs consisted of untreated and hydrogenated ND particles, 250 nm in size, sprayed on alluminum substrates. We show photoemission measurements performed by a KrF nanosecond excimer laser (λ = 248 nm) in a vacuum chamber at 10−6 mbar and the study of emittance. This last was carried out by the pepper pot method using radiochromic films, HD-810 Gafchromic, as sensible detectors for electrons. The study of the emittance was performed by varying the laser spot onto the PC surface and the accelerating voltage (5, 10 and 15 kV). The obtained results showed quantum efficiencies of the ND-based photocathodes higher than that of the reference Cu one, which instead exhibited the lowest value of …