[en] The beam emittance and brightness from a mildly relativistic (200 to 400 kV) high current density (0.5 to 3.5kA/cm²) planar, field emission diode provided with a velvet covered cathode have been studied experimentally as a function of the applied electric field (100 to 600kV/cm). Transverse beam spreading has been measured using a conventional pinhole arrangement followed by a fluorescent screen and open shutter camera. Good turn-on, and a high normalized beam brightness (B/sub n/ = 300kA/cm²-rad²) have been observed. The results are compared with those obtained with a graphite cathode. 11 refs., 6 figs