Films of aligned multi-walled carbon nanotubes (MWNT) are produced by two different methods, thermal chemical vapour deposition (thermal CVD) and plasma chemical vapour deposition (plasma CVD), on silicon substrates. Electron field emission measurements on these films show that the thermal CVD produced films have excellent emission properties, while the plasma CVD films seem to give a lower electron emission with lower threshold and turn-on fields on the initial voltage scan. The electron emission for some of the films is accompanied by light emission from the carbon nanotubes at high emission current densities. The light emission is a result of strong ohmic heating and can be explained in terms of the one dimensional heat equation. This heating effect in the nanotube film is more important for the thermal CVD films than for the plasma films and can be qualitatively explained by considering the nanotube morphology in each case.