This article presents a model of electron field emission from quantum states arising from the tight confinement of quasi-free electrons on a nanotube hemispherical cap. The model outlines the possibility of inhomogeneous electron field emission for very thin carbon nanotubes at high emission levels and the appearance of peculiar ring-shaped field emission images. The conclusions qualitatively agree with existing experimental evidence, therefore supporting the hypothesis that part of the electrons on the cap of the emitter may behave as quasi-free in a high emission level/high-temperature regime. (C) 2004 American Vacuum Society.