A visual prosthesis for the blind that is based on electrical stimulation of the visual cortex requires the development of an array of electrodes. To establish design specifications for such an electrode array, psychophysical experiments were concluded with normally sighted subject wearing a portable phosphene simulator. The simulator consists of a small video camera, a monitor masked by an opaque perforated film, and optical lenses. The visual angle subtended by the masked monitor is 1.7° or less. Visual acuity and reading rate were measured as function of the number of pixels and their spacing. The results indicate that a phosphene image produced by 600 electrodes implanted in a 1-cm2 area near the foveal projection on the visual cortex should provide a limited but useful visual sense for the profoundly blind.