A device is described to display continuously on a CRT the emittance of an ion beam. The system uses electrostatic deflectors and small apertures to scan the beam’s phase−space position and momentum coordinates. Measurements are reported of the emittance and brightness of the following beams produced by five different ion sources: H−, D−, He+, He−, C−, D2−, O−, NH2−, A+, and Cu−. Also investigated was the emittance of an H−beam from a duoplasmatron direct−extraction source as a function of the radial displacement of thezwischenelectrode, and the phase−space particle density distribution of an H−beam was measured.