We have determined the size and location of the ‘‘distributed virtual source’’ of an electron-beam evaporation source using a mask/shadowing technique. By depositing copper through slit masks onto glass witnesses, then measuring the width of the deposition pattern it was possible to calculate the size and location of the virtual source. We determined the source size to be 3.9±1.0 cm side to side, 4.8±0.5 cm front to back, 4.4±0.5 cm across one diagonal and 4.4±0.5 cm across the other diagonal while its location was 9±3 cm above the hearth. The size of the virtual source is dramatically larger than the area created by the electron beam rastering across the surface of the muffin. This data may be useful when high-tolerance vapor deposition into porous materials is desired (as in microchannel plates). In this application, the angle between the virtual source and the substrate may be critical for effective coating of the channels.