A virtual protractor is used to measure angles between objects perceived in stereo image pairs using virtual reality technology. Each image is generated from a scanning electron microscope, and stereo pairs are obtained by either manipulating the specimen or the electron beam. Images from slightly different viewpoints are combined using stereoscopic rendering to create the illusion of a 3D scene. Virtual reality user interface technology is used to manipulate a virtual measuring device with the goal of accurately measuring spatial characteristics of 3D objects perceived while viewing the stereo image pairs. This approach relies on the human observers' ability to successfully fuse stereo geometry and image-based data. A paper was presented at IEEE Vis99 that describes our work. More information.