1. A computer-based method for designing a free space reflective optical surface for use in a head-mounted display that produces a virtual image of a display surface for viewing at a preselected spatial location by a user's eye, the method comprising using one or more computers to perform the steps of: (a) representing the display surface by a plurality of display objects; (b) representing the free space reflective optical surface by a plurality of surface elements, each surface element being characterized by (i) a spatial location relative to the display surface, a nominal user's eye, and the preselected spatial location of the virtual image, (ii) a normal, and (iii) a radius of curvature; (c) associating each display object with at least one surface element in the direction of which, during use of the head-mounted display, a virtual image of the display object at the preselected spatial location will be displayed to a nominal user's eye, each surface element being associated with one and only one display object; (d) for each surface element: (i) defining an initial spatial location of the element; (ii) calculating an initial direction of the element's normal using the element's initial spatial location, the location of the display object with which the element is associated, and the location of a center of rotation of a nominal user's eye so that light from the display object that reflects off of the element will pass through said center of rotation; and (iii) calculating an initial radius of curvature for the element so that the virtual image of the display object is at the preselected spatial location; and (e) for each surface element, calculating a final spatial location of the element, a final direction of the element's normal, and a final radius of curvature for the element and a set of surrounding elements by iteratively adjusting the spatial locations of the elements until an error function satisfies a predetermined criterion.