The Explorer version of Google Glass pairs a Himax HX7309 nHD LCOS microdisplay with optics to project a “display [that] is the equivalent of a 25 inch (63.5 cm) [diagonal], high definition screen [viewed] from eight feet away.” and the UI-guidelines say to “target a 640×360 resolution” using a default text size of 40 px.
When viewed through the Explorer’s optics, display appears like a 25 inch, 16:9 screen. is 20 inches (51 cm) wide and when viewed from 8 feet (244 cm) away, each pixel subtends just over 1 arcminute in your visual field (67 arcseconds = 3600*degrees(2*atan(20/640/2/(8*12)) ). Typical 20/20 vision has a resolution of about 1 arcminute (60 arcseconds) and so Glass pixels are very close to qualifying as a “retina” display. The whole screen, however, fills just 14 degrees of your visual field. For reference, an iPhone held screen 11 inches (28 cm) away fills 18 degrees (or 20.6 degrees for an iPhone 5).