There are two main components to the HMD eyepiece. The wearer looks through a right angle prism that is 16x17mm on two sides and mirrored on the hypotenuse. The round side of the eyepiece holds a 22mm diameter plano-convex shaped lens. The prism is rotated 35 degrees to bring the viewing plane closer to the eye. The rounded side of the lens faces outward toward the OLED display and focuses perfectly on the display at 55mm from the edge of the eyepiece. The prism was obtained as part of a lot purchase on ebay, and the lens was salvaged from a little pair of binoculars. The software needs to draw to the display in reverse to make it appear correct in the eyepiece.