1. An interactive head-mounted device, comprising: a modular optical assembly through which a surrounding environment is viewable, the modular optical assembly including a curved partially reflecting mirror optically intermediate the surrounding environment and a user's eye, and the modular optical assembly further including a beam splitting layer having an optically flat film disposed at an angle optically intermediate the user's eye and the curved mirror; a processor for handling virtual content for display, and a modular image source for introducing the virtual content to the modular optical assembly, wherein the modular image source generates and delivers image light to the beam splitting layer, wherein the beam splitting layer reflects said image light from the modular image source away from the user's eye to the curved partially reflecting mirror, wherein the curved partially reflecting mirror reflects a portion of the image light from the image source back to the user's eye and transmits a portion of scene light from the surrounding environment through the beam splitting layer to the user's eye, so that a combined image comprised of portions of the reflected image light and the transmitted scene light is transmitted through the optically flat film to the user's eye, wherein the modular image source is mounted relative to a frame of the eyepiece such that a position of the modular image source with respect to the user's eye is adjustable.