Head-worn computers may include a rigid optical chassis mechanically configured to provide a stable optical mounting reference plane with an image source reference plane, a first image source mounted on the image source reference plane and configured to project a first image light from the stable optical mounting reference plane to a combiner positioned in front of a user's eye when the head-worn computer is worn by the user, an outer frame configured to hold the optical chassis such that, when worn by the user, the first image light is aligned with the eye of the user, and an arm rotatably mounted on the outer frame and adapted to be positioned on a user's ear, wherein the arm comprises a battery compartment and a wire connected to at least the first image source.