Apparatus are described herein including an imaging device and a multi-pixel display disposed in a head-mountable device (HMD). The apparatus includes an optical system configured to optically couple the multi-pixel display and the imaging device to a retina of a wearer of the head-mountable device, such that the retina is at a focal plane that is conjugate to both a first focal plane at the multi-pixel display and a second focal plane at the imaging device. The direction of gaze of a user of the HMD could be determined from the retinal image. The determined gaze direction could be used to operate the HMD. The pattern of vasculature in the retinal image could be used to identify the user of the HMD. Information in the retinal image could be used to determine the medical state of the user and diagnose disease states.