Abstract: An apparatus and method comprising: including an image source; an optical arrangement including at least one light guide and an exit pupil where the exit pupil is arranged to be positioned proximate to an eye of a user and the at least one light guide is arranged to convey a beam of light from the image source to the exit pupil to enable an image from the image source to be projected onto the user's eye; a detector arranged to detect the image projected onto the user's eye; and means for enabling the detected image to be analysed so as to identify changes in the projected image.