A system for determining a gaze direction of a user of a wearable device is disclosed. The system may include a primary lens, an illuminator, an image sensor, and an interface. The illuminator may include a light guide, may be disposed at least partially on or in the primary lens, and may be configured to illuminate at least one eye of a user. The image sensor may be disposed on or in the primary lens, and may be configured to detect light reflected by the at least one eye of the user. The interface may be configured to provide data from the image sensor to a processor for determining a gaze direction of the user based at least in part on light detected by the image sensor.