In order to enable collection of data from a head-mounted inward-facing camera, a clip-on device is attached to eyeglasses. The clip-on device optionally weighs less than 40 g and includes: (i) a body that may be attached and detached, multiple times, from a pair of eyeglasses in order to secure and release the clip-on device from the eyeglasses, (ii) an inward-facing camera fixed to the body, and (iii) a wireless communication module fixed to the body.
