An apparatus, comprising: a light-transmitting substrate having at least two parallel major surfaces (104, 106) for guiding light by internal reflection, a first of the major surfaces being deployed in facing relation to an eye; an optical element associated with the first of the major surfaces, the optical element configured for applying optical power to incident light in accordance with at least one property of the incident light, wherein the at least one property is the wavelength, such that the optical element applies non-zero optical power to incident light of a first type returned from the eye within a first not visible optical spectrum so as to collimate the incident light of the first type and such that the optical element applies substantially zero optical power to incident light of a second type within a second visible optical spectrum; an optical coupling configuration associated with the substrate and configured for: coupling-in a proportion of light of the first type, collimated by the optical element and incident on the first of the major surfaces, so as to propagate within the substrate by total internal reflection, and coupling-out a proportion of light of the second type propagating within the substrate; optics associated with the substrate and configured for converting the collimated light of the first type into converging beams of captured light; an optical sensor deployed for sensing the captured light; and at least one processor (130, 132) electrically associated with the optical sensor and configured to process signals from the optical sensor to derive a current gaze direction of the eye wherein the optical element is positioned between the light-transmitting substrate and the eye. a light-transmitting substrate having at least two parallel major surfaces (104, 106) for guiding light by internal reflection, a first of the major surfaces being deployed in facing relation to an eye; an optical element associated with the first of the major surfaces, the optical element configured for applying optical power to incident light in accordance with at least one property of the incident light, wherein the at least one property is the wavelength, such that the optical element applies non-zero optical power to incident light of a first type returned from the eye within a first not visible optical spectrum so as to collimate the incident light of the first type and such that the optical element applies substantially zero optical power to incident light of a second type within a second visible optical spectrum; an optical coupling configuration associated with the substrate and configured for: coupling-in a proportion of light of the first type, collimated by the optical element and incident on the first of the major surfaces, so as to propagate within the substrate by total internal reflection, and coupling-out a proportion of light of the second type propagating within the substrate; coupling-in a proportion of light of the first type, collimated by the optical element and incident on the first of the major surfaces, so as to propagate within the substrate by total internal reflection, and coupling-out a proportion of light of the second type propagating within the substrate; optics associated with the substrate and configured for converting the collimated light of the first type into converging beams of captured light; an optical sensor deployed for sensing the captured light; and at least one processor (130, 132) electrically associated with the optical sensor and configured to process signals from the optical sensor to derive a current gaze direction of the eye wherein the optical element is positioned between the light-transmitting substrate and the eye.