An optical touch-sensitive device (600, 700, 800, 900, 1300) for use with a display, the touch-sensitive device (600, 700, 800, 900, 1300) comprising: a planar optical waveguide (604, 704, 804, 904, 1004, 1304) extending over a surface of the display, the waveguide having a top surface and a bottom surface; emitters and detectors (606, 706, 806, 906, 1006) arranged along a periphery of the display and below the bottom surface of the waveguide; an optical coupler assembly (602, 702, 802, 902, 1002) positioned along the periphery of the display below the waveguide, the optical coupler assembly coupling optical beams produced by the emitters into the waveguide and out of the waveguide to the detectors, the optical beams propagating through the waveguide via total internal reflection wherein touches on the top surface of the waveguide disturb the optical beams, the touch-sensitive device (600, 700, 800, 900, 1300) determining touch events based on the disturbances, the optical coupler assembly comprising: an upper surface and a lower surface, both formed parallel to the bottom surface of the waveguide, the upper surface forming an interface at which light enters from or exits to the waveguide, a surface joining both the upper and lower surfaces and formed at an acute angle to the lower surface and an obtuse angle to the upper surface, measured within the optical coupler assembly (602, 702, 802, 902, 1002), and wherein one of the surface and the lower surface forms an interface at which light enters from or exits to an emitter or a detector, the other one of the surface and the lower surface being a surface upon which total internal reflection of light occurs within the optical coupler assembly (602, 702, 802, 902, 1002). a planar optical waveguide (604, 704, 804, 904, 1004, 1304) extending over a surface of the display, the waveguide having a top surface and a bottom surface; emitters and detectors (606, 706, 806, 906, 1006) arranged along a periphery of the display and below the bottom surface of the waveguide; an optical coupler assembly (602, 702, 802, 902, 1002) positioned along the periphery of the display below the waveguide, the optical coupler assembly coupling optical beams produced by the emitters into the waveguide and out of the waveguide to the detectors, the optical beams propagating through the waveguide via total internal reflection wherein touches on the top surface of the waveguide disturb the optical beams, the touch-sensitive device (600, 700, 800, 900, 1300) determining touch events based on the disturbances, the optical coupler assembly comprising: an upper surface and a lower surface, both formed parallel to the bottom surface of the waveguide, the upper surface forming an interface at which light enters from or exits to the waveguide, a surface joining both the upper and lower surfaces and formed at an acute angle to the lower surface and an obtuse angle to the upper surface, measured within the optical coupler assembly (602, 702, 802, 902, 1002), and wherein one of the surface and the lower surface forms an interface at which light enters from or exits to an emitter or a detector, the other one of the surface and the lower surface being a surface upon which total internal reflection of light occurs within the optical coupler assembly (602, 702, 802, 902, 1002). an upper surface and a lower surface, both formed parallel to the bottom surface of the waveguide, the upper surface forming an interface at which light enters from or exits to the waveguide, a surface joining both the upper and lower surfaces and formed at an acute angle to the lower surface and an obtuse angle to the upper surface, measured within the optical coupler assembly (602, 702, 802, 902, 1002), and wherein one of the surface and the lower surface forms an interface at which light enters from or exits to an emitter or a detector, the other one of the surface and the lower surface being a surface upon which total internal reflection of light occurs within the optical coupler assembly (602, 702, 802, 902, 1002).