An optical assembly (1, 20, 30, 40, 60) comprising: an image projector comprising: a light source for providing a light beam; a MEMS mirror which is arranged such that it can receive the light beam, and which can oscillate about at least one oscillation axis (7a,b) to scan said light beam; one or more multiple-beam-generators (8, 80, 41,61,62) each of the one or more multiple-beam-generators comprising, a planar beam splitter which is arranged to receive the light beam reflected by the MEMS mirror, and a planar reflector which can receive the part of the light beam which is transmitted through the planar beam splitter, so that each of the one or more multiple-beam-generators (8,80,41) can generate multiple beams (15a-c), and wherein the planar beam splitter and planar reflector are arranged to be in parallel; and a focusing lens which is arranged to receive multiple beams (15a-c) which are generated by the one or more multiple-beam-generators (8, 80, 41, 61, 62); wherein, in each of the one or more multiple-beam-generators (8, 80, 41, 61, 62), the distance (h) between the planar beam splitter and planar reflector is such that the optical distance between the planar beam splitter and planar reflector is greater than, or equal to, half the coherent length of the light beam; and a screen arranged to receive light which has passed through the focusing lens, wherein the screen is configured to diffuse light it receives, and a head-up-display which is arranged to receive light which is diffused by the screen, wherein the screen comprises a microlens array and wherein the microlens array comprises different sized microlenses so that the microlens array has a plurality of different pitch lengths between the microlenses, or wherein the screen comprises a micromirror array wherein the micromirror array comprises different sized micromirrors so that the micromirror array has a plurality of different pitch lengths between the micromirrors, and wherein the optical assembly is configured to have a numerical aperture which is greater than 0.016, wherein the numerical aperture of the the optical assembly is defined as (d / 2) / (F.M), wherein M is the magnification of the head-up-display, and F is the focal length of the of the focusing lens, and d is the distance between two consecutive multiple light beams (15a-c) which are output from the multiple beam generator and prior to said multiple light beams (15a-c) being incident on the focusing lens. an image projector comprising: a light source for providing a light beam; a MEMS mirror which is arranged such that it can receive the light beam, and which can oscillate about at least one oscillation axis (7a,b) to scan said light beam; one or more multiple-beam-generators (8, 80, 41,61,62) each of the one or more multiple-beam-generators comprising, a planar beam splitter which is arranged to receive the light beam reflected by the MEMS mirror, and a planar reflector which can receive the part of the light beam which is transmitted through the planar beam splitter, so that each of the one or more multiple-beam-generators (8,80,41) can generate multiple beams (15a-c), and wherein the planar beam splitter and planar reflector are arranged to be in parallel; and a focusing lens which is arranged to receive multiple beams (15a-c) which are generated by the one or more multiple-beam-generators (8, 80, 41, 61, 62); wherein, in each of the one or more multiple-beam-generators (8, 80, 41, 61, 62), the distance (h) between the planar beam splitter and planar reflector is such that the optical distance between the planar beam splitter and planar reflector is greater than, or equal to, half the coherent length of the light beam; and a light source for providing a light beam; a MEMS mirror which is arranged such that it can receive the light beam, and which can oscillate about at least one oscillation axis (7a,b) to scan said light beam; one or more multiple-beam-generators (8, 80, 41,61,62) each of the one or more multiple-beam-generators comprising, a planar beam splitter which is arranged to receive the light beam reflected by the MEMS mirror, and a planar reflector which can receive the part of the light beam which is transmitted through the planar beam splitter, so that each of the one or more multiple-beam-generators (8,80,41) can generate multiple beams (15a-c), and wherein the planar beam splitter and planar reflector are arranged to be in parallel; and a focusing lens which is arranged to receive multiple beams (15a-c) which are generated by the one or more multiple-beam-generators (8, 80, 41, 61, 62); wherein, in each of the one or more multiple-beam-generators (8, 80, 41, 61, 62), the distance (h) between the planar beam splitter and planar reflector is such that the optical distance between the planar beam splitter and planar reflector is greater than, or equal to, half the coherent length of the light beam; and a screen arranged to receive light which has passed through the focusing lens, wherein the screen is configured to diffuse light it receives, and a head-up-display which is arranged to receive light which is diffused by the screen,