Optical system for a luminaire, in particular for a downlight, comprising: - a light guide element having a light entry surface for the entry of a light and having a light exit region for the exiting of the light and - a light passage element , wherein the design is such that the light passes through the light passage element after exiting the light guide element , wherein, for the exiting of the light from the light exit region of the light guide element , a structured surface region is formed on one of the following two components: the light passage element or the light guide element , which structured surface region forms a contact surface by means of which the light guide element and the light passage element are connected to each other, and wherein the structured surface region has structural elements which have, in a first approximation, a trapezoidal shape when viewed in a cross section, characterized in that the structural elements , when viewed in the cross section, have a lateral flank region having an inflection point (W), such that a first flank sub-region having a concave curvature and a second flank sub-region having a convex curvature are formed. - a light guide element having a light entry surface for the entry of a light and having a light exit region for the exiting of the light and - a light passage element , wherein the design is such that the light passes through the light passage element after exiting the light guide element , wherein, for the exiting of the light from the light exit region of the light guide element , a structured surface region is formed on one of the following two components: the light passage element or the light guide element , which structured surface region forms a contact surface by means of which the light guide element and the light passage element are connected to each other, and wherein the structured surface region has structural elements which have, in a first approximation, a trapezoidal shape when viewed in a cross section, characterized in that the structural elements , when viewed in the cross section, have a lateral flank region having an inflection point (W), such that a first flank sub-region having a concave curvature and a second flank sub-region having a convex curvature are formed. wherein the design is such that the light passes through the light passage element after exiting the light guide element , wherein, for the exiting of the light from the light exit region of the light guide element , a structured surface region is formed on one of the following two components: the light passage element or the light guide element , which structured surface region forms a contact surface by means of which the light guide element and the light passage element are connected to each other, and wherein the structured surface region has structural elements which have, in a first approximation, a trapezoidal shape when viewed in a cross section, characterized in that the structural elements , when viewed in the cross section, have a lateral flank region having an inflection point (W), such that a first flank sub-region having a concave curvature and a second flank sub-region having a convex curvature are formed. <EOS>