Abstract Three-dimensional background-oriented schlieren (3D-BOS) is an effective method for reconstructing 3D density fields from optically measured data, but it has limitations in measuring near-wall regions, where most of the light paths are blocked. This paper proposes a new extension, 3D-BOS using Mirror, which uses a wall as a mirror to provide sufficient light paths. In this paper, first, the conventional formulations are modified for the proposed method to handle the mirror reflections of the light paths. …