Projected moving patterns for reference-free background-oriented schlieren technique (BOS)

Abstract A background-oriented schlieren technique (BOS) using projected background patterns is presented. The projected backgrounds can be scaled to the measurement requirements, providing greater flexibility than printed backgrounds. The evaluation of the BOS image pairs using cross-correlation or optical flow analysis was demonstrated to produce equivalent results. The background pattern can dynamically be shifted by a rotating mirror, allowing evaluation using the reference-free shadowgraphy or reference-free BOS methods. The forward BOS technique is demonstrated, in which a projected speckle pattern is illuminated through a density object and recorded by a camera focused on the screen. In contrast to standard shadowgraphy, where the local image intensity is the parameter proportional the second derivative of density, this technique is robust with respect to varying lighting of the field as long as the displacement of small-scale image structures can be determined.