Generation and evalutaion of projected moving patterns for reference-free BOS with increased spatial resolution

Abstract

A method of taking background oriented schlieren technique (BOS) images using projected backgrounds is shown. The projected backgrounds can be scaled to the measurement requirements, providing more flexibility than printed backgrounds. The evaluation of the BOS image pairs using cross correlation or optical flow analysis was shown to produce equivalent results. The background can be dynamically moved using a rotating mirror, allowing evaluation using the reference-free shadowgraphy or reference-free BOS methods. Forward BOS is shown, where the projected speckle pattern shines through the density object onto a screen, and is recorded by a camera focused on the screen. In contrast to standard shadowgraphy, this technique is robust when the lighting of the field is uneven, and the point movement is related to the first derivative of density.