Double-pass imaging background-oriented schlieren technique for focusing on measurement target

This paper introduces the double-pass imaging background-oriented schlieren (DPBOS) technique to overcome a defocusing problem of conventional background-oriented schlieren (BOS) systems. In the conventional BOS system, a camera focuses on a background situated behind a measurement target, which inevitably causes a blurred image of the target. The advantage of the proposed technique, using a digital projector with proper optical alignment, is that the camera focuses on both the target and the background image at the same time. Therefore, measurement in the vicinity of the target can be achieved with higher sensitivity than that of the conventional BOS system. For validation, a test target (lens) and the density field near a heat sink are measured using the DPBOS. The results show good agreement with theoretical prediction and exhibit higher sensitivity than the conventional and telecentric BOS system. In addition, the accuracy of DPBOS was assessed by comparing the calculated surface temperatures from the displacement of DPBOS and conventional BOS with the corresponding theoretical temperature. As the results, the DPBOS and conventional BOS have errors of 3% and 9%, respectively. The proposed method clearly shows the advantage of DPBOS over BOS in overcoming the defocus problem and achieving high-accuracy measurements.