Simultaneous measurement of temperature by Background-oriented Schlieren and speckle interferometry: Comparison and validation

Abstract

The application of Background-oriented Schlieren (BOS) technology for measuring air flow temperatures is becoming increasingly popular due to its simplicity and practicality. However, there is still a lack of synchronous experimental measurement techniques to verify its measurement accuracy. Electronic Speckle Pattern Interferometry (ESPI), as a high-precision measurement technique, is currently being employed in the field of temperature field measurements. In this study, to verify the accuracy of BOS, an innovative optical setup was designed to achieve the synchronous measurement of temperature field by BOS and ESPI. A plate is set in the object beam path of ESPI, and the laser is irradiated on the plate to form a BOS measurement laser speckle background pattern, which forms a BOS measurement system with the camera. The two measurement systems can simultaneously capture speckle patterns before and after the hot air flow field is introduced into the optical path, enabling the calculation of air temperature. This approach avoids errors that may arise from slight variations in the flow field when comparing results from non-synchronous measurements. The quality of the laser speckle is optimized by adjusting the aperture, beam expander and laser power. The measurement accuracy, uncertainty and sensitivity of BOS are evaluated in the reconstruction of the temperature field of the hot air flow above the alcohol lamp flame. The results show that the temperature field measured by BOS is consistent with the temperature field measured by speckle interferometry, and its measurement sensitivity is related to the flow field position and the focal length of the camera.