Two examples are presented of interferometric measurements in hostile environments.
In the first example a modified shearing interferometer. The optical approach is novel in that while being based upon conventional interferometry; modifications provide a whole field, non-invasive, quantitative measurements of density gradients of 300Atmos in a supercritical CO2 flow. This an almost 2 orders of magnitude greater than previously presented by an interferometric approach.
The combination of the area of view (40mmx70mm) and the strong localised density gradient within the flow was beyond the image capture resolution of conventional interferometry. This necessitated the need for a modified Twyman-Green interferometric shearing approach. The images recorded had a resolution of 5,000x3,000 pixels and provided sub-wavelength measurements, with an average of 10 pixels/fringe.
A Fourier based phase unwrapping approach has been used to measure the differential path change in the gas to 1/20 of an optical wavelength. The phase unwrapping process used was critical in its ability to successfully process images of 15M pixels to an accuracy of 0.05kg/m3. The results from the unwrapped images obtained have been numerically integrated and validated against conventional pressure measurements. They have also been compared with numerical prediction.
The second example is a high resolution holographic interferometric measurement of a propane flame. To increase the burning efficiency of the flame a vortex mixing process had been augmented in the burner between mixing gases of propane and air. This experimental requirement was both to image the high degree of turbulence and provide a whole field visualization of the complete flame. This was achieved using double exposure, image plane, pulse laser, holographic interferometry. The optical resolution of the technique both preserved fringe data and provided a near instantaneous whole field visualization of the burning process. The burner was 0.1m in diameter and the flame extent was 0.2m long.
In this case a photographic reconstruction has been made using a conventional CCD camera. To increase the digital image resolution the camera was mounted on a traversing system and moved in a step and repeat manner, each image being 4,000x3,000 pixels. The 20 individual frames were stitched to create a contiguous 240Mbytes image. This resolution was essential in visualizing the whole complex highly turbulent burning structure of the flame visual. However, initially such high-resolution image presented an intractable processing problem, conventional, Fourier processing had insufficient spatial bandwidth to be applicable. However, recently a globally searching algorithm has yielded a solution. Further, a tomographic reconstruction approach has been applied to the resulting unwrapped map which utilizes the overall symmetry in the flame to augment a simple transform.
Peter Bryanston-Cross and Brenda Ms Timmerman, "Interferometric measurements in hostile environments ," Proc. SPIE 10677, Unconventional Optical Imaging, 1067723 (Presented at SPIE Photonics Europe: April 26, 2018; Published: 24 May 2018); https://doi.org/10.1117/12.2309531.5789402760001.
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