Measurement of dynamic flow field by optical computed tomography with shearing interferometers

A system of interferometric computed tomography (CT) for measuring temporal variation of refractive index distribution in a transparent medium such as a how held has been developed. The system adopts shearing interferometers with double-period diffraction gratings to obtain path-length differences b etween sheared rays. In order to obtain projection images simultaneously si x shearing interferometers are placed in six different directions. The imag e data for the six interferometers are stored in image memories at the rate of 30 frames per second. Refractive index distribution in the sectional pl ane of the flow field is calculated with the data using the algorithm of CT modified for shearing interferometers. In experiments temporal variations of refractive index distributions due to a flame and gas flow from a nozzle are measured. These experiments verified that the shearing interferometers are suitable for interferometric computed tomography because they are simp le in arrangement and insensitive to disturbances such as mechanical vibrat ion. Potential of the shearing technique and comparison between the CT with the shearing type of interferometer and that with another type of wavefron t sensor are discussed.