Nondestructive three-dimensional measurement of gas temperature distribution by phase tomography

This study presents a nondestructive three-dimensional (3D) measurement of gas temperature distribution around a heater. The distribution is obtained by a coupling method of optical interferometry and computed tomography (CT). Since the gas temperature is related to refractive index, once a series of two-dimensional (2D) phase modulation that is an integral of refractive index along an optical path is obtained, the 3D gas temperature distribution can be ideally determined in the same way as the widely-used CT to determine a distribution of attenuation factor. However, the series of 2D phase images is not complete; phase images from certain directions cannot be obtained because of limitations of the measurement system. Furthermore, the 2D images of phase modulation are not observed directly, since the interferometer can only detect a 2D image of intensity distribution called fringe pattern. To retrieve the phase modulation from the fringe pattern, both digital holography and phase unwrapping algorithm are applied. To obtain 3D gas temperature distributions with such incomplete data sets, we apply a method using localized compensator for phase unwrapping algorithm to obtain 2D modulation maps, and a maximum-likelihood tomography for a 3D reconstruction. Accuracy of each method is compared with that of conventional methods.