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This study proposes a highly phase-sensitive imaging polariscope with a common-path heterodyne configuration for the mapping of spatially distributed cell gap of a twisted-nematic liquid crystal (TNLC). The polariscope adopts an electrooptic modulator incorporated with a quarter-wave plate to modulate the linear polarized light. By applying an integrating-bucket method, the two-dimensional (2-D) phase distribution of the detected heterodyne signal is extracted for the inverse calculation of the TNLC cell thickness. The proposing system has no need to remove or to rotate any optical elements in the measuring process and makes it possible to perform high-speed 2-D mapping. The dynamic range of retardation is around 0.7 mu m without ambiguity in the proposing method. By using a standard 8-bit gray level charged-coupled device (CCD), the experimental result shows that the standard deviation of 1.8 nm in the 2-D measurement of cell thickness has been obtained. The retardation resolution of 0.5 nm of the proposed method is also achieved and is better than that of the Stokes parameters determination method with high gray level CCD.