Multi-subzone algorithm for absolute phase retrieval in digital fringe projection profilometry

Codewords are important in encoded absolute phase retrieval techniques such as two-frequency, gray-code, and phase-coding. Each sinusoidal fringe is marked by a unique codeword so that an absolute fringe order can be determined by decoding the codeword. However, due to the limited number of unique codewords, sinusoidal fringe patterns do not contain high-frequency fringes without the use of additional patterns. A multi-subzone coding and decoding algorithm is thus proposed to overcome this limitation. Three multi-subzone coding methods based on two-frequency, gray-code, and phase-coding techniques are presented. The coding creates multiple subzones of unique codewords and the decoding enables it to use non-unique codewords to identify absolute fringe order. Specifically, the range of fringe order is estimated by the use of a wrapped phase map and the absolute fringe order is identified by a codeword. Experimental studies demonstrate the advantages of the proposed algorithm over existing coding methods. The proposed algorithm is suitable to measure objects with large step-height surface discontinuities.