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A novel phase-unwrapping algorithm is proposed by using interlaced fringe in optical 3-D measurement. In the measurement, a four-step phase-shift method including four sinusoidal fringes with 90° phase shift is used. On top of these four fringes, an interlaced fringe is encoded. In the measurement, all these designed fringes are projected onto the tested object, and fringe images are acquired by charge-coupled device cameras. Then, four fringe-different images with zero-valued fringes can be obtained by subtracting the interlaced fringe image from the other four sinusoidal fringe images; thus, the position and the characteristic of each fringe can be distinguished. Combined with the composition of the encoding interlaced fringe, the order of the fringes can be obtained. The whole process needs only one assistant interlaced fringe image. Experiment result shows that the proposed algorithm can get the 3-D information rapidly and exactly and has more robustness.