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Abstract: In view of the problem that the traditional endoscopic digital speckle pattern interferometry (DSPI) can only measure the in-plane deformation indirectly, a method of direct measurement of the in-plane deformation using a dual-beam-illumination endoscopic DSPI is proposed to avoid the influence of the out-of-plane deformation on the in-plane deformation. High-accuracy measurement of the in-plane deformation of surface inside cavity, deep hole, or hidden object is achieved by combining the phase-shifting DSPI with an endoscope and using symmetrically distributed multimode fibers to generate a dual-beam illumination. In the experiment, a direct measurement of the in-plane deformation inside a cavity with inlet diameter of 40 mm is realized. The measuring area can reach 300 mm2 at a working distance of 20 mm and the measurement resolution reaches 68 nm.
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