Another application of HI is the generation of a fringe pattern corresponding to contours of constant elevation with respect to a reference plane (see Fig. 7.1). Such contour fringes can be used to determine the shape of a 3D object. Holographic contour interferograms can be generated by the two-illumination-point method (discussed in Section 5.2); the two-refractive-index technique, which is generally not practical because the refractive index of the medium wherein the object is located must be changed; and the two-wavelength method, which is discussed in this chapter.
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