Interferometry has been a time-honored technique for surface topography measurement. Interferometric measurements of surface shape are relative measurement techniques in which the shape of a known surface is compared with that of an unknown surface, and the difference is displayed as a series of interference fringes. Noise attached in the interference fringes can have catastrophic effects on the phase-unwrapping process, so denoising is essential before reconstruction. Some noise may be generated due to vibrations when multiple images over a finite time period are captured for reconstruction by phase-shifting technique. This harmful noise is drastically reduced when fast phase shifting–based single-shot parallel four-step combined with Fizeau interferometer is applied. Measuring the shape of strongly curved surfaces using two-beam interferometry is very complicated due to the higher fringe density. This problem may be solved by multiple-beam interferometry, thanks to the very sharp interference fringes. The experimental results show the feasibility and high precision of multiple-beam interferometry.
Part of the book: Optical Interferometry
Digital holography (DH) is an attractive measuring optical technique in the fields of engineering and science due to its remarkable accuracy and efficiency. The holograms are recorded by an interferometer and reconstructed by numerical methods such as Fresnel transform, convolution approach, and angular spectrum. Because harmful coherent noise often arises when long coherent lengths are used, bright femtosecond pulse light with ultrashort coherent length may be the solution to reduce both spurious and speckle noises. Since the usual DH uses a visible light, it is difficult to visualize 3D internal structure of visibly opaque objects due to their limited penetration depth. The terahertz (THz) radiation has a good penetration capability; thus, 3D visualization of both surface shape and internal structure in visibly opaque object can be achieved via THz-DH technique.
Part of the book: Holographic Materials and Optical Systems