EDITORIAL

Abstract

This is a special issue on ‘Digital Holography and Speckle Interferometry’. The papers of this issue were presented at the symposium on ‘Digital Holography – Exploring Possibility of Using Digital Holography for Experimental Mechanics’, organised by the authors at the ‘13th International Conference on Experimental Mechanics’ (ICEM13), Alexandroupolis, Greece, 1–6 July, 2007, chaired by Emmanuel E. Gdoutos, School of Engineering, Democritus University of Thrace, Xanthi, Greece. The call for papers of the symposium is explained in the following: “Recent progress of digital holography makes it possible to measure various mechanical and geometrical parameters accurately and quickly without film development. This technique has the potential to replace other current techniques in the near future. This session focuses on new developments and techniques of digital holography as well as their applications to various engineering problems, industrial fields and real structures. The contributions on similar techniques such as moiré, conventional holography, speckle interferometry and related techniques will also be presented in this symposium. Please join this symposium if you have interests in digital holography and related techniques.” The history of digital holographic interferometry is not so long. Schnars and Jüptner developed direct recording of Fresnel holograms with a CCD sensor in 1993. Yamaguchi and Zhang developed phase-shifting digital holography in 1997. The system is very simple, capable of whole-field analysis and high process speed. The system is similar to the system of speckle interferometry, which is widely used in the market. It is valuable to compare these techniques. This gave impetus for the publication of this special issue on ‘Digital Holography and Speckle Interferometry’. This issue contains five papers on digital holography and three papers on speckle interferometry. The first paper on the digital holography by I. Yamaguchi et al. deals with the measurement of both position and shape of an object using dual wavelengths. Yamaguchi developed the phase-shifting digital holography. In this paper they also proposed a new method to measure object shapes using both the amplitude and phase of the averaged conjugated product of complex amplitudes recorded before and after wavelength shift of a laser diode. The position determination is related to the fourth paper by M. Fujigaki et al. The second paper by G. Sheoran et al. proposes shape measurement using dual index immersion. It also proposes speckle noise reduction using 5 by 5 median filtering which is compared with the third paper. The third paper by Y. Morimoto et al. presents the effect of window size on accuracy and spatial resolution in windowed phase-shifting digital holographic interferometry. The accuracy is of the sub-nanometre level by decreasing the effect of speckle noise, though the spatial resolution becomes worse according to the number of windows. The fourth paper by M. Fujigaki et al. proposes a method to determine the reconstruction distance using the standard deviation of the intensity on the reconstructed image. It also proposes a method to obtain the orientation of the specimen. The fifth paper by Y. Zhang et al. concerns with the terahertz digital holography for the future. It provides useful applications to biology and medical sciences. The first paper on speckle interferometry by A. Moreau et al. deals with vibration measurement by time average speckle interferometry and by Doppler vibrating. The second paper by G. Kalogiannakis et al. investigates the feasibility of shearographic imaging of acoustic cross-modulation. It was motivated by the latest advances in nonlinear laser Doppler vibrometry and nonlinear ultrasonic, The third paper by F. Chiang et al. presents a new approach to 3D surface countering and surface strain measurement using speckle gap effect. Although it is not an ordinary speckle interferometry, the digital speckle photography technique applied to a flat surface is extended to 3D surface countering and strain measurement of a human heart. In addition to the above eight papers, a plenary paper of ICEM13 on ‘Three-Dimensional Displacement Analysis by Windowed Phase-Shifting Digital Holographic Interferometry’ by Y. Morimoto et al. should be mentioned. This paper extends the method of a windowed phase-shifting digital holographic interferometry mentioned in the third paper of digital holography to analyse three-dimensional displacement components in a microscope. This paper was published in the January 2008 issue of Strain. Digital holography and speckle interferometry are useful methods for measurement of whole-field displacement and strain distributions in ordinary rough surface of structures. They are superior of strain gauges, which measure point strains. Use of whole-field methods requires standardization. The symposium on ‘Digital Holography and Speckle Interferometry’ was initially organized with Dr. Satoru Yoneyama of Aoyama Gakuin University. The authors would like to acknowledge his support. Unfortunately, he could not attend the symposium because of his movement to a new position.