This article describes the principles and major applications of digital recording and numerical reconstruction of holograms (digital holography). Digital holography became feasible since charged coupled devices (CCDs) with suitable numbers and sizes of pixels and computers with sufficient speed became available. The Fresnel or Fourier holograms are recorded directly by the CCD and stored digitally. No film material involving wet-chemical or other processing is necessary. The reconstruction of the wavefield, which is done optically by illumination of a hologram, is performed by numerical methods. The numerical reconstruction process is based on the Fresnel–Kirchhoff integral, which describes the diffraction of the reconstructing wave at the micro-structure of the hologram. In the numerical reconstruction process not only the intensity, but also the phase distribution of the stored wavefield can be computed from the digital hologram. This offers new possibilities for a variety of applications. Digital holography is applied to measure shape and surface deformation of opaque bodies and refractive index fields within transparent media. Further applications are imaging and microscopy, where it is advantageous to refocus the area under investigation by numerical methods.

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Digital recording and numerical reconstruction of holograms

The surface to be studied is illuminated with laser light and two images are formed, displaced relative to one another in the image. Speckle pattern interferometry is used to measure phase changes between the two images resulting from deformation of the surface. A fringe system is obtained showing contours of constant slope resolved in the direction of image displacement. A comparison is made with result obtained by holographic interferometry.

https://iopscience.iop.org/article/10.1088/0022-3735/6/11/019/pdf

An image-shearing speckle-pattern interferometer for measuring bending moments

This paper discusses the possibilities of recording electrooptic holograms using a TV camera and digital frame storage for applications to hologram interferometry. The technique of reference beam phase stepping is proposed to allow determination of object beam amplitude and phase for every pixel element. The use of binary levels for both picture display and camera readout is discussed along with estimates of computation times.

Electrooptic holography and its application to hologram interferometry.

Three different image-processing methods based on the time-averaged technique were compared by the electronic speckle pattern interferometry (ESPI) technique for vibration measurement The three methods are the video-signal-addition method, the video-signal-subtraction method, and the amplitude-fluctuation method Also, errors introduced by using the zero-order Bessel function directly into the analysis of the fringe pattern were investigated The video-signal-addition method has been the most generally used ESPI technique for vibration measurement However, without additional image and/or signal-processing procedures, the fringe pattern obtained directly by the video-signal-addition method is rather difficult to observe The reason for poor visibility of the experimentally obtained fringe pattern with this method is explained To increase the fringe pattern's visibility without additional image and/or signal processes, we tried two video-signal-subtraction methods One of the two methods is the video-signal-subtraction method that has normally been used in the static applications The other method, called the amplitude-fluctuation method, and its associated theory are reported here

Vibration measurement by the time-averaged electronic speckle pattern interferometry methods

Historically, the concept of replacing the photographic emulsion by video recording and display occured to several independent groups almost simultanously. Macovsky(1) in USA, Schwomma(2) in Austria and Kopf(3) in West-Germany considered the technique to be based on pure holography. The Lougborough group in England, headed by Butters and Leendertz(4) viewed the technique more as an off-spring of their speckle work and they also introduced the name ESPI as an abbreviation for electronic speckle pattern interferometry. Although the ESPI-name might be considered appropriately descriptive, its association with coarse, annoying speckles has undoubtedly hampered the general acceptance of the technique. Video holography might be a name with better sales appeal, but might be confused with techniques for direct transmission of three-dimensional holographic images(5).

http://iopscience.iop.org/article/10.1088/0305-4624/11/1/303/pdf

Electronic speckle pattern interferometry

In a brief period, lasers have developed from curiosities of the physics laboratory to tools suited to incorporation in complex instrumentation systems. One line of progress has been through the applications of holography and holographic interferometry to engineering metrology where the potential of a precision comparator is realized without a convenient means of using it. The process, however, has formed a basis for development and the progress of holographic metrology is reviewed leading to more general concepts better described as “Coherent Optical Metrology”.The authors' latest developments in this area are described where, by means of a coherent optical sampling technique, three dimensional data is coded onto an electronic video waveform. The method is based on speckle pattern interferometry with optical to electronic interfacing by means of a modified closed-circuit television arrangement. With this instrumentation, most of the measurement facilities of holography are retained without many of the in...

J A Leendertz

Papers

Holographic and Video Techniques Applied to Engineering Measurement