Yunlu Zou

Digital double pulse-TV-holography

Abstract A digitial double-pulsed holographic system will be described. Two separate holograms of an object under test are recorded within a few microseconds using a CCD-camera and stored in a frame grabber. Different recording arrangements for Fresnel hologram, quasi-Fourierand image-plane hologram are discussed and compared. The holograms are reconstructed digitally using a computer. The phases of the two reconstructed wave fields are calculated from the complex amplitudes. The deformation information is obtained from the phase difference. In the case of the image-plane hologram the deformation can be obtained by using a sinusoid-fitting method without reconstruction of the wavefront. Experimental results are presented.

Digital Double-pulsed Holographic Interferometry for Vibration Analysis

Abstract In this paper we describe a digital double-pulsed holographic system. Two separate holograms of an object under test are recorded within a few microseconds using a charge-coupled device camera and stored in a frame grabber. The holograms are digitally reconstructed using a computer, by simulation of the Fresnel diffraction of the hologram illuminated by the reference wave. The phases of the two reconstructed wave fields are calculated from the complex amplitude and the deformation is obtained from the phase difference. Experimental results are presented.

Simultaneous quantitative evaluation of in-plane and out-of-plane deformations by use of a multidirectional spatial carrier.

An optical system for the parallel evaluation of in-plane and out-of-plane deformations is described. The object is illuminated from two different directions and imaged onto a CCD sensor. Each illumination interferes with its corresponding reference beam. This produces two sensitivity vectors. The references have different directions in order to produce two-directional spatial carriers. Two separate interferograms of an object under test in its undeformed and deformed states are recorded. The Fourier method is used for the quantitative evaluation. The measurements along different sensitivity vectors are separated in the Fourier domain. The phases of the two interferograms are obtained from the complex amplitudes, and the two-dimensional deformation is calculated from the phases. Two different arrangements (with and without a lens system) are presented together with some experimental results.

Surface contouring in a video frame by changing the wavelength of a diode laser

Two-wavelength surface contouring systems using either electronic speckle pattern interferometry (ESPI) or an electronic holography procedure by modulating directly the current of a laser diode are reported. Two states of the test object corresponding to each wavelength of the laser can be recorded in both fields of a single video frame. This is achieved by synchronizing the interline-transfer CCD (charge-coupled device) camera in a noninterlaced mode to the current modulation of the diode. Environmental disturbances at low frequencies such as air turbulence, slow object drift and stress in the object have no influence on the measurements. The test object does not need to be on a vibration isolated table. Quantitative experimental results are presented.

Geometry for contouring by electronic speckle pattern interferometry based on shifting illumination beams

When the method of contouring an object surface by electronic speckle pattern interferometry is based on shifting the illumination beams, the shifted phase of the interference speckle pattern has a new relationship with the depth of the test surface. Therefore the contour interval as well as the fringe sensitivity of this method has new forms. The geometry of such a situation, which differs from that of either the method of two-wavelength contouring or the method of contouring by tilting the test object is presented. The requirements on the experimental conditions for this method are also presented. Experimental results are in agreement with these analyses.

Derivatives obtained directly from displacement data

Two flexible shearing methods taking the derivative information directly from the data for displacement measurement are reported. The displacement information is obtained using a holographic recording directly on a CCD (charge-coupled-device) chip. Besides the advantage of taking the displacement information and the derivative information from a same set of data, other characteristics of these methods are that the derivative sensitivity can be controlled after recordings and displacement derivatives along arbitrary directions can be chosen easily. One method is based on the electronic recording which keeps the complex amplitudes of the wave fronts in a computer. The other needs only the phase modulo 2π of the displacement.

Speckle size of digitally reconstructed wavefronts of diffusely scattering objects

Abstract A hologram of an object is recorded using a CCD-sensor and stored in a frame grabber. The hologram is reconstructed digitally using a computer by simulation of Fresnel diffraction. The complex amplitudes of the reconstructed wave fields and the coherence factor between two points in space is then calculated. The mean 3D speckle size in a digitally reconstructed wavefront is calculated by autocorrelation. By simulating ring apertures it is possible to reconstruct wavefronts containing speckle clusters.

Two-wavelength contouring with a pulsed ruby laser by employing TV-holography

Abstract A two-wavelength surface contouring system based on electronic speckle pattern interferometry (ESPI) with a pulsed ruby laser is reported. Two states of the test object were recorded at different wavelengths of the laser. The wavelength change was controlled by altering the temperature of the output etalon. The phase information of the surface contour can be obtained by using either the sinusoid fitting method or the Fourier transform method. Contour intervals of about 14 mm and 7 mm are available.

Contouring by Electronic Speckle Pattern Interferometry Employing Divergent Dual Beam Illumination

Abstract For contouring of large object surfaces by means of electronic speckle pattern interferometry divergent illuminations were used. A method to shift dual illumination beams was employed to obtain contour fringes. The relationship between the fringes and object depth does not have the same form as in the case of collimated illuminations. It shows that the original measurement data can be corrected. Theoretical analysis and experimental results are presented which are in agreement with each other.

A novel approach to determine decorrelation effect in a dual-beam electronic speckle pattern interferometer

An intrinsic decorrelation effect in a dual-beam ESPI system for contouring application is quantified by simple image processing techniques incorporating experimental data of speckle patterns. Practical limits for the range of application on contouring an object are also considered from the point of view of automatic fringe analysis. An acceptable degree of decorrelation due to the tilt of illuminating beams has been established. Mit Hilfe einer einfachen Bildverarbeitungstechnik und experimentellen Daten von Specklemustern wird der intrinsische Dekorrelationseffekt in einem Zweistrahl-ESPI-System fur die Erzeugung von Hohenlinien quantifiziert. Praktische Grenzen bei der Anwendung zur Hohenlinienerzeugung werden aus der Sicht der automatischen Streifenanalyse diskutiert. Ein akzeptabler Dekorrelationsgrad bei der Verkippung der Beleuchtungsstrahlen wurde definiert.