Digital Image Plane Holography Research Articles

Image plane digital holography for simultaneous measurement of temporal and spatial coherence

In this work, we propose and validate the Image Plane Digital Holography (IPDH) with laminar illumination technique for measuring the complete coherence degree, the temporal coherence length and the spatial coherence area (transversal coherence radius), of a light source at the same time and from a single measurement. This holographic technique has been used before as a fluid velocimetry technique that allows measuring the three components of the fluid velocity in a plane. Since its performance is limited by the temporal and spatial coherence of the laser, we can take advantage of this and make IPDH with laminar illumination a means of obtaining these laser properties. The proposed method could be of interest for the optics community since it could be used for any type of light source, presenting some advantages by comparing it with other well-established techniques. It has been tested by using a high repetition rate laser commonly used in velocimetry and the results have been compared to those obtained with other standard techniques, verifying the well-functioning of the IPDH-based technique.

Large field Digital Image Plane Holography with a double cavity high speed laser

The three velocity components in a fluid plane can be measured by applying Digital Image Plane Holography. This technique is limited by the laser coherence length, which reduces its application with high speed lasers that, generally, have a very short coherence length. In addition, the use of a double cavity can also imply a small wavelength difference between the two laser beams. In this work, we present an improved Optical Path Length Enlarging Device that allows the velocity measurement, in a 2D field whose width is four times larger than the laser coherence length. The optical set-up and the procedure for measuring in a larger field (ten times the laser coherence length) were optimized, and the issues derived from the laser spatial and temporal coherence and wavelength changes were analyzed and solved. Digital Image Plane Holography with the Optical Path Length Enlarging Device and Particle Image Velocimetry were applied for measuring the whole velocity field in the central plane of a cylindrical cavity with a rotating lid, for two Reynolds numbers (800 and 2000), showing both of them a very good agreement with the numerical simulations.Graphical abstract

Complete Detection and Reconstruction of Object Image in Image Plane Digital Holography

Complete Detection and Reconstruction of Object Image in Image Plane Digital Holography

Digital Image Plane Holography to Characterize High-Speed Laser Temporal and Spatial Coherence

Digital Image Plane Holography to Characterize High-Speed Laser Temporal and Spatial Coherence

Phase-shifting image-plane digital holography with elliptical vortex sieve

Phase-shifting image-plane digital holography is considered as a widely-used measuring method for observation of micro-scale phase objects and for nondestructive testing of phase morphology. The elliptical vortex sieve is a kind of amplitude-only diffractive lenses which can be used for focusing and imaging from coherent X-rays to terahertz regime. Here an elliptical vortex lens is introduced into phase-shifting image-plane digital holography, and verifies the validity of phase shift by measuring a test target and the multiplanar focal spots of a Taiji lens. The experimental results agree well with the theoretical analysis. Owing to amplitude-only diffractive lenses, they will have great potential in X-rays holography and biochemical microscopy for the next generation of synchrotron radiation and free electron laser in the future.

Measurement of pile-up around spherical indentation by image-plane digital holography.

Instrumented indentation has been used to measure mechanical properties, residual stresses, hardness, and viscoelastic properties, among others, due to its fast and nondestructive procedure. The accurate determination of the contact area between the indenter and the material is important for material property measurements and usually is masked by the pile-up or sink-in phenomenon. This paper presents the application of dual-wavelength image-plane digital holography in order to achieve a full-field determination of the topography around the indentation mark. The optical configuration was able to identify pile-up with magnitudes close to 25μm. Measurement results were compared with the references performed by a commercial focus-variation device showing positive agreement between the obtained profiles. Additionally, most of the points presented differences between measurements lower than 3μm.

Time-resolved image plane off-axis digital holography

In this work, we demonstrate off-axis image-plane digital holography for measuring ultrafast processes with high temporal resolution. The proposed image-plane holographic configuration in conjunction with numerical post-processing procedures allows us to neglect the walk-off effect in the off-axis arrangement by synthesizing spatial phase distribution with the whole field of view from separate fragments and to increase the spatial resolution by means of a telecentric system with adjustable magnification. We have analyzed temporal resolution taking into account all dispersing elements that increase the duration of the pulses being propagated through the optical setup. The technique was approved with experiment on measuring the dynamics of the refractive index, induced by laser filamentation in air.

Perfect digital holographic imaging with high resolution using a submillimeter-dimension CCD sensor

In order to improve the resolution of digital holography with a common-dimension charge-coupled device (CCD) sensor, the point spread functions are briefly derived for the commonly used and practical post-magnification, pre-magnification, and image-plane digital holographic microscopic systems. The ultimate resolutions of these systems are analyzed and compared. The results show that the ultimate lateral resolution of pre-magnification digital holography is superior to that of post-magnification digital holography in the same conditions. We also demonstrate that the ultimate lateral resolution of image-plane digital holography has no correlation with the photosensitive dimension of the CCD sensor, and it is not significantly related to the pixel size of the sensor. Moreover, both the ultimate resolution and the imaging quality of image-plane digital holography are superior to that of pre- and post-magnification digital holographic microscopy. High-resolution imaging, whose resolution is close to the ultimate resolution of the microscope objective, can be achieved by image-plane digital holography even with a submillimeter-dimension sensor. This system, by which perfect imaging can be achieved, is optimal for commonly used digital holographic microscopy. Experimental results demonstrate the correctness of the theoretical analysis.

Resolution enhancement phase-contrast imaging by microsphere digital holography

Microsphere has shown the superiority of super-resolution imaging in the traditional 2D intensity microscope. Here a microsphere digital holography approach is presented to realize the resolution enhancement phase-contrast imaging. The system is designed by combining the microsphere with the image-plane digital holography. A microsphere very close to the object can increase the resolution by transforming the object wave from the higher frequency to the lower one. The resolution enhancement amplitude and phase images can be retrieved from a single hologram. The experiments are carried on the 1D and 2D gratings, and the results demonstrate that the observed resolution has been improved, meanwhile, the phase-contrast image is obtained. The proposed method can improve the transverse resolution in all directions based on a single exposure. Furthermore, this system has extended the application of the microsphere from the conventional 2D microscopic imaging to 3D phase-contrast microscopic imaging.

Vibratory strain field measurement by transverse digital holography.

A method is presented for measuring vibratory strain fields using phase-stepped, image-plane digital holography. An object surface is observed along its normal vector while illuminated at equal and opposite angles by two mutually coherent laser beams. One beam is phase stepped by quarter-wavelength increments between TV frames, and the resulting images are processed to yield holographic images. Object vibrations result in zero-order Bessel function fringes in the display. The second beam is modulated at the same frequency of the object vibration and is used to shift the fringes in a manner analogous to phase step interferometry. The resulting images are processed to yield a wrapped phase map, which is unwrapped and corrected for the error associated with using zero-order Bessel functions in place of cosine functions. The unwrapped images are processed to obtain the average slopes for image segments, and these slopes are multiplied by a scale factor to convert them to strain. The analysis program used here divides the field of view into five horizontal by four vertical segments, which provide a map of the vibratory strain field.

Point spread function and lateral resolution analysis of digital holographic microscopy system

A major issue so far for digital holography is the low lateral resolution. In order to improve the resolution, we deduce the point spread function of the most common and practical digital holographic microscopy system. Based on the derived formulas, the determinant factor of lateral resolution of the system is analyzed. The results show that the optimal resolution of this system is limited by the numerical aperture of microscope objective, and the resolution can be further deteriorated by the recording media parameters and recording distance. By decreasing the recording distance the resolution can be improved dramatically. In addition, by choosing large photosensitive area and small pixel size camera the resolution of the holographic microscopy system can be improved a little. Moreover, the highest resolution and the best image quality can be achieved by image-plane digital holography. Experiment results demonstrate the correctness of the theoretical analysis.

Analysis and improvement of reconstruction phase error in the image plane digital holography

The image plane digital holography is commonly used in digital holographic measurement and imaging. The methods of the discrete Fourier transform and the frequency filtering are usually used for reconstructing the object wave in image plane digital holography, and the influence of these algorithms on phase reconstruction is discussed in this paper. Firstly, the influence of the frequency spectrum leakage on phase reconstruction error is analyzed. The results show that the error of the phase reconstruction is very small when the sampling periodicity is an integer, so high accuracy of phase reconstruction can be achieved in this case. But when the sampling periodicity is not an integer, the phase reconstruction error is increased obviously. In order to reduce the phase error caused by the frequency spectrum leakage, the Hanning window is used in the pretreatment of the digital hologram. The result shows that the Hanning window can improve the accuracy of the reconstruction phase effectively.

Use of optical fiber bundle in digital image plane holography

Use of optical fiber bundles (OFBs) in digital image plane holography systems is limited due to, first, poor quality of transmitted images through the OFB and, second, further image degradation caused by overlappings in the image (also hologram) plane. Existing systems are aimed at near-distant (order of a centimeter) objects and use demanding procedures to achieving acceptable quality of holographic reconstructions. This paper presents an OFB based holographic system capable of achieving fast and good-quality holographic and interferometric reconstructions of far-distant (order of a meter) objects. The proposed system is based on performing subtraction operations through the image acquisition interface buffers directly accessed to process the holograms and instantly display the output. The system is analyzed theoretically and its effectiveness is validated by experimental results.

Optimized digital micro-holographic imaging system

Based on the analysis of the recording and reconstructing of hologram and the point spread function of pre-magnification digital micro-holographic system (PMDMHS), the performances of the six common PMDMHSs are compared in imaging resolution, imaging quality and conveniences, for the first time to our knowledge. The results show that the digital image-plane holography (DIPH) has the highest imaging resolution and the best imaging quality; the resolution of DIPH is independent of the photosensitive surface size of the recording device; this system can completely record the information about the object transmitted through the micro-objective (MO); it is not needed to consider the size of illuminated region of object. Moreover, the reconstruction process of DIPH is very simple. DIPH is an optimized digital holographic imaging system. The DIPH with equal curvature of reference wavefront and object wavefront is very conducive to phase unwrapping and phase aberration compensation; therefore DIPH is more suitable for phase microscopy. The experimental results demonstrate the validation of the theoretical analysis.

High-resolution digital holographic microscopy realized by small size CCD camera

In order to achieve high-resolution imaging by reducing the cost of digital holographic microscopy system, the recording and reconstruction process of image-plane digital holographic system (IPDHS) and its point spread function are analyzed. Then, the determination factor of lateral resolution of IPDHS is investigated. It is concluded that the dependence of lateral resolution of IPDHS on the photosensitive surface size of the recording device is very weak, which is completely different from the common knowledge that the lateral resolution of digital holographic imaging system can be improved by using large size recording device. Then the imaging characteristics of an IPDHS are analyzed. The results show that the information about the tested object can be completely recorded and reconstructed by image-plane digital holography, and that the lateral resolution of IPDHS is higher than that of pre-magnification digital holography. Two IPDHS with plane and spherical reference wave are built. The experimental results are in accordance with the theoretical analysis.

Comparison between Digital Fresnel Holography and Digital Image-Plane Holography: The Role of the Imaging Aperture

Optical techniques are now broadly used in the field of experimental mechanics. The main advantages are they are non intrusive and no contact. Moreover optical techniques lead to full spatial resolution displacement maps enabling the computing of mechanical value also in high spatial resolution. For mesoscopic measurements, digital image correlation can be used. Digital holographic interferometry is well suited for quantitative measurement of very small displacement maps on the microscopic scale. This paper presents a detailed analysis so as to compare digital Fresnel holography and digital image-plane holography. The analysis is based on both theoretical and experimental analysis. Particularly, a theoretical analysis of the influence of the aperture and lens in the case of image-plane holography is proposed. Optimal filtering and image recovering conditions are thus established. Experimental results show the appropriateness of the theoretical analysis.

基于像面数字全息术的中药饮片细胞定量成像技术研究

基于像面数字全息术的中药饮片细胞定量成像技术研究

Imaging Analysis of Image Plane Digital Holography

Digital holography microscopy has been widely used as a testing technology in many areas for the ability of real-time three-dimensional imaging. In this study we focus on understanding the system imaging mechanism and recording capacity of the off-axis image plane digital holography (IPDH) which has several advantages. Wigner distribution function and space-bandwidth product are utilized to examine the minimum requirement for the recording capacity of the CCD camera. The discussion provides a good understanding and a useful guide to the formation of practical IPDH systems.

Cross-talk prevention in optical dynamic measurement

In interferometry-based optical dynamic measurement, single high-speed camera or photodetector is normally adopted to avoid critical alignment and synchronization problems. Due to the rapid development of high-speed cameras and photodetectors, the signal obtained is sufficient to encode different dynamic informations. However, cross-talk problem always exists. In order to avoid the influence of cross-talk, well-designed carriers must be adopted in either spatial or temporal domain. In this paper, we will present two applications on cross-talk prevention in optical dynamic measurement. The first one is a camera-based dual-wavelength image-plane digital holography. In this application, the direction of two spatial carriers is designed to encode information of two wavelengths at different areas of the spectrum. Two phase maps can be retrieved from one hologram. The second application is a spatially encoded multi-beam laser Doppler vibrometry using a single detector. It encodes vibration information of 20 points on separated frequency ranges. Three methods that can bypass the cross-talk are discussed. The experiment verifies it is possible to do a precise vibration measurement on a 4×5 matrix simultaneously using a single photodetector.

Three-component velocity field measurement in confined liquid flows with high-speed digital image plane holography

In the last years, several techniques have been developed for the measurement of the three velocity components in a fluid plane or volume. Techniques as stereoscopic particle image velocimetry (SPIV) or tomographic PIV need a complex set-up and present serious restrictions when applied to confined liquid flows. Other like digital holographic PIV has some limitations in the particle concentration that can be measured. In this work, high-speed digital image plane holography has been applied for the measurement of the three velocity components in a complex geometry brain aneurysm model, using a two-cavity high-speed laser, one double frame camera and normal visualization, like in regular PIV. A portable and compact system has been built for adapting the high-speed laser short coherence length to the measurement of larger areas.