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Biomedical subjects

Giancarlo Pedrini

Publications and source records attributed to Giancarlo Pedrini.

13 recordsLinked to original sources
PubMed2006

Complete wavefront reconstruction using sequential intensity measurements of a volume speckle field.

The recording of the volume speckle field from an object at different planes combined with the wave propagation equation allows the reconstruction of the wavefront phase and amplitude without requiring a reference wave. The main advantage of this single-beam multiple-intensity reconstruction (SBMIR) technique is the simple experimental setup because no reference wave is required as in the case of holography. The phase retrieval technique is applied to the investigation of diffusely transmitting and reflecting objects. The effects of different parameters on the quality of reconstructions are investigated by simulation and experiment. Significant enhancements of the reconstructions are observed when the number of intensity measurements is 15 or more and the sequential measurement distance is 0.5 mm or larger. Performing two iterations during the reconstruction process using the calculated phase also leads to better reconstruction. The results from computer simulations confirm the experiments. Analysis of transverse and longitudinal intensity distributions of a volume speckle field for the SBMIR technique is presented. Enhancing the resolution method by shifting the camera a distance of a half-pixel in the lateral direction improves the sampling of speckle patterns and leads to better quality reconstructions. This allows the possibility of recording wave fields from larger test objects.

Journal Article↗
PubMed2006

Simultaneous two-dimensional endoscopic pulsed digital holography for evaluation of dynamic displacements.

An endoscope is used in pulsed digital holography to simultaneously evaluate in-plane and out- of-plane transient and harmonic displacements on a flat metallic plate. The plate is illuminated from two different directions. The optical path for each illumination direction is matched to its corresponding reference beam, but also in such a way that each object-reference beam pair optical path is mismatched such that they are incoherent and can be stored in a single CCD frame. As is typical in these types of interferometric arrangement, two digital holograms are needed to compare two different states of the plate. Each hologram is Fourier transformed and due to the incoherence introduced, two separate spectra are readily identified, each belonging to an object-reference beam pair. On comparing by subtraction the phase obtained from the two pulsed digital holograms, it is possible to gather quantitative in-plane and out-of- plane results from transient and harmonic displacements.

Endoscopes↗
PubMed2006

Reconstruction algorithm for high-numerical-aperture holograms with diffraction-limited resolution.

A fast algorithm is proposed for the reconstruction of digital holograms that are recorded at high numerical aperture. The method directly evaluates the Rayleigh-Sommerfeld diffraction integral by use of a fast convolution algorithm. A shift parameter that accounts for the coordinate system's transverse displacement of the object plane and the hologram plane is introduced in a discrete representation of the diffraction kernel. Combination of the samplings reconstructed with different shift values yields diffraction-limited resolution over the full field of view. The algorithm is suitable for various applications such as holographic microscopy and metrology. Simulation and experimental results are presented.

Journal Article↗
PubMed2006

High-speed digital holographic interferometry for vibration measurement.

A system based on digital holographic interferometry for the measurement of vibrations is presented. A high-power continuous laser (10 W) and a high-speed CCD camera are used. Hundreds of holograms of an object that has been subjected to dynamic deformation are recorded. The acquisition speed and the time of exposure of the detector are determined by the vibration frequency. Two methods are presented for triggering the camera in order to acquire at a given phase of the vibration. The phase of the wavefront is calculated from the recorded holograms by use of a two-dimensional digital Fourier-transform method. The deformation of the object is obtained from the phase. By combination of the deformations recorded at different times it is possible to reconstruct the vibration of the object.

Journal Article↗
PubMed2006

On-line surveillance of a dynamic process by a moving system based on pulsed digital holographic interferometry.

A method based on pulsed digital holographic interferometry for the measurement of dynamic deformations of a surface by using a moving system is presented. The measuring system may move with a speed of several meters per minute and can measure deformation of the surface with an accuracy of better than 50 nm. The deformation is obtained by comparison of the wavefronts recorded at different times with different laser pulses produced by a Nd:YAG laser. The effect due to the movement of the measuring system is compensated for by digital processing of the different holograms. The system is well suited for on-line surveillance of a dynamic process such as laser welding and friction stir welding. Experimental results are presented, and the advantages of the method are discussed.

Journal Article↗
PubMed2005

Applications of short-coherence digital holography in microscopy.

We present an optical system based on short-coherence digital holography suitable for the imaging of three-dimensional microscopic objects. The short temporal coherence properties of the light source allow optical sectioning of the sample. Proper reconstruction of different layers within biological samples is possible up to a depth of a few hundred micrometers, but multiple scattering and inhomogeneities in the refractive index reduce the imaging quality for deeper layers. We have studied the possibility of numerically correcting sample-induced aberrations, and we now propose a method of improving image quality. Numerical simulations and preliminary experimental results show that compensation of these aberrations is possible to some extent.

Algorithms↗
PubMed2004

Reconstruction of in-line digital holograms from two intensity measurements.

We propose a new method based on in-line digital holography for the reconstruction of a wave front from only two intensity recordings. The simulation result shows that this method works well when the object wave is weak compared with the reference wave. This technique can be employed for real-time imaging.

Journal Article↗
PubMed2004

Applications of fractional transforms to object reconstruction from in-line holograms.

We propose a method for the digital reconstruction of an object whose diffraction pattern has been recorded on a hologram. The fractional Fourier transform is used for the object reconstruction. To determine the position of the object, the fractional order is scanned. The fractional cosine and fractional Hartley transforms are also employed for object reconstruction. These two transforms are real valued and allow the reconstruction to be done with lower computing complexity. Simulations and experimental results are presented.

Journal Article↗
PubMed2003

Image reconstruction for in-line holography with the Yang-Gu algorithm.

A new approach to the numerical reconstruction of wave fronts stored by in-line holography is presented. The new algorithm can achieve good reconstructed results in both unitary and nonunitary systems. The influences of recording distance and noise as well as of digitalization errors on the quality of reconstruction are numerically investigated. The experimental results demonstrate the validity of this new approach.

Journal Article↗
PubMed2003

Temporal phase unwrapping of digital hologram sequences.

A method for recording and evaluating digital image-plane holograms is presented. Hundreds of holograms of an object that has been subjected to dynamic deformation (e.g., vibrations) are recorded. The phase of the wave front is calculated from the recorded holograms by use of a two-dimensional digital Fourier-transform method. By temporal phase unwrapping it is possible to determine the absolute deformation (included the direction of motion) of the object. Experimental results are presented, and the advantages of temporal phase unwrapping compared with spatial phase unwrapping are discussed.

Journal Article↗
PubMed2002

Short-coherence digital microscopy by use of a lensless holographic imaging system.

An optical system based on short-coherence digital holography suitable for three-dimensional (3D) microscopic investigations is described. The light source is a short-coherence laser, and the holograms are recorded on a CCD sensor. The interference (hologram) occurs only when the path lengths of the reference and the object beam are matched within the coherence length of the laser. The image of the part of the sample that matches the reference beam is reconstructed by numerical evaluation of the hologram. The advantages of the method are high numerical aperture (this means high spatial resolution), detection of the 3D shape, and a lensless imaging system. Experimental results are presented.

Journal Article↗
PubMed2002

Digital-holographic interferometry with an image-intensifier system.

A method for recording digital holograms on an image intensifier coupled with a CCD sensor is presented. The advantage of the image intensifier is that it can be gated (electronic shutter action produced by controlling of the image intensifier's photocathode voltage). This allows us to record holograms with a short exposure time. Two holograms of an object submitted to dynamical displacements (e.g., vibrations) are recorded by two short exposures. The phase of the wave front recorded at different times is calculated from the recorded intensity by use of a digital Fourier-transform method. By comparison of the phases recorded it is possible to get the displacement of the object during a short interval. Experimental results are presented, and the problems related to the noise and to the spatial resolution are discussed.

Journal Article↗