THREE-DIMENSIONAL CONTOURING WITH DIGITAL HOLOGRAPHY

Presentamos una técnica para determinar contornos tridimensionales de objetos con dimensiones de al menos cuatro órdenes de magnitud mayor que la longitud de onda de la iluminación. Nuestra propuesta esta basada en la reconstrucción numérica del frente de onda objeto a partir de un holograma registrado digitalmente. El mapa de fase módulo-2π requerido en cualquier proceso de contorneado es obtenido por medio de la substracción directa de dos imágenes de fase (imagen de diferencia de fase) de un objeto bajo diferentes ángulos de iluminación. La obtención de la imagen de diferencia de fase es posible gracias a la capacidad de reconstrucción numérica del campo óptico complejo proporcionado por la holografía digital. Esta característica particular nos proporciona un robusto, confiable y rápido procedimiento que sólo requiere de dos imágenes para su ejecución. La propuesta es soportada con el análisis teórico del sistema de contorneado y verificado por medio de resultados numéricos y experimentales.We report on a technique to determine the 3D contour of objects with dimensions of at least four orders of magnitude larger than the illumination optical wavelength. Our proposal is based on the numerical reconstruction of the object optical wave field of digitally recorded holograms. The re-quired 2π-module phase map in any contouring process is obtained by means of the direct subtraction of two phase-contrast images (phase-difference image) of a still object under different illumination angles. Obtaining phase difference images is only possible by using the capability of numerical reconstruction of the complex optical field provided by the digital holography. This unique characteristic leads us to a robust, reliable, and fast procedure that only requires of two images. We support our proposal with the theoretical means of numerical and experimental results.Presentamos una técnica para determinar contornos tridimensionales de objetos con dimensiones de al menos cuatro órdenes de magnitud mayor que la longitud de onda de la iluminación. Nuestra propuesta esta basada en la reconstrucción numérica del frente de onda objeto a partir de un holograma registrado digitalmente. El mapa de fase módulo-2π requerido en cualquier proceso de contorneado es obtenido por medio de la substracción directa de dos imágenes de fase (imagen de diferencia de fase) de un objeto bajo diferentes ángulos de iluminación. La obtención de la imagen de diferencia de fase es posible gracias a la capacidad de reconstrucción numérica del campo óptico complejo proporcionado por la holografía digital. Esta característica particular nos proporciona un robusto, confiable y rápido procedimiento que sólo requiere de dos imágenes para su ejecución. La propuesta es soportada con el análisis teórico del sistema de contorneado y verificado por medio de resultados numéricos y experimentales

Phase-difference images by digital holography applied to 3D surface contouring

The problem to determine the 3D-contour of objects with all sizes and shapes is a hot topic in diverse fields of optical research and engineering. Through the use of the numerical reconstruction of digital recorded holograms, we propose a technique to evaluate the 3D contour of objects at least four orders of magnitude larger than the optical wavelength. The required 2π-module phase map in any contouring process is obtained by means of phase-difference image of two digital reconstructed holograms. These holograms are interferometric recordings from a still object under different angle illumination. Our proposal is supported on the theoretical analysis of the contouring system, verified by means of numerical results.El problema de determinar el contorno de objetos 3D de diferentes tamaños y formas es un tópico de gran estudio en diversos campos de investigación en óptica e ingeniería. Por medio del uso de la reconstrucción numérica de hologramas registrados digitalmente, proponemos una técnica para evaluar el contorno de objetos que al menos son cuatro órdenes de magnitud más grandes que la longitud de onda óptica. El mapa de fase módulo-2π requerido en cualquier proceso de contorneado es obtenido por medio de la diferencia de imágenes de fase de dos hologramas reconstruidos digitalmente. Estos hologramas son registros interferométricos de un objeto estático bajo diferentes ángulos de iluminación. Nuestra propuesta es soportada por el análisis teórico del sistema de contorneado verificado por medio de resultados numéricos.Universidad EAFITUniversidad Nacional de Colombia Sede Medellí

Improvement of the signal-to-noise ratio in digital holography

Un problema fundamental en holografía óptica y digital es la presencia de ruido speckle en el proceso de reconstrucción, el cual disminuye considerablemente la relación señal ruido (SNR). Para muchas aplicaciones, esta reducción de la SNR hace que la holografía digital sea impráctica, de modo que gran número de alternativas se utilizan con el propósito de superar este problema. Estas alternativas van desde modificar la coherencia espacial de la iluminación a la aplicación de técnicas del procesamiento de imágenes. En este artículo proponemos el acople de técnicas del procesamiento digital de imágenes con la intención de incrementar esta SNR por medio de la reducción del ruido speckle en la reconstrucción digital de hologramas de Fresnel registrados ópticamente. Las técnicas propuestas son ilustradas con resultados experimentales.A fundamental problem in optical and digital holography is the presence of speckle noise in the reconstruction process, which essentially diminishes the signal to noise ratio (SNR). For many applications, that reduction of the SNR makes the digital holography impractical, so great number of approaches have been carried out in order to overcome such a problem. They range from modifying the spatial coherence of the illumination to the application of image processing techniques. In this paper we propose the merged use of digital image processing techniques in order to increase this SNR by reducing the speckle noise in the digital reconstruction of optically recorded Fresnel’s holograms. The proposed techniques are illustrated with experimental results.Un problema fundamental en holografía óptica y digital es la presencia de ruido speckle en el proceso de reconstrucción, el cual disminuye considerablemente la relación señal ruido (SNR). Para muchas aplicaciones, esta reducción de la SNR hace que la holografía digital sea impráctica, de modo que gran número de alternativas se utilizan con el propósito de superar este problema. Estas alternativas van desde modificar la coherencia espacial de la iluminación a la aplicación de técnicas del procesamiento de imágenes. En este artículo proponemos el acople de técnicas del procesamiento digital de imágenes con la intención de incrementar esta SNR por medio de la reducción del ruido speckle en la reconstrucción digital de hologramas de Fresnel registrados ópticamente. Las técnicas propuestas son ilustradas con resultados experimentales

Fast and robust phase-shift estimation in two-dimensional structured illumination microscopy

A method of determining unknown phase-shifts between elementary images in two-dimensional Structured Illumination Microscopy (2D-SIM) is presented. The proposed method is based on the comparison of the peak intensity of spectral components. These components correspond to the inherent structured illumination spectral content and the residual compo- nent that appears from wrongly estimated phase-shifts. The estimation of the phase-shifts is carried out by finding the absolute maximum of a function defined as the normalized peak intensity difference in the Fourier domain. This task is performed by an optimization method providing a fast estimation of the phase-shift. The algorithm stability and robustness are tested for various levels of noise and contrasts of the structured illumination pattern. Furthermore, the proposed approach reduces the number of computations compared to other existing techniques. The method is supported by the theoretical calculations and validated by means of simulated and experimental results

Evaluation of fringe projection and laser scanning for 3d reconstruction of dental pieces

The rapid prototyping and copying of real 3D objects play a key role in some industries. Both applications rely on the generation of appropriated computer aided manufacturing (CAM) files. These files represent a set of coordinates of an object and can be understood by a computer numerically controlled (CNC) machine. Non-contact techniques, like laser scanning and fringe projection, are among the possibilities for obtaining such CAM files. In this work, a comparison between the two aforementioned non-contact techniques is presented. The comparison is made based on their performance as candidates for generating CAM files of objects of high reflectivity and maximum lateral dimensions of the order of 15 mm The parameters tested are the quality of the 3D reconstruction, the processing time, and the possibility of these being implemented in industrial scenarios, among others. Under the scope of these parameters, it is concluded that laser scanning offers superior performance for the kind of objects here considered. The techniques are evaluated with dental pieces in order to validate these methodologies in the rapid prototyping and copying of teeth

Resolution limit in opto-digital systems revisited

The resolution limit achievable with an optical system is a fundamental piece of information when characterizing its performance, mainly in case of microscopy imaging. Usually this information is given in the form of a distance, often expressed in microns, or in the form of a cutoff spatial frequency, often expressed in line pairs per mm. In modern imaging systems, where the final image is collected by pixelated digital cameras, the resolution limit is determined by the performance of both, the optical systems and the digital sensor. Usually, one of these factors is considered to be prevalent over the other for estimating the spatial resolution, leading to the global performance of the imaging system ruled by either the classical Abbe resolution limit, based on physical diffraction, or by the Nyquist resolution limit, based on the digital sensor features. This estimation fails significantly to predict the global performance of opto-digital imaging systems, like 3D microscopes, where none of the factors is negligible. In that case, which indeed is the most common, neither the Abbe formula nor the Nyquist formula provide by themselves a reliable prediction for the resolution limit. This is a serious drawback since systems designers often use those formulae as design input parameters. Aiming to overcome this lack, a simple mathematical expression obtained by finely articulating the Abbe and Nyquist formulas, to easily predict the spatial resolution limit of opto-digital imaging systems, is proposed here. The derived expression is tested experimentally, and shows to be valid in a broad range of opto-digital combinations

Reconstrucción de hologramas de microscopía holográfica digital en línea a velocidad de video

La Microscopía Holográfica Digital en Línea (MHDL) es quizás la metodología más simple para obtener información del mundo micrométrico: una fuente puntual y un dispositivo de registro digital es el equipo necesario. En esta técnica de microscopía, la recuperación de la amplitud compleja dispersada por el espécimen bajo estudio, se hace por medio de algoritmos muy robustos operando sobre registros bidimensionales de intensidad, que contienen millones de pixeles; el tiempo para llevar a cabo este procesamiento de información ha limitado las aplicaciones de la MHDL. Usando la capacidad de cómputo que proveen las unidades de procesamiento gráfico (GPU) unido a un número reducido de operaciones, en este artículo se presenta una metodología para la reconstrucción numérica de hologramas a velocidad de video adquiridos en MHDL. Esta estrategia permite la reconstrucción de hologramas de 1 megapixel a 32 cuadros por segundo sin cambiar el desempeño de la MHDL en términos de la resolución espacial de esta arquitectura de microscopía. Los resultados experimentales presentados de reconstrucción un holograma registrado de una monocapa bidimensional auto-organizada de esferas de tamaño micrométrico, revelan las capacidades tridimensionales de la MHDL operando a velocidad de video

Off-axis Digital Holographic Microscopy: practical design parameters for operating at diffraction limit

The utilization of microscope objectives (MOs) in digital holographic microscopy (DHM) has associated effects that are not present in conventional optical microscopy. The remaining phase curvature, which can ruin the quantitative phase imaging, is the most evident and analyzed. As phase imaging is considered, this interest has made possible the development of different methods of overcoming its undesired consequences. Additionally to the effects in phase imaging, there exist a set of less obvious conditions that have to be accounted for as MOs are utilized in DHM to achieve diffraction-limit operation. These conditions have to be considered even in the case in which only amplitude or intensity imaging is of interest. In this paper, a thorough analysis of the physical parameters that control the appropriate utilization of MOs in DHM is presented. A regular DHM system is theoretically modeled on the basis of the imaging theory. The Fourier spectrum of the recorded hologram is analyzed to evaluate the performance of the DHM. A set of the criteria that consider the microscope features and the recording parameters to achieve DHM operation at the diffraction limit is derived. Numerical modeling and experimental results are shown to validate our findings

Optical sectioning with a Wiener-like filter in Fourier integral imaging microscopy

Non-scanning, single-shot, 3D integral microscopy with optical sectioning is presented. The method is based on the combination of Fourier-mode integral microscopy with a 3D deconvolution technique. Specifically, the refocused volume provided by a regular back-projection algorithm is 3D deconvolved with a synthetic 3D impulse response function that takes into account the number and positions of the elemental images. The use of this hybrid technique provides a stack of true-color depth-refocused images with significant gain of optical sectioning. The stack can be used, among other applications, to inspect inside the thick microscope specimen, to calculate collections of perspective views with fine angular resolution and extended full parallax, and also to display 3D images in an integral monitor. The method here presented is validated with both simulation and experimental data

Handheld and cost-effective Fourier lightfield microscope

In this work, the design, building, and testing of the most portable, easy-to-build, robust, handheld, and cost-effective Fourier Lightfield Microscope (FLMic) to date is reported. The FLMic is built by means of a surveillance camera lens and additional off-the-shelf optical elements, resulting in a cost-effective FLMic exhibiting all the regular sought features in lightfield microscopy, such as refocusing and gathering 3D information of samples by means of a single-shot approach. The proposed FLMic features reduced dimensions and light weight, which, combined with its low cost, turn the presented FLMic into a strong candidate for in-field application where 3D imaging capabilities are pursued. The use of cost-effective optical elements has a relatively low impact on the optical performance, regarding the figures dictated by the theory, while its price can be at least 100 times lower than that of a regular FLMic. The system operability is tested in both bright-field and fluorescent modes by imaging a resolution target, a honeybee wing, and a knot of dyed cotton fibers