J. Martinez-Carranza

Optimum measurement criteria for the axial derivative intensity used in transport of intensity-equation-based solvers

For several years, scientific, industrial, and biological fields have benefited from knowledge of phase information, which allows for the revealing of hidden features of various objects. An alternative to interferometry is single-beam phase retrieval techniques that are based on the transport of intensity equation, which describes the relation between the axial derivative of the intensity and the phase distribution for a given plane in the Fresnel region. The estimation of the axial intensity derivative is obtained from a series of intensity measurements, where the accuracy is subject to an optimum separation between the measurement planes depending on the number of planes, the level of noise, and the actual object phase distribution. In this Letter, a quantitative analysis of the error in estimated axial derivative is carried out and a model is reported that describes the interdependence between these parameters. The results of this work allow for estimation of the optimum separation between measurement planes with minimal error in the axial derivative.

Multi-filter transport of intensity equation solver with equalized noise sensitivity.

Phase retrieval based on the Transport of Intensity Equation (TIE) has shown to be a powerful tool to obtain the phase of complex fields. Recently, it has been proven that the performance of TIE techniques can be improved when using unequally spaced measurement planes. In this paper, an algorithm is presented that recovers accurately the phase of a complex objects from a set of intensity measurements obtained at unequal plane separations. This technique employs multiple band-pass filters in the frequency domain of the axial derivative and uses these specific frequency bands for the calculation of the final phase. This provides highest accuracy for TIE based phase recovery giving minimal phase error for a given set of measurement planes. Moreover, because each of these band-pass filters has a distinct sensitivity to noise, a new plane selection strategy is derived that equalizes the error contribution of all frequency bands. It is shown that this new separation strategy allows controlling the final error of the retrieved phase without using a priori information of the object. This is an advantage compared to previous optimum phase retrieval techniques. In order to show the stability and robustness of this new technique, we present the numerical simulations.

Optimum plane selection for transport-of-intensity-equation-based solvers

Deterministic single beam phase retrieval techniques based on the transport of intensity equation (TIE) use the axial intensity derivative obtained from a series of intensities recorded along the propagation axis as an input to the TIE-based solver. The common belief is that, when reducing the error present in the axial intensity derivative, there will be minimal error in the retrieved phase. Thus, reported optimization schemes of measurement condition focuses on the minimization of error in the axial intensity derivative. As it is shown in this contribution, this assumption is not correct and leads to underestimating the value of plane separation, which increases the phase retrieval errors and sensitivity to noise of the TIE-based measurement system. Therefore, in this paper, a detailed analysis that shows the existence of an optimal separation that minimizes the error in the retrieved phase for a given TIE-based solver is carried out. The developed model is used to derive analytical expressions that provide an optimal plane separation for a given number of planes and level of noise for the case of equidistant plane separation. The obtained results are derived for the widely used Fourier-transform-based TIE solver, but it is shown that they can also be applied to multigrid-based techniques.

Effect of imposed boundary conditions on the accuracy of transport of intensity equation based solvers

The transport of intensity equation (TIE) describes the relation between the object phase and the intensity distribution in the Fresnel region and can be used as a non-interferometric technique to estimate the phase distribution of an object. A number of techniques have been developed to solve the TIE. In this work we focus on one popular class of Poisson solvers that are based on Fourier and the Multigrid techniques. The aim of this paper is to present an analysis of these types of TIE solvers taking into account the effect of the boundary condition, i.e. the Neumann Boundary Condition (NBC), the Dirichlet Boundary Condition (DBC), and the Periodic Boundary Condition (PBC). This analysis, which depends on the location of an object wave-front in the detector plane, aims to identify the advantages and disadvantage of these kinds of solvers and to provide the rules for choice of the best fitted boundary condition.

Fast and accurate phase-unwrapping algorithm based on the transport of intensity equation

The phase information of a complex field is routinely obtained using coherent measurement techniques as, e.g., interferometry or holography. The obtained measurement result is subject to a 2π ambiguity and is often referred to as wrapped phase. Phase-unwrapping algorithms (PUAs) are commonly employed to remove this ambiguity and, hence, obtain the absolute phase. However, implementing PUAs can be computationally intensive, and the accuracy of those algorithms may be low. Recently, the transport of intensity equation (TIE) has been proposed as a simple and practical alternative for obtaining the absolute phase map. Nevertheless, an efficient implementation of this technique has not yet been made. In this work, we propose an accurate solution for the TIE-based PUA that does not require the use of wave-propagation techniques, as previously reported TIE-based approaches. The proposed method calculates directly the axial derivative of the intensity from the wrapped phase when considering the correct propagation method. This is done in order to bypass the time-consuming wave-propagation techniques employed in similar methods. The analytical evaluation of this parameter allows obtaining an accurate solution when unwrapping the phase map with low computational effort. This work further introduces the use of the iterative TIE-PUA that, in a few steps, improves significantly the accuracy of the final absolute phase map, even in the presence of noise or aliasing of the wrapped data. The high accuracy and utility of the developed TIE-PUA technique is proven by both numerical simulations and experiments for various objects.

Comparison of phase retrieval techniques based on the transport of intensity equation using equally and unequally spaced plane separation criteria

Phase retrieval techniques based on the Transport of Intensity Equation (TIE) use a sequence of through-focus intensity images in order to recover the phase information. Classically, the capturing of these images have been made using equally spaced plane separations. Recently, it has been shown that the phase retrieval techniques based on TIE can be carried out using unequally plane separations. In this work we compare quantitatively the phase reconstruction of various TIE solvers using the equal and unequal plane separation strategy.

Height reconstruction algorithm for multi-incidence digital holography

The aim of this work is to develop a holographic method that provides the shape reconstruction with high, interferometric accuracy and an extended measurement range. The method requires recording of a set of n holograms obtained for selected combination of illumination angles θ𝑛. The difference between the optical phases corresponding to on-axis φ0 and offaxis φ𝑛 fields allows calculating the object height. To maintain high-accuracy evaluation of height the dedicated shape reconstruction algorithm is proposed. The algorithm consists of n steps, each with several substeps. Each substep is divided into: (1) calculation of the height from φ0 and φ𝑛 ; (2) propagation of the optical fields. In this paper, the proposed algorithm is numerically validated using three types of objects.

Enhanced lateral resolution for phase retrieval based on the transport of intensity equation with tilted illumination

Quantitative Phase Imaging based on the Transport of Intensity Equation (TIE) has shown to be a practical tool for retrieving the phase information of biological and technical samples. When recovering the phase information with the TIE, the maximum lateral resolution that can be obtained is limited by the numerical aperture (NA) of the optical system. In order to overcome this limitation, a system that combines structured illumination and TIE-like techniques have been proposed. These methodologies enlarge synthetically the NA of the optical system, and thus, the lateral resolution of the retrieved phase can be improved. However, the employment of structured illumination may bring error amplifications in the retrieved phase due to its sensitiveness to phase discontinuities and shot noise. In this work, we propose a new methodology that improves the lateral resolution of the retrieved phase beyond the diffraction limit avoiding the problems related with the structured illumination. The methodology presented here uses tilted illumination and a TIE solver. We show that when using this configuration, we can extend the set of recovered frequencies by adjusting the angle of the tilted wave-front. Further, our methodology is designed to extend the NA by employing less tilted angles than other similar techniques. Hence, the final retrieved phase will have an enhanced lateral resolution without amplifying the numerical errors and employing a few tilted angles. Moreover, we show that the algorithm presented here can be combined with other TIE algorithms that are used for suppressing the Low Frequency Artifacts (LFAs) usually present when using TIE based techniques.

Optical methods for measurements of surface shape in optical components for high power laser beam forming

The paper presents modifications of full-field optical methods commonly used to test the surface quality of optical components used for forming a high power laser beam and tests of a final wavefront. The modifications in reference to surface measurements rely on implementation of the novel fringe pattern processing methods including the quality improvement of initial interferogram and analysis of a reconstructed phase based on Hilbert-Huang transform aided by the principal component analysis. Also the Point Diffraction Interferometer as the efficient tool for high quality measurements of elements with high NA is introduced. In reference to a wavefront quality measurements two solutions are discussed: the use of a lateral shear interferometer and the system employing Transport of Intensity Equation method. The pros and cons for both methods are discussed.

Solution to the Boundary problem for Fourier and Multigrid transport equation of intensity based solvers

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP