Ryszard Hypszer

The optimal construction of fiber-optic Fabry-Perot interferometer

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

Modeling of broadband light source to use with optical coherent tomography system

The spectral shape of a light source in optical coherence tomography imaging is of prime importance because it determines resolution and quality of the image. Spectra and axial point spread function of photonic crystal fiber light source TB-1550 from Menlosystems GmbH before and after optical spectral shaping are presented. Low-pass and high-pass filters are simulated to shape the irregularities in light spectra of the source. Full-spectrum shaping results with use of spectral processor are calculated. Results show that shaping of a light source improves meaningly axial resolution and inhibits sidelobes of the point spread function.

An optical low-coherence system for 2-dimensional visualization of thin polymer layers

An Optical Low-Coherence Tomography (OCT) is a novel optical measurement technique, which enables non-destructive and non-contact investigation of multilayer structures. Nowadays, this method is highly applied in medical diagnostics. Despite of great progress in optoelectronics and optical measurement methods there is lack of studies on the OCT for non-medical application. In this paper authors present a laboratory OCT system for surface and subsurface investigation of scattering technical objects such as polymer layers. Preliminary test results on subsurface technical objects investigation using OCT system have been presented and discussed.

Fiber-optic low-coherence tomography system for visualization of internal structure of ceramic materials

Fiber-optic low-coherence tomography system with low-coherent reflectometry has been used to non-destructive characterization of ceramics. We presents experimental results showing that the measurements of ceramics internal structure can be done with suitable resolution.

White-light interferometric temperature sensor for biomedical diagnostic

A monomode and multimode fiber white-light interferometric temperature sensor for biomedical applications is presented. The sensor uses a low-finesse Fabry-Perot interferometer working in reflective mode. Dependence of material parameters on fringe visibility is discussed and both construction of the sensor has been optimized with respect to fringe visibility. This new sensor system is expected to be a practical approach for many medical applications.

Synthesized light source for optical coherence tomograph

The influence of low-coherence source parameters on the optical coherence tomography imaging has been studied. Experimentally and theoretically it is demonstrated, that by summing autocorrelation function of two superluminescent diodes with different wavelength, the effective coherent length of the source and signal-to-noise ratio required to identify the central fringe position can be greatly reduced.

Measurement of selected characteristics of low-coherence optical signal sources for optical coherence tomography

Optical Coherence Tomography (OCT) is a new type of an imaging technique of the internal microstructure of the material. Among several detection techniques which can be used to implement OCT, Optical Low-Coherence Reflectometry (OLCR) is the most common and the most promising one. OLCR uses a broadband source, which parameters: the spectral characteristics and the noise property, determine metrological abilities of the designed OCT system. Authors measured the power spectral density of the intensity noise, as well as the spectral characteristic of various broadband sources. Obtained results has been used to determine metrological properties of the design OCT system.

Ultrahigh-resolution detection techniques for biomedical applications of optical coherent tomography

Optical Coherent Tomography (OCT) is an imaging techniqu that enables a range of clinical applications where imaging can be carried out in situ and in real time. In OCT, cross-section images are generated by transverse scanning light beam across investigated tissue and by measuring the delay and magnitude of the back-scattered light. Among several detection techniques which can be used to implement OCT, Optical Low-Coherent Reflectometry (OLCR) or Optical Frequency Domain Reflectometry (OFDR) are the most promising. The paper presents merits of both techniques.

Tunable semiconductor laser application for interferometric optical fiber sensors

Methods of operating point stabilization of interferometric sensors using tunable lasers are presented. These methods employ circuits which control the wavelength of an external cavity tunable semiconductor laser. Modes of operation of the control circuits and problems related to their design are discussed. The limitations restricting the use of discussed control circuit types are analyzed. An interferometric force sensor was built and tested. Results of measurements carried out using the sensor are presented.

Optimization of white-light interferometric temperature sensor

White-light interfermetry, also referred to as low-coherence interferometry, is an interferometric technique employing broadband light sources in order to make absolute, high-resolution measurements of several physical quantities, such as position, displacement, temperature and pressure. In the paper a model of a low-coherence interferometric temperature sensor using a Fabry-Perot sensing interferometer is presented and optimization of the sensor is discussed. The sensor uses an inexpensive source such as LED or VCSEL, standard telecommunication fibers and components, which makes it a potentially low-cost solution.