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Preface to Second Edition ................................................................................................................ vii
Chapter 1 Image Processing and Computer Vision for MEMS Testing ....................................... 1 Markus Hüttel
Chapter 2 Surface Features ......................................................................................................... 51
Xiangqian Jiang
Chapter 3 A Metrological Characteristics Approach to Uncertainty in Surface Metrology 73
Richard Leach, Han Haitjema, and Claudiu Giusca
Chapter 4 Image Correlation Techniques for Microsystems Inspection 93
Dietmar Vogel and Bernd Michel
Chapter 5 Light Scattering Techniques for the Inspection of Microcomponents and Structures ................................................................................................................. 139
Angela Duparré and Sven Schröder
Chapter 6 Characterization and Measurement of Microcomponents with the Atomic Force Microscope (AFM) 155
F. Michael Serry and Joanna Schmit
Chapter 7 Optical Profiling Techniques for MEMS Measurement 177
Klaus Körner, Johann Krauter, Aiko Ruprecht, and Tobias Wiesendanger
Chapter 8 Grid and Moiré Methods for Micromeasurements 207
Anand Asundi, Bing Zhao, and Huimin Xie
Chapter 9 Grating (Moiré) Interferometry for In-Plane Displacement and Strain Measurement of Microcomponents .......................................................................... 245
Leszek Salbut and Malgorzata Kujawinska
Chapter 10 Interference Microscopy Techniques for Microsystem Characterization ...............
Alain Bosseboeuf, Philippe Coste, and Sylvain Petitgrand
Chapter 11 Measuring MEMS in Motion by Laser Doppler Vibrometry .................................
Christian Rembe, Georg Siegmund, Heinrich Steger, and Michael Wörtge
Chapter 12 An Interferometric Platform for Static, Quasi-Static, and Dynamic Evaluation of Out-of-Plane Deformations of MEMS and MOEMS 349
Christophe Gorecki, Michał Józwik, and Patrick Delobelle
Chapter 13 Optoelectronic Holography for Testing Electronic Packaging and MEMS ............
Cosme Furlong
Chapter 14 Digital Holography and Its Application in MEMS/MOEMS Inspection ...............
Wolfgang Osten and Pietro Ferraro
Roland Höfling and Petra Aswendt
Ingrid
Preface to Second Edition
2007 was a good year to publish a book dedicated to optical methods for the inspection of microsystems. The technological basis for the design and fabrication of microcomponents was meanwhile mature, and many devices and applications were already implemented or under test. Consequently, reliable procedures for quality assurance gained importance, and the search for efficient inspection methods was on the agenda of many vendors. Full-field optical methods established an alternative approach in comparison with conventional tactile and/or pointwise sampling methods, owing to their noncontact, nondestructive, and areal working principle. Thus, this book was very well received by the readers and translated to Chinese1 a couple of years after its first edition. However, 10 years later, it was time to think about an update. As the technology progressed, naturally higher expectations were placed on quality assurance and thus the inspection techniques used. For instance, challenges such as checking the system’s internal parts after packaging came up. New requirements for the calibration of measurement tools, new rules for the estimation of uncertainty in measurement, and meaningful surface features had to be considered.
Consequently, the publisher and editor decided to prepare an updated and extended edition, taking into account the progress in the field. This second edition is now available. Three new chapters were added, focusing on the following:
• Micro-Electro-Mechanical Systems (MEMS) surface features characterization
• Metrological characteristics to uncertainty in surface metrology
• Sensor fusion strategies for microsystems inspection
All these new chapters were written by leading experts in the particular fields. Nine of the original chapters have been updated, while only five chapters have retained the content of the first edition. Thus, the reader finds an almost completely updated and extended book. Publisher, authors, and editor hope that the second edition will find an even well-interested audience and help to solve the even more challenging problems in microsystems technology.
The editor is very grateful to all contributors and the publisher for the repeated fruitful cooperation. Special thanks goes to Michelle Kazensky of Lumina Datamatics and Marc Gutierrez and Kari Budyk at CRC Press.
Wolfgang Osten Lilienthal, 2019
1 China Machine Press, Beijing, China, 2014, ISBN 978-7-111-45837-1.
Preface to First Edition
The miniaturization of complex devices such as sensors and actuators is one of the biggest challenges in modern technology. Different manufacturing technologies—for instance, the so-called LIGA technique and UV lithography—allow the realization of nonsilicon and silicon microparts with a high aspect ratio and structural dimensions in the range from nanometers to millimeters. LIGA is an acronym standing for the main steps of the process, i.e., deep x-ray lithography, electroforming, and plastic molding. These three steps make it possible to mass-produce high-quality microcomponents and microstructured parts, in particular from plastics, but also from ceramics and metals at low cost. Techniques based on UV lithography or advanced silicon etching processes (ASE) allow for direct integration of electronics with respect to the realization of advanced microelectromechanical systems (MEMS) devices. Further technologies such as laser micromachining, electrochemical milling (ECF), electrodischarge machining, and nanoimprint lithography (NIL) offer, meanwhile, an economical, high-resolution alternative to UV, VUV, and next-generation optical lithography. Increased production output, high system performance, and product reliability and lifetime are important conditions for the trust in a new technology and deciding factors for its commercial success. Consequently, high quality standards are a must for all manufacturers. However, with increasing miniaturization, the importance of measurement and testing is rapidly growing, and therefore the need in microsystems technology for suitable measurement and testing procedures is evident. Both reliability and lifetime are strongly dependent on material properties and thermomechanical design. In comparison to conventional technologies, the situation in microsystems technology is extremely complicated. Modern microsystems (MEMS and MOEMS) and their components are characterized by high-volume integration of a variety of materials and materials combinations. This variety is needed to realize very different and variable functions such as sensor and actuator performance, signal processing, etc. Still, it is well known that the materials´ behavior in combination with new structural design cannot be easily predicted by theoretical simulations. A possible reason for wrong predictions made by FEM calculations with respect to the operational behavior of microdevices is, for instance, the lack of reliable materials data and boundary conditions in the microscale. Therefore, measurement and testing procedures are confronted with a complex set of demands. In general, the potential for the following is challenged:
• Microscopic and nanoscopic measurement and testing on wafer scale
• Fast in-line measurement of various dimensional and functional properties of highly heterogeneous hybrid systems
• Verification of system specifications including geometrical, kinematical, and thermomechanical parameters
• Fast and reliable recognition of surface and subsurface defects, with the possibility for review and repair
• Measurement of complex 3-D structures with high aspect ratio
• Determination of material properties well defined for the bulk but to be specified for microscale
Measurement and inspection techniques are required that are very fast, robust, and relatively low cost compared to the products being investigated. The reason for this demand is obvious: properties determined on much larger specimens cannot be scaled down from bulk material without any experimental verification. Further on, in microscale, materials’ behavior is noticeably affected by production technology. Therefore, simple and robust methods to analyze the shape and deformation of the microcomponents are needed. Together with the knowledge of the applied load and appropriate physical models, these data can be used for the derivation of material parameters and various
system properties. It is obvious that neither a single method nor a class of measurement techniques can fulfill these requirements completely. Conventional tensile test techniques (e.g., strain gauges) are unable to test specimens from submillimeter-sized regions because of their limited local resolution and partly unwanted tactile character. Other approaches, such as, for instance, microhardness measurements, do not reveal directional variations.
However, full-field optical methods provide a promising alternative to the conventional methods. The main advantages of these methods are their noncontact, nondestructive, and fieldwise working principle; fast response potential; high sensitivity and accuracy (typical displacement resolution of a few nanometers, strain values of 100 microstrain); high resolution of data points (e.g., 1000 × 1000 points for submillimeter field of view); advanced performance of the system, i.e., automatic analysis of the results; and data preprocessing in order to meet requirements of the underlying numerical or analytical model. Thus, this book offers a timely review of the research into applying optical measurement techniques for microsystems inspection. The authors give a general survey of the most important and challenging optical methods such as light scattering, scanning probe microscopy, confocal microscopy, fringe projection, grid and Moiré techniques, interference microscopy, laser Doppler vibrometry, holography, speckle metrology, and spectroscopy. Moreover, modern approaches for data acquisition and processing (for instance, digital image processing and correlation) are presented.
The editor hopes that this book will significantly push the application of optical principles for the investigation of microsystems. Thanks are due to all authors for their contributions, which give a comprehensive overview of the state of the art in the fascinating and challenging field of optical microsystems metrology. Finally, the editor is grateful for the cooperation shown by CRC Press represented by Taisuke Soda, Preethi Cholmondeley, Gerry Jaffe, and Jessica Vakili.
Wolfgang Osten Stuttgart, 2007
Editor
Wolfgang Osten earned an MSc/Diploma in physics at the Friedrich Schiller University Jena in 1979. From 1979 to 1984, he was a member of the Institute of Mechanics in Berlin, working in the field of experimental stress analysis and optical metrology. In 1983, he earned a PhD at the Martin Luther University Halle-Wittenberg for his thesis in the field of holographic interferometry. From 1984 to 1991, he was employed at the Central Institute for Cybernetics and Information Processing ZKI in Berlin, making investigations in digital image processing and computer vision. Between 1988 and 1991, he headed the Institute for Digital Image Processing at the ZKI. From 1991 until 2002, he joined the Bremen Institute of Applied Beam Technology (BIAS) to establish and direct the Department Optical 3D Metrology. From September 2002 until October 2018, he was a full professor at the University of Stuttgart and director of the Institute for Applied Optics. From 2006 until 2010, he was the vice rector for research and technology transfer of the Stuttgart University, and from 2015 until 2018, he was the vice chair of the university council. His research work is focused on new concepts for industrial inspection and metrology by combining modern principles of optical metrology, sensor technology, and image processing. Special attention is directed to the development of resolution-enhanced technologies for the investigation of micro- and nanostructures.
Contributors
Anand Asundi
School of Mechanical and Aerospace Engineering
Nanyang Technological University
Singapore
Petra Aswendt
ViALUX GmbH
Chemnitz, Germany
Alain Bosseboeuf
Centre de Nanosciences et de Nanotechnologies
UMR CNRS 9001
Université Paris-Sud Université Paris-Saclay Orsay Cedex, France
Philippe Coste
Centre de Nanosciences et de Nanotechnologies
UMR CNRS 9001
Université Paris-Sud
Université Paris-Saclay Orsay Cedex, France
Patrick Delobelle
Department LMA, FEMTO-ST Université de Franche-Comté Besançon, France
Ingrid De Wolf
imec, Department of 3D & Optical I/O Technologies and KULeuven, Department of Materials Engineering Leuven, Belgium
Angela Duparré
Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) Jena, Germany
Pietro Ferraro
Director of ISASI
CNR - Institute of Applied Sciences & Intelligent Systems Pozzuoli NA, Italy
Cosme Furlong
Center for Holographic Studies and Laser MicroMechatronics
Mechanical Engineering Department Worcester Polytechnic Institute Worcester, Massachusetts
Claudiu Giusca
Cranfield University Cranfield, United Kingdom
Christophe Gorecki
Département LOPMD, FEMTO-ST Université de Franche-Comté Besançon, France
Marc Gronle
Institut für Technische Optik Universität Stuttgart Stuttgart, Germany
Han Haitjema
Mitutoyo Research Center Europe Leuven, Belgium the Netherlands
Roland Höfling ViALUX GmbH Chemnitz, Germany
Markus Hüttel
Head of Department Machine Vision and Signal Processing Fraunhofer Institute for Manufacturing Engineering and Automation IPA Machine Vision and Signal Processing Stuttgart, Germany
Xiangqian Jiang
EPSRC Future Metrology Hub Centre of Precision Technologies University of Huddersfield Huddersfield, United Kingdom
Michał Józwik
Faculty of Mechatronics Warsaw University of Technology Warsaw, Poland
Klaus Körner
Institut für Technische Optik Universität Stuttgart Stuttgart, Germany
Johann Krauter
Institut für Technische Optik Universität Stuttgart Stuttgart, Germany
Malgorzata Kujawinska
Institute of Micromechanics and Photonics Warsaw University of Technology Warsaw, Poland
Richard Leach
Professor of Metrology Manufacturing Metrology Team Faculty of Engineering University of Nottingham Nottingham, United Kingdom
Bernd Michel
Micro Materials Center Berlin Fraunhofer Institute for Reliability and Microintegration (IZM) Berlin, Germany
Wolfgang Osten
Institute of Electrical Information Technology
Clausthal University of Technology Stuttgart, Germany
Sylvain Petitgrand Fogale Nanotech Nîmes, France
Christian Rembe
Institute of Electrical Information Technology Clausthal University of Technology Clausthal-Zellerfeld, Germany
Aiko Ruprecht
Institut für Technische Optik Universität Stuttgart Stuttgart, Germany
Leszek Salbut
Institute of Micromechanics and Photonics Warsaw University of Technology Warsaw, Poland
Joanna Schmit
Principal Optical Engineer 4D Technology Business of Nanometrics Tucson, Arizona
Sven Schröder
Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) Jena, Germany
F. Michael Serry
Veeco Instruments, Inc. Santa Barbara, California
Georg Siegmund Polytec GmbH Waldbronn, Germany
Heinrich Steger Polytec GmbH Waldbronn, Germany
Dietmar Vogel
Micro Materials Center Berlin Fraunhofer Institute for Reliability and Microintegration (IZM) Berlin, Germany
Tobias Wiesendanger
Institut für Technische Optik Universität Stuttgart Stuttgart, Germany
Michael Wörtge Polytec GmbH Waldbronn, Germany
Huimin Xie
School of Mechanical and Production
Engineering Nanyang Technological University
Singapore
Bing Zhao
School of Mechanical and Production
Engineering Nanyang Technological University
Singapore
1 Image Processing and Computer Vision for MEMS Testing
1.1 INTRODUCTION
Not only is there a requirement for testing the electrical and dynamic behavior of MEMS, but there is also a considerable demand for methods for testing these systems during both the development phase and the entire manufacturing phase. With the aid of these test methods, it is possible to assess such static properties as dimension, shape, presence, orientation, and the surface characteristics of microsystems or their components. Using an optical measurement and testing technique based on image processing and computer vision, a wide range of procedures can be applied, which enable such properties to be recorded rapidly and in a robust and noncontact way.
If measurement and testing means are not based on special optical procedures but rather on illumination with normal light and imaging with normal and microscopic optics, their resolution capabilities extend only to just below the micrometer range. This is due to the diffraction of light and the dimensions of imaging sensor elements in a lateral direction. Such a degree of resolution is inadequate, as it is unable to cover the entire scale of microsystem structure sizes, which ranges from just a few nanometers (e.g., surface roughness) to a few millimeters (e.g., external contours). In order to measure sizes in the nanometer range, special imaging measurement and testing means are required. These include interferometers, spectrometers, near-field/scanning electron/atomic force microscopes (AFMs), and specialized reconstruction and analysis processes such as fringe processing and scanning techniques, which are described in the following chapters.
The main advantages of implementing optical testing equipment using simple light sources and normal and microscopic optics are the speed with which images can be recorded and analyzed and the fact that they can be easily integrated into the manufacturing process, thus making the errorprone removal of components from and reintroduction into the clean environment for test purposes superfluous. For this reason, despite their limited resolution capabilities, they are still ideally suitable for testing large piece numbers, that is, in the manufacturing process in the areas of assembly, function, and integrity testing and packaging.
Furthermore, the algorithms developed for image processing and computer vision are not only suitable for analyzing images recorded using a video camera but can also be applied to the fields of signal analysis, data analysis and reconstruction, and many other areas.
This chapter deals not only with the technical aspects of illumination and image recording techniques but also with image processing and computer vision processes relevant to the optical measurement and testing technique and their implementation in typical measurement and testing tasks. Finally, several software products that are available commercially for image processing and computer vision will also be described. However, first, a classification of typical measurement and testing tasks in the field of microsystem development and production is given.
1.2 CLASSIFICATION OF TASKS
The tendency that has been observed for many years toward miniaturization in the electronics industry has also been affecting the field of optics and mechanics over the last few years. The origins of miniaturizing mechanical systems, with dimensions in the micrometer range, can be found in the laboratories of research institutes concerned with semiconductor manufacturing and also in the laboratories of the semiconductor manufacturers. They possess not only expertise in processing delicate silicone structures but also the equipment required to produce them. Most of the microelectromechanical systems or microoptoelectromechanical systems available today (so-called MEMS/MOEMS) have been created using the combination of electronic, optical, and mechanical functional groups. Today, MEMS/MOEMS can be found in a wide range of technical devices used in our everyday lives. In ink-jet printers, MEMS-based microinjectors transfer the ink to paper. Digital micro mirror devices (DMDs)—a matrix of thousands of electrically adjustable micromirrors—are responsible for producing digital images in digital light projection (DLP) technology projectors. Sensors for measuring pressure, force, temperature, flow rates, air mass, acceleration, tilting, and many other values are
constructed as MEMS and are utilized especially in the automotive industry—a mass market with high safety requirements (Airbag, Electronic Stability Program, Bosch, Germany [ESP®]). However, MEMS and MOEMS are being used more and more in many other fields such as in the medical industry or in biological and chemical diagnostics. The “Lab-on-a-Chip” is capable of carrying out complete investigation processes for chemical, biological, or medical analyses. Microspectrometers, just a few millimeters in size, enable extremely small and cheap analysis devices to be constructed, which can be used in flight equipment for monitoring terrestrial biological and climatic processes and also in a wide range of technical applications in the form of hand-held devices. Using micromotors, microdrives, micropumps, and microcameras, instruments can be constructed in the medical field for keyhole diagnosis and surgical interventions.
Although, in comparison with microelectronic components, microsystems possess a clear threedimensional structure (e.g., microspectrometers, electrical micromotors, and gears for drives made by using microinjection molding techniques), classic MEMS structures, especially sensor and mirror systems, are essentially two-dimensional in shape. This feature is the result of constructing MEMS based on semiconductor materials and on the manufacturing processes used in conjunction with such materials. Another conspicuous characteristic of MEMS-based systems is the common use of hybrid constructions, where the drive and analysis electronics are located on a semiconductor chip and the actual MEMS (e.g., the sensor element) is located on a separate chip.
Both these properties influence the tests realizable for MEMS, using image processing and computer vision. These are essentially performed using incident light arrangement, where both image recording and illumination take place at the same angle. The transmissive light arrangement (which can be much better controlled), where the object to be tested is situated between the illumination source and the image recording system, can be used to advantage if MEMS are more threedimensional in shape. This is becoming more and more the case.
From the point of view of image processing and computer vision, the solutions listed below are possible for the following examples of testing tasks:
• Test object or regions of interest: Test objects need to be located if the position of the objects to be tested varies either in relation to each another or in relation to the reference system of the observation area. This is often the case with hybridly constructed systems, where individual components are located relatively inaccurately next to one another. Regions of interest need to be located, for example, in cases where MEMS components are processed or adjusted individually (e.g., when measuring the cross-section of lasertrimmed resistances and capacitors) or if a measuring area needs to be determined using a small measurement range for a high-resolution measuring or test system (multiscaled measuring/testing).
• Position recognition: The position of components needs to be recognized if they are not aligned for a test or if their position, when installed, shows degrees of freedom (e.g., resistance, diode, and gear). Another example is when tests need to be carried out on the components themselves, such as the recognition of component coding and the measurement of geometric features. In some cases, it is necessary to merge the best two or more images taken from the same scene (e.g., with different sensors or from different positions) in one image. This very important task in image processing is called “image registration.” There are a lot of approaches and methods to realize this “best fit” of images in medical, remote sensing, and industrial applications [1].
• Measuring geometric features: Investigating a component from a metrological point of view shows whether production tolerances have been adhered to; this affects both the mechanical and electrical behavior of the object. The location of geometric features using contour or edge recognition and the calculations based on them to obtain straightness, circularity, length, enclosed surfaces, angle, distance, diameter, etc, form the principles of optical metrology.
• Presence verification: The monitoring of production and assembly processes often requires a purely qualitative statement regarding certain features, without specific knowledge of their geometrical characteristics. For example, in production processes, tool breakage can be monitored by checking a work piece for the presence of bore holes, grooves, etc. With assembly processes, the focus of interest is usually on the assembled result, that is, the presence of all components requiring assembly.
• Fault detection: In contrast with presence verification, in the case of fault detection, features are checked for deviations from the required standards. To detect faults, for example, text recognition is used for reading identification markings on components, color checking for verifying color-coded components, and texture analysis for investigating structured surfaces for flaws (e.g., scratches).
In order to solve these examples of measuring and verification tasks based on two-dimensional image data, image processing and computer vision have a whole range of proven processing and interpretation methods available. Shading correction, the averaging of image series, spatial and morphological filtering, edge detection, pattern and geometric feature matching, segmentation, connectivity analysis, and the metrology of geometric features denote some of these methods. Other techniques that are required, for example, in interferometry, spectroscopy, and holography, and that also permit imaging metrology, are described in the following chapters.
1.3 IMAGE PROCESSING AND COMPUTER VISION COMPONENTS
The elementary task of digital image processing and computer vision is to record images, to process/ improve and analyze the image data by using appropriate algorithms, to supply results derived from them, and to interpret these results. Figure 1.1 shows a typical scenario for an image-processing system. Images of an object illuminated by a light source are recorded by an electronic camera. After having been converted by a frame grabber into digital values, the analog, electrical image signals from the camera reach the memory of a computer. The digital representation of the scene obtained in this way forms the starting point for subsequent algorithmic processing steps, analyses, and conclusions, otherwise known as image processing and computer vision.
In addition to requiring the knowledge of the theory of digital image processing (which represents the fundamentals for the algorithms used in image processing and computer vision), it becomes immediately clear from the scenario that expertise in the fields of light physics, optics, electrical engineering, electronics, and computer hardware and software is also necessary. The first steps towards finding a successful image-processing solution are not in the selection and
FIGURE 1.1 Components of a computer vision scenario.
implementation of suitable algorithms but rather much earlier on, that is, in the depiction of the images. Particular attention must be given to the illumination of a scene. A type of illumination well-adapted to the task in question produces images that can be analyzed using simple algorithmic methods. Correspondingly badly illuminated scenes may produce images from which even the most refined algorithms are unable to extract the relevant features. It is also equally important to take the dynamic behavior of imaging sensors into consideration. If illumination is too bright, sensor elements may reach saturation range and produce overilluminated images, which could lead to false conclusions. As far as metrological tasks are concerned, it is essential to understand the imaging properties of lenses. For these reasons, the following sections are concerned with the most important aspects of illumination, imaging sensors, camera technology, and the imaging properties of lenses.
1.3.1 Behavior of Light, CoLors, and fiLters
The range of the electromagnetic waves (see Figure 1.2) that are of interest as far as image processing is concerned lies between ultraviolet (UV) and near infrared (IR)—a very small part of the entire electromagnetic spectrum. The human eye is capable of recognizing only a part of the spectrum, that is, of visible light.
The intensity I [W/m 2] of a light source is defined by the quantity of photons radiated per unit of area. If the distance is increased between a spot-light source illuminating diffusely and a surface element, the intensity of the light decreases proportionally to the square of the distance. The areas of constant intensity are present as concentric spheres around the light source (see Figure 1.3). As described later on, with light collimators, the conformity with this law leads to an inhomogeneous density of light, which decreases from the center toward the periphery.
Light from the sun or a light bulb is seen by the human eye as white light. This is due to the fact that these light sources radiate electromagnetic waves that cover the entire spectrum of visible light. As depicted in Figure 1.4, this becomes clear when light propagates through a prism made of glass or similar transparent material. Owing to the higher optical density of glass, expressed as
FIGURE 1.2 Electromagnetic spectrum.
FIGURE 1.3 Illustration of the inverse-square law.
FIGURE 1.4 Refraction of white light by a prism.
the refraction index n, the electromagnetic waves of the light are refracted at different degrees, depending on their wavelength λ, and disintegrate into the colors of the rainbow. This is known as dispersion.
If light refracted into the colors of the rainbow is merged again using a lens, white light results. This observation leads to the phenomenon of mixing colors. If lights of different colors are mixed, the colors are added together and form white. If, however, colored substances (e.g., pigments) illuminated by white light are mixed, the colors are subtracted from one another and form black (Figure 1.5).
From this, the following questions arise: Why objects appear colored if they are illuminated by white light? What are the effects of colored lights in conjunction with colored objects?
If an object appearing red to the human eye is illuminated by white light, the electromagnetic waves of the spectrum corresponding to the color red are reflected and reach the eye. The light from all other wavelengths is adsorbed by the object’s pigments and is transformed into warmth. Naturally, the same also applies for objects made up of several colors, as shown in Figure 1.6.
As monochrome cameras are often applied in image processing, colored light can be used advantageously to highlight colored objects. If, as shown in Figure 1.7, a red-and-green-colored object is illuminated by green light, essentially only the green-colored areas of the object reflect the light, leading to pale gray values in an image taken by a monochrome camera.
Additive and subtractive colors.
Response of colored objects to white light.
Response of colored objects to colored light.
FIGURE 1.5
FIGURE 1.6
FIGURE 1.7
Today, light-emitting diodes (LEDs) are often used as colored light source in image processing if the area to be illuminated is not too large. Because of their specific color types, LEDs cover the entire frequency range of visible light and the adjacent areas of near-IR and UV, induce minimal loss of warmth, and can be switched on and off very quickly. However, if intensive light sources are required, sources of white light such as halogen lamps are implemented and combined with optical filters. The filters are generally made of colored glasses, which selectively allow light of a certain wavelength to pass through and either adsorb or reflect all other light wavelengths.
If a scene is illuminated with colored light but recorded with a monochrome camera, it often makes sense in practice to place the filter directly in front of the camera lens rather than to filter the light from the source of white light. In this way, any stray light from the environment that does not possess the wavelength of the transmitted filter light is also filtered out and is therefore unable to reach the camera’s sensors (see Figure 1.8).
In cases where the colors of an object are irrelevant, near-IR light sources are often used in conjunction with a filter that transmits only this light. As a result, light conditions where the object is illuminated are almost completely independent of visible light. If the IR light source is also monochromatic, that is, only light from a narrow range of the spectrum is transmitted—or if the filter is constructed as a narrow band pass (see figure 1.9)—the color distortions of the lens cause fewer chromatic errors in the images recorded, which can be used advantageously, especially where metrological tasks are concerned.
As far as the transmission of light is concerned, the behavior of optical filters is characterized using spectral response plots. To do this, in general, the relative transmission of the filter material is plotted as a function of the wavelength of the electromagnetic spectrum (see Figure 1.9).
In the same way, it is also possible to characterize the spectral fractions of irradiated light from a light source and the spectral sensitivity of light-sensitive sensors.
FIGURE 1.8 Effect of filter location on the resulting grayscale image.
Spectral response plot of: (a) edge filters, (b) band pass filters, (c) light sources, and (d) CCD sensor.
1.3.2 iLLumination
The type of illumination plays a crucial role in image processing. By using an illumination adapted to the special task in question, the features of a test object requiring measurement or testing can often be highlighted better. Thus, the processing of images is simplified drastically in many cases. Illumination can essentially be classified into two types in accordance with the arrangement of the light source, test object, and image-recording system. If the test object is situated between the light source and the image-recording system, this is known as a transmissive light arrangement Correspondingly, if the light source and the image-recording system are situated on the same side as the test object, this is known as an incident light arrangement. The simplest realization of a transmissive light arrangement is represented by a light panel (see Figure 1.10), in which the light source (usually consisting of several lamps, tube lamps, or LEDs)
FIGURE 1.10 Transmissive light arrangement: (a) principle of a light panel and (b) silhouette of an object.
FIGURE 1.9
(a) (b)
is positioned behind a ground-glass screen, which scatters light diffusely. This arrangement is especially used to determine and measure the contours of flat, nontransparent test objects, because the camera sees the shadowed image of the object. In cases where objects are translucent or transparent, this arrangement enables internal structures to be recognized. However, the diffusely scattered light from a ground-glass screen is disadvantageous if the dimensions of test objects are particularly large in the direction of the optical axis of the camera. This is because surfaces that are almost parallel to the axis could also be illuminated, thus falsifying the true silhouette of the test object and the resulting measurement data obtained.
This disadvantage of diffusely scattered light can be reduced if an illumination system composed of a concave lens and a spotlight source (see Figure 1.11)—a so-called collimator—is used instead of ground-glass illumination. By placing the spotlight source in the focal point of the lens, the light emerges as almost-parallel light rays. The LEDs (without lens optics) are especially useful as light sources for this, because the light-emitting semiconductors are so small that they are almost punctiform.
Despite the advantage of parallel light rays, illuminating collimators have the disadvantage of producing inhomogeneous illumination. In accordance with the law regarding the decrease in light intensity of a spotlight source, the light intensity of light rays passing through the lens decreases from the optical axis toward the periphery, known as shading or vignetting. This effect, which is disadvantageous as far as image processing is concerned, can be avoided if a homogeneously illuminated diffuse ground-glass light source, placed at the image level of a lens, is projected, instead of using a spotlight source in the focal point of a collimator. If a telecentric lens (see Section 1.3.3) is used instead of a normal lens to project the ground-glass light source, homogeneous illumination with parallel light rays results.
As many MEMS or their components are based on silicon wafers, which are—according to their nature—essentially two-dimensional in shape, the main type of illumination used in image processing to test such elements is that of incident light arrangement. In contrast with transmissive light arrangement, by using this type of illumination, both the test features of the object and all other areas are equally illuminated. This results in the fact that features in the images being recorded are often more difficult to differentiate in the subsequent image processing. From this point of view, a type of illumination that is adapted to the task is of particular importance.
With incident light arrangements, an essential difference is made between dark field arrangement and bright field arrangement, because they highlight very different aspects of a test object.
As far as the angle of observation is concerned, with dark field arrangement (see Figure 1.12), the test objects are illuminated from an almost-perpendicular angle. In an ideal situation, as a result, no or only a small amount of light falls onto the object surfaces, which are parallel to the angle of
FIGURE 1.11 Transmissive light arrangement: (a) collimator and (b) projection of a homogeneous light source.
(a)
(b)
incidence and which are thus perpendicular to the angle of observation. These fields are seen by the observer as being dark areas. In contrast, all other nontransparent object details are highlighted, provided they are not positioned in the shadows of other object details. With nontransparent test objects, dark-field illumination is therefore advantageous if raised sections of an object have to be highlighted. Another example of using dark-field illumination to advantage is for detecting particles on smooth, even surfaces. Transparent test objects such as glass bodies and plastics can also be examined for inclusions or edge defects, as these features stand out well against a dark background, especially when the latter is matt black in color. Generally, spot and linear, diffuse and directed light sources are used for dark-field illumination. In many cases, low-angle glass fiber or LED ring lights reduce the problem of shadow formation.
In contrast with dark field arrangement, in the case of bright field arrangement test, objects are illuminated almost from the angle of observation. In simple cases, the light source for bright field arrangement is realized using either one or several spotlight sources, high-angle glass fiber lights, or LED ring lights placed around the camera lens. Using this illumination arrangement, a good differentiation of features can be obtained where object surfaces are diffusely dispersive, provided the colors of the features to be tested contrast well against their surroundings, such as in the case of dark print markings and barcodes on pale or colored markings.
The bright field arrangement described earlier becomes problematic if object surfaces are shiny or reflective. This is because the light sources may be projected in the recording camera system in the form of reflections, thus impeding or preventing reliable feature analysis. Homogeneous illumination can be achieved in these cases if all visible sides of an object are illuminated equally by a diffuse light source. The light dome (half sphere) shown in Figure 1.13 is capable of doing this. Light sources fixed at the edge of the light dome illuminate its internal surface, which is matt and coated white.
In the case of flat, shiny surfaces, reflex-free bright-field illumination can be achieved if the light is coupled coaxially to the angle of observation. As shown in Figure 1.14a, diffuse light from a homogeneously illuminated surface—such as that used in transmissive light arrangements—is deflected by 90° toward the angle of observation by using a semitransparent mirror. Owing to the (a) (b)
FIGURE 1.12 Principles of dark-field illumination: (a) single sided and (b) circular light.
homogeneity of the light source and the evenness of the object surface, object areas possessing an equal degree of reflection are depicted in the camera image with the same level of brightness, making this a reliable method for imaging and analyzing features on such surfaces.
A special form of coaxial bright-field illumination is utilized for telecentric lenses (see Section 1.3.3). As shown in Figure 1.14b, light from a spotlight source is coupled to the beam path of the lens by using a semitransparent mirror. Ideally, the light emitted from the lens is made up purely of parallel light rays, which are reflected toward the telecentric lens from object surfaces only (a)
(b)
(a)
(b)
if those surfaces are perpendicular to the optical axis. The properties of telecentric lenses prevent the light of all other surfaces from reaching the camera, with the result that these appear as dark areas in the recorded image. This feature can be used to make the edges of flat, three-dimensional structures visible.
Besides the types of illumination described here, other forms of illumination can also be implemented in image processing. These include types of structured illumination such as fringe projection and laser scanning for recording and measuring objects in three dimensions (see Chapter 6).
1.3.3 Lens systems
To project a real scene on the imaging sensor of a camera, a wide range of various lenses is available. Depending on the task, normal, macro-, and microscopic lenses can be used. In principle, the imaging properties of these lenses do not differ much from the imaging behavior of a single thin lens shown in Figure 1.15, with the imaging equation:
As can be seen in Figure 1.15a, the geometric construction of the image Im results from the object O as follows: a line is drawn from each object point through the center of the lens; a second line parallel to the optical axis is bisected with the principal plane of the lens; and starting at this intersection, a third line is drawn through the focal point of the lens. The intersection of the first and third line then gives the corresponding image point.
As shown in Figure 1.15b, it can be seen from the lens equation that both the position and the size of the image Im alter if the object is moved along the optical axis. In the case of lenses, in order to depict the object in focus, the displacement of the image Im needs to be corrected. This is achieved by moving the lens along the optical axis, so that the focal plane (where the imaging sensors of the camera are situated) is always in the same place. Owing to the limitation of the depth of field, it can be deduced directly that the objects that are particularly large in the direction of the optical axis and that are close to the lens cannot be depicted completely in focus. To be more precise, this results in part of the image to be in focus and the surrounding areas to be slightly out of focus. This characteristic may considerably impair metrological image analysis or even render it impossible.
The cause of this blurred imaging can be explained by the following observation (see Figure 1.16a). Each point of the object O emits light homogeneously in all free directions in the form of spherical waves. Part of the light reaches the lens, and this then depicts it as an image point in the zone of sharp focus. Circles of confusion are formed outside the zone of sharpness instead of image points, thus resulting in a blurred image.
FIGURE 1.15 Imaging behavior of a thin lens: (a) geometric construction of the image of an object, and (b) if an object moves toward a lens, the image moves away from the lens and gets enlarged.
(a) Formation of image points from object points and (b) telecentric imaging.
As the geometric construction shows, the light rays running parallel to the optical axis possess two characteristic features. Independent of the distance between the object and the lens, they always run parallel, and when they are projected, they always pass through the focal point F of the lens. This results in the concept of generating a sharp image, independent of distance, by utilizing a screen with a small aperture at the height of the focal point (focal pupil) in the beam bath of the lens (see Figure 1.16b). Using this measure, a so-called object-sided telecentric lens is created.
As illustrated in Figure 1.17, telecentric lenses possess the following properties:
1. The field of view is equal to the size of the lens, because light rays running parallel to the optical axis are not imaged outside the lens area.
2. The imaging of an object O (see Figure 1.17a) always results in (within limits) a sharp image Im, independent of the distance between the object and the lens. This characteristic is limited by the finite dimensions of the focal pupil, because almost no light passes through an infinitely small point.
3. For all distances from the lens, the object is imaged with the same magnification as far as the focal plane remains fixed.
4. Owing to the focal pupil, telecentric imaging is of low light intensity.
5. The displacement of the focal plane (see Figure 1.17b) along the optical axis alters the size of an image but not its sharpness.
FIGURE 1.17 Behavior of one- and double-sided telecentric lenses: (a) within limits, object movement always result in a sharp image; (b) within limits, movement of the focal plane changes size of image but not sharpness; and (c) in case of double-sided telecentric lenses, sharpness and size of image (within limits) do not alter if focal plane moves along the optical axis.
FIGURE 1.16
This characteristic predestines telecentric lenses to being used for tasks where precise metrological image analysis is required.
The transition from an object- to a double-sided telecentric lens is achieved by introducing a second lens behind the focal pupil (see Figure 1.17c) in such a way that the focal points of the two lenses coincide. As a result, the focal rays are imaged into rays running parallel to the optical axis, and subsequently, the alteration in image size, which takes place when the focal plane is moved along the optical axis, no longer occurs. Double-sided telecentric lenses can be used to advantage if imaging sensors are equipped with microlenses (one lens per pixel) to give a better light yield. In this case, light entering obliquely may lead to vignetting, thus resulting in inhomogeneous grayscale value distributions in the images recorded.
1.3.4 sensors
Imaging sensors built into today’s cameras are based on semiconductor materials. The formation of “electronic” images by using such sensors utilizes the principle of the internal photo effect. With this principle, light quanta (photons) possessing a minimum material-specific energy release atomic electrons from their bonded state (see Figure 1.18a). However, in the process, the freed electrons remain in the material.
The external photo effect describes the process where electrons are emitted from a material. Photomultipliers (not discussed here) are based on this effect, for example.
The light-sensitive elements of imaging sensors based on semiconductor materials are constructed as photodiodes and use the depletion-layer photo effect. The functioning principle of such diodes is explained briefly below.
A silicone substrate is p -doped with boron atoms; owing to the three free valence electrons of boron, there are insufficient electrons to bond with the four valance electrons of silicone. The substrate is then coated with a thin layer of silicone n-doped with fluoride. As fluoride atoms possess five free valence electrons—that is, one electron more than necessary in order to bond with silicone—some of the free surplus electrons fill in the missing electrons (holes) in the p -substrate of the junction zone (see Figure 1.18b). Through this recombination, the p -substrate in the junction zone becomes negatively charged and the n-doped silicon layer positively charged, thus preventing additional electrons from migrating from the n-doped silicone to the p -doped silicone. The junction zone is thus transformed into a depletion layer.
FIGURE 1.18 Illustration of the principle of (a) the photo effect and (b) of the photodiode.
(a)
(b)
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The Project Gutenberg eBook of Address of President Roosevelt on the occasion of the celebration of the hundredth anniversary of the birth of Abraham Lincoln, Hodgenville, Ky., February 12, 1909
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Title: Address of President Roosevelt on the occasion of the celebration of the hundredth anniversary of the birth of Abraham Lincoln, Hodgenville, Ky., February 12, 1909
Author: Theodore Roosevelt
Release date: June 20, 2022 [eBook #68350]
Language: English
Original publication: United States: Government Printing Office, 1909
Credits: Donald Cummings and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/American Libraries.)
*** START OF THE PROJECT GUTENBERG EBOOK ADDRESS OF PRESIDENT ROOSEVELT ON THE OCCASION OF THE CELEBRATION OF THE HUNDREDTH ANNIVERSARY OF THE BIRTH OF ABRAHAM LINCOLN, HODGENVILLE, KY., FEBRUARY 12, 1909 ***
Address of President Roosevelt on the occasion of the Celebration of the Hundredth Anniversary of the Birth of Abraham Lincoln Hodgenville, Ky.
February 12, 1909
Washington Government Printing Office 1909
We have met here to celebrate the hundredth anniversary of the birth of one of the two greatest Americans; of one of the two or three greatest men of the nineteenth century; of one of the greatest men in the world’s history. This rail splitter, this boy who passed his ungainly youth in the dire poverty of the poorest of the frontier folk, whose rise was by weary and painful labor, lived to lead his people through the burning flames of a struggle from which the nation emerged, purified as by fire, born anew to a loftier life. After long years of iron effort, and of failure that came more often than victory, he at last rose to the leadership of the Republic, at the moment when that leadership had become the stupendous world-task of the time. He grew to know greatness, but never ease. Success came to him, but never happiness, save that which springs from doing well a painful and a vital task. Power was his, but not pleasure. The furrows deepened on his brow, but his eyes were undimmed by either hate or fear. His gaunt shoulders were bowed, but his steel thews never faltered as he bore for a burden the destinies of his people. His great and tender heart shrank from giving pain; and the task allotted him was to pour out like water the life-blood of the young men, and to feel in his every fiber the sorrow of the women. Disaster saddened but never dismayed him. As the red years of war went by they found him ever doing his duty in the present, ever facing the future with fearless front, high of heart, and dauntless of soul. Unbroken by hatred, unshaken by scorn, he worked and suffered for the people. Triumph was his at the last; and barely had he tasted it before murder found him, and the kindly, patient, fearless eyes were closed forever.
As a people we are indeed beyond measure fortunate in the characters of the two greatest of our public men, Washington and Lincoln. Widely though they differed in externals, the Virginia landed gentleman and the Kentucky backwoodsman, they were alike in essentials, they were alike in the great qualities which made each able to render service to his nation and to all mankind such as no other man of his generation could or did render. Each had lofty
ideals, but each in striving to attain these lofty ideals was guided by the soundest common sense. Each possessed inflexible courage in adversity, and a soul wholly unspoiled by prosperity. Each possessed all the gentler virtues commonly exhibited by good men who lack rugged strength of character. Each possessed also all the strong qualities commonly exhibited by those towering masters of mankind who have too often shown themselves devoid of so much as the understanding of the words by which we signify the qualities of duty, of mercy, of devotion to the right, of lofty disinterestedness in battling for the good of others. There have been other men as great and other men as good; but in all the history of mankind there are no other two great men as good as these, no other two good men as great. Widely though the problems of to-day differ from the problems set for solution to Washington when he founded this nation, to Lincoln when he saved it and freed the slave; yet the qualities they showed in meeting these problems are exactly the same as those we should show in doing our work to-day.
Lincoln saw into the future with the prophetic imagination usually vouchsafed only to the poet and the seer. He had in him all the lift toward greatness of the visionary, without any of the visionary’s fanaticism or egotism, without any of the visionary’s narrow jealousy of the practical man and inability to strive in practical fashion for the realization of an ideal. He had the practical man’s hard common sense and willingness to adapt means to ends; but there was in him none of that morbid growth of mind and soul which blinds so many practical men to the higher things of life. No more practical man ever lived than this homely backwoods idealist; but he had nothing in common with those practical men whose consciences are warped until they fail to distinguish between good and evil, fail to understand that strength, ability, shrewdness, whether in the world of business or of politics, only serve to make their possessor a more noxious, a more evil member of the community, if they are not guided and controlled by a fine and high moral sense.
We of this day must try to solve many social and industrial problems, requiring to an especial degree the combination of indomitable resolution with cool-headed sanity. We can profit by the
way in which Lincoln used both these traits as he strove for reform. We can learn much of value from the very attacks which following that course brought upon his head, attacks alike by the extremists of revolution and by the extremists of reaction. He never wavered in devotion to his principles, in his love for the Union, and in his abhorrence of slavery. Timid and lukewarm people were always denouncing him because he was too extreme; but as a matter of fact he never went to extremes, he worked step by step; and because of this the extremists hated and denounced him with a fervor which now seems to us fantastic in its deification of the unreal and the impossible. At the very time when one side was holding him up as the apostle of social revolution because he was against slavery, the leading abolitionist denounced him as the “slave hound of Illinois.” When he was the second time candidate for President, the majority of his opponents attacked him because of what they termed his extreme radicalism, while a minority threatened to bolt his nomination because he was not radical enough. He had continually to check those who wished to go forward too fast, at the very time that he overrode the opposition of those who wished not to go forward at all. The goal was never dim before his vision; but he picked his way cautiously, without either halt or hurry, as he strode toward it, through such a morass of difficulty that no man of less courage would have attempted it, while it would surely have overwhelmed any man of judgment less serene.
Yet perhaps the most wonderful thing of all, and, from the standpoint of the America of to-day and of the future, the most vitally important, was the extraordinary way in which Lincoln could fight valiantly against what he deemed wrong and yet preserve undiminished his love and respect for the brother from whom he differed. In the hour of a triumph that would have turned any weaker man’s head, in the heat of a struggle which spurred many a good man to dreadful vindictiveness, he said truthfully that so long as he had been in his office he had never willingly planted a thorn in any man’s bosom, and besought his supporters to study the incidents of the trial through which they were passing as philosophy from which to learn wisdom and not as wrongs to be avenged; ending with the
solemn exhortation that, as the strife was over, all should reunite in a common effort to save their common country.
He lived in days that were great and terrible, when brother fought against brother for what each sincerely deemed to be the right. In a contest so grim the strong men who alone can carry it through are rarely able to do justice to the deep convictions of those with whom they grapple in mortal strife. At such times men see through a glass darkly; to only the rarest and loftiest spirits is vouchsafed that clear vision which gradually comes to all, even to the lesser, as the struggle fades into distance, and wounds are forgotten, and peace creeps back to the hearts that were hurt. But to Lincoln was given this supreme vision. He did not hate the man from whom he differed. Weakness was as foreign as wickedness to his strong, gentle nature; but his courage was of a quality so high that it needed no bolstering of dark passion. He saw clearly that the same high qualities, the same courage, and willingness for self-sacrifice, and devotion to the right as it was given them to see the right, belonged both to the men of the North and to the men of the South. As the years roll by, and as all of us, wherever we dwell, grow to feel an equal pride in the valor and self-devotion, alike of the men who wore the blue and the men who wore the gray, so this whole nation will grow to feel a peculiar sense of pride in the man whose blood was shed for the union of his people and for the freedom of a race; the lover of his country and of all mankind; the mightiest of the mighty men who mastered the mighty days, Abraham Lincoln.
*** END OF THE PROJECT GUTENBERG EBOOK ADDRESS OF PRESIDENT ROOSEVELT ON THE OCCASION OF THE CELEBRATION OF THE HUNDREDTH ANNIVERSARY OF THE BIRTH OF ABRAHAM LINCOLN, HODGENVILLE, KY., FEBRUARY 12, 1909 ***
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