GEOMATICS & VOCABULARY

Transcription

1 MATERIAŁY DYDAKTYCZNE DO PRZEDMIOTU GEOMATICS & VOCABULARY Training and Project Materials Wydział Inżynierii Środowiska, Geomatyki i Energetyki Opracował - Ryszard Florek Paszkowski 1 / 65

2 CONTENTS 1. THE GEOMETRIC AND INTERPRETATIVE ASPECTS OF MICROSCOPE STEREOPHOTOGRAMMETRY The basic theory of electron microscope Description of a simple microscope electron gun SEM and photogrammetry SEM users and photogrammetrists Similarities and differences between transmission and scanning electron microscopes Production of stereo micrographs Remarks on tilt angles Distortions which affect a micrograph image Importance of microscope calibration Final conclusions PHOTOGRAMMETRIC MAPPING OF THE ARCHEOLOGICAL SITE WITH THE USE OF AIR-PHOTOGRAPHS AND DIGITAL STEREO RESTITUTORS Introduction Terrestrial photos with respect to archaeology Aerial photos with respect to archaeology Stereo restitution with digital photogrammetric station Leica DVP Problems experienced with the research Conclusions and recommendations A COMPARISION OF A METRIC AND NON-METRIC CAMERA FOR THE PURPOSE OF DOCUMENTATION OF BUILDINGS Introduction Major characteristics of metric and non-metric cameras Documentation of buildings Review of applications of documentation of buildings Applications of photogrammetry and surveying for the documentation of buildings Conclusions 65 2 / 65

3 1. THE GEOMETRIC AND INTERPRETATIVE ASPECTS OF MICROSCOPE STEREOPHOTOGRAMMETRY Users of Scanning Electron Microscopes seldom apply stereo measurements to the data which they obtain from the SEM. The Scanning Electron Microscopes which are utilized are often not calibrated properly, therefore the necessary corrections are not known (Florek 1994). These investigations were performed as a result of the cooperation within University of Cape Town between the Electron Microscope Unit and the Department of Surveying and Geodetic Engineering, namely between Prof. T Sewell and Dr. R Florek. The object of this work was to evaluate the accuracy of the StereoScan 440 Scanning Electron Microscope which has been purchased by UCT for the Electron Microscope Unit. The accuracy of the instrument needs to be known as many users extract information from the micrographs which a Scanning Electron Microscope produces. For the accuracy of the SEM to be determined it was necessary that a number of different magnifications were utilized. For this purpose the magnifications chosen were 250x, l000x and x. The following topics are presented: 1) The basic principles surrounding Electron Microscopes and especially the Scanning Electron Microscope. The manner in which one creates a stereo pair of micrographs. 2) A discussion of the various calibration techniques and the manner in which these techniques are utilized to determine the accuracy of the data obtained from the StereoScan ) The distortion parameters associated with the micrograph images, taken by the StereoScan ) An application in which stereo microscopy can be used with the StereoScan 440. The application undertaken was a brief analysis of Ferritic Stainless Steel, in which the Department of Materials Engineering at UCT is interested THE BASIC THEORY OF ELECTRON MICROSCOPE This chapter serves as an introduction to the subject of stereo electron micrography. It can provide the basis for further, more detailed studies into some of the areas of stereo electron microscopy. The chapter covers such topics as: 1) principles of the electron microscope; 2) selection of micrographs (electron microscope photographs) to create stereo images; 3) distortions which affect a micrograph; 4) theory and procedures to be followed in order to calculate 3-dimensional coordinates of points on a specimen, and 5) recommendations to improve the accuracy of the calculated coordinates. The very common and well known forms of photogrammetry are terrestrial and close range photogrammetry. Pictures of large objects are taken from a distance, not only from an airplane but also from a ground station or even from a satellite. These photographs are at a very small scale. If a user wished to photograph a small object at a larger scale, a camera fitted with macro lenses or special 3 / 65

4 devices could be used. Users were able to achieve photographs with a magnification of around 3x. At first, the cameras were partial metric cameras, but more recently CCD cameras have also been used. Characteristics OLM TEM SEM Lens System Glass or Quartz Electromagnetic Electromagnetic Image Display Eye Retina or Film Fluorescent viewing CRT for viewing and plate or film photography Energy Uses radiation from Thermonic emission UV through visible of electrons which spectrum to IR are accelerated by 50 to loookv Wave Lengths nm > 0.1 nm > 0.1 nm Resolution 100 nm 0.3 nm 3.0 nm Depth of Field Poor Good Excellent Sample Requirements Thin Sections lyccm thick sections or only replica, 2-3 mm in diameter Magnification 15x x loox OOOx (stepped) Tab Comparison of OLM, TEM and SEM Thermonic emission of electrons which are accelerated by 1 to 30kV There is no thickness limit. Non-Metals (better) metal coated (Au, Pd or Al) up to 25 mm in diameter lox OOOx (continuous) Work which needs even greater magnification and larger scales requires the use of an optical light microscope. By fitting a camera to the microscope, photographs at magnifications up to 3000x are possible. If even greater magnification is required, electron microscopes are the only instruments capable of satisfying the user. Magnifications of up to 200,000x or 500,000x are possible depending on the type of electron microscope used. Electron microscopes have given scientists greater detection and were ever previously obtainable with optical light microscopes (OLMs). They have become indispensable in the fields of microbiology, metallurgy, criminology, microelectronics and many other sciences. For most users, single, two-dimensional micrographs (electron microscope photographs) have been suitable for their requirements. These users are generally interested in their micrograph's descriptive qualities and they only require low accuracy measurements of their specimens, if any. There is however, a growing demand for highly accurate, three-dimensional information of microscopic objects. These measurements can only be made from stereo pairs of micrographs. In most instances, this appears to be no more than a microscopic version of the macroscopic examples with which photogrammetrist regularly deals. However, the projection geometry as well as certain deformations, are different in electron microscopes than in any other photogrammetric situations (Ghosh 1989). Standard photogrammetric procedures can be applied if certain modifications are made to the measurement process and certain corrections are applied during calculations. There are two distinct types of electron microscopes; Scanning Electron Microscopes (SEM) and Transmission Electron Microscopes (TEM). Both can be used to create stereo micrographs and to measure three-dimensional characteristics of specimens. A distinction between the two will be made in a later part. This chapter can be broadly divided into two areas: I. the study of the production of a pair of stereo micrographs. II. the study of the measurement of micrographs to provide three-dimensional coordinates of points on the specimen. 4 / 65

5 These two areas can be further subdivided to investigate: 1) how a scanning electron microscope functions. 2) the procedures involved in obtaining a stereo pair of micrographs. 3) the effects that various degrees of rotation of the specimen stage have on the stereo effect of the micrographs. 4) the geometry of electron micrographs. 5) distortions to the micrographs. 6) procedures to measure the micrographs. 7) factors affecting measurement accuracy. For the purpose of studying the effects of stage rotation, micrographs of ostracods (bivalved micro crustaceans) at 200x and 500x were used.the micrographs were produced using a JEOL JSM-5200 scanning electron microscope at the South African Museum in Cape Town shown in Fig Fig The JEOL JSM-5200 scanning electron microscope Electron microscopes are used in preference to Optical Light Microscopes when very small objects need to be examined. An electron microscope offers greater resolutions at higher magnifications than optical light microscopes. This is possible because the method by which electron microscopes create an image of a specimen is different from the way in which optical light microscopes do. The fundamental principles behind electron and optical microscopes are similar. In both optical and electron microscopes, the specimen is illuminated by a form of radiation. This radiation interacts in some way with the specimen. The resulting radiation that is transmitted or reflected by the specimen is detected by a sensor and an image of the specimen is formed. The main differences between the principles of electron and optical microscopes, lie in the source of the radiation, how it is controlled, and how it is later detected by a sensor to form an image. The illuminating radiation in an optical light microscope is visible light emitted from a bulb. The light is controlled by optical lenses and directed towards the specimen. Once it has interacted with the specimen, the light is detected by a camera or the operator's eye. The illuminating radiation of an electron microscope is a beam of electrons emitted from an electron gun. This necessitates a different method of focusing the radiation than in optical light microscopes. Instead of optical lenses, magnetic lenses are used to direct the electron beam. Once the electron beam has interacted with the specimen, transmitted or reflected electrons cause an image to be formed on a screen. The remainder of this chapter will deal with two of the main differences between optical and electron microscopes; the electron gun and magnetic lenses DESCRIPTION OF A SIMPLE MICROSCOPE ELECTRON GUN The simplest electron gun consists of three components shown in Fig. 1.2.: 5 / 65

6 the filament - f the shield s the anode a, Fig The three components of a simple electron gun Filament The filament in its simplest form is usually a V-shaped piece of wire. It is very thin, about 0.1 mm in diameter, and can therefore be electrically heated to incandescence. When a current is passed through the wire, the tip of the V becomes the hottest part of the filament and therefore the greatest source of electrons. This method of causing the filament to discharge electrons makes the filament a thermionic cathode. There are two characteristics which determine the suitability of a metal for use as a filament. The first is that the metal must have a low work function (measured in volts); it must supply a large number of electrons at as low a temperature as possible. Secondly, it must have a long working life. The working life of the filament can be extended by preventing oxidation and evaporation of the metal when it is heated up. This is done by heating the wire just high enough to provide satisfactory electron emission. Tungsten is the metal most commonly used since it has all the necessary properties. However, although the operating temperature is kept as low as possible, it is still near enough to the boiling point of the metal to cause some evaporation. This results in an average filament life of hours (Wischnitzer 1970) Shield The shield is a cylinder, with an aperture, which is in front of, and centered over the tip of the filament. The aperture is usually 1-3 mm in diameter (Wischnitzer 1970). The shield helps to ensure that only a narrow beam of electrons leaves the electron gun. 6 / 65

7 Anode The anode is a disk with an aperture in its center. The anode is grounded and attached to the positive terminal of the power supply. The rest of the microscope is also grounded. The cathode (filament) is therefore held at a large negative potential with respect to the anode and the microscope. This causes electron to be accelerated from the cathode towards the anode, through the aperture and down the column of the microscope. The path of the electrons is controlled by magnetic lenses. In reality, the operation of the electron gun is more complicated than has been mentioned in this section. However, although various types of electron guns have been developed and refined, they all share the same basic principles THEORY OF MAGNETIC LENSES In 1924, de Broglie proposed a hypothesis of the wave nature of a beam of moving electrons. He stated that the wavelength of the beam was dependent on the potential through which it had been accelerated. It later became apparent that if electron beams could be accelerated to a high velocity, then their small wavelength would make them suitable for illuminating microscopic objects. The small wave length (slightly greater than 0.1 nm) of electron beams would make them capable of providing a high resolution. Although de Broglie's hypothesis indicated the potential of electron beams, he did not propose ways in which electron beams could be controlled. In 1926, Busch stated that a magnetic field could be used as a lens for an electron beam (cited in Wischnitzer 1970). If a single electron enters a large magnetic field perpendicularly, the force of the magnetic field will cause the electron to move in a circular path. Should the electron escape the magnetic field, its velocity will be the same as on entering the magnetic field. An electron which moves parallel to a magnetic field will not be affected or deviated in any way. Fig Electron vectors in a magnetic field The theory can be developed further when the magnetic field is created by a solenoid. A solenoid's magnetic field not only runs from one end to the other, but also circles the solenoid. When an electron passes at a slight angle through the magnetic field of a solenoid, the electron's path will have two components (or vectors) with respect to the solenoid's magnetic field. 7 / 65

8 One of the electron's components is parallel to the magnetic field which runs the length of the solenoid. The other vector is at right angles to the magnetic field which circles the solenoid shown in Fig. 1.3 where V electrons initial path, V1 parallel component, V2 perpendicular component. The magnetic field has no effect on the vector that is parallel to the magnetic field; the vector remains constant and parallel to the long axis of the solenoid. The vector at right angles to the magnetic field causes the electron to move in a circular path. The combined effect of these two components is to cause the electron to trace a path which is a spiral. The axis about which the electron will spiral is an extension of the axis which runs lengthways through the solenoid. The spiral is ever decreasing, so the electron converges on the axis at a set distance from the solenoid. In reality, there are many electrons in an electron beam. The effect of the solenoid is to force the electrons into a narrow, focused beam at a set distance from the solenoid. The solenoid acts as a magnetic lens. The lens can be used to direct a narrow, high intensity beam or to magnify a narrow beam. By adjusting the strength of the magnetic field, the focusing point of the magnetic lens can be shifted. Magnetic lenses allows the electron beam within an electron microscope to be controlled and directed towards the specimen. First serial scanning electron microscopes were made by Cambridge Scientific Instruments Ltd. in 1965 and Japan Electron Optics Laboratory Ltd. (JEOL) in Scanning electron microscopes (SEM) have become well known research instruments for scientists in many domains. However, users of SEM techniques surprisingly seldom apply stereo photogrammetric methods for interpretation and measurement of geometric data. The procedures of stereophotogrammetric recording, interpreting and measuring, whilst conceptionaly easy, are not widely accepted by SEM users. They still continue to obtain third height Z co-ordinate by approximate methods which are less accurate and more uncertain. The experiments has been performed using Cambridge, JEOL and Tesla SEM microscopes in cooperation with different professions. e.g. from metallurgy, material engineering, biology, crystallography and criminology. The conclusions on problems of implementation of photogrammetric methods to SEM users research tools, are discussed in this chapter SEM AND PHOTOGRAMMETRY The SEM imagery can be characterized by an excellent depth of field. By tilting the specimen, the stereo micrographs pairs may be produced in perspective or almost parallel projection, depending on the image magnification. The angle of tilt of the specimen depends on many factors and sometimes the optimal value can be found to be out of the expected range. One of the major problems in using SEMs for stereo photogrammetric measurements is the need for accurate measurement of the tilt angle of the specimen stage. It can be solved by applying an optical level (Boyde, Ross) or other devices with satisfactory accuracies about 30''- 60". The TEM imagery taken with the transmission electron microscope can also be arranged in stereo mode, but depth of field is significantly limited by the nature of TEM. A procedure of analytical self-calibration of SEM was developed to solve for the tilt, rotation, magnification and systematic distortion parameters, in the department of Geodetic Science, Ohio State University (Ghosh, 1975). A new "spiral" distortion, resulting from non-linear electron scanning, was mathematically defined, in addition to an affine deformation, radial and tangential distortions (Maune). The accuracy of 3D coordinates obtainable from a given photogrammetric procedure depends on: stability of the system, observational error, geometry of intersection (angle of convergence), image 8 / 65

9 quality, shape of the object and the kind of instrument applied for observations (Ghosh, Nagaraja, 1976). More information can be obtained by using the combination of stereo models with TEM and SEM. Image quality obtainable from the TEM is very high, while the resolving power of the SEM is a limiting factor in the data acquisition (Ghosh et al, 1978, 1980). The SEM and TEM micrographs can be visualized as analogous to digital or ordinary photographs and X-ray images respectively. By combining their features in 3-D for the same sample object, the scientist would obtain information which is impossible with any other system. Data captured from a SEM stereo model represents the surface features of the object while data from TEM plotting reveals the internal structure of the object (Elghazali 1984a). Together, SEM and TEM, they offer a unique tool of complete analysis of micro objects. Elghazali (1984b) presented the mathematical models together with results of system calibration for the SEM micrographs. In regard to SEM applications, the author carried out or supervised some projects concerning SEM stereo photogrammetry: 1) Stereo-photogrammetric analysis of the topography of the surface of mill balls whilst being abraded; stereo micrographs taken with SEM JEOL 50A (Florek, et al, 1977 and research reports); 2) An investigation of the metallurgic fracture of steel thermally processed; on the base of micrographs taken with SEM Cambridge Stereoscan S4-10 (research project and supervised by Dr Florek thesis: Szpak & Szwaja); 3) An Application of stereo SEM to an investigation of the crystallographic processing of analcym Na(AlSi 2 O 6) H 2 O; micrographs taken from SEM Tesla BS-300; 4) A stereophotogrammetric analysis of cell membrane structures of frog cardiac myocytes in freeze-fracture replicas (Kordylewski & Florek, 1990); 5) One of the objectives of the thesis project (Dingle, 1993) was to determine the relative coordinates of selected points of a specimen. This work was carried out with assistance from the marine micropalaeontology unit of the South African museum in Cape Town. The specimens selected were ostracods (sea shell approximately 500 micrometers in length). A JEOL JSM-5200 SEM was used to create the images. An overall discription of systems and applications of electron microscopy is published in chapter 13, Non - Topographic Photogrammetry (Ghosh, 1989). There are seven contributions recommended for meaningful three-dimensional mapping with SEM micrography. These concern: a) magnification & parallactic angle; b) distortions in the micrographs; c) mathematical model; d) taking a pair of micrographs; e) instrumental /digital orientations; f) x,y,z digital or plotted output; g) other necessary data output. Should the following two points also not be considered 1) analysis of reliability of specimen; 2) an interpretation study of images, before one commence any measurements? 9 / 65

10 A new tool for scientists, the atomic force microscope AFM, may be used in association with TEM, to examine the same identified details of specimen. It allows comparison of the surface topography by both techniques (Kordylewski et al, 1994). At the other extreme, macro photographic methods offer a range of magnification 1:1 to 25:1, which are still best for some projects SEM USERS AND PHOTOGRAMMETRISTS The reasons why stereo photogrammetric analysis of SEM micrographs has not become very popular is not only the lack of suitable instruments but because of the lack of realization of SEM users to that possibility as well as weak inter field cooperation. Photogrammetrists involved in SEM projects solve geometric, and seldom interpretive aspects, using their air survey and close range photogrammetry background. They do not see the problems from the SEM user point of view and they present tendency to exaggerate the problem of measurement accuracy whilst neglecting the reliability of their data source. Often, comparative study, preliminary or relative results are most important. Unfortunately, when an interpretative aspect is neglected then precise and corrected measurements may be derived from wrong data source. The accuracy of the final results is not only affected by the work specifically undertaken by the photogrammetrist. The high accuracy of the measuring instruments available to the photogrammetrist may lead us to believe that provided we exercise extreme care and apply any corrections applicable to the measuring machine, extremely accurate relative coordinates of points on the object can be obtained. However, the stereo pair of images which the photogrammetrist works with are the end product of a long process in which the specimens are selected, prepared and photographed or recorded digitally. This process offers many opportunities to devalue the accuracy claimed by the photogrammetrist. The images should be suitable for photogrammetric measuring. There should be sufficient contrast and resolution to enable the photogrammetrist to distinguish between the points to be measured and their surroundings. In most cases SEMs are used to produce paper prints; however, the photogrammetrist requires the negatives or digital records. In order to produce negatives which are suitable to measure from, the contrast of the SEM screen and camera setting may need to be different from what would usually be used. It is important that the regular SEM operator be made aware of this. Within the SEM, distortions can occur due to the nature of the electron beam itself as well as due to the way in which the electron signal is amplified for the purpose of magnification. If these distortions are not predetermined and corrected for by the photogrammetrist, it is pointless for the photogrammetrist to strive for a exceptionally high accuracy. The calibration data can usually only be obtained for a microscope if a person with photogrammetric knowledge carries out an extensive calibration of the SEM (the camera and the SEM should be seen for this purpose as a single unit, not two separate parts of a system). It would be unlikely that a photogrammetrist provided with stereo images from, for instance, a geology department, could expect to be provided with calibration data of the SEM used. Furthermore, unless the SEM is specifically designed with stereo microscopy in mind, it is likely that the camera and film will be both fairly standard and will introduce distortions of their own to the image. The selection and preparation of the specimen has an effect on the final obtainable accuracies. If the specimen is dirty, it may be difficult under high magnification to distinguish between dirt and the specimen's features. 10 / 65

11 1.5. SIMILARITIES AND DIFFERENCIES BETWEEN TRANSMISSION AND SCANNING ELECTRON MICROSCOPES As was mentioned previously, there are two different types of electron microscopes; the transmission electron microscope TEM and the scanning electron microscope SEM. The two microscopes employ different principles to produce their images, and therefore produce different types of images with different applications. Although the transmission electron microscope is not the subject of this thesis, a differentiation between the two different types of microscopes is desirable Similarities between transmission and scanning microscopes The two types of electron microscopes have certain features in common. In both types of electron microscopes the specimen is placed on a specimen stage inside the microscope. This stage can generally be rotated in all three axis. The specimen on the stage is then illuminated by an electron beam generated from an electron gun. The complete path of the electrons needs to be free of any interference except that caused by the specimen. Consequently, the interior of the microscope needs to be exhausted to remove vapors and gas. A near total vacuum must be created by pumps and valves attached to the microscope. Special care also needs to be taken in the selection of the material for the stage to hold the specimen. The material should not be of a type that will cause excessive scattering of electrons. If this occurs, it may be difficult to distinguish between the stage and the specimen in the final image (Ghosh 1988) Differences between transmission and scanning microscopes Two basic operating principles of Transmission Electron Microscope are as follows: 1) An electron beam is allowed to impinge upon the specimen. The specimen is a very thin (0.1/µm to 1.0/µm) slice of the sample. This is to allow the electron beam to pass through the specimen. As the electron beam passes through the specimen the electrons are scattered. 2) The final image is a measure of the scattering power of each point of the specimen. The image is formed by a magnetic objective lens, magnified and then displayed on a fluorescent screen. Operating principles of Scanning Electron Microscope are: 1) An electron beam is allowed to strike the specimen, but because the beam has less strength than that in a transmission electron microscope, and because the specimen is far thicker, the electrons do not pass through the specimen. 2) The beam of electrons is moved over the specimen in a grid like pattern by a scan generator (hence the name scanning electron microscope). The electron beam interacts with the surface of the specimen to produce various signals. 3) One of these signals is a beam of secondary electrons. These are electrons which are released from the atoms of the specimens surface (Ghosh 1989). These secondary electrons are drawn to a collector which has a positive charge. The collector is also known as an electron probe. 11 / 65

12 4) The image which is formed on a cathode ray tube is a measure of the strength of the secondary electron beam after it has been amplified. The strength of the secondary electron beam is a function of the angle between each point on the specimen and the primary electron beam. The image of the specimen is built up point by point; each scan of the electron beam adding to the image. Fig. 1.4 Representation of how the major components of an electron microscope work together. The different methods by which transmission and scanning electron microscopes form their images result in different types of images. A transmission electron microscope penetrates through the specimen resulting in an image which is similar to an X-ray. Transmission electron microscopes are capable of greater resolution at greater magnifications than scanning electron microscopes. A scanning electron microscope shows surface the detail of a specimen. They are capable of magnifications greater than 100,000x. They have good depth of field and resolution at even these magnifications. This gives them great potential for forming stereo pairs to carry out three-dimensional measurements of the surface of microscopic specimens PRODUCTION OF STEREO MICROGRAPHS This chapter describes the procedure followed for the production of a pair of stereo micrographs from a specimen within a scanning electron microscope SEM. A series of micrographs was produced to study the influence of specimen stage rotation on the quality of the stereo effect of pairs of micrographs. The results of this study will be presented in the next chapter. The specimens photographed are ostracods which are being studied and researched at the South African Museum. The researchers view stereo pairs of micrographs to interpret the surface of the ostracods. Although the specimens which were photographed are ostracods, the general technique is applicable to virtually all other types of specimens. Ostracods belong to the class Crustacea and live on the sea floor. They are bi-valves which gives them a certain external similarity to clams, but the creature which lives inside is more analogous to a crab. Fig. 1.5 is intended to give the reader an impression of how the creature stood. Although they occur abundantly in the modern world, the specimens which are being studied by the museum are approximately 20 million years old. 12 / 65