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An Application of Differential Interference Contrast In View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Directory of Open Access Journals Vol. 2 No. 1, Feb. 2005 CHINA FOUNDRY An application of differential interference contrast in metallographic examination *Xiang CHEN , Yanxiang LI (Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P. R. China) Abstract: As one of the most exciting inspection and powerful analysis methods in modern materials metallographic examinations, the difference interference contrast (DIC) method has many advantages, including relatively low requirement for specimen preparation, obvious relief senses observed under microscope. Details such as fine structures or defects that are not or barely visible in incident-light bright field, could be easily revealed and thus make materials analysis more reliable. Differential interference contrast produces an image that can be readily manipulated using digital and video imaging techniques to further enhance contrast. But, studies of material metallography based on DIC method have rarely carried out. Based on the fundamental principle of the DIC method combing with the computer image analysis, applications of DIC method in materials metallographic examination were investigated in this study. Keywords: difference interference contrast method;metallographic examination; and image analysis CLC number: TG14, TG115.2 Document: A Article ID: 1672-6421(2005) 01-0014-07 1. Introduction Like polarized light, DIC was also primarily developed The difference interference contrast (DIC) method is as an analytical method to determine various optical one of the most exciting inspection and powerful analysis properties of crystals. Thus, it is a method of measure- methods in modern materials metallographic exami- ment, requiring specific knowledge and specially- nations [1], which has many advantages. Specimens designed microscopes of considerable complexity. Appro- preparation is relatively simple. For certain specimens, priate applications of the DIC method can yield fine their microstructure could be observed without etching details of microstructure. under the microscope by using the DIC method as the Differential interference contrast has found wide polished specimen surface is preserved. The obser- application in biology due to its simplicity in use and vation of the specimen surface via the microscope has emergency of commercially-available microscopes. obvious accidented senses, taking the form of relief. The However, little study of materials metallography based relative locations of different features in a specimen could on the DIC method has been carried out. This is because be distinguished readily. The particles, crackles, caves, most laboratories for materials research presently are not slopes, valleys, and other discontinuities could be judged equipped with metallographic microscopes including DIC correctly, with the improved accuracy of the metallo- optical components. Few materials researchers have graphic examination, in which the contrast of the features realized the potential and capability of the DIC method are enhanced. With the DIC method, details such as microstructure analysis. some fine structures or defects that are not or barely The basic principles of the DIC method are introduced visible in incident-light bright field could be easily based on the Neophot32 microscope in this paper. observed. The DIC method based on the traditional Combining with the technology of computer image polarized light with a polarizer and specialized beam analysis, applications of the DIC method in materials splitting prisms, named Wollaston or Nomarski prisms, metallographic examination are presented. adds pseudo-color which improves visual contrast between different phases. In addition, differential 2. The fundamental principles of DIC interference contrast produces an image that can be easily Method manipulated using digital and video imaging techniques to The DIC method is based on the theory of differential further enhance contrast. interference contrast (DIC) by using a Nomarski prism in the polarized light fields. The optical path is very sensitive *Xiang CHEN: Associate professor, engaged in research of materials to small height difference of light interference in processing E-mail: xchen @ mail.tsinghua.edu.cn nanometer size. [Received date] 2004-11-16; [Accepted date] 2004-12-06 The basic DIC system, first devised by Francis Smith in Vol. 2 No. 1 An application of differential interference contrast [2] 1955 ,is a modified polarized light microscope with two wavefronts of plane-polarized light are split (or sheared) additional Wollaston prisms added, one at the front focal into mutually perpendicular (orthogonal) polarized plane of the condenser and the second at the rear focal components (termed ordinary and extraordinary plane of the objective. Several years later, Georges wavefronts) by the Nomarski prism, and then pass through Nomarski, modified the standard Wollaston prism the specimen. The material of the prism is normally configuration to enable these exceedingly thin optical quartz. components to be physically located away from the Analyzer, the second linear polarizer, is installed conjugate planes of an aperture. behind the Nomarski prism, usually in an intermediate The optical components required for differential inter- tube between the microscope nosepiece and observation ference contrast microscopy do not mask or otherwise (eyepiece) tubes. Termed an analyzer, this polarizing obstruct the objective and condenser apertures, thus element is positioned in the optical pathway before the enabling the instrument to be employed at full numerical tube lens (for infinity-corrected microscopes) and image aperture. The result is a dramatic improvement in plane. The analyzer is oriented with the transmission axis resolution (particularly along the optical axis), elimination of the electric field vector perpendicular (North-South) to of halo artifact [2]. Moreover, differential interference that of the substage polarizer. Components of circular and contrast can produce images that can be easily elliptically polarized light arriving from the objective manipulated using digital and video imaging techniques to prism pass through the analyzer and subsequently undergo further treatments or quantitative analysis. interference to generate the DIC image at the microscope [3] 2.1 The basic components of DIC method intermediate image plane . Because only the second series have rich hue where the In general, only four basic components are required to optical path difference is between 560 and 1 120 nm. configure a research or standard laboratory bright field Normally, a piece of full-wave plate should be added in microscope for observation in differential interference the optical path of polarized microscope. This full-wave contrast: linear polarizer, sensitive wave plate (also plate can produce an optical path difference of 576 nm. termed λ-a compensator), DIK module and analyzer. Regardless of the polarization state changes, it can extend Linear polarizer is inserted into the optical pathway the wide range of color contrast nuances gradually and between the microscope light port (or anywhere after the always produce bright color changes. This full-wave plate illumination source collector lens) and the condenser lens is called the sensitive plate or λ-a compensator. assembly. This component is designed to produce the 2.2 The imaging mechanisms of DIC method necessary plane-polarized light for interference imaging. The vibration plane transmission axis for the electric Fig. 1 presents the basic Nomarski prism differential vector component is oriented in an East-West direction interference contrast device. When light passes through an (right to left when standing in front of the microscope), optically homogeneous medium, it produces one typical of a standard polarized light microscope. Some refraction of light. But when it passes through a differential interference contrast designs incorporate a non-homogeneous material, the refracted light will be rotating polarizer combined with a quarter-wavelength split into two bundles, which is called the birefringence, retardation plate at this position in the microscope [3]. or double refraction. The vibration directions of the two DIK module is provided with a Nomarski prism which lines produced by the birefringence are perpendicular to consists of two prisms at right angle slidable in its each other. Their speed is different and their refractive working position and two set-screws. By actuating the index is different [5]. smaller set-screw towards the optical axis of the When a bundle of parallel light passing through the microscope, an optimum position of the prism relative to polarizer, it will become plane-polarized light. Then the the objective's exit pupil; while actuating the larger light after polarizing passes through the DIC prism, it set-screw, the relative position of the two prisms of the splits into two lights. One, being called ordinary light or o Nomarski prism resulting a slight change of the optical light, propagates along its original direction. The other, path difference that producing the interference effect being called extraordinary light or e light, deviates slightly could be adjusted and the contrast could be varied from its original direction. Both the o and e lights delicately. The Nomarski prism separates the polarized produced by the birefringence are polarized lights. light emanating
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