The principle of microscopic research (in the applied sense, of course) is based on the answer to the question: how does visible light interact with the surface of the material under study? How to use this interaction? How to register it? And how to interpret it?

The structure of a sample can only be seen when different parts of it reflect or refract light in different ways. These properties determine the difference in the amplitudes and phases of light waves reflected from different parts of the surface. That difference, in turn, determines the contrast of the image. Therefore, observation methods in microscopy are selected depending on the nature and properties of the objects under study. Objects of research in metallography are often structurally complex. Therefore, it can be difficult to determine in advance which analysis technique (lighting method) is optimal. Even a skilled researcher is often forced to try different lighting techniques and filters to get the best possible image of the structure. Therefore, it is important to show the modification of the image of the structure of the material when using illumination by the methods of bright field, dark field, polarized light, etc. to demonstrate the capabilities of the analysis.

The structure of a sample can only be seen when different parts of it reflect or refract light in different ways. These properties determine the difference in the amplitudes and phases of light waves reflected from different parts of the surface. That difference, in turn, determines the contrast of the image. Therefore, observation methods in microscopy are selected depending on the nature and properties of the objects under study. Objects of research in metallography are often structurally complex. Therefore, it can be difficult to determine in advance which analysis technique (lighting method) is optimal. Even a skilled researcher is often forced to try different lighting techniques and filters to get the best possible image of the structure. Therefore, it is important to show the modification of the image of the structure of the material when using illumination by the methods of bright field, dark field, polarized light, etc. to demonstrate the capabilities of the analysis.

The illumination of the sample by the methods of dark and bright fields is the most informative in the study of materials. Analysis methods based on polarized light are used less frequently, but they also are taking place.

Currently, the Internet contains a lot of information about dark-field microscopy. Most of it is devoted to biological research. Such as, for example, the use of dark-field illumination for the study of blood preparations (hemoscanning) . There are numerous advertisements for dark-field biological microscopes. But little information can be found on the application of the dark field method for metallographic studies. It is especially difficult to find illustrations of the capabilities of that method, performed on modern equipment. The reason for this is probably that the method of dark-field microscopy in metallography has been known for a long period of time and people/researchers? managed to get used to and then to forget about it.

Illumination by bright and dark field methods in reflected light

The bright field method is primary in metallography and is used to observe opaque objects that reflect light. Under brightfield illumination, the object is illuminated by a cone of rays passing through the microscope objective. In this case, the entire cone of light participates in the formation of the image. The central rays of the cone, parallel to the axis of the lens, are predominant. They, in essence, determine the nature of the illumination of the object.

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Figure 1. Scheme of object illumination when using light (a) and dark (b) fields.

Figure 1 shows the light and dark field lighting schemes. With the bright-field illumination, a spot of light is visible on the sample surface. In dark field lighting, the center is darkened. Real images of a light spot on white paper under both illumination methods are shown in Figure 2. For greater clarity, the surface of the paper is below the focus.

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Figure 2. Illumination of an object using a light (a) and dark (b) field.

Therefore, when illuminating some objects using the dark field method, a specific image is formed. Figure 3 shows an image of an air bubble below the surface of the epoxy. The bubble burst. In the bright field, a hole and a white mass of displaced material can be seen in the center (Fig. 3a). In a dark field (Fig. 3 b), an image of a circle, consisting of three segments, is visible. It is formed by the interaction of the incident light and the attachment of the optical system (the so-called "helicopter"). An air bubble is a perfect spherical object and a circle is perfect. If the object does not have an ideal shape, then the image of the "helicopter" is not round (Fig. 4).

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Figure 3. Image of a bubble in epoxy resin: a - bright field, b - dark field.

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Figure 4. Image of starch particles: a - bright field, b - dark field.

Figure 5 shows a diagram of light reflection from the surface of a metallographic section under bright-field illumination. In the case of a smooth polished surface, the light is reflected and enters the lens (1). In this case, the surface is uniformly illuminated. After etching, grooves are formed at the junctions of adjacent grains (or phases). Therefore, the reflection occurs from the curved surface. Reflected light does not enter the lens (2). The grain border is visible as a dark line. If there is no etching or the boundary has not been etched out for some reason (3), then the light is reflected by the surface similarly to option 1 and the grain boundary is not visible.

Figure 5. Scheme of light reflection by a metal surface when illuminated by the bright field method: 1 - reflection from a flat surface; 2 - reflection from the grain boundary after etching; 3 - reflection from the area with the grain boundary in the absence of etching.

Figure 6. Scheme of light reflection by a metal surface when illuminated by the dark field method: 1 - reflection from a flat surface; 2 - reflection from the grain boundary after etching; 3 - reflection from the area with the grain boundary in the absence of etching.

With the dark-field illumination method, the formation of an image of the sample surface occurs according to the scheme in Figure 6. A flat area of the surface (1) will be unlit since the reflected light does not enter the microscope objective. In this case, the body of the grain is dark. The sloped area (etched grain boundary) is in a reflective position and is visible as a light stripe against a dark background (2). In area (3), the surface also looks dark.
A comparison of the bright field and dark field images of the steel structure is shown in Figure 7. The image shows a bearing steel after deep etching to reveal the former austenite grain. In the bright-field image (Fig. 7, a), well-etched austenite grain boundaries look dark against a light background (option 2 in Fig. 3). The grain body is illuminated to the limit (option 1 in Fig. 3). Dark spots are the preserved areas of martensite. The image formed under illumination by the dark field method (Fig. 7, b), in this case, is perceived as negative in relation to the image in Fig. 7, a. The body of the grain is not illuminated, the borders "glow" according to how much they are etched. Martensite looks as a dark spot.

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Figure 7. Metal section after etching on the microstructure: a - bright field, b - dark field.

Scratches and various inclusions in the bright field look dark against the background of the illuminated grain body (Fig. 5, a). In a dark field (Fig. 5, b), scratches and inclusions are illuminated. Figure 8 shows the structure of gray pearlitic cast iron. Ferrite and phosphide eutectic are brightly illuminated in a bright field. In a dark field, they are dark; brightly lit cementite in the composition of perlite. Small luminous points are also noticeable in the dark field. This is dust on the surface of the sample.

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Figure 8. Section of cast iron after etching on the microstructure: a - bright field, b - dark field.

When observing an uneven surface in a bright field, it can hardly be seen due to the strong scattering of light. Lighting in a dark field of view creates contrasting images and maintains the natural appearance of painted objects (Fig. 9).

Figure 9. Crystals of potassium dichromate in a dark field.

Figure 10 shows an example of a cast metal surface that has been partially ground. Surface "1" is molded. It has its own undulating relief. Surface "2" is flat, formed by grinding. In a bright field it looks light, the traces of grinding partially scatter the light (Fig. 10, a) and look like dark stripes on a light background. The cast surface "1" appears dark in the bright field. When using dark-field illumination, the surface illumination patterns are mutually inverse (Fig. 10, b). It should be noted that this is not always the case. The ratio of the type of such images depends on the surface morphology, structure dispersion, and phase composition features.

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Figure 10. Metal surface of complex configuration: a - bright-field image; b - dark-field image.

When observing using the dark field method, the surface is illuminated through a special ring system located around the lens and called an epi-condenser. In fact, the dark field principle consists in blocking the central beam of light rays. The sample is illuminated by a hollow cone of light. Only the rays oriented obliquely to the sample surface are involved in the formation of the image. Dark-field illumination expands the capabilities of a metallographic microscope in the study of non-planar objects.