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Comparison of bright and dark field metallographic microscopes and its application in metallographic analysis

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Comparison of bright and dark field metallographic microscopes and its application in metallographic analysis

1. Bright field and dark field imaging principle

The principle of dark field imaging is the same, but the effect of imaging is opposite to that of bright field.

2. Comparison of bright field and dark field characteristics

⑴ Bright field--------The beam of light illuminates the surface of the sample vertically

⑵ Dark field--------light obliquely irradiates the surface of the sample

3. Optical stroke of dark field illumination:

Dark-field lighting is significantly different from bright-field lighting in terms of optical path arrangement and lighting effects. To illustrate the effect of dark field lighting, we must first study the optical arrangement of the dark field (see the figure below). The parallel light from the light source is blocked by the circular shading plate, and the central part of the light is blocked. The light passing through the circular shading plate enters the vertical illuminator as a hollow cylindrical beam. The vertical illuminator for dark-field illumination is a circular reflector, which reflects the cylindrical beam upwards and casts it on the metal arc reflecting surface of the reflecting condenser along the objective lens shell. Its reflection makes the light focus on the ground surface. on.


The light reflected by the reflective condenser is projected on the metallographic grinding surface because of its extremely large inclination angle. If the sample is a polished mirror surface, the light reflected by the sample is still reflected in the opposite direction at a large inclination angle , It is impossible to enter the objective lens. So only pitch black can be seen in the eyepiece tube. Only the recesses of the sample can have light enter the objective lens. The tissue on the sample will be reflected in the dark field of view with a bright white image. Because of its name, it is called \"dark field\" (dark field) illumination. In most cases, the bright part of the white image on black background obtained by dark field illumination is suitable for the black part of the black image on white background obtained by bright field illumination.

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(Metallographic bright field image) (Metallographic dark field image)

Four, dark field lighting mainly has the following three advantages

1. Due to the extremely large inclination angle of the incident beam in the dark field, the effective numerical aperture of the objective lens increases, so the discrimination ability of the objective lens also increases. Observed under dark field illumination, even very fine wear scars are easy to identify.

2. Unlike bright field lighting, the light incident on the ground surface does not pass through the objective lens first, thus significantly reducing the

The reflection and glare caused by the line passing through the glass-air interface multiple times improves the contrast of the final image.

3. Dark field observation can accurately identify the color of transparent non-metallic inclusions. For example, copper oxide exhibits a light blue hue when observed under a bright field of white light illumination, while a true ruby ​​red color can be seen under dark field observation. Therefore, dark field observation is extremely important when identifying non-metallic inclusions.

To perform dark field observations,Metallurgical microscopeMust have a circular reflector and a special dark-field reflective condenser or refraction condenser. The reflective condenser is a metal arc-shaped reflective surface, which is set outside the objective lens when in use; some designs combine the objective lens and the reflective light mirror into one.

Dark-field illumination must use parallel light. When using dark-field observation, it is usually necessary to re-adjust the light source system, change the position of the collecting lens, and insert the circular shutter behind the light barrier. In dark field observation, due to the low brightness of the object image, it is necessary to be very careful when photographing the light, and choose a film with a higher sensitivity speed, and the exposure time will increase accordingly.

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