What inspection techniques are available for metallographic microscopes?
Ⅰ. Bright Field Observation
Bright field microscopy is a widely familiar inspection method, commonly applied in pathology and examination to observe stained sections. This function can be realized by all microscopes, so it will not be elaborated further here.
Ⅱ. Dark Field Observation
Dark field observation adopts dark-ground illumination. It differs greatly from bright field observation. In metallographic microscopes, the illuminating light itself is not observed directly; instead, the reflected or diffracted light from the specimen is detected. As a result, the field of view appears dark, while the specimen shows a bright image.
The principle of dark field is based on the Tyndall effect. When strong light shines straight onto fine particles, they are invisible to the human eye due to light diffraction. If light strikes the particles obliquely, light reflection makes the particles appear enlarged and visible. The essential accessory required for dark field observation is a dark-field condenser. For this observation mode of the metallographic microscope, the light beam does not pass vertically upward through the specimen. Instead, the light path is redirected so that light strikes the specimen at an angle. The illuminating light is prevented from entering the objective lens directly, and the bright image is formed by the light reflected or diffracted from the specimen surface. The resolution of dark field observation is much higher than that of bright field observation, reaching 0.004–0.02
mm.
Ⅲ. Phase Contrast Microscopy
In the development of optical microscopy, the invention of phase contrast microscopy represents a major achievement in modern microscopy technology. The human eye can only distinguish the wavelength (colour) and amplitude (brightness) of light waves. When light passes through colourless and transparent biological specimens, the wavelength and amplitude barely change, making the specimens difficult to observe under bright field.
Phase contrast microscopy performs inspection based on the optical path difference of the specimen. It effectively uses the interference of light to convert undetectable phase differences into distinguishable amplitude differences. Even colourless, transparent substances can be seen clearly. This greatly facilitates the observation of living cells, so phase contrast microscopy is widely used with inverted microscopes.
