Several Observation Methods of the Microscope
1. Bright-field Observation
Bright-field microscopy is a familiar inspection method and the most widely used microscope observation mode. It is extensively applied in metallography, pathology, inspection and other fields. All microscopes support this function.
2. Dark-field Observation
Dark-field observation adopts dark-field illumination. Unlike bright-field observation, the illuminating light is not viewed directly; only the light reflected or diffracted by the specimen is detected. As a result, the field of view presents a dark background while the specimen appears bright. The principle of dark-field observation is based on the Tyndall effect. Dust particles under direct intense light are invisible to the human eye due to light diffraction. When light strikes the particles obliquely, light reflection makes the particles appear enlarged and visible. The special accessory required for dark-field observation is a dark-field condenser. It prevents the light beam from passing vertically through the specimen. Instead, it redirects light to illuminate the specimen at an oblique angle. The illumination light does not enter the objective lens directly; bright images are formed by reflected or diffracted light from the specimen surface. The resolution of dark-field observation is far higher than that of bright-field observation, reaching 0.02–0.004 μm.
3. Polarizing Microscope
A polarizing microscope is used to identify the optical properties of the fine microstructure of materials. Substances with birefringence can be clearly distinguished under a polarizing microscope. Some of these substances can also be observed by staining, while others cannot and must be examined with a polarizing microscope. The feature of a polarizing microscope is that it converts ordinary light into polarized light for microscopic examination, to identify whether a substance is isotropic (single refraction) or anisotropic (birefringent). Birefringence is a fundamental characteristic of crystals. Therefore, polarizing microscopes are widely used in mineralogy, chemistry, biology and botany.
4. Fluorescence Microscopy
Fluorescence microscopy uses short-wavelength light to irradiate specimens stained with fluorochromes. The fluorophores are excited and emit longer-wavelength fluorescence for observation.
