As a key tool in the field of materials science, the illumination technology of the upright metallographic microscope directly affects the imaging quality and analysis efficiency. The current mainstream lighting technologies include bright field lighting, dark field lighting, polarized lighting, and differential interference (DIC) lighting, each of which achieves specific analysis needs through unique optical path design.
Bright field illumination is the fundamental mode of upright microscopy, where light is focused vertically on the sample by a condenser lens, and the reflected light forms a bright field of view through the objective lens. This technology is suitable for observing metal samples etched after polishing, and can clearly present structures such as grain boundaries and phase boundaries. By adjusting the aperture stop to control the beam aperture angle, the balance between resolution and depth of field can be optimized.
Dark field illumination uses an annular aperture to obliquely illuminate the sample, scattering only the light into the objective lens, creating a high contrast dark background image. This technology has advantages in detecting surface defects and small precipitates, for example, inclusions with a diameter less than 1 μ m can be clearly observed in aluminum alloys. In dark field mode, the field of view aperture needs to be closed to reduce stray light interference.
Polarized illumination is achieved by inserting polarizing and analyzer mirrors, and analyzing the phase composition of materials using the birefringence properties of crystals. In the research of steel materials, polarization technology can distinguish anisotropic phases such as ferrite and austenite, and combined with angle rotation measurement, the phase ratio can be quantitatively calculated.
Differential interference illumination (DIC) divides a beam of light into two polarized beams through a Wollaston prism, which are reflected by the sample to produce a phase difference and create a relief effect image. This technology is sensitive to surface morphology and can identify step height changes of 0.1 μ m in semiconductor wafer inspection.
Application Guide:
Prioritize bright field mode for basic observation, and use 5X-100X objective lens to achieve continuous observation from macro to micro level.
Switch defect detection to dark field mode to enhance the visibility of low contrast defects.
Crystal analysis uses a polarizing device and a rotating stage to complete phase identification.
The surface morphology adopts DIC technology, and the surface roughness is quantified through three-dimensional reconstruction function.
Attention should be paid in practical use: Kohler illumination calibration can ensure the uniformity of the field of view; Color filter selection (such as green filter) can enhance the contrast of specific structures; Regular cleaning of the spotlight and aperture components can maintain system performance. By combining multi-mode illumination, the upright metallographic microscope can meet diverse needs from basic research to industrial quality inspection.