Metallographic Microscope: Light Path Differences and Selection
A metallographic microscope features four common light path options: Bright Field (BF), Dark Field (DF), Polarized Light (PL), and Differential Interference Contrast (DIC). These four light paths are selected based on the different samples observed by the user. Below is an explanation of the differences between each light path and how to choose a microscope with the appropriate light path.
Bright Field (BF)
Illumination light irradiates the sample vertically through the objective lens. After the sample absorbs the light, it is reflected back vertically and transmitted to the eyepiece or camera through the light path, forming a bright field image.
Dark Field (DF)
A dark field stop is used to allow only the scattered light from microscopic details to pass through the microscope, while blocking the direct light on the background from entering the microscope. This increases contrast, thus forming an image with bright details against a dark background.
DF can clearly display the morphology and distribution of defects in metallic materials, such as blowholes, inclusions, cracks, etc.
Polarized Light (PL)
Contrast of the sample is enhanced through the interaction between transmitted light and polarizers. When light passes through metallic or alloy samples, it is scattered by the sample’s microstructure and crystal morphology. The polarizer functions by allowing only light with a specific direction to pass through, aligning the direction and vibration direction of the scattered light.
Differential Interference Contrast (DIC)
A polarizer and a birefringent prism are used to split a single beam of light into two coherent beams with a slight optical path difference. After irradiating the sample, the two beams recombine and interfere, converting the height differences on the sample surface into brightness contrast and forming a three-dimensional embossed image. It is suitable for observing tiny height differences, surface undulations, and film thickness variations on the sample surface, and can clearly present the three-dimensional topographical details of the sample without staining.
