1、 Principles of Optics

The upright metallographic microscope uses an optical magnification and imaging system to observe the microstructure of metal materials, and its core principle is as follows:

Optical amplification

Using a combination of short focal length objective lens and long focal length eyepiece to achieve two-stage magnification. The object is located near the focal point in front of the objective lens, forming an inverted magnified real image through the objective lens. This real image is then magnified twice by the eyepiece to become a virtual image, and finally presented to the observer at a visible distance.

Imaging path

The light emitted by the light source is focused by a condenser and vertically illuminates the surface of the sample. The sample transmits the light to the objective lens through reflection or scattering. The objective lens collects light and forms an image, and the eyepiece further magnifies to form a visible image. Due to the differences in the microstructure of the sample surface and the varying intensity of reflected light, a contrast between light and dark is formed, highlighting features such as grain size and phase boundaries.

Upright imaging technology

By using a prism group to invert the image, the final imaging direction is aligned with the actual sample direction, avoiding the image inversion problem of traditional inverted microscopes and improving the intuitive operation.

2、 Optimization strategy for imaging quality

Lighting system optimization

Choose the appropriate lighting method: Select transmissive lighting (transparent samples), reflective lighting (opaque metals), or polarized lighting (enhancing internal structural contrast) based on the characteristics of the sample.

Adjust light source parameters: calibrate the uniformity of the light source through Kohler illumination, control the incident light angle using aperture stop, and optimize image contrast and resolution. For example, dark field illumination can highlight grain contours, while bright field illumination is suitable for detailed observation.

Lens and parameter adjustment

Keep the lens clean: Use professional cleaning solution or soft cloth to regularly wipe the objective lens and eyepiece to avoid dust or stains that may reduce imaging clarity.

Choose high magnification objective lens: For fine-grained materials (such as cold-rolled steel), a 50 × or 100 × objective lens should be selected to balance field of view and resolution; Coarse grained materials (such as cast iron) can be used with 10 x or 20 x objective lenses.

Fine tuning the focal length: First, adjust the image to clear at low magnification, then switch to high magnification objective for fine tuning to prevent crushing of the sample.

Sample preparation and corrosion process

Fine polishing: After gradually polishing to 1200 # sandpaper, use 0.5 μ m diamond polishing paste for mechanical polishing or electrolytic polishing (for hard and brittle materials) to eliminate scratches and pseudo structures.

Precision corrosion: Select a corrosion agent based on the type of metal (such as 4% nitric acid alcohol solution to corrode carbon steel), determine the corrosion time through preliminary experiments (usually 5-30 seconds), and clearly display grain boundaries and phase boundaries. Immediately rinse with alcohol and blow dry after corrosion to prevent excessive corrosion.

Image acquisition and analysis

Using a high-resolution camera: equipped with a high pixel camera and a large dynamic range sensor to enhance the ability to capture image details.

Software post-processing: Optimize image quality through image enhancement, filtering, and grayscale transformation, and use ImageJ and other software for grain size grading (such as ASTME112 standard) or quantitative analysis of phase composition.