Observing Method of Metallurgical Microscope

Observation Methods of Metallographic Microscope

For the physical characteristics of metallographic samples, metallographic microscopes generally have four common observation methods: bright field, dark field, polarized light, and differential interference. In the past ten years, the latest laser confocal method has appeared.

According to the design idea of "modular design, building block structure" of modern instruments, these four functions are not necessarily solidified on every metallographic microscope, but with bright field as the basic core, other functions can be like "building blocks" "Assembled onto this microscope.

Bright Field

Brightfield illumination is the main observation method in metallographic research. The incident light is irradiated on the sample surface vertically or approximately vertically, and the reflected light from the sample surface enters the objective lens for imaging. If the sample is a mirror, it is a bright piece in the field of view, and the tissue on the sample will reflect the colored image in the bright field of view, which is called "bright field" illumination.

Advantages: high brightness and uniform field of view; wide range of applications; simple operation and low price.

Disadvantages: low-contrast specimens have low contrast; specimens have no three-dimensional effect.

Dark Field

The sample is illuminated by the periphery of the objective lens, and the illumination light does not enter the objective lens, and the image formed by the diffuse reflection light on the surface of the sample can be obtained. If the sample is a mirror surface, the light reflected from the sample is still reflected in the opposite direction at a large inclination angle, and it is impossible to enter the objective lens, and it is pitch black in the field of view. The objective lens, the tissue on the sample will be reflected in the dark field of view with a bright white image, just like the stars in the night sky, which is called "dark field" illumination.

Advantages: observe extremely small objects with a resolution of 0.02-0.004um (0.4um in bright field).

Disadvantages: only the existence, movement and external form of objects can be observed.

Polarizing

1. Natural light and polarized light

Natural light (or natural light) refers to sunlight and electric light. Its light vibration is balanced in all directions and is perpendicular to the direction of propagation. If the light vibration is limited to a certain direction in the plane perpendicular to the direction of propagation, it is called for polarized light.

2. The acquisition of polarized light

There are two common methods: polarizing prism or artificial polarizer

Polarizing Prism

Polarizing prism is a polarizing device made of crystal birefringence. Whether it is natural light or polarized light passing through a polarizing prism, it becomes linearly polarized light whose vibration direction is determined by the polarization direction of the prism.

There are many types of polarizing prisms, and common polarizing prisms include Nicol prisms, Glan prisms, and Wollaston prisms, etc.

Polarizer

When the natural light hits the artificial polarizer, the natural light becomes polarized light. At present, the artificial polarizer is generally used in the microscope.

Differential Interference(DIC)

Differential interference uses the principle of polarized light interference. The illumination light changes from a differential interference contrast prism to two beams of diffracted light, and the difference in height of the sample causes a small difference in the optical path, while the optical path difference becomes a differential interference contrast prism and a polarizer. contrast of light and dark.

Advantages: it can make the object under inspection produce a three-dimensional sense; the observation effect is more intuitive; no special objective lens is needed, and it works better with fluorescence observation; the color changes of the background and object can be adjusted to achieve the ideal effect.

Disadvantages: high light intensity is required, birefringent substances cannot achieve DIC microscopy effects, and cannot be applied to the observation of cultures in plastic containers. The microscopy sensitivity is directional and the adjustment is complicated.