Structure and Operation Instructions for Metallurgical Microscopes
1. Basic Principle of Metallurgical Microscopes
A metallurgical microscope is an optical microscope designed for examining the internal microstructure of metals and alloys.
A convex lens magnifies an object to form an image, but a magnifier composed of a single lens or one lens group has limited magnification. Higher magnification can be achieved by using a second lens group to further magnify the initially formed image. Metallurgical microscopes are designed on this principle, and their magnification imaging mechanism is illustrated in Figure 1-1.
A metallurgical microscope is fitted with two sets of magnifying lenses. The set close to the specimen is called the objective lens, and the set close to the observer's eye is called the eyepiece. The specimen AB under observation is placed slightly beyond the front focal point F₁ of the objective lens. Light reflected from the specimen passes through the objective lens and is refracted to form an enlarged, inverted real image A₁B₁, which falls exactly within the front focal point F₂ of the eyepiece. The eyepiece then magnifies this image a second time into an inverted virtual image A₂B₂ - this is the twice-magnified image observed through the microscope.
2. Performance of Metallurgical Microscopes
The core performance metrics of a microscope mainly include magnification, resolution and depth of field.
(1) Magnification (M): defined as the product of the objective lens magnification (M₁) and the eyepiece magnification (M₂). M = M₁ × M₂
(2) Resolution (d): reflects the microscope's ability to resolve the minimum distance between two points on a specimen when imaging fine microstructures. A smaller d value indicates higher resolving power, making this a key performance indicator of a microscope.
where:
λ = wavelength of incident light
A = numerical aperture (NA) of the objective lens
(3) Depth of field: also referred to as the vertical resolution of a microscope, it describes the microscope's ability to form sharp images of specimens with surface height variations or uneven topography.
3. Structure of Metallurgical Microscopes
Metallurgical microscopes are available in a variety of designs, primarily categorized into three types: bench-top, upright and horizontal. Despite their differences in form, they share essentially the same basic structure, typically consisting of three major systems: the optical system, illumination system and mechanical system. Some microscopes are further equipped with photographic units, polarized light attachments and other accessories to extend their functionality.
The structure of the domestic Model 4X bench-top metallurgical microscope is shown in Figure 2. This type of microscope is widely applied thanks to its compact structure and straightforward operation.
