Definition
An optical microscope is an optical instrument that uses visible light and a lens system to magnify images of tiny objects. It is widely used in materials science, biology, geology, electronics industry and food safety testing and other fields, and is the basic and core observation tool of the laboratory.
Principle
Optical microscopes work based on the principles of geometric optics. Light is emitted from a light source and illuminated by a condenser to illuminate the sample. After light passes through or reflects off the sample surface, it carries the sample structure information into the objective. The objective lens acts as a key optical component to produce a magnified real image. The real image is further magnified by the eyepiece to form a virtual image that can be observed by the human eye. The total magnification is the product of the magnification of the objective lens and the magnification of the eyepiece. Its resolution is limited by the Abbe diffraction limit formula: d = λ / (2NA), where d is the minimum resolving distance, λ is the illumination wavelength, and NA is the numerical aperture of the objective lens. This formula suggests that increased resolution can be achieved by using objectives with shorter wavelengths of light or larger numerical apertures.
Measurement method
Depending on the illumination and imaging methods, optical microscopy derives a variety of observation methods to adapt to the characteristics of different samples. Brightfield observation is the most commonly used method, where light is directly transmitted or reflected, and is suitable for stained samples or high-contrast samples. Darkfield observation uses a special condenser to prevent direct light from entering the objective lens, allowing only scattered light from the sample to image, suitable for observing edges, scratches, or transparent particles. Phase contrast observation uses the phase difference created by light passing through different regions of the sample and converts it into an amplitude difference (chiaroscuro) to observe live cells or clear specimens without staining. Differential interference phase difference observation can produce images with a three-dimensional relief feeling, which is sensitive to surface height differences. Polarization observation uses polarized light and is used to study anisotropic materials such as minerals, crystals, or polymer fibers. Fluorescence observation uses specific wavelengths of excitation light to irradiate fluorescently labeled samples to detect the fluorescence emitted by them, with strong specificity.
Influencing factors
The final imaging effect of the microscope is influenced by a combination of factors. Resolution determines the ability to distinguish between two adjacent points, primarily determined by the numerical aperture of the objective lens and the wavelength of illumination light. Contrast is the difference in light or color between the target and the background, depending on the sample itself, the staining method, and the observation technique used. The numerical aperture of the objective lens affects not only the resolution and light collection ability, but also the working distance and depth of field. Lighting uniformity, intensity, and color temperature stability are essential for observation and photography. In addition, the quality of sample preparation, such as slice thickness, flatness, staining effect, is a prerequisite for obtaining clear images. Vibration and dust in the operating environment, as well as the operator's proficiency in focusing, can also have a direct impact on the results.
Application:
In life science research, light microscopy is used to observe cell morphology, tissue structure, and microbial activity. In materials science and metallurgy, it is used to analyze the metallographic structure, grain size, inclusions, and structure of composites. In geology and mineralogy, it is used to identify the mineral composition and structural characteristics of rock flakes. In the electronics manufacturing and semiconductor industries, it is used to inspect circuit board printing, solder joint quality, and silicon wafer surface defects. In the textile industry, it is used to analyze the morphology and damage of fibers. In the field of food safety, it can be used to detect foreign objects, microorganisms and starch particle morphology in food. The core of its application is to provide intuitive morphological and structural information for subsequent analysis.
Instrument selection
When choosing an optical microscope, the system needs to be evaluated based on the primary application needs. Start by defining the core observation method, such as whether fluorescence, phase contrast, or polarization functions are required. The objective lens is at the heart of determining optical performance, focusing on its numerical aperture, correction level (e.g., flat-field achromatic), and desired magnification. The mechanical structure of the microscope, such as the movement mode of the stage and the accuracy and stability of the focusing mechanism, are related to the convenience of operation and long-term reliability. In terms of imaging modules, it is necessary to consider the appropriate digital camera interface and pixel requirements according to the recording needs. For scenarios where multiple people need to share or teach, consider a trinocular tube to facilitate camera connection or display. In addition, the scalability of the system, such as the ability to add new illumination modules or specialized objectives, also leaves room for possible changes in requirements in the future. Ultimately, a balance between core performance, suitability, and long-term cost should be sought within budget.