What techniques can be used to increase the microscope's resolution?
One of the primary testing tools is the microscope, and resolution is a crucial metric for assessing the tool's performance. Resolution is the capacity to discern between two lines or small points distinctly at close range. The eye itself functions as a microscope. The resolution of the human eye at the distance of vision, which is universally accepted to be 25 cm, is around 1/10mm under normal lighting circumstances. Because straight lines can excite a number of nerve cells, the resolution of the eyes can be increased while watching two straight lines.
Since the human eye's resolution is only 1/10mm, it cannot discern between two extremely small objects that are closer than 1/10mm apart. So the development of the optical microscope for microscopic inspection came first, followed by the development of the electron microscope. The shortest distance between two tiny spots that can be clearly distinguished on a specimen is referred to as the microscope resolution. D=0.61/NA is its calculating formula.
In the formula: D is the resolution (um); λ is the wavelength of the light source (um); NA is the numerical aperture of the objective lens (also called the aperture ratio).
It can be obtained from the formula that the resolution of the microscope depends on the wavelength of the incident light source and the numerical aperture of the matched objective lens. It can be seen that the method of improving the optical microscope:
1. Reduce the wavelength of the light source.
The shorter wavelength of visible light is 390nm. If ultraviolet light of this wavelength is used as the illumination source, the resolution of the optical microscope can be reduced to 0.2um. However, since the glass of most common materials absorbs a large amount of light with a wavelength below 340nm, the ultraviolet light cannot form a clear and bright image after a large amount of attenuation. Therefore, expensive materials such as quartz (which can pass through ultraviolet light as low as 200nm) and fluorite (which can pass through ultraviolet light as low as 185nm) have to be used, and the ultraviolet light microscope cannot be observed with the naked eye, and even the observed sample Due to the limitations of the microscope, coupled with the high cost, this method of improving the resolution of the microscope is not widely used due to its own limitations.
2. Increase the numerical aperture NA of the objective lens.
Numerical aperture NA=n*sin(u)
In the formula, n is the refractive index of the medium between the objective lens and the specimen; u is the half aperture angle of the objective lens. Therefore, from the perspective of optical design, appropriately adopting a larger aperture angle or increasing the refractive index has become a common method to improve the resolution of an optical microscope. Generally, the medium of low magnification objective lens such as below 10X is air, and its refractive index is 1, that is, dry objective lens; the medium of water immersion is distilled water, and its refractive index is 1.33; the medium of oil immersion objective lens is cedar oil or other transparent oil, its refractive index is average Around 1.52, close to the refractive index of the lens and glass slide, such as the 100X oil lens of Olympus. The water immersion objective lens and oil immersion objective lens not only have high magnification but also improve the resolution of the objective lens due to the use of high refractive index media.
