How can I improve the resolution of my microscope?
Microscope as one of the main equipment for testing equipment, and the important index to judge the performance of the microscope is the resolution. Resolution refers to the ability to clearly distinguish between two small points or the smaller distance between two lines. The human eye itself is a microscope, in the standard lighting conditions, the human eye in the visual distance (internationally recognized as 25cm) on the resolution of about 1 / 10mm. for the observation of two straight lines, due to a series of straight lines can stimulate a series of nerve cells, the resolution of the eye can be increased a bit.
The resolution of the human eye is only 1/10mm, so the human eye can not distinguish objects smaller than 1/10mm or the distance between two tiny objects closer than 1/10mm. Therefore, from the simple macroscopic magnifying glass to the microscopic observation of the optical microscope, followed by the electron microscope. The resolution of a microscope is defined as the smaller distance between two small points that can be clearly distinguished on the specimen. Its calculation formula is: D = 0.61λ / NA
Where: D for the resolution (um); λ for the wavelength of the light source (um); NA for the numerical caliber of the objective lens (also known as the rate of the mirror mouth).
From the formula can be obtained, the resolution of the microscope is determined by the wavelength of the incident light source and the matching objective lens numerical aperture. It can be seen that the method of improving the optical microscope:
1, reduce the wavelength of the light source.
Visible light shorter wavelength of 390nm, if the wavelength of the ultraviolet light as an illumination source, the resolution of the optical microscope can be reduced to 0.2 um. But due to the majority of common materials, glass will be a large number of absorption of wavelengths of 340nm or less, the ultraviolet light after a large number of attenuation can not be formed after a clear, bright image. Therefore, we have to use quartz (transmits ultraviolet light as low as 200nm), fluorescent stone (transmits ultraviolet light as low as 185nm) and other expensive materials, and ultraviolet microscopy can not be observed with the naked eye, and even subject to the limitations of the observed samples, coupled with the expensive cost, so this way of improving the resolution of the microscope due to its own limitations and is not widely used.
2 The objective lens is used to increase the numerical aperture NA.
Numerical aperture NA=n*sin(u)
Where n is the refractive index of the medium between the objective lens and the specimen; u is the one-half aperture angle of the objective lens. Therefore, from the optical design of the appropriate use of a larger aperture angle, or increase the refractive index has become more common to improve the resolution of the optical microscope. General low-magnification objective such as 10X below its medium using air, refractive index of 1, that is, the dry system objective; water immersion medium is distilled water, its refractive index of 1.33; oil-immersed objective medium is cedar oil or other transparent oil, its refractive index is generally in the vicinity of 1.52, close to the refractive index of the lens and the slide, such as Olympus's 100X oil lens. Water-immersion objective and oil-immersion objective is not only high magnification but also due to the use of high refractive index of the medium, thus improving the resolution of the objective.
