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Microscope Optical System - Objective Lens
The objective lens is an important component that determines the performance of a microscope. It is installed on the objective lens converter and is c
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objectiveIt is an important component that determines the performance of a microscope. It is installed on the objective converter and is close to the observed object, so it is called an objective lens or a connecting lens. http://www.shhx17.com/shhx17/com_pro.asp
1. Classification of objective lenses
Objective lenses can be divided into dry objectives and immersion objectives according to different usage conditions;
Among them, immersion lenses can be divided into water immersion lenses and oil immersion lenses (commonly with a magnification of 90)—100 times).
Among them, immersion lenses can be divided into water immersion lenses and oil immersion lenses (commonly with a magnification of 90)—100 times).
According to different magnification factors, it can be divided into low magnification objectives（10Below magnification), medium magnification objective lens（20High magnification objective lens（40—65 times).
According to the aberration correction situation, it is divided into achromatic objective lenses (commonly used, which can correct the color difference of two colors in the spectrum) and chromatic aberration objective lenses (which can correct the color difference of three colors in the spectrum, expensive, and rarely used).
2. Main parameters of objective lens:
The main parameters of the objective lens include magnification, numerical aperture, and working distance.
① ThemagnificationIt refers to the ratio of the size of the image seen by the eyes to the corresponding specimen size. It refers to the ratio of length rather than the ratio of area. Example: Magnification factor of 100X refers to the length of1μmWhat is the length of the enlarged image of the specimen100μmIf calculated by area, it enlarges10000 times.
The total magnification of a microscope is equal to the product of the magnification of the objective lens and eyepiece lens.
② TheNumerical aperture, also known as aperture ratio, abbreviated as NAperhapsA, is the main parameter of the objective lens and condenser, directly proportional to the resolution of the microscope. The numerical aperture of the dry objective lens is0.05-0.95The numerical aperture of the oil immersed objective lens (cedarwood oil) is1.25.
③ Thework distanceIt refers to the distance from the front lens of the objective lens to the top of the specimen cover glass when the observed specimen is clear. The working distance of the objective lens is related to its focal length, with longer focal length/lower magnification/longer working distance of the objective lens.
Example: 10The objective lens is marked with10/0.25and160/0.17Among them10The magnification of the objective lens;0.25Numerical aperture;160The length of the lens barrel (in units)mm）；0.17The standard thickness of cover glass (in units)mm）.
10The effective working distance of the magnifying lens is6.5mm，40The effective working distance of the magnifying lens is 0.48mm.
Example: 10The objective lens is marked with10/0.25and160/0.17Among them10The magnification of the objective lens;0.25Numerical aperture;160The length of the lens barrel (in units)mm）；0.17The standard thickness of cover glass (in units)mm）.
10The effective working distance of the magnifying lens is6.5mm，40The effective working distance of the magnifying lens is 0.48mm.
3. The function of objective lensIt is the first magnification of the specimen, and it is an important component that determines the performance of the microscope - the level of resolution.
Resolution, also known as resolution or resolution power. The magnitude of resolution is represented by the value of resolution distance (the minimum distance between two object points that can be resolved). At a visible distance of 25cm）Places that can be clearly seen by normal human eyes0.073mmThe two object points, thisThe value of 0.073mm is the resolution distance of a normal human eye. The smaller the resolution distance of a microscope, the higher its resolution and the better its performance.
The resolution of a microscope is determined by the resolution of the objective lens, which in turn is determined by its numerical aperture and the wavelength of the illuminating light.
When using the ordinary central illumination method (bright illumination method that evenly transmits light through the specimen), the resolution distance of the microscope is d=0.61λ/NA
In the formula, d——Resolution distance of objective lens, unit: nm
λ - wavelength of illumination light, unit: nm
λ - wavelength of illumination light, unit: nm
NA——Numerical aperture of objective lens
For example, the numerical aperture of an oil immersed objective lens is1.25The visible light wavelength range is400—700nmTake its average wavelength550 nm, thend=270 nmApproximately equal to half the wavelength of the illuminating light.Generally, the limit of resolution for microscopes illuminated with visible light is0.2μm。
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