A common optical microscope is a precision optical instrument. In the past, the simplest microscope consisted of only a few lenses, and the microscope currently in use consisted of a set of lenses. Ordinary optical microscopes typically magnify objects by 1500-2000 times.
(1) Construction of the microscope
The structure of a common optical microscope can be divided into two parts: one is a mechanical device, and the other is an optical system. The two parts are well matched to play the role of a microscope.
1. Microscope mechanical device
The mechanical mechanism of the microscope includes a lens holder, a lens barrel, an objective lens converter, a stage, a pusher, a coarse spiral, a micro-screw, and the like.
(1) Mirror base The mirror base is the basic support of the microscope. It consists of two parts: the base and the mirror arm. Attached to it is a stage and a lens barrel that serves as the basis for mounting the components of the optical amplification system.
(2) Lens barrel The eyepiece is connected to the eyepiece and connected to the converter to form a dark room between the eyepiece and the objective lens (mounted under the converter).
The distance from the trailing edge of the objective lens to the end of the barrel is called the length of the barrel. Because the magnification of the objective lens is for a certain length of the barrel. The change in the length of the lens tube not only changes the magnification, but also the image quality. Therefore, when using a microscope, the barrel length cannot be arbitrarily changed. Internationally, the standard cylinder length of the microscope is set to 160 mm, and this number is marked on the outer casing of the objective lens.
(3) Objective lens converter The objective lens converter can be equipped with 3-4 mirrors, usually three objective lenses (low magnification, high magnification, oil mirror). The Nikon microscope is equipped with four objective lenses. By rotating the converter, any one of the objective lenses and the lens barrel can be turned on as needed, and an eyepiece mirror on the lens barrel constitutes an amplification system.
(4) Stage There is a hole in the center of the stage for light passage. A Spring specimen holder and a pusher are mounted on the table to fix or move the position of the specimen so that the microscopic object is located at the center of the field of view.
(5) The pusher is a mechanical device for moving specimens. It is composed of a metal frame with two horizontal and one longitudinal propellers. A good microscope is engraved with a scale on the longitudinal and transverse poles to form a very precise plane coordinate. system. If we have to repeatedly observe a part of the specimen that has been inspected, the value of the vertical and horizontal scale can be recorded in the first inspection, and then the position of the original specimen can be found by moving the pusher by numerical value.
(6) Coarse-moving spiral The coarse-moving spiral is a moving piece that adjusts the distance between the objective lens and the specimen. The old-fashioned microscope has a thick spiral forward and the lens descends close to the specimen. When a newly produced microscope (such as a Nikon microscope) is microscopically examined, the right hand twists the stage up, causing the specimen to approach the objective lens, and vice versa, and the specimen is separated from the objective lens.
(7) Micro-motion spiral The coarse-motion spiral can only adjust the focal length extensively. To obtain the clearest image, it needs to be further adjusted with a micro-screw. The microscopic spiral moves 0.1 mm (100 μm) per revolution of the lens barrel. The coarse-grained spiral and the micro-motion spiral of the newly produced higher-grade microscope are coaxial.
Magnifying glass imaging principle
An optical lens made of curved glass or other transparent material can magnify and image the object. The optical path diagram is shown in Figure 1. The object AB, located within the object focus F, has a size of y, which is magnified by a magnifying glass into a virtual image A'B' of size y'.
Magnifying glass magnification
Γ=250/f'
250--the apparent distance, in mm
f'--Magnifying glass focal length in mm
The magnification refers to the ratio of the angle of view of the object image observed with a magnifying glass within a distance of 250 mm to the angle of view of the object observed without the magnifying glass.
2, the optical system of the microscope
The optical system of the microscope consists of a mirror, a concentrator, an objective lens, an eyepiece, etc. The optical system magnifies the object to form an enlarged image of the object. See Figure 1-2.
(1) Mirrors Earlier ordinary optical microscopes used natural light to inspect objects, and mirrors were mounted on the mirror mounts. The mirror consists of a plane and another concave mirror that reflects the light projected on it into the center of the concentrator lens to illuminate the specimen. When a concentrator is used, a concave mirror can be used, and the concave mirror can converge light. When using a concentrator, a flat mirror is generally used. The newly produced higher-grade microscope mount is equipped with a light source and a current-regulating spiral that adjusts the light intensity by adjusting the current.
(2) Condenser The concentrator is under the stage, which is composed of a collecting lens, an iris diaphragm and a lifting spiral. The concentrator can be divided into a bright field concentrator and a dark field concentrator. The common optical microscope is equipped with a bright field concentrator, and the bright field concentrator has an Abbe concentrator, a Qiming concentrator and a shake concentrator. Abbe concentrators exhibit excellent difference and spherical aberration when the numerical aperture of the objective lens is higher than 0.6. The Qiming concentrator has a high degree of correction for chromatic aberration, spherical aberration and coma. It is the best concentrator in bright field inspection, but it is not suitable for objective lenses up to 4 times. Shaking out the concentrator can shake the lens on the concentrator out of the optical path to meet the needs of low magnification objective (4×) large field of view illumination.
The concentrator is mounted under the stage, and its function is to focus the light reflected by the light source through the mirror on the sample to obtain the strongest illumination, so that the object image has a bright and clear effect. The height of the concentrator can be adjusted so that the focus falls on the object to be inspected for maximum brightness. The focus of the general concentrator is 1.25 mm above it, and its rise limit is 0.1 mm below the plane of the stage. Therefore, the thickness of the required slide should be between 0.8 and 1.2 mm, otherwise the sample to be inspected is not in focus, which affects the microscopic effect. The front of the front lens group of the concentrator is also equipped with an iris diaphragm, which can be enlarged and reduced, which affects the resolution and contrast of the image. If the iris aperture is opened too large and exceeds the numerical aperture of the objective lens, a spot is generated; if the iris is contracted The aperture is too small, the resolution is reduced, and the contrast is increased. Therefore, when observing, through the adjustment of the iris aperture, the field diaphragm (microscope with field diaphragm) is opened to the outer edge of the field of view, so that objects not in the field of view can not get any light. Illumination to avoid interference from scattered light.
(3) Objective lens The objective lens mounted on the front end of the lens barrel uses light to make the object to be inspected for the first time. The quality of the objective lens imaging has a decisive influence on the resolution. The performance of the objective lens depends on the numerical aperture of the objective lens (abbreviated as NA). The numerical aperture of each objective lens is marked on the outer casing of the objective lens. The larger the numerical aperture, the better the performance of the objective lens.
There are many types of objective lenses, which can be classified from different angles:
According to the medium between the front lens of the objective lens and the object to be inspected, it can be divided into:
1 Drying objective lens Air is used as the medium. For example, the commonly used objective lens of 40× or less has a numerical aperture of less than 1.
2 oil immersion objective lens often uses cedar oil as the medium, this objective lens is also called oil lens, its magnification is 90×-100×, and the numerical hole value is greater than 1.
According to the magnification of the objective lens, it can be divided into:
1 low magnification objective lens means 1 × - 6 ×, NA value is 0.04 - 0.15;
2 medium objective lens means 6 × -25 ×, NA value is 0.15 - 0.40;
3 high magnification objective lens means 25 × - 63 ×, NA value is 0.35 - 0.95;
4 Oil immersion objective refers to 90×—100×, and the NA value is 1.25—1.40.
According to the degree of correction of the object image difference, it can be divided into:
1 Achromatic objective lens is the most commonly used objective lens, the shell is marked with the word "Ach", which can remove the chromatic aberration formed by red and cyan. Microscopic examinations are often used in conjunction with Huygens eyepieces.
2 Apochromatic objective lens The objective lens housing is marked with the word "Apo". In addition to correcting the chromatic aberration of the red, blue and green lights, it can also correct the phase difference caused by the yellow light, usually in combination with the compensation eyepiece.
3 special objective lens Based on the above objective lens, the objective lens is manufactured to achieve certain observation effects. Such as: with correction ring objective lens, with field diaphragm objective lens, phase difference objective lens, fluorescent objective lens, no strain objective lens, no cover objective lens, long working distance objective lens. At present, the objective lenses commonly used in research are: semi-apochromatic objective lens (FL), flat field objective lens (Plan), flat field apochromatic objective lens (Plan Apo), super flat field objective lens (Splan, super flat field apochromatic Objective (Splan Apo) and so on.
(4) Eyepiece The purpose of the eyepiece is to magnify the real image magnified by the objective lens and reflect the object image into the observer's eye. The structure of the eyepiece is simpler than that of the objective lens. The eyepiece of an ordinary optical microscope usually consists of two lenses. One lens at the upper end is called an "eyepiece", and the lens at the lower end is called a "field mirror." Between the upper and lower lenses or below the two lenses, an annular diaphragm made of metal or a "field stop" is mounted, and the enlarged intermediate image of the objective lens falls on the plane of the field diaphragm, so that it can be Place the eyepiece micrometer.
The eyepiece commonly used in ordinary optical microscopes is the Huygens eyepiece. For research purposes, eyepieces with better performance are generally used, such as compensating eyepieces (K), flat field eyepieces (P), and wide-field eyepieces (WF). ). A photographic eyepiece (NFK) is used for photography.
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