Detailed explanation of ordinary optical microscope knowledge: structure
Ordinary optical microscope is a precise optical instrument. Whereas the simplest microscopes in the past consisted of just a few lenses, the microscopes in use today consist of a set of lenses. Ordinary optical microscopes can usually magnify objects 1500-2000 times.
(1) The structure of the microscope
The structure of an ordinary optical microscope can be divided into two parts: one is a mechanical device and the other is an optical system. Only when these two parts cooperate well can the microscope function.
1. The mechanical device of the microscope
The mechanical device of the microscope includes the lens holder, the lens barrel, the nosepiece, the stage, the pusher, the coarse motion screw, the fine motion screw and other components
(1) Mirror base The mirror base is the basic bracket of the microscope, which consists of two parts: the base and the mirror arm. The stage and the lens barrel are attached to it, and it is the basis for installing the components of the optical magnification system.
(2) Lens barrel The eyepiece is connected to the top of the lens barrel, and the converter is connected to the bottom to form a dark room between the eyepiece and the objective lens (installed under the converter).
The distance from the trailing edge of the objective lens to the rear end of the lens barrel is called the mechanical barrel length. Because the magnification of the objective lens is relative to a certain length of the lens barrel. The change in the length of the lens barrel not only changes the magnification, but also affects the image quality. Therefore, when using a microscope, the length of the lens barrel cannot be changed arbitrarily. The standard barrel length of the microscope is set at 160mm internationally, and this number is marked on the shell of the objective lens.
(3) Objective lens converter The objective lens converter can be installed with 3-4 objective lenses, generally three objective lenses (low magnification, high magnification, oil lens). Nikon microscopes are equipped with four objective lenses. By rotating the converter, any one of the objective lenses and the lens barrel can be connected as required, forming a magnifying system with the eyepiece on the lens barrel.
(4) Stage There is a hole in the center of the stage, which is the light passage. There are spring specimen clamps and pushers on the stage, which are used to fix or move the position of the specimen, so that the microscopic object is just in the center of the field of view.
(5) The pusher is a mechanical device for moving the specimen. It is composed of a metal frame with two propelling gear shafts, one horizontal and one vertical. A good microscope has a scale scale engraved on the vertical and horizontal frame rods, which constitutes a very precise plane coordinate. Tie. If we need to observe a certain part of the inspected specimen repeatedly, in the first inspection, we can write down the value of the vertical and horizontal scales, and then move the pusher according to the value to find the position of the original specimen.
(6) Coarse-moving screw Coarse-moving screw is a mechanism that moves the lens barrel to adjust the distance between the objective lens and the specimen. In old-fashioned microscopes, the coarse screw is twisted forward, and the lens descends to approach the specimen. When a newly produced microscope (such as a Nikon microscope) is used for microscopic inspection, the stage is twisted forward with the right hand to raise the stage to allow the specimen to approach the objective lens, and vice versa, the specimen falls away from the objective lens.
(7) Micro-movement screw The coarse-movement screw can only adjust the focal length roughly. To get the clearest object image, the micro-movement screw needs to be used for further adjustment. The lens barrel moves 0.1 mm (100 microns) per revolution of the micro-spiral. The coarse- and fine-moving helices are coaxial in newer, higher-grade microscopes.
The imaging principle of magnifying glass
An optical lens made of glass or other transparent materials with a curved surface can magnify and image objects. The optical path diagram is shown in Figure 1. The object AB located within the focal point F of the object side, and its size is y, is formed into a virtual image A'B' of size y' by the magnifying glass.
magnification of magnifying glass
Γ=250/f’
In the formula, 250--photopic distance, the unit is mm
f'-- the focal length of the magnifying glass, in mm
The magnification refers to the ratio of the viewing angle of the object image observed with a magnifying glass to the viewing angle of the object observed without a magnifying glass within a distance of 250mm.
2. The optical system of the microscope
The optical system of the microscope consists of a reflector, a condenser, an objective lens, an eyepiece, etc. The optical system enlarges the object and forms an enlarged image of the object. See Figure 1-2.
(1) Reflector The earlier ordinary optical microscope used natural light to inspect the object, and the reflector was installed on the mirror base. The reflector consists of one flat and another concave mirror that reflects light projected on it to the center of the condenser lens, illuminating the specimen. Concave mirrors are used when the condenser is not used, and the concave mirrors can condense the light. When using a condenser, a flat mirror is generally used. The newly produced higher-grade microscope lens holder is equipped with a light source and a current adjustment screw, which can adjust the light intensity by adjusting the current size.
(2) Condenser The condenser is under the stage, which is composed of a condenser lens, an iridescent aperture and a lifting screw. Condenser can be divided into bright field condenser and dark field condenser. Common optical microscopes are equipped with bright-field condensers. Bright-field condensers include Abbe condensers, Zimmer condensers and shake-out condensers. Abbe condensers exhibit chromatic and spherical aberrations at objective numerical apertures higher than 0.6. The Ziming condenser has a high degree of correction of chromatic aberration, spherical aberration and coma aberration, and is the condenser with the best quality in bright field microscopy, but it is not suitable for the objective lens below 4 times. Swing out the condenser can shake the upper lens of the condenser out of the light path to meet the needs of low magnification objective (4×) large field of view illumination.
The condenser is installed under the stage, and its function is to focus the light reflected by the light source through the reflector on the sample, so as to obtain the strongest illumination, so that the object image can be bright and clear. The height of the condenser can be adjusted so that the focus falls on the object to be inspected for maximum brightness. The focal point of a typical condenser is 1.25mm above it, and its rise limit is 0.1mm below the plane of the stage. Therefore, the thickness of the required glass slide should be between 0.8-1.2mm, otherwise the sample to be inspected will not be in focus, which will affect the effect of microscopic inspection. The front of the condenser front lens group is also equipped with an iridescent aperture, which can be opened up and down, which affects the resolution and contrast of imaging. If the aperture is too small, the resolution decreases and the contrast increases. Therefore, when observing, through the adjustment of the iris diaphragm, the field diaphragm (microscope with the field diaphragm) is opened to the circumscribing of the periphery of the field of view, so that objects not in the field of view can not get any light. Lighting to avoid interference from scattered light.
(3) Objective lens The objective lens installed on the converter at the front end of the lens barrel uses light to image the inspected object for the first time. The imaging quality of the objective lens has a decisive influence on the resolution. The performance of the objective lens depends on the numerical aperture (numerical aperture abbreviated as NA) of the objective lens. The numerical aperture of each objective lens is marked on the housing of the objective lens. The larger the numerical aperture, the better the performance of the objective lens.
